Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power travels from the power plant to your house through an amazing system called the power distribution grid. All commercial electrical generators of any size generate what is called 3-phase AC power. The 3-phase power leaves the generator and enters a transmission substation at the power plant. For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. The voltage from transmission grid is stepped-down to mains voltage (120V or 230V AC) before it enters typical house. A typical house needs only one of those phase (in some countries 3 phases to house is common).
Basically mains wiring (110/120V or 230V) is relatively simple to wire and connect and does not require a lot of special equipment or handling. But that wiring needs to be right, because wrong wiring can cause fires and kill people. You can virtually eliminate most dangers with a little knowledge and proper safety practices. Safety is of utmost importance when working with electricity. Be very careful with electricity. So if you are not confident in wiring up mains sockets, get some one who is, or better still, get a qualified electrician in to do it for you. And even if you know how to do it correctly, then getting a qualified electrician to do that might be a good idea for potential liability and legal reasons (in many countries making electrical installion is very regulated who can do what).
Electrical codes codes around the world require wire that is well insulated, right size for the application (generally thick enough that it does not heat too much), has enough physical protection for the application and has right wire colors in it. Many different wires are used in mains wiring for different applications. The size of the wire determines the current it can carry and the insulation the maximum operating voltage. The most commonly used interior wiring in USA is a 12 or 14-gauge NM (nonmetallic) sheathed cable, sometimes called "Romex." Within the cable are plastic-coated copper wires, colored for each function. In Europe the wiring is genrally done using quite similar cable which has wires with thickness of 1.5mm^2 or 2.5mm^2.
Safe electrical installation demands many safety devices. Fuses and circuit breakers are safety devices. Overloading electrical circuits is extremely dangerous and should not be permitted at any time. The main job of the fuse is to protect the wiring. Fuses should be sized and located to protect the wire they are connected to. Circuit breakers and fuses are protective devices that control the power going to a particular route of wiring. In case of an overload or a short on that circuit, the breaker trips and automatically shuts off power to that circuit. Ground fault circuit breakers offer protection against more than just overloads.
Any house that has been properly wired by a qualified electrician will have a circuit breaker panel that are used to shut circuits off in the event that they draw too much current. It is the current capacity of circuit breaker (in amperes) that determines how much current a circuit can supply. The breaker size is chosen relative to the type of cabling and connector used for the circuit, as each have different capacities.
When making repairs to the electronic wiring system or equipment that part of wiring of equipment needs to be disconnected from the mains voltage so that there are no dangerous voltage present when the work is done. This is the general case in all electronics and electrical repairs (there are very few exceptions what is allowed to be done when power is applied).
In in order to disconnect the some form of reliable method of isolation between the mains network and place to be repaired needs to be provided. There are many specific safety related regulations on this kind of power disconnecting devices. The disconnecting device can be a detecheable plug (like in normal mains equipment), a special dafety switch of some form or in some cases a removable fuse.
No electronic switching device fulfils this safety isolation requirement. The reasons for this is that in those there may also be a measurable and tangible voltage at the motor terminal even if the switching device is switched off. In additional a typical failing mode of most electronic switching devices is that the output remains energized even if the control signal is removed!
Remember the safety when using electricity. Do not do anything you do not have total confidence in your ability to do when working with electicity. There are many potential dangers. When wiring up electrical outlets, if you reverse the hot and the neutral lines, you can actually create a lethal voltage potential between the outlets. If you should ever run wiring in your house, you need to make sure that the breaker that you use matches the capabilities of the wiring.
When wiring anything the power must be turned off to work safely. So when doing the wiring, turn of the power and make sure it is off with measuring instruments to be sure that the wires do not have power in them. Make also sure that nobody else can turn on the power while you or somebody else is working with the wiring. Professional electricians will put a little warning sign over any breakers, switches, etc. that are shut off that says essentially, "if you turn on power here, you'll kill someone." Make sure you have one. In many plases the electricians hang a lock to the electrical panel to lock out the main power switch and/or the breaker of the circuit they are workign with. The sign describe is attached to the lock and has your picture on it, contact information, and contact information for your supervisor and employer. The technicians would put padlocks that only they had keys for on switches when they powered something down. If you have not had the correct training, you can not safely lock out equipment.
For safety reasons you should stay far away from the main electrical distribution panel unless you are knowledgeable in this kind of things. That thing is dangerous, there are many non obvious mistakes you can make if you are not an experienced electrician. You can hurt yourself, burn down the building, damage stuff attached to the electrical system, and can hurt even someone working for the power company outside the house. There is a reason why good industrial electricans charge a lot for their services: they work with dangerous stuff, and they know what they are doing. It is illegal in most places to do the work with electrical wiring and panels unless you're a licensed electrician.
Single phase power has the mains voltage (typically 120V AC or 230V AC depending on the country) between two wires: live and neutral. The frequency of DC voltage is 50 or 60 Hz depending on the country. Single-phase power is used in very many applications, for example to power all typical home electrical appliances. You get single-phase power from normal elecrical outlet in home. Distributing single-phase power takes two wires: live and neutral. In some cases an extra safety ground wires is used to provide increased user safety.
In three phase power system the generator the generates electricity produces three voltages. Each voltage rises and falls at the same frequency (50 Hz or 60 Hz depending on the country). However, the phases are offset from each other 120 degrees. Electrical utilities generate and transmit three-phase power. Commercial electrical generators of any size generate three-phase AC power. The 3-phase power leaves the generator and enters a transmission substation at the power plant. Three phase power is commonly found in industrial applications and electrical distribution. Three-phase electrical generation is very common and is a more efficient use of conductors than other systems. Three phase power is particularly useful in AC motors, where it can be used to generate a rotating magnetic field easily and efficiently. Practically all large electrical motors used in heavy industry use three phase power.
Three phase power distribution saves copper for the following reason: At the load end of the circuit the return legs of the three phase circuits can be coupled together at the neutral point, where the three currents sum to zero. This means that the currents can be carried using only three cables, rather than the six that would otherwise be needed. In practical applications the three phase power is wired either with only three phase wires or three phase wires plus neutral wire systems. In addition to those there can be a separate safety ground wire.
Three phase 230V/400V is the standard way for three phase power distribution in Europe to homes. The ouput from mains transformer is Y-conneted. There is 230V AC from each phase to neutral and 400V AC from phase to phase. The normal 230V electrical outlets are wired between neutral and one phase. Large high power loads use all three phases (the indivudual loads in such equipment can be phase to neutral or phase to phase as needed).
Within the European Community the mains voltage is currently 230V +10/-6% (50Hz) between the LIVE and the NEUTRAL terminals, together with a separate protective EARTH terminal. In USA two live (hot) wires each separately provide 120 volts (60 Hz) relative to the neutral wire and go to wall outlets to run low power devices (lights, TVs etc.). In USA permantly wired power hungry devices like electric stoves, water heaters and some air conditioners which require 240 volts are connected across the two live or hot wires.
In the rest of the world various supply schemes are employed, ranging in voltage from about 100V to around 250V. The mains frequency can vary being typically 50 Hz or 60 Hz. The grounding arrangements and some other safety details vary from country to country.
European standards are different from US standards because they are intended for use in different overall regimes. Often the concepts for safety in US standards and European standards are simply different, and rely on differences in the surrounding environments for even similar products. Wiring, earthing, field terminations, power distribution schemes etc. are simply different, and are not under the control of the organizations who write standards.
There are historical reasons for those differences. In the mains frequency issues the reasons for the following: Many frequencies were used in the 19th Century for various applications, with the most prevalent being the 60 c/s supplied by Westinghouse-designed central stations for incandescent lamps. The development of a synchronous converter which operated best at 60 cycles encouraged convergence toward that standard. Around 1900, the introduction of the high-speed turbine led to settlement on two standards: 25 cycles for transmission and for large motors (this had been a compromise decision at Niagara Falls), and 60 cycles for general purpose systems. Meanwhile, in Germany, AEG (which used 50 cycles) had a virtual monopoly, and this standard spread to the rest of the continent.
The selection of mains voltage: It appears that the 120 were chosen somewhat arbitrarily. Edison came up with a high-resistance lamp filament he thought desirable to keep distribution losses down. The voltage of the original electrical systems were determined by the number of light bulbs in a string, obviously because at that time the only thing connected to the electrical system were light bulbs. So around 110V (110-120V) was chosen because it was a convenient number of lights. In 1882, he applied for patents on a 3-wire system which gave 220v transmission with 110v lamps. The Japanese took it one notch lower, they standardized on 100VAC.
In Europe it happened so that 220V was considered to be suitable to be distributed directly to the consuming devices. UK happened to choose a little bit higher voltage 240V. European standardization has lead now to situation that the whole Europe has migrated to 230V standard (230V +- so much that both 220V and 240V stay within the limits).
Both 120V and 230V systems works in real ligfe use and have proven to be good and safe enough. 120VAC works just fine as general purpose distribution system, it just need somewhat more copper to transfer the same power. And it's safer, withless potential for shock. 230V system wil build the electrical distribution wiring somewhat cheaper and is better in powering high power equipment. Besides, in the U.S. anyone can have 240VAC just by installing both phases to the same socket (special 240V socket used typically by air conditiong devices).
When connecting equipment to outlets on different country you need to check the voltage available before plugging the device in. Usually different countries have different types of electrical outlets so uusally you can't plug your equipment in without a suitable plug adapter. But when thinking of using a plug adapter, be sure to knwo what you do so that you don't try to plug an equipment to a wrong voltage outlet. This means that when appliances made for use in North/South America (for 120V AC) are plugged into a 220-240V outlet, the universal motors in many appliances go faster than it was designed to, damaging or destroying the appliance. Also the equipment that are designed to heat something will heat up at much higher power than they are designed to meaning damage to the device. Devices with electronics in then can also be severely damaged because much higher voltages than they are designed to gets to the device. Depeding on the case 120V AC equipment plugged to 220V will cause burned fuse and/or severu damaged equipment. order to use a North/South American 110/125V appliance abroad, it is necessary to convert (or Stepdown) the 220/250 volt electricity to 110/125 volts with either a converter or transformer.
Things do not work to other way. If you plug equipment designed for 220-240V operation to 120V AC outlet in USA, it will not work properly. Usually the equipment does not get damaged in the same way as if ypu plug equipment to higher voltage, but damages are possible. Appliances made for use in countries other than the Americas and rated 220-240 Volts AC when used in a 110-120 Volt Alternating Current Country will need to convert (or Step Up) the 110-120 Volt electricity to 220-240 Volts with either a Step Up Converter or Transformer.
An increasing number of hair dryers, clothes steamers, computers, and travel irons are dual voltage (110/240V). These dual voltage appliances can be used in countries with either 110-120 or 220-240V AC currents. You do not need to use a converter or transformer with these appliances. You may still need an adapter plug to plug the appliance into the wall outlet. To determine if your appliance is dual voltage, look for a 110/220 voltage switch or a label on the back of the appliance that reads 110/220V or 120/240V. The rating plate on your appliance will also indicate 110/220V or 120/240V if it is a dual voltage unit. If there is a voltage switch, always select the proper voltage setting for the country you're in before plugging the appliance into the electrical outlet. Nowadays there are also meny small electronics appliance power supplies that can handle both 110V and 220V voltages without any manual switching. Those devices are designed in such way that the devices will either automatically to switch to right voltage seting or are capable of taking any voltage from 100V to 240V AC (many laptop and digital camera power supplies are built in this way).
If you are bringing equipment from USA to Europe, remember the following things:
Today many information technology equipment use an IEC (CEE22) 10 amp rated three pin power input connector. This connector is then wired to the mains plug through suitable mains cable which plugs to mains plug in the country the device is operated. So those devices can be easily adpated form cointry to country which uses same mains voltage by simply changing the mains cable. The equipment which have switchable or automatic multi voltage power supply can operate in both 120V and 230V AC coutries. Most popular Notebook, Laptop and Handheld PCs have mains adaptors or chargers, with removable power-cords. On laptop adapters the connector for mains cords are usually IEC (CEE22) 10 amp connector or smaller IEC320 C5 type connector (used by IBM and Compaq).
For most other equipment many companies sell power-plug adaptors designed to provide complete inter-changeability between power plugs and sockets all around the world. Those are ideal for equipment with fixed power-cords, or for people who simply prefer the convenience of an adaptor, they are equally suitable for visitors from abroad, as well as for travellers going overseas. Nowadays there are many small gadgets with the universal wall converters, you can plug into either voltage and it will work without any hassles.
Electrical system design is a compromise between safety and cost. Much of the world considers 220 V (220-240V) to be safe enough for standard residential outlets and lighting. Within the European Community the mains voltage is currently 230V +10/-6% (50Hz) between the LIVE and the NEUTRAL terminals, together with a separate protective EARTH terminal. When this high voltage is developed across the human body it could gives rise to a fatal electric shock. Therefore you MUST NOT under any circumstances simultaneously touch both the LIVE and the NEUTRAL terminals or you are very likely to die.
Those countries which use 120V considered that 220V to be to dangerous for most residental uses. For example USA, Canada and many other countris have selected 120V AC. This 120V AC voltage is still high enough to be able to cause fatal electric shock if you touch both live and neutral wires at the same time.
Remember that electric shocks can be fatal, even for voltages of 50 V, and that most of the resistance of the body is in the skin, so do not handle electrical apparatus with wet or even damp hands. Electricity kills a great many people worldwide every year. A current of 50mA (barely enough to make a low wattage lamp even glow) is sufficient to send your heart into a state called "ventricular fibrillation", where the heart muscles are all working out of synchronisation with each other. Little or no blood is pumped, and you will die within about 3 minutes unless help is immediately at hand. To avoid this kind of things to happen, the electrical installations and devices should be built in such way that people don't come in touch with the dangerous voltages. Different safety measures and standard exist for this.
Insulation and grounding are two recognized means of preventing injury during electrical equipment operation. Conductor insulation may be provided by placing nonconductive material such as plastic around the conductor. Grounding may be achieved through the use of a direct connection to a known ground such as a metal cold water pipe.
The sole purpose of a safety ground in electrical wiring is to protect against hazardous fault currents - if there can be no fault than a ground is never needed. In theory the safest electrical supply is one that is totally isolated from its environment. In such a case you can safely connect yourself to any part of the live circuit since there is no return path to carry a current through your body. When you touch that isolated circuit, it is no longer isolated but is tied to ground at the point of contact, with your body as the potential fault current path. If a fault has previously occured that caused another part of the circuit to be shorted to ground then a return path will already exist, you will complete the circuit and a current will pass through your body. Whether the effect will be negligible, painful or fatal will depend only upon the fault impedences, potential difference and the current capacity of the supply.
Floating supplies are permissible in certain circumstances. For example in some places bathroom shaver sockets are isolated or even use this system - but the supply is provided by a current limiting isolation transformer. Floating supply is also recommended for medical life-support equipment where risks to human life due to an interruption of the supply are dominant. Floating supply in form of safety isolation transformer is also used in electronics laboratories to isolate the electronics equipment being tested or repaired from the mains supply.
There is one wirign methid that uses ungrounded power supply, it is called TT wiring. In TT wirign system you have to fit an alarm that detects the first fault to earth & an RCD system to cope with any further faults to earth. You normally only bother with this system in very special situations.
Leakage to 'ground' is a very common occurrence and can arise due to insulation failure, cable damage, water ingress, breakdown of capacitors etc etc, all of which are particularly likely in a mobile, temporary installation with a large amount of equipment and huge quantities of cabling running over metal edges on a truck. It can also occur capacitively - insulation acts as a dielectric and capacitance increases with area and so may become significant with large cable runs. Because of those risks, the normal electrical distribution safety is generally based on grounding. Most of the time, earthing everything in sight will work. Very occasionally, it doesn't. It depends on the detailed design of the earthing system. Sometimes the safety level is expanded with other safety devices. RCDs detect an imbalance in the live and neutral currents. 30mA is usual, as not being life threatening. This is always indicative of a fault situation, but the current may be going anywhere.
In most cases you puff and bluster to your hearts content about the theoretical safety of a totally isolated power installation but the fact remains that insulation faults can and do occur and if, as a result, someone were to be injured or electrocuted then you as the specifier, installer or user would be morally, legally and financially liable.
Operator Exposure safety details. Operators shall not be exposed to:
The operator(s) shall be protected from electrical and mechanical hazards by one or more of the following:
Here are some tips for good electrical safety:
European standards are different from US standards because they are intended for use in different overall regimes. Often the concepts for safety in US standards and European standards are simply different, and rely on differences in the surrounding environments for even similar products. Wiring, earthing, field terminations, power distribution schemes etc. are simply different, and are not under the control of the organizations who write standards.
When working with electronic devices (repairing etc.) switch then off and disconnect from the mains. When you need to test live circuits, use properly sheathed probes and power the device through protection device such as isolation transformer. When working with mains voltage or higher voltage, make sure that there is someone else in the room and that he or she knows what you are doing.
In normal operation electronics devices are designed such that they are safe to use. The insulation inside electronics devices must be good enough to withstand the majns voltage and overvoltage links. Even though there is insulation, there is always some leakages and potential for failures. Class I devices are designed to have grounded metal case, which keep the leakage out of reach and burns mains fuse if there is short circuit to case. Class II equipment are designed to work without grounding. They have thicker insulation in wires and components connected to mains. Leakage current from Class II equipment is limited low so that it is safe to touch, and I think we don't have to care of electric shock too much when using correctly designed Class II equipment alone.
Electrical safety cannot be over emphasised. When workign with electricity, make sure that you find out the legal requirements in your country, and don't do anything that places you at risk - either from electrocution or legal liability. Neither is likely to be a pleasant experience.
The circuits which you are making connection must not be energized when you wirk with them. So you need to turn off the power before workign with the wiring. If you are working on the wiring of a circuit (not just connecting a device to outler or simila), in repairing it, you should use a lock-out/tag-out system for your safety. Turn off the breaker(s) or remove fuses that affect the circuit and using a proper lock device with a key that only you have, lock it and tag it with your name, date and reason why it is locked out. No one can randomly turn it back on w/o your knowledge since they would have to get the key from you. I which case, YOU are the only one that should be the one to reenergize that circuit. You are then the one responsible for whether or not the circuit is safe to reenergize.
A number of years ago an awareness and concern about the effectsof power frequency EMFs arose. For a number of years it had been known that electrical workers that were in close proximity to very strong magnetic or electric fields sometimes "saw" flashing lights or patterns that were supposed to be due the action of these fields on the nervous system. This was obviously evidence of the fact that EMFs could have some direct effects on a human directly, but little attention was paid other than as a curiosity. Today the average home or office is literally full of field producing devices. While the jury is still out on the biological effects of these fields on the human body, there is still sufficient evidence, both observed and anecdotal, that may be significant. It might be wise to take at least some precautions to try to minimize the production of these fields in new designs, and to check existing equipment for the presence of EMFs.
Electric fields are not very strong in most parts of a house. High electric-field areas are found near TVs, computer monitors (including laptop computers), fluorescent lights, light dimmer controls, and improperly grounded equipment. Electric fields are measured in (V/m). Also the frequency of the field (Hz) is important.
Magnetic fields are much more common in the home than are electric fields. Most of the recent health concerns have been about magnetic fields. Any wire that carries an AC electrical current produces magnetic fields. However, two wires are required to carry power to an appliance, and if the two wires are bundled parallel and very close together, the magnetic field from one will exactly cancel the field from the other. Thus, an extension cord rarely produces much magnetic field. Electromagnetic fields are measured in Tesla (nT) or Gauss (G). Also the frequency of the field (Hz) is important.
Many experts nowadays agree there could a link between magnetic fields and some diseases. Laboratory studies have shown that electrosmog (electrical and magnetic fields) can affect living cells but it is unclear whether these effects are harmful. Some epidemiological studies have reported a possible link between electrosmog exposure and cancer. Other studies indicate that continuous exposure to levels as low as 2 Milligauss (mG) may be harmful. Current research is expected to provide more answers about potential health effects within the next few years (answers is there effect or not). There are some thoughs that magnetic fields from power lines could linked with cancer.
A wide variety of EMF protective devices are on the market these days, despite lots of medical advice saying there is no hard evidence to prove the problem even exists.
The purpose of the electrical system in a house is to distribute the power safely to all of the different rooms and appliances.
The mains wiring is generally built using insulated copper cables. The choice of conductor material is a compromise among electrical properties, mechanical properties, and price. From the start, copper has been the material of choice for household branch circuits. Aluminum is softer than copper and weaker, and a poorer electrical conductor, so is not widely used in small sizes for home wiring. Aluminium cable material is sometimes used (for economical reasons) for thick mains feeder cables coming from electrical utility to the mains distribution panel.
Many precautions are needed to make occurence of short circuits unlikely, because the very high currents caused by short circuit situation can cause lots of damage to electrical installation. Protective devices are needed to break short-circuit and overload currents.
One type of situation that wiring needs to be protected against is overcurrent. The electrical wiring is rated for certain maximum current. If you try to pull more current through it, the wiring will heat considerably. When the wiring heats too much, it will cause the meltíng of cable insulation, cause fire if there is something flammable near cable and even melt the copper conductors in the cable. So protection is needed to guarantee that in case of something tries to pull too much current through mains wiring, this cannot happen for any long time until the fuse blows and stops the current. Fuses and circuit breakers protect nicely agains overcurrent.
Many people are familiar with a “short circuit,” which is a type of fault that occurs when two conductors of an electric circuit touch each other. The current flow caused by a short circuit is usually high and rapid and is quickly detected and halted by conventional circuit protective devices, such as fuses or circuit breakers.
Ground faults are one type of problem when the insulation fails. When mains power line connects directly to ground, its goal in life is to pump as much electricity as possible through the connection. If there is nothing to stop that, either the device or the wire in the wall will burst into flames in such a situation. A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation, thus stopping the current from flowing through the wire (thus stops the dangerous short circuit situation).
An arc fault is a high power discharge of electricity between two or more conductors. The arc faults of our concern occur in major electrical distribution systems where there is considerable current available. While a low power arc of a few amps may initiate an arc fault, a true arc fault will rapidly increase in current up to several hundred amps or even thousands of amps. A large arc fault can cause a large electrical switchboard to be reduced to an empty shell in a few seconds. This is caused by the fact that in this kind of short circuit situation there is very much power available from electrical distribution system, and this power will generate lots of heat, that will burn anything near (here withing the electrical switchboard). The chance of Arc Faults in electrical switchboards can be reduced by proper design.
In home wiring an arc of a hundred watts can set wallboard, carpet, cloth, wooden studs, etc on fire. This kind of arc can be caused by bad wiring (broken isulator in the wiring, bad wire terminations etc.) This kind of arc fault can be defined as an unintentional electrical discharge characterized by low and erratic current that may ignite combustible materials. The US Consumer Produce Safety Commission states "Problems in home wiring, like arcing and sparking, are associated with more than 40,000 home fires each year". An arc fault, however, is characterized by the low and erratic flow of electricity. Due to these types of characteristics, arc faults occurring in damaged electrical cords or cable can continue undetected by conventional circuit protective devices. This leads to hazardous situations such as igniting of nearby combustible materials. Fuses and circuit breakers cannot detect low level arcs, so special protection devices are sometimes used to detect them and top current flowing when arc is detected. The UL code states "By recognizing characteristics unique to arcing and functioning to de-energize the circuit when an arc-fault is detected, AFCI's further reduce the risk of fire beyond the scope of conventional fuses and circuit breakers."
Circuit breakers and fuses are protective devices that control the power going to a particular route of wiring. In case of an overload or a short on that circuit, the breaker or fuse trips and automatically shuts off power to that circuit. Fuses are the commonly used protection devices to protect components like wires, transformers electronics circuit modules against overload. The general idea of the fuse is that it "burns fuse link" when current gets higher than it's rating and thus stops the current flowing. A device called Miniature Circuit Breaker (MCB) is is used to replace fuses in electrical distribution systems (mains electrical panel). Miniature circuit Breaker can be is used in lighting distribution system or motor distribution system for protecting overload and short-circuit in the system. Miniature circuit breaker has a "switch" on it, so it cam be used for overload and short-circuit protection as well as for unfrequent on-and-off switching electric equipment and lighting circuit in normal case.
For MCBs there are two characteristics that determine at what point it will trip. The first is the rated current, and the second is the class of breaker. The breaker will hold the rated current forever. The class of breaker determines how large a surge current the breaker will allow without tripping. Breakers are defined into classes by EN 60898. Type B and C breakers are likely to be suitable for general use (but an electrician will need to do the calcs to check), with B being the more sensitive type.
The breakers used in mains distribution networks need to have very high current breakage rating because the short circuit currents supplied by the mains network (distirbution wiring, transformers etc.) can be very high. Most transformer fuses will 'let through' whatever the transformer and system impedance will allow. Fuses and circuit breakers (excepting some special types) take longer than 1/2 cycle to interrupt a fault, so they will see the full fault current available through the source impedance. In low distance distribution wiring going to the house this short circuit current can be easily in the range of 5-10 kA. The main fuse and breakers mut be capable of stopping this current when needed.
Ground fault circuit breakers (combination of circuit breaker and RCD) offer protection against more than just overloads. The Residual Current Device(RCD) is a special electronic/electromechanical protection device that cuts off the fault circuit immediately on the occasion of shock hazard or earth leakage of trunk line. Earth Leakage Circuit Breaker (ELCB) is mainly prevent eletric fire and personal casualty accident caused by personal electric shock or leakage of electrified wire netting. Residual Current Circuit Breakers (RCCB) is similar to Earth Leakage Circuit Breaker (ELCB). In mains wiring earth leakage circuit breaker is used for the protection,against electrical leakage in the circuit of 50Hz or 60Hz. When somebody gets an electric shock or the residual current of the circuit exceeds the fixed value, the ELCB/RCCB can cut off the power.
RDC's are rated by the current differential required to trip them. RCD trips if the difference between line and neutral current exceeds a preset value (30mA is common, but 5 mA, 10mA, 30mA, 100mA, 300mA and 3A types are available). Trip times are usaually specified as less than 30ms, but delay types (to provide sub circuit discrimination) are available. RCDs are implemented usually in such way that phase (live) and neutral wires pass through a sensor coil. If currents are equal there is no net magnetic field in the coil. There should be 0A differential between hot and neutral if there is no leakage to ground. If live ant neutral currents are uneqal because some current is leaking to earth, a voltage is induced in the coil and that activates a circuit breaker. Simplest RCDs have just a toroidal transformer, the L and N being monitored being fed through the middle, with the secondary feeding a trip solenoid that trip the switching element. Some more complicated ones have electronics in them to process the input signal (for example very sensitve ones and ones which sense also pulsatung DC faults). Sometimes different RCD types are classified to different classes: AC, A and B. 'normal' Class AC devices are intended for use with pure sinusoidal residual currents. Class A devices should be used if the residual current includes pulsating DC components. Class B is used when the current is DC or impulse DC.
RCD with 30mA rating is typical for a single 230V circuit. RCDs with 30mA residual current sensitivity are generally used for property and person protection. RCDs with a residul sensitivity of 100mA to 300mA are recommended for property and equipment protection (particarly where numerous items of equipment are supplied through the protected circuits). Please note that RCDs cannot protect people from serious shock which could occur if they contact both live and noutral conductors at the same time (RCs protect only against live to earth faults). RCDs cannot substitute for care, commonsense and regular maintenace. RCDs are no substitute for fuses or circuitbreakers (for complete protection both a combination of RCDs and other protection devices are needed).
A ground fault circuit interrupter or GFCI, is an electronic device for protecting people from serious injury due to electric shock. GFCIs constantly monitor electricity flowing in a circuit. If the electricity flowing into the circuit differs by even a slight amount from that returning, the GFCI will quickly shut off the current flowing through that circuit. The advantage of using GFCIs is that they can detect even small variations in the amount of leakage current, even amounts too small to activate a fuse or circuit breaker. GFCIs work quickly, so they can help protect consumers from severe electric shocks and electrocution. Even if the GFCI is working properly, people can still be shocked. However, the GFCI can act quickly to prevent electrocution. All GFCIs work in the same manner to protect people against ground faults. However, unlike the receptacle GFCI, the circuit breaker type GFCI also provides overload protection for the electrical branch circuit. GFCIs are necessary even if the product has a third wire to ground it. GFCIs provide very sensitive protection to consumers against electric shock hazards. Under some conditions, a shock hazard could still exist even if a product has a grounding wire. Consumers are encouraged to use a qualified and certified electrician to install circuit breaker-type GFCIs. Individuals with strong knowledge of electrical wiring practices, who can follow the instructions accompanying the device, may be able to install receptacle-type GFCIs. The portable GFCI requires no special knowledge or equipment to install. Some equipment have GFCI type shock protectors nowadays. Appliances that have built-in shock protectors, as now required for hair dryers, may not need additional GFCI protection (but having extra protection does not cause any problems, better be safe than sorry).
Note: You cannot use most surge protectors or even some devices that have a surge circuit in them, with or downstream of, a GFCI / RCD. It will constantly "trip" if you encounter some overvoltages. There are also some devices which have pretty high gorund leakage and can cause occasional "trip" on some situations.
An AFCI (Arc Fault Circuit Interrupter) uses electronics to recognize the current and voltage characteristics of arcing faults, and interrupts the circuit when the fault occurs. This kind of special device is needed for arc protection when it is needed, because fuses and circuit breakers cannot detect low level arcs. Since cables between the electrical panel and the end appliance are subject to arc faults, protection is needed at the source of the electrical supply. AFCI protective devices are now available as part of the circuit breaker construction. AFCI is useful here you get significant number of fires caused by bad contacts and terminal connections, particularly relevant for aluminium wiring. The UL code states "By recognizing characteristics unique to arcing and functioning to de-energize the circuit when an arc-fault is detected, AFCI's further reduce the risk of fire beyond the scope of conventional fuses and circuit breakers." Effective January 1, 2002, NFPA 70, The National Electrical Code (NEC), Section 210-12, requires that all branch circuits supplying 125V, single phase, 15- and 20-ampere outlets installed in dwelling unit bedrooms be protected by an arc-fault Circuit interrupter. Installing AFCIs in your home will require a qualified electrician. The AFCIs snap into the electrical panels. And at this time, only a few electrical panel manufacturers make AFCIs. A word of caution about the overselling of AFCIs. The risk of fires caused by arcing is real and the total US loss to electrical fires is large. AFCIs can help to reduce that risk, but it will not completely solve the problems.
AFCIs should not be confused with ground-fault circuit interrupters or GFCIs. Typically AFCI circuit breakers look similar to GFCI circuit breakers. While both AFCIs and GFCIs are important safety devices, they have different functions. AFCIs are intended to address fire hazards; GFCIs address shock hazards.
The large box-like device found on the ends of some appliance cords can be either an appliance leakage circuit interrupter (ALCI), an immersion detection circuit interrupter (IDCI) or a ground fault circuit interrupter (GFCI). They work in different ways, but they are all intended to shut off the power to an appliance under an abnormal condition such as immersion of the appliance in liquid. Just because you have an appliance with one of these devices doesn't mean that it is okay to drop the appliance in water and retrieve it while it's plugged in. If you should happen to drop an electrical appliance in water, shut off power to the circuit into which the appliance is plugged, unplug the appliance, drain the water and retrieve the appliance. The rule that "electricity and water don't mix" still applies.
In any real-world situation where there is any chance of an insulation fault, or any other resistive, capacitive or inductive path to local 'ground', proper safety earth bonding is desirable.
Today electrical installation standards are highly developed and cover all major aspects for a safe installation. Standard maker have paid particular attention to the measures to be implemented to guarantee protection of of the personnel and property. This concern has resulterd in standardization of three Earthign Systems.
Many mains distribution networks between buildings use Multiple earth grounding principle. In multiple earth grounding there is a ground rod at the service entrance panel of the building and there are other ground rods connected at other buildings to the neutral in a similar manor.
One grounding approach used in sensitive electrical installation is "single point grounding", which is tying all of the equipment together to one point and all cables coming in at that point so as not to have a "ground loop" between any equipment. This single point that all of the equipment is tied to then goes to ground and often multiple ground rods and or radial wires buried in the ground. Single point grounding principle is used inside some equipments and some in house mains wiring practices.
Good grounding practices are essential to prevent ground transients. Ground Transients are a common cause of system crashes, lock-ups, loss of information or permanent system failure in computers, Point of Sale terminals, PLC’s (programmable logic controllers) and medical electronic devices.
There are different earthing systems in used and used around the world. The criteria for selection of earthing systems has changed. Different earthing systems are standardized in IEC 364 standard.
Sometimes you might ask why the electrical distirbution system is grounded anyway ? There have been differing philosophies about grounded and ungrounded systems. If the circuits downstream of an isolation transformer are small enough, there isn't enough capacitance and leakage resistance to shock you, then ungrounded system could work in theory. But who's going to check the circuits for unintentional grounds? A significant leakage path from one leg to ground would compromise the safety of the ungrounded system.
One of the major reasons for grounding the low voltage systems supplied by hi-voltage systems is to protect the low voltage 'stuff' from an accidental high voltage. Transformer insulation breakdown, a high-voltage line falling across a low-voltage line are at least two ways that high-voltages could be put onto a low voltage circuit. With the circuit grounded, the high-voltage line will see ground-fault current and trip. Also with grounded low voltage, the maximum voltage seen by the low-voltage circuits under such a situation would be limited. This isn't so much for personnel safety as to avoid burning down houses.
As to running a grounding wire to every outlet, the idea there is if the casing of all tools/appliances are grounded by a third prong and the outlet, then a fault in the appliance (such as a hot lead shorting to the case) will short to the grounding conductor and back to the service entrance panel, tripping the breaker. The voltage developed on the case will be mild and unless you're grounded by a separate circuit, you won't be seriously injured. Double insulated tools/appliances are excused from third prong IIRC because they would require two faults (line to frame, frame to casing) and that is considered very remote. Experience shows that this works, but it is not infallible due either to insulation failure, or more importantly the human factor (either stupidity or error).
Yes, there are certain specific situations/accidents where the 'safety' device (grounding) actually makes things worse, but generally the grounded mains system is the best.
In some special cases isolation transformers are used instead of grounding. For example in some hospital and electronics labortory applications they use isolation transformers to float everything - can't shock if there is no return path. This work as long as every device has their own isolation transformer and it does not fail.
The mains wiring ground is used also as ground reference for many electronic devices. In those case you must remeber that the electrical system ground is not a perfect ground potential that is same in all grounding points. There can be considerable ground potential differences that can cause problems (common mode voltage to signals, currents to ground conductors etc) if not taken in account in the electronic devices that are connected to electrical system ground and each other. Voltage measurements from ground point to ground point are typically 0.2 to 5V rms and, though rarely, as high as 65 V rms between widely separated grounds. Remeber that in case of lighting strikes nearby, very high potential differences can occur from "point a to point b" on the same ground system due to the ground system/earth's combined impedance at the strike's higher frequencies.
When installing ground connection to ground, the quality of it needs to be measured that it is good enough to fullfill the needs of that particular installation. Ground resistance is usually measured using the 3-stake fall of potential method. Theoretically, the final measurement achieved on the completed ground system is the same resistance to any other ground system on earth. A good ground system measurement would be between 5 and 10 Ohms. A well designed 5 Ohm ground system is usually considered optimal for a lightning ground system. A 4-stake resistivity measurement should be done ahead of the actual ground system installation. This procedure tells the engineer which areas within the system.s geographic confines have the most conductive soil and at what depth this occurs. The results will be expressed as resistance (in Ohms - cm/m) and will determine the ground system.s design. The ground system.s final 3 stake fall of potential ground resistance reading is the impedance of the system measured with approximately 100 to 300 Hz source potential. This measurement is how well the ground system will handle electric utility ground faults. There must be enough current flow in to the earth to trip the applicable ac circuit breaker.
Different countries and different environments use differnt mains connectors. In USA NEMA Plug & Receptacle configuarations are common. In Europe the normal single phase domestical power plugs are mainly at least somewhat country specific, and for higher current & three phase power CeeForm (CEE 17 / IEC 309) connectors are used.
Most commonly used mains connector models:
There are also many other connector types in use in different countries and special applications.
Some most commonly used mains connectors used in equipment end are based on IEC 320 standard. A system to power accessories (such as monitors, disk drives, printers etc.) almost anywhere in the world can be designed by the use of an accessory power system based on the international IEC 320 connector pattern. The most commonly used IEC 320 connectors are the following:
Here are some links on mains connector information:
Electrical safety and wiring regulations are here to help to make the electrical systems and installations safe. They give the basic safety needs, define which kind of circuits to use, material to use and generally specify amins wiring colors. This standardization makes is easier for electrical installers to make wiring right, make them safe and for somebody else to fix them later safely if that is needed.
Electrical code is a compromise between safety and cost. Much of the world considers 220 V (220-240V) to be safe enough for standard residential outlets and lighting, and they can wire a house with about one-half the copper compared to countries that use 120 V. Those countries which use 120V considered that 220V to be to dangerous for most residental uses.
Some other wiring practices have their good and bad sides. Using a common neutral, saves copper (or aluminium), but carries some risk, that if done improperly, an overloaded or open neutral situation may occur. Using aluminium in wiring saves costs in wiring material, but if connections done to aluminium wiring are not done carefully they can become loose and cause fire danger (nowadays aluminium is not generally used in residential installations, usually only on large power feed cables going to distribution board power input).
The National Electrical Code is a set of regulations which specify the wiring nad safety practices in use in USA.
Domestic power in North America is variously referred to as: 110, 115, 117, 120 V. Likely varies even wider than that. The National Electrical Code speaks to a nominal line voltage of 120 volts with a +/- 5% tolerance for a low of 114 volts and a high of 126 volts. Most appliances and electronic equipment, etc. is designed to work within that voltage range without problems. At higher and lower voltages, risk of damage increases although most modern appliances are remarkably tolerant.
ANSI C84.1 "Electric Power Systems and Equipment - Voltage Ratings (60 Hz)" sets the preferred nominal voltage at 120/240V and lists limits as:
Maximum Utilization and Service Voltage 126/252 Minimum Service Voltage 114/228 Minimum Utilization Voltage 110/220Equivalent Canadian spec is CAN3-C235.
One historical claim for selection of 110V voltage: The US has 110 volts because the original light bulb (invented by Thomas Edison in the USA) ran on 110 volts DC, and when we converted to AC we kept that voltage so that everyone wouldn't have to buy new light bulbs. The voltage was also suitable because a shock from 120 V supply it is not so likely to be fatal.
The mains power frequency used nowadays in USA is 60 Hz. Many frequencies were used in the 19th Century for various applications, with the most prevalent being the 60 Hz supplied by Westinghouse-designed central stations for incandescent lamps. The development of a synchronous converter which operated best at 60 cycles encouraged convergence toward that standard. Around 1900, the introduction of the high-speed turbine led to settlement on two standards: 25 cycles for transmission and for large motors (this had been a compromise decision at Niagara Falls), and 60 cycles for general purpose systems.
In USA homes get two-phase 120v. In a typical home in the states you have 3 cables coming into your panel from the service. Basically, there's a center-tapped step-down (few kV distribution voltage to 120V+120V AC) transformer on the electrical line pole, with the tap earthed (at least in theory) and each socket connected across one side of the transformer. Larger devices (electric stoves, central air conditioning units, electric dryers, etc.) are wired across the entire transformer, receiving 240v. How much current is fed to the house varies on the needs of the house. Feeds up to 200 amp electrical service can be seen often in American homes (200A seems to be a norm for new houses nowadays).
The power distribution in typical residental house in USA is implemented so that locally (near the house) a single phase transformer provides a 240 volts center tapped output. This center tap is grounded at the transformer and called the neutral wire , then the three wires are run into each house along the street. The two live (hot) wires each separately provide 120 volts relative to the neutral wire and go to wall outlets to run low power devices (lights, TVs etc.). Power hungry devices like electric stoves and water heaters which require 240 volts are connected across the two live or hot wires. This two 120V hot lines with 240V between them wiring system is sometimes called "two phase power".
US practice is that neutral and earth are bonded together at one place in each structure. This is normally at the main power panel, but the code allows for exceptions (in some cases house has local ground and neutral is only grounded at sub-station). Each house also has a good local ground which is connected to input mains power neutral wire. For example typical small house could use system such as an 8 ft ground rod or a cold water pipe which also goes to each power outlet as the ground pin. The neutral connections to ground in all the buildings are part of the lightning protection scheme for the power distribution network. There are also ground stakes on the power poles on the street bonded to the neutral conductor. These multiple earth ground connections safely drain off the enormous energy of a lighting strike to the earth.
Inside house all the safety ground wires ( green ) are bonded to the earth stake ground or earthing system in a properly installed power system. What comes in from the street is the hots and the neutral.
The typical wall outlet in home in USA outputs 120 volts AC. The maximum current that should be allowed to be drawn from a normal outlet is 15 amps. That means that nothing over 1800 watts should be plugged into that circuit. The 1800 watts is the total for all devices on that same circuit fed by one breaker in the supply panel. There are grounded and ungrounded power outlets. In grounded outlets the National Electric Code (NEC) requires the ground pin to be first-make, last-break. That's why they're longer in mains connectors than the other contacts, and I doubt you'd disconnect a ground pin without disconnecting the blades. This connector in the wall is wired in the the following way: The ground pin should be at the bottom, the "hot" blade should be on the right and the neutral blade (the wider one) should be on the left. The ungrounded outlet is wired in the same way, just without the ground pin.
For special uses there are somtimes higher current outlets available. Sometimes you can see a 20A 120V AC outlet (a little different AC connector than normal 15A). In 20A plug one blade is rotated 90 degrees to the plane of the other as compared to normal 15A plug where blades are in parallel. Nowadays there are many 20A outlets that have one "T" shaped hole for neutral blade. Thus, for a 20 Amp outlet, you can plug in a 15 Amp male plug, or a 20 Amp male plug. But for a 15 Amp outlet, the 20 Amp male plug will not be insertable. It keys the load to the supply capability.
Some very high power loads like air conditioners usually use 240V two phase outlets (15A or 20A).
3-phase power is not typically available in homes in the US. 3-phase power is it is common in commercial and industrial installations.
Most household circuits are 15 amp (15 amp receptacles, 14 guage wire, 15 amp breaker or fuse). The wiring in the walls is at least 14 gauge (better yet 12) as that's the minimum allowed by the NEC for 15 Amp breaker circuits. Today's code requires 20 amp (12 guage wire and 20 amp recepticles) in kitchens and dining rooms. Today'd code needs 12 guage wire to bathroom outlets, although these are usually 15 amp outlets so one should not exceed 15 amps.
Copper wire resistance table for some wires used in main wiring applications:
AWG Feet/Ohm Ohms/100ft Ampacity* mm^2 Meters/Ohm Ohms/100M 10 490.2 .204 30 2.588 149.5 .669 12 308.7 .324 20 2.053 94.1 1.06 14 193.8 .516 15 1.628 59.1 1.69 16 122.3 .818 10 1.291 37.3 2.68 18 76.8 1.30 5 1.024 23.4 4.27These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77 degrees Fahrenheit or 25 degrees Celsius.
Breakers are designed to trip not so much at the nominal amperage rating (15,20 etc) but when they get hot - pulling too much current through a wire heat it up, too hot and the breaker will trip. It the wire is over 100 feet from the panel to the point of use, the amperage rating is dropped down one, or else the wire is upped to next thicker wire thickness. It is typical that electrical panel manufacturers also make the circuit breakers that go into their panels. Circuit breakers are not interchangeable in other manufacturers' electrical panels.
Circuits are designed so that under normal conditions (whatever those are) that the load will not be more than 80% of the rating - so a 15 amp circuit (120volts x 15 amps = 1800 watts x 80% = 1440 watts) should not regularluy exceed 1440 watts. That is why today each bathrooom outlet gets it's own 20 amp circuit, a kithcen should have at least 2 small applaince circuits (for outlets) the disposal, dishwasher, fridge and microwave should each be on their own individual 20 amp circuit.
Ground fault interrupters (GFIs) are designed to provide reliable protection from line-toground faults, while other types of overcurrent protection may see the ground fault only as a load current. GFIs detect a ground fault by using a current transformer with the line and neutral conductors passing through the center of the transformer. ). A ground fault on any one of the conductors causes an unbalance in the circuit. The current transformer senses this unbalance and trips the circuit breaker quickly. The NEC requires all receptacles installed in bathrooms, garages, and outside areas to be protected by GFI circuit breakers or GFI receptacles. An exception to this rule applies to receptacles installed in garages to supply power to dedicated appliances, such as refrigerators, freezers, or gas dryers. Also, receptacles that are not readily accessible are not required to have GFI protection; for example, the receptacle for an overhead garage door motor. All GFIs manufactured after 1 January 1976 are required to trip open automatically when the fault current reaches 5 milliamps.
GFI-protected receptacles are required for all 120-volt, single-phase, 15-amp and 20-amp circuits used by personnel on construction sites. This requirement can be fulfilled by installing GFI breakers on all receptacle circuits or by installing GFI receptacles at the jobsite.
Electrical work must be done so that it meets the local and national codes. For most works it means that you must be a licensed persons and the work may need to be inspected before it is taken to use.
Enclosures, raceways, wireways, and wiring ducts shall be suitable for the application and compatible with the environment that the equipment is to be operated in. Electrical enclosures shall be NEMA type or equivalent.
The National Electrical Code says that electrical equipment be "suitable" for use, and says that "suitability of equipment MAY be evidenced by listing or labeling". However many localities by local authorities DO require appropriate listing. Most all local and national building codes require that eligible equipment be "safe", and that one of the ways demonstrate safety is for the product to have earned a safety certification from a Nationally Recognized Testing Lab ( like UL ). In practice this usually requires that all equipment must be listed, if listed equipment is available. Listing is done by 3rd party safety testing agencies, two of which are Underwriters Laboratories (UL) and Electrical Testing Laboratories (ETL). These agencies list a product if it has successfully passed the required testing. Standards for testing and listing are established by the agencies. Standards are not available for all products. UL listing on electrical equipment is required for code compliance in most areas. UR listing is usually acceptable if the component is being used in a UL approved panel or piece of equipment.
The wire thickness used in USA for mains wiring are specified in unit called AWG. Here is some data on different
AWG Feet/Ohm Ohms/100ft Ampacity* mm^2 Meters/Ohm Ohms/100M 10 490.2 .204 30 2.588 149.5 .669 12 308.7 .324 20 2.053 94.1 1.06 14 193.8 .516 15 1.628 59.1 1.69 16 122.3 .818 10 1.291 37.3 2.68 18 76.8 1.30 5 1.024 23.4 4.27 20 48.1 2.08 3.3 0.812 14.7 6.82 22 30.3 3.30 2.1 0.644 9.24 10.8 24 19.1 5.24 1.3 0.511 5.82 17.2 26 12.0 8.32 0.8 0.405 3.66 27.3 28 7.55 13.2 0.5 0.321 2.30 43.4These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77 degrees Fahrenheit or 25 degrees Celsius.
The size of wire inside wall:
Gauge Amps
14 15
12 20
10 30
8 40
6 65
US practice of power distribution is often to distribute at high voltage (several kilovolts) and provide small transformers to individual properties or small groups of properties. There are a couple of likely reasons for distributing at medium voltage rather than at utilization voltage in the US. One is the fact that historically the population in the US has been much less concentrated. Houses are further apart, increasing voltage drop and losses for secondary distribution. Also, the norm in the US now is for a 200A service. Serving many homes at secondary voltage, each with a 200A service, would require very large conductors.
The electrical power distribution in USA/Canada is built roughtly in the following way: The power company distributes at different high voltages, usually 7.2kV or higher (single phase) and can be as high as 19.9kV. Basically there are three "classes" of medium voltage that are now common in the US for distribution: 15kV (12.47, 13.2, 13.8 etc), 25kV, and 35kV (these are all three phase, multigrounded wye). 4kV three phase was once popular but has been replaced in most areas. In a home or smaller commercial building the transformer at the building or near the building lowers that voltage to 220 (or 240, depends on where you are) volts with a centre tapped neutral refrenced (and tied) to ground. In other words, you have 220 volts between the two secondary outputs of your transformer or 110 between either secondary and neutral. All normal circuits (for lighting or electrical outlets) are one hot and one neutral (plus another ground for safety) thus 110 volts. Large appliances such as the stove or clothes dryer use both hots for 220 volts.
Things are nowadays somewhat changing. Now, it is common in urban residential areas to have large transformers serve many residences. It would not be uncommon to see a one or two large transformers serve a city block in a residential area.
If we are talking about a three phase service (practically never seen in a home situation), the transformer is set up in a "y" configuration with 208 volts between any two secondaries and 110 between any one secondary and the centre tapped neutral. In some areas, you get 2 legs of a 120/208 wye instead of the usual 120/240 split phase service. A large three phase bank will serve perhaps a block of a small city with 120/208 three phase. Any single phase services get two legs of this. This is done because of the mix of small three phase commercial and single phase residential customers. You'll also see this in a fair number of apartmemnt complexes. The building has three phases coming in, and four wires will run up to the roof (the three phases plus neutral, plus a safety ground) to run heavy equipment such as air conditioning. Each apartment along the way will get three wires (two hot ones and a neutral. And a ground) tapped off of two hot wires. The problem here is that the hot-to-hot voltage is 208 rather than 240, and while most appliances can deal with either, some will complain.
As buildings get bigger, the system will be set up differently. The main service transformers will will deliver higher voltage three phase power to the building. This can be for example 575/600 volts between secondaries with 347 volts available between any one secondary and neutral in Canada. Most lighting and other large loads will then run at 347 single phase or 600 volts three phase. USA seems to usually use 460 volts between phases, but the idea is same. There will be another transformer in the building to lower the voltage to 208/110 for the outlets for normal use. In even larger buildings there will be many of these smaller transformers distributed around the building feeding the 110 loads in there area.
There are certain standard on the sizes etc. on different electrical installation products. For example the most commonly used electrical box in USA is single gang, standard US. Standard single-wide wallplate for it is (WxH) 2.8 x 4.5 inches.
In most of the US, at least if you're in a city or a medium-heavily-populated county, there's probably a building code electrical code that says who's allowed to work on what kind of electricity. Usually in a home, you're allowed to work on sockets and switches inside existing electrical boxes, and almost everywhere you're not allowed to touch the main power feed yourself, and in some jurisdictions you can install new electrical boxes and plug-in circuit breakers yourself and in some you can't. In commercial buildings, you're more likely to need a license to do anythign with the electricity. If you're required to use a licensed electrician for something, and you do it yourself, various bad things can happen, and if you do it your self and something goes wrong, more bad things will happen. You do not want this. If you have fire insurance or liability insurance, those contracts probably also require licensed electricians to do the electrical work.
Safety is of utmost importance when working with electricity. Develop safe work habits and stick to them. Be very careful with electricity. It may be invisible, but it can be dangerous if not understood and respected. The electrical installations should be properly designed, properly installed and use the necessary safety devices needed on this type of application.
Fuses and circuit breaker are devices which protect wiring and devices agains short circuits and overloads. Circuit breaker is a protective device for each circuit, which automatically cuts off power from the main breaker in the event of an overload or short. Only a regulated amount of current can pass through the breaker before it will "trip." The fuses and circuit breakers generally connect to the live wires (the ungrounded phase conductor). Where circuit breakers are used to protect sub-fused circuits, the circuit breakers will almost always trip before the fuses blow. Circuit breakers also make accidental short circuits less violent.
Ground fault circuit breakers offer protection against more than just overloads, Ground fault protection devices (RCD, GFI, GFCI) are good protection devices to give extra protection agains accidents. GFCI's are most often used for protection from hazards associated with "portable" appliances in wet damp areas. This kind of devices are generally used in dangerous environments like places near water (electrical outlets outside, bathroom outlets, kitchen outlets), in consrution sites and in work shops for example. As far as the RCD's / GFI's (Residual Current Device) / (Ground Fault Interuptor) go, they are not infallible. It is still possible to pass enough current to kill without reaching the tripping current, and it's also still possible to get a live to neutral shock which will look like a normal load to the breaker. These things should also be tested quite regularly since they can fail. For personal protection 30 mA offers a high degree of protection and will operate by cutting off the earth fault current well within the time specified in the IEC Publication 1008/1009. IEC Standard 1008/1009 of 30 mA sensitivity for domestic and personal protection with the tolerance of 30 mA plus zero and minus 50%, that is, a range from 15 mA to 30 mA.
Within the European Community the mains voltage is currently 230V +10/-6% (50Hz) between the LIVE and the NEUTRAL terminals, together with a separate protective EARTH terminal. The history for 50 Hz frequency is form Germany. At the beginning of 1900 in Germany, AEG had a virtual monopoly on lectrical power systems. AEG decided to use 50 Hz and this standard spread to the rest of the continent.
The mains connections and wiring practices vary somewhat from country to country. In Europe, two wire (ungrounded) wall outlets supply maximum of 6A (10A in some countries). Three wire (grounded) outlets, maximum of 15A or 16A depending on the country (sometimes fused only with 10A fuse). All mains wall outputs are fused at distribution point in house. In modern installations in Northern Europe the wall outputs are grounded outlets often 16 A per circuit. In most countries system uses a star arrangement in which a cable from the fusebox feeds, for example all of the wall outlets in one room only. The fuses or more commonly, circuit breakers, are designed to protect not just the wiring inside the wall but also the wiring from mains plug to device devices and devices. So, there are no fuses in the plugs. The houses/apartments in Europe can be supplied by single phase power or three phase supply. If three phase supply is used, separate rooms in the same apartment may be on different phase.
In most European countries the electrical mains connectors are not polarized. This means that generally common 2 pin and 3 pin mains connector plugs may be inserted either way to the wall, thus interchanging neural and live wires going to equipment. The design philosophy of e.g. the German system (Schuko) is that a room (or a small number of rooms) has a 10 A or 16A fuse in the consumer unit, and all leads and plugs are designed to withstand any short-circuit current that will not yet blow the fuse (today usually circuit breakers are used, not fuses). If a fault occurs, a circuit breaker is trivial to reset, The fuses are generally in the main distribution panel.
The CEE 7/7 ("Schuko") grounded plug is used as a standard in Germany, Austria, Norway, Sweden, Finland, the Netherlands, Belgium and France; it is also used in Portugal and Spain. A variation of the CEE 7/4 plug with 4mm contacts is used in Eastern European countries and in some Soviet republics. Variations of the CEE 7 plug are also used in Northern Africa, the Middle East and Brazil.
Many small electronics appliances in Europe use small flat "Europlug" connector. The Europlug originated as CEE 7 and has been around for almost 30 years. It is a clever design that fits all the historic national sockets in all European countries, except for the UK. IEC 884 is one spec for the "Europlug" (BS EN 50075), which is the today commonly used plug in all EU countries for small devices that need no protective ground and consume less than 2.5 A. It is flat, has thin round pins and the pins are not exactly parallel (slightly closer together at the front).
Grounding in European mains wiring installations can vary somewhat. There is a number of different earthing (= US 'grounding') schemes in use. In almost all cases neutral and earth are run separately within an installation, with all accessible metalwork and earth terminals within the premises tied back to a common earthing point at the entrance to the property. How that earthing point is provided varies from place to place.
Three common earthing schemes:
The power to most houses in Europe (mainland Europe) are provided either as single phase power (230V AC, phase wire + neutral wire) or three phase power (230V AC from phase to neutral, 400V between phases, three phase wires + neutral wiring). Which one method is used depends somewhat on the country, site of load and sometimes from electrical company used. Usually larger sites and houses are powered with three phase power. Three phase power is sometimes supplied to high power appliances (large electrical motor, powerful heaters, ovens etc.) directly through permanent wiring or three phase power socket (nowadays most often CEE 17/ IEC 309 socket). Generally the neutral of the mains input power is bonded to the house main grounding point (connected to house plumbing and metal paets) at the main electrical panel or separate house grounding bar.
On many European countries (for example UK and Finland) it is typical that transformer sub-station will supply a large number of properties at normal mains voltage (230V AC single phase or 230/400V three phase). The available current varies on the needs on the house. A 3x25A feed (230/400V three phase power, 25 amperes per phase) is quite typical in houses in Finland. In house where there are many apartment in one building, each apartment typically has either one phase power (often 25A or 32A on older houses) or three phase power (for example 3x16A, 3x25A etc.).
Different countries have different customs. All have been devised by good engineers to suit the country specific situation. Three-phase supplies are not usually provided for individual residential consumers in most parts of Europe (three phase power to house seems quite common in Finland and Germany, in thoise countries for example electrical stoves for kitchen cooking are build for connection to 3 phases, but can be also wired to 1 phase only if needed). Generally you get a single phase supply, unless you exceed the current which is deemed the maximum for a single phase supply in which case you get a 3 phase supply. Now what does vary significantly is what this single phase limit is. In the UK, it's 100A so 3-phase is unusual in houses, but in some EU countries, it's as low as 20A, so 3-phase is the norm. The thresholds for 1 vs 3 phase supply would vary between networks and countries. Supplying houses at two or three phase improves load balancing and reduces voltage drop, but at the cost of more conductors and more expensive metering.
In Europe, street distribution is typically done in 3-phase at the normal mains voltage (230 / 400V) in urban areas. The power to this street distribution comes three phase transformer nearby the street (typical power 30 - 500kVA). Houses then get typically one, two or three phases as needed.
Typical current rating for wires used in mains wiring inside wall:
Cross- Overload sectional current area rating 0.5mm² 3A 0.75mm² 6A 1mm² 10A 1.25mm² 13A 1.5mm² 16A
In European countries power cords all have to be sheathed, which means there are always two layers of insulation around the conductors (or one extra thick layer).
UK is somewhat special case. The UK is unusual in having fused plugs as standard. 13A max in each plug, and 30A at the panel for each ring. Maximum Current at wall outlet in the UK is for 13A. The plugs carry a fuse holder and the fuse should be rated to suit the appliance used (fuse ratign from 1A up to 13A exist). The fuse in the plug is for protecting the cable to the appliance, not the appliance itself. For the latter, the appliance would have its own fuse (or other suitable protection means). Neutral is neither switched nor "protected". UK mains plugs are polarized. In the UK, a wiring system known as a ring mains is used. UK standard (for the last 30 years or so) has been the ring -main (domestic and commercial) rated at 30/32A @230V. A single cable runs all the way round part of a house interconnecting all of the wall outlets. This will be protected by one large fuse in the fuse box. A typical house will have three or four such rings. The power rings are normally protected by a 30 amp fuse and the lighting rings by 5 or 10 amp fuses. Those fuses protect the wiring, not the appliances so, every appliance carries its own fuse in the plug. In the UK, you will normally only ever see a single phase supply in a house. In industrial installations, a variation on the Ring Main may be used (as well as 415V 3 phase for equipment etc). In UK the Neutral will generally be grounded at the local substation, not locally in the house. The Earth is local to the house.
Other special cases: There are also some special wiring used in some places. Another much rarer scheme is 230V between phases and no neutral in the supply. Houses are then fed two phase wires, neither of which is necessarily anywhere near earth potential. This is used in at least in Norway coutries in some locations. Also in some places in Belgium three phase 220 across the phases (= 127 phase to earth/neutral) is used on older domestic dwellings (new installations are 400/230 3 phase, neutral, earth). For this reason all Belgian fuseboards (whether actual fuses or circuit breakers) protect both current carrying wires irrespective of supply type.
Typical modern mains voltage distribution panel in Europe consists of DIN-rail mounted components like switches, miniature circuit-breakers (MCBs) and residual-current circuit-breakers (RCCBs).
When automation is added, European Installation Bus (EIB) is commonly used for modern control application (like light level). But this is not the only automation system in use.
The wiring inside building has the power fused in the live side always. Fusing the neutral is not allowed in the building wiring.
Typically the devices devices connected to mains outlet have one fuse to stop the excess current flowing. Because most mains connectors on devices used in European countries you can plugged in two ways to power connector, you ever know if the fuse inside equipment really goes to live or neutral wire (this is not considered to be a major problem when some other safety things are taken into account). The problem with fusing the neutral is that some countries have a designated neutral, such as the UK, whereas others treat it as reversible phase conductor. This means that some products are produced with the only fuse in the neutral hence, if it blows, all the circuitry in the product is still live, even with the fuse removed. In some special cases double fusing is used in equipments (one fuse for live wire and nother for neutral). Although it is allowable to use double fusing, it should be accompanied with a warning label on some countries (wording is in the standard EN60950).
Residual current device is generally used in new installations to protect outlets on dangerous places like wet locations and outside. RCD with 30mA rating is typical for a single 230V circuit (maximum residual current for human protection). In some cases the whole house three phase feed is protected with RCD for whole house fire protection with maximum of 300 mA RCD.
The protective components, wires and connectors must be designed to handle the high currents that may occur because of short circuits and similar faults. For example UK 13 A plug fuses are tested to break 3000 A without exploding and why HBC fuses ought to be used in almost all mains circuits. For household and light industrial 230 V 13 A or 16 A branch circuits, the prospective short-circuit current is approximately 200 A, but in industrial premises it may be much higher. Wires used for the circuit can normally carry 200 A for a few tens of milliseconds without any ill effects.
Safety of temporary power systems vary somewhat from country to country. In most parts of Europe the sandard means of protection are the use of good grounding and residual current devices is the way to operate electricity safely. In UK big yellow boxes are common, which contain serious power 240V : 110V balanced isolation transformers. These are intended for building sites, so that if you damage the power cable of your tool, the maximum voltage you get applied is 55V above Earth potential, thus safe.
The overvoltage categories for the electrical equipment are as follows:
Since the middle of 1994 a new European quality standard for the product "Electric Energy" has been in effect. EN50160 tries to standard the electrical supply you can get. The purpose of the EN 50 160 standard " Voltage characteristics of electricity supplied by public distribution systems" is to specify the characteristics of the supply voltage with regard to the course of the curve, the voltage level, the frequency and symmetry of the three phase-network at the interconnecting point to the customer. The goal is to determine limiting values for regular operating conditions. EN50160 is not exactly a standard, it's a bedtime story by the supply industry, telling you want you can expect to get for your energy payment. Assuming this is for the synchronous network, the figures are: 50 Hz +/-1% for 95% of a week, and +4/-6% for 100% of a week. The power companies generally try to keep the number of mains power cycles in 24 hours correct within a few hundred, so that clocks keep good time.
One problem in making electrical installations in differerent countries in Europe has been that the color coding standard have varied from country to country. A long time ago the European standardization organisations have standardized the color code for safety ground wire to be yellow+green stiped (other colors for this are not allowed). The color of the neutral wire has been standardized to be blue.
After the approval of the harmonisation document review CENELEC HD 308, new rules about identification of cores in cables and flexible cords will be applied to all cables. The cables matching the new rules are marked with the brand <HAR>. This standard has been applied around the Europe for flexible cords for a long time.
The new standard version of HD 308 published in 2001 standardizes also the fixed wiring colors up to 1000V operation. In this new standard described thw following color codes to be used:
The transfer time for the new colors on fixed wiring could be long and there is a transfer time where both colro coding are used. It can take up to year 2006 when it is used in whole Europe uses only the new codes in new installations.
Because those wiring documents consider the sitation in Finland most of the documents listed in this section are written in Finnish.
AC is one of those things we never think about-even most electronics textbooks consider it to be a simple, 60-Hertz 120-volt RMS sine wave or 50-Herz 230-volt RMS sine wave (depends on where you live). Many books mention any impurities only in passing. But it's not that simple. The mains power can contain radio frequency interference, fluctuations in the voltage, high voltage spikes and even short breaks in power distribution. Proliferation of sensitive devices makes good electrical power a must-have.
Voltages other than nominal affect the equipment they power, reducing performance or increasing wear. Power quality problems that plague many modern offices and factories are largely preventable. Copper-intensive solutions include using larger neutral conductors to handle harmonic loads, better grounding systems to dissipate transients and lightning, and fewer outlets per circuit to lessen interaction between office equipment and computers.
The term power quality simply describes how well the power available at the outlet conforms to the electrical standards we all take for granted. The power should be continuous, uninterrupted by gaps, "notches" or other outages, even momentarily; it should be delivered at what electrical systems are standardizes to provide. In USA this 120 V, plus or minus only a few volts; its frequency, 60 Hz, and waveform should likewise be invariant and undistorted. In Europe this is 230V (+-10V) and frequency is 50 Hz. Except for occasional hiccups, electric utilities generally do a good job of supplying high quality power.
Brief (1 sec or shorter) power outages are common in developing countries and seem to be increasing in the United States. They are generally nondestructive, so power-protection equipment should not trip and disconnect the load. Electric motors do not appreciably slow, and electronic equipment can usually ride through the outages on the energy stored in filter capacitors. As a point of reference, a 10-msec outage makes an incandescent light bulb flicker. At 100 msec, you can see the bulb go completely dark.
Power quality is an issue that is ordinarily associated with commercial structures such as office buildings that contain large numbers of computers, printers and other electronic equipment, along with heavy machinery that wouldn't be found in a home. Still, the proliferation of electronic equipment in home settings has come to the point where power quality considerations are becoming increasingly germane.
If you're building a home or office building, now is the ideal time to make certain your electrical system is designed and installed such that your sensitive electronic equipment receives the power quality it needs. If you're upgrading the electrical system in an older house, you can usually quite easily and relatively economically accomplish the same goal.
A wide variety of natural disasters or electrical system failures can cause short-term power outages. We are all dependent on electricity, so a power outage of more than a few minutes becomes pretty annoying. A power outage of very short time can cause problems to computer systems (most often causes your PC to boot and loose some data). An Uninterruptible Power Supply is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from the supply from adversely affecting the performance of the device. There really is no standard definition of what a UPS is. The UPS industry is made up of many manufacturers, and there is a lack of standard terms within the industry. There are basically three different types of devices, all of which are occasionally passed off as UPSs:
Genrally an UPS device / system consists of array of switches, rectifiers, inverters and particularly batteries which ensures a continuos supply of power to the consumers in case of power failure and possibly improves energy quality.
A wide variety of natural disasters can cause long-term power outages. Emergency power system are used to generate electricity when mains power is missing. To generate normal (120-volt or 230-volt) mains power on an emergency basis, you have two options: an engine-powered generator which converts some fuel (gasoline, diesel or propane) to mains power or you can use an inverter which converts power from card battery (or similar) to mains power.
In order to choose the right emergency power source and to size it properly, you need to understand something about the power requirements of the devices you plan to operate with it.
One warning for UPS users: Keep in mind that devices aren't designed to let you keep working for a long period of time; they're designed to give you enough time to shut your system down in the normal manner.
An electric generator is a device used to convert mechanical energy into electrical energy. The operation of generator is based on the principle of "electromagnetic induction": moving the wire through the magnetic field causes electric current to flow in the wire.
Large generators are used by electrical utilieties to generate the power to the mains power grid. When you need power on places where normal electrical distribution grids do not reach, motor powered generators are generators are convient sources of mains power. The generator itself in those motor powered generaotrs is rotated by a suitable motor (usually diesel motor).
When selecting a generator, go for a generator with a rating higher than you need, so that you don't run it at full load (not good for fuel efficiency), or risk tripping the thing out or stalling it (motor generators can not generally take much overload). The power factor needs to be taken into account when driving inductive loads (be sure you have ebough watts and VA:s). Generally take the total KW load of the load, add at least 20-30 % as safety margin. When selecting generator you will need to know roughly how many hours per day the generator will be running, and rough load estimates of those hours. When renting a generator, tell the rental company those numbers, and ask them for their recommendations.
Power (kW) ratings of an AC generator are based on the ability of the prime mover to overcome generator losses and the ability of the machine to dissipate the internally generated heat. Typical name plate data for an AC generator includes: manufacturer name, serial number and type number, speed (rpm), number of poles, frequency of output, number of phases, and maximum supply voltage, capacity rating in KVA and kW at a specified power factor and maximum output voltage, armature and field current per phase; and maximum temperature rise.
Generators are perfectly safe provided they are set up and used correctly. If you plan to hire a geenrator, be sure to hire from a reputable company. They should be able to provide you with suitable cables, and site the generators for maximum efficiency. Define your need and ask the rental company for their solution.
When installing generator, make sure that the generator has a good earth/ground connection. How this earthign shoudl be done depends somewhat on local electrical regulations and instructions on generator.
The effectiveness of the multiple earth neutral wiring for lightning protection is quite limited. In multiple earth grounding there is a ground rod at the service entrance panel of the building and there are other ground rods connected at other buildings to the neutral in a similar manor. But as far as lightning is concerned the wire between those ground points has too much inductance to look like much of a ground to lightning Any appreciable length of ground wire looks like a large inductor to the lightning (most energy at DC to 1 MHZ). Then don't forget that the hot wires carrying the ac are not grounded.
Lighting facts from Polyphaser "grounds for lightning and EMP protection" book: The average lightning strike will have a peak current of 18kA for the first impulse (stroke) and less (about half) for the second and third impulses. Three strokes is the average per lightning strike. 50% of all strikes will have a first strike of at least 18kA, 10% will exceed a 65kA level and only 1% will have over 140kA. The largest strike ever recorded was almost 400kA.
Disconnecting everything when a storm is approaching is of course one way to avoid damage but that doesn't always get done. There are many "plug in" surge devices sold. They are not very effective for proecting agains direct strikes. The biggest problem being that there is no ground for them. The ground lead at the ac outlet is too far away from the ground rods to do any good. The impedance of the ground line is too high in normal mains wiring. To be effective they need a short good ground connection. Plug-in devices without very good groundign can protect somewhat devices agains small small overvoltage spikes and devices connected only to mains (no telephone line connection etc.).
When protecting sensitive electronics like PC, look for a suppressor that offers a response time (the time it takes for the suppressor to react to a surge) of 10 nanoseconds or less and energy dissipation rating of 200 joules or more. A failure indicator light that tells you when the suppressor is on the blink is also important. If you are still using a dial-up modem, you want to be sure the suppressor blocks electricity that can come in from the phone lines, too. Finally, be sure that the suppressor you buy meets the local safety ratings (has compliance stamps). In USA be sure that the suppressor you buy has a UL compliance stamp and look for a suppressor that meets the UL 1449 specifications. Some brands of surge suppressor include an equipment protection guarantee that covers components damaged if the device fails to do its thing.
When designing larger systems, the system grounding needs to be considered also. All equipment involved in a system should physically be located as close as possible to one another. This reduces the potential that is developed between the ground site and the individual components of the system during a lightning strike. This single point grounding greatly reduces the potential for lightning damage to electrical equipment. If you are unable to achieve single-point grounding due to large distances between equipment or other variables, other means of lightning protection must be considered. Consult a reputable lightning protection company.
Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly. Electricity can be generated anywhere-once generated, it is immediately entered into the transmission system, sometimes called the Grid. Those huge high-tension tower lines hum with just-created electricity at extremely high voltage. It must be changed into a usable form and delivered before it dissipates.
Electrical power starts at the power plant. Power travels from the power plant to your house through an amazing system called the power distribution grid. The electrical power distribution used nowadays is based on alternating current (AC). Alternating current (AC) is electric current which repeatedly changes polarity from negative to positive and back again. The most commonly used form of alternating current does so in a sine wave pattern, and that is the one used in mains power distribution. Alternating-current electric power is a form of electrical energy that uses alternating currents to supply electricity commercially as electric power. AC can be stepped up by a transformer to a higher voltage or stepped down to lower voltage as needed. The electricla power is converted to high voltage for long distance transmission (to avoid resistive losses in the wires, because at higher voltage lower current is needed to carry same power).
The electrical is generally distributed using polyphase electrical systems that supply electrical power in overlapping phases, the most common example being three phase power. These systems exhibit three-phase induction or more. Nikola Tesla discovered these types of systems. Polyphase power is particularly useful in AC motors, where it can be used to generate a rotating magnetic field.
Three phase power is commonly found in industrial applications and electrical distribution. Three-phase electrical generation is very common and is a more efficient use of conductors than other systems (like single phase, two phase or more than three phases). Three-phase power is common only in industrial premises and many industrial electric motors are designed for it. Three current waveforms are produced that are 120 degrees out of phase with each other. At the load end of the circuit the return legs of the three phase circuits can be coupled together at the neutral point, where the three currents sum to zero. This means that the currents can be carried using only three cables, rather than the six that would otherwise be needed.
In many situations only a single phase is needed to supply street lights or residential consumers. When distributing three-phase electric power, a fourth or neutral cable is run in the street distribution to provide a complete circuit to each house. Different houses in the street are placed on different phases of the supply so that the load is balanced, or spread evenly, across the three phases when a lot of consumers are connected.
Commercial electrical generators of any size generate what is called 3-phase AC power. The 3-phase power leaves the generator and enters a transmission substation at the power plant. For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. The power goes from the substation transformer to the distribution bus which distributes the power to the lines which leave the substation. This voltage is usually several thousand volts and is converted to normal mains voltage somewhere nearer to house. Past a typical house runs a set of poles with one phase of power and a ground wire (although sometimes there will be two or three phases on the pole depending on where the house is located in the distribution grid). There can be a transformer attached to the pole to give power to the house or the line can carry mains voltage and the step-down transformer is located some short distance away. In many suburban neighborhoods, the distribution lines are underground and there are transformer boxes at every house or set of houses. When the electrical power is inside the house, the inside wiring takes care of distributing it where it is needed.
The power from the local distribution transformer comes generally in three common confgurations:
Two and three phase power are used in wiring to house because using them saves copper: more than one live wires can share same neutral wire.
There has been several different voltages and ways to distribute the "low voltage" power to the homes and end users (some historical, some use still today):
The systems where normal electical outlets are wired between neutral and ground, typically only the phase is fused. In three phase system all there phases are fused and neutral is not fused. In system where one outlet gets two pahases of power, both of the phase wires going to outlet are fused.
On the AC systems the 50 Hz and 60 Hz frequencies are the most commonly used. Other frequencies are only used for special applications. Large aircraft typically use 400 Hz because this frequency allows for the use of smaller and lighter motors and transformers.
In many occasions three phase power is divided to single phase power by feeding one phase an neutral to each single phase outlet. When using this kind of power feed with some modern equipment, be prepared to take into consideration some three phase power wiring practices used. If you have a 3-phase supply, and are going to install lots of PCs, you need to check the size of neutral conductors. Harmonic currents (3rd, 9th, 15th etc.) add up in the neutral and can be at least as large as the fundamental phase current, whereas the neutral conductors may be smaller than the phase conductors on some systems (in normal well balanced three phase loads there is practically no current on neutral wire).
In Europe power is distributed extensively as 3-phase 230/400 V. There may be 500 customers on one MV/LV or HV/LV transformer. In US, power is distributed extensively at MV (several kV) and small transformers feed 1 to maybe 4 customers.
There are safety considerations. The modern practices nowadays demand the use of en extra safety wire from electrical outlets/loads to the main distribution panel. This wire wire is known in Britain and most other English-speaking countries as the earth wire, whereas in America it is the ground wire. At the main switchboard the earth wire is connected to the neutral wire and also connected to an earth stake or other convenient earthing point (to Americans, the "grounding point") such as a water pipe. In the event of a fault, the earth wire can carry enough current to blow a fuse and isolate the faulty circuit. The earth connection also means that the surrounding building is at the same voltage as the neutral point and prevents a person from receiving an electric shock from the appliance. As many parts of the neutral system are connected to the earth, balancing currents, known as earth currents, may flow between the generator and the consumer and other parts of the system, which are also earthed, to keep the neutral voltage at a safe level. This system of earthing the neutral point to balance the current flows for safety reasons is known as a multiple earth neutral system. Multiple earth neutral system is used widely hrougout electrical distribution networks, because it also provides good lightning protection for the distribution network.
Electrical installatuin safety testing includes many different tests:
The electrial testing of mains outlet wiring with the modern testing instruments includes typically the following tests to make sure that the installation is safe (related to tests in finland, 230v single phase outlet):
Insulation resistance tests should be made on a dead circuit and any electronic equipment which might be damaged by application of the test voltage must be disconnected or isolated. The insulation resistance should be measured between each live conductor and earth and should not be less than 0.5 megohm. Where practicable the tests are applied to the whole of the installation with all fuse links in place and all switches closed. Alternatively, the installation may be tested in parts.
The resistance of the phase-neutral loop or phase-phase loop is needed to ensure that adequate over-current protection has been provided in the case of a short circuit developing between the two live conductors or live to neutral. Generally, circuits protected by fuses need to have very low loop resistances. The phase-ground loop resistance is also needed to ensure adequate over-current protection, but also to ensure that, in the case of a ground fault, an adequately large current will flow to activate any protective devices. Depending on the situation the resistances are measured with resistance measurement device that supplies suitable measurement current to the wire loop or by using a short circuit current measuring meters (enough short circuit current guarantees low resistance).
Where protection is provided by an R.C.D., the effective operation of each R.C.D. shall be verified by a test simulating an appropriate fault condition independent of any test facility incorporated in the device, followed by operation of the integral test device.
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