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Electrical Wiring Page

    General information

    Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Electricity is a form of energy. Electricity is the flow of electrons. When electrons are "lost" from an atom, the free movement of these electrons constitutes an electric current. Electricity is a basic part of nature and it is one of our most widely used forms of energy. We get electricity, which is a secondary energy source, from the conversion of other sources of energy, like coal, natural gas, oil, nuclear power and other natural sources, which are called primary sources. Many cities and towns were built alongside waterfalls (a primary source of mechanical energy) that turned water wheels to perform work. Commercial scale generation began slightly over 100 years ago. Beginning with Benjamin Franklin's experiment with a kite one stormy night in Philadelphia, the principles of electricity gradually became understood. In the mid-1800s, Thomas Edison changed everyone's life by introducing the electric light bulb. Edison's invention used electricity to bring indoor lighting to our homes. Power travels from the power plant to your house through an amazing system called the power distribution grid. It consists of lots of wire, transformers and controlling equipment. The power transformer deveoped by George Westinghouse allowed electricity to be efficiently transmitted over long distances. Long distance power transfer uses very high voltages (up to hundreds of kilovolts) to avoid power (at higher voltage you need less current so supply same amount of power). This made it possible to supply electricity to homes and businesses located far from the electric generating plant

    An electric utility power station uses either a turbine, engine, water wheel, or other similar machine to drive an electric generator or a device that converts mechanical or chemical energy to electricity. Steam turbines, internal-combustion engines, gas combustion turbines, water turbines, and wind turbines are the most common methods to generate electricity. 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 (normal outlets, lighting etc.) is generally done using quite similar cable which has wires with thickness of 1.5mm^2 (10A circuits) or 2.5mm^2 (16A circuits). In USA you most wiring us done using 14 AWG (15A circuits) or 12 AWG (20A) circuits.

    There are two considerations when selecting the right wire size for electrical installation: voltage drop and heat buildup. The smaller the wire is, the higher the resistance is. When the resistance is higher, the wire heats up more, and there is more voltage drop in the wiring. The installation enviroment and wire ratings can have effect on the minimum wire thickenss that can be used: Inside wall you need to use thicker wires (heats less) than in ssytems where the freely circulating air can cool down the wires. Also the short circuit situations needs to be considered, the wire must be thick enough to burn the fuse (or trip the beaker) before it gets danegrously hot. Electrical wiring regulations give the figures what are the minimum wire thicknesses to use in different situations.

    There are some very specific exceptions, where use of smaller than the normally needed wire thickness is allowed. The obvious one is the line cord on most lamps. Neither heating or voltage drop effect is very significant over very short distances. Don't try to use thin wires unless you're certain that your use fits one of the allowed exceptions. You can never go wrong by using larger "full size" wire or thicker.

    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 or circuit breaker 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 presentwhen the work is done. This is the general case in all electronicsand 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 orin 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.

    Electricity is measured in units of power called watts (named to honor James Watt, the inventor of the steam engine). One watt is is a quite small amount of power. A kilowatt represents 1,000 watts. A kilowatt-hour (kWh) is equal to the energy of 1,000 watts working for one hour. Around 750 watts is equal one horsepower.

    • 3 Phase Alternating Current    Rate this link
    • Glossary of Technical Terms    Rate this link
    • How Power Distribution Grids Work - Power travels from the power plant to your house through an amazing system called the power distribution grid.    Rate this link
    • IEEE Historical FAQs - Why did the US choose 120v for household current and Europe choose 220v? Why does US use 60 cycles and Europe use 50 cycles?    Rate this link
    • How Nuclear Power Works Nuclear power plants provide about 17 percent of the world's electricity. Some countries depend more on nuclear power for electricity than others. In the United States, nuclear power supplies about 15 percent of the electricity overall, but some states get more power from nuclear plants than others. There are more than 400 nuclear power plants around the world, with more than 100 in the United States. Have you ever wondered how a nuclear power plant works or how safe nuclear power is? In this edition of HowStuffWorks, we will examine how a nuclear reactor and a power plant work.    Rate this link
    • What is Electricity? - Electricity is a form of energy. Electricity is the flow of electrons. Electricity is a basic part of nature and it is one of our most widely used forms of energy. We get electricity, which is a secondary energy source, from the conversion of other sources of energy, like coal, natural gas, oil, nuclear power and other natural sources, which are called primary sources.    Rate this link

    Wiring systems

    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 industrialapplications and electrical distribution. Three-phase electricalgeneration is very common and is a more efficient use of conductorsthan 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 phasecircuits can be coupled together at the neutral point, where the threecurrents sum to zero. This means that the currents can be carriedusing only three cables, rather than the six that would otherwise beneeded. 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 powerdistribution in Europe to homes. The ouput from mains transformer isY-conneted. There is 230V AC from each phase to neutral and 400V ACfrom phase to phase. The normal 230V electrical outlets are wiredbetween neutral and one phase. Large high power loads use all threephases (the indivudual loads in such equipment can be phase to neutralor phase to phase as needed).

    International Electricity

    The electricity is not the same in all the countries. There are differences in the mains connector type, voltage, mains frequency and electrical safety practices in different countries. The nominal voltage is typically at 100-120V or 220-240V range on normal electrical outlets. 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.

    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.

    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 ofthe 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 thenumber 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 life 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 severe damaged equipment.

    In order to use a North/South American 110/125V appliance abroad in country that uses 220-240V voltage, 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 on other way either. 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.

    The needed conversion transformer are available from electronics and electrical supplies. Before goign to buy a trnasformer you need to determine what kind of conversion do you need to do (from 220V to 110V or other way aound), how much your equipment you want to power with it need power (the covnerter needs to be rated for this power or higher power), what type of equipment it is (is it electronic equipment or simple heater type device), what are the connectors you need to have on yout transformer, is the device planned to be used continuously for long time and is the conversion anyway feasible (check that the equipment can work at the mains frequency on the site). The most common needs for converter are for people who come from 110V couintry with their equipment to 220V country. They have a selection of following conversion transformer types:

    • Safety isolation transformer with 110V output
    • Autotransformer type 220V to 110V converters
    • Electrical 220V to 110V converters
    Most cheap trveller converter transformes are autotransformer type devices, because with this construction you can get the most power out of the given transformer size. The downside is that the input and output sides of the transformer are directly connected to each other. With an autotransformers, if you plug them in backwards then the 110 volt outlet is actually 220 volts on one pole and 110 volts on the other. It looks like 110 volts to the appliance (220 minus 110), but the appliance is floating 110 volts about ground (can be a safety hazard if equipment insulation is only rafed for that lower voltage in mind). If it's plugged in correctly, then you should get zero or neutral (maybe a few volts) on one pole and 110 volts on the other pole. A safety isolation transformer with 110V output will completely isolate the 110V output from the input mains power. The completely isolated output will give good safety (you will not get killed if you accidentally touch one side of that 110V output), but the downside is usually considerably bigger and more expensive transformer construction. If you can afford a true safety isolation transformer type converter, this is the best choice to use. When selecting the conversion tranformer, you need to look at the power rating of the transformer. This needs to match or be higher than the power rating (input power) of the equipment you are using. Please also note that many cheap travel converter transformers are not designed for continuous operation in mind, meaning that if you use them for long time they will heat much and may fail quicly (at worst case carch fire). If you plan to operate an equipment continuously, then buy a proper tranformer rated for continuous use and avoid those cheap "traveller converters". A tranformer rated for continuous use and high power is a big device and weights easily many kilograms. A proper transformer can be used with any kinf of electronics equipment that can operate at the mains requency you have. The output of transformer is the same nice sinewave power as your incomign mains power.

    In addition to the transformer type converters there are also "electronic converters" that boast having over 1000W power ratings in a very small and lightwirght package. Those are not real transformers. They are devices that take the input 220V and convert the output to some signal that has similar average voltage as 110V voltage, but the waveform usually something entirely different. The converter could work like a diode (passes only positive half of mains power to output, not ood for mains distribution system and will nto give accurate 110V output) or like a mains dimmer (passes only around half of each mains half cycle to output). This kind of converter can work with simple heater type loads (coffee maker for example, simple hair dryer), but should not be used for anything else. If you try to connect any equipment with transformers or electronics in it, the result could be permanently damaged equipment!

    When you need to run 220V equipment at USA (normal maisn power 110V), you have the following options:

    • Safety isolation conversion transformer with 110V input and 220V output
    • Autotransformer type conversion tranformer from 110V to 220V
    • Use two live "phases" coming to electrical panel directly (this gives 220V voltage but instead of normal live and neutral wires, you have two live wires, this might not suit for all appliances and might need extra fuses etc..)

    An increasing number of hair dryers, clothes steamers, laptop 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).

    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.

    If you are bringing equipment from USA to Europe, remember the following things:

    • First: The 220/240 - 110 Volt thing. Most equipment which is destined (or orginates from) the US, is dedicated 110VAC stuff. This means you will require a step-down transformer, in order for your equipment to operate correctly at 220-240V AC voltage.
    • Second: The US uses 60Hz mains frequency. This MAY be a problem, depending on how smart the manufacturer is. Many US manufacturers (and far fewer Japanese manufacturers) employ power transformers which will only operate satisfactorily on 60Hz. When operating at 50Hz, they will often exhibit various problems, which may or may not be objectionable. If your equipment has mains voltage operated motors, the frequency difference usually causes lots of problems.
    • Third: The US electrical safety standards are not the same as European ones. Some products may not operate correctly, or to the requisite safety required by your local authorites. Don't forget: The US mains Voltage, is far less lethal than European mains supplies. As a consequence, safety standards in the US are not as high. For safety reasons it is recommended that if you run equipment from USA in Europe, connect it through a safety isolation transformer to mains power feed (isolation transformer that gives 110V AC output).
    • Fourth: There could be problems with radio equipment. FM de-emphasis and frequency steps may be different to your local area, thus FM reception may not be ideal. AM frequency steps may also be different. Many other radio equipment that have transmitter use different frequency ranges than European counterparts and are not type-approved for use in Europe (which means that you are not alloed to use them in Europe).
    If you are bringing equipment from Europe to USA, remember the following things:
    • First: The 220/240 - 110 Volt thing. Most equipment which is destined for European markets, is dedicated for 230VAC stuff. This means you will require a step-up transformer, in order for your equipment to operate correctly from 110V AC outlet. Some European equipment have options inside equipment to adjust the voltage, so check the manufacturer for this.
    • Second: The US uses 60Hz mains frequency where Europe uses 50 Hz. The transformers designed for 50 Hz operation generally work also well with 60 Hz mains power, so the frequency change to this direction is not a major problem. If your equipment has mains voltage operated motors, the frequency difference usually causes lots of problems. Many equipment have been damaged by using them on the 50 Hz mains, when they were designed for 60 Hz only.
    • Third: The US electrical safety standards are not the same as European ones. Usually the European safety standard are higher, so generally this is not a problem, but there can be special cases where the differences in regulations can be a problem.
    • Fourth: There could be problems with radio equipment. FM deemphasis and frequency steps may be different to your local area, thus FM reception may not be ideal. AM frequency steps may also be different. Many other radio equipment that have transmitter use different frequency ranges than USA counterparts and are not type-approved for use in USA (which means that you are not alloed to use them in USA).
    • Fifth: be careful with the selection of travel converter and the use of it. Be very careful using a travel converter that is not a real transformer. The electronic ones damage a lot of equipment. Many devices have been damaged by both these solid state travel converters.

    Whatever you are doing, you need to be very careful when using equipment abroad. When moving electrical appliances from one country to another, you need to check many thigns to make judgement if it is feasible or not. One thing that needs to be considered is that electrical equipment apprived in one country might not be approved for use in some other country. Using appliances that are not approved in the country you use them can have effect on your liability effect, for example using them could be illegal and/or the insurance company might not pay for any damages if anything goes wrong (for example your equipment brough from abroad bursn down your house).

    Electrical safety

    While standards of safety in most areas of life are constantly improving, the safety of domestic electrical installations is not keeping pace. People expect to be at their safest when in their own homes and tend not be aware of the risks that face them there. Incidents resulting from unsafe installations - deaths and injuries from fire and electrical shock - are preventable.

    Electrical wiring system design is always a compromise between safety and cost. The practical systems are designed in such way that the wiring is "safe enough" in normal use, but is not horribly expensive. The safety requirements on the new installations are more strict than on old systems, but there are many quite old electrical installations in use and will be in use for long time. The current completion rate for new build dwellings implies the average lifetime of a European dwelling is 200 years and the majority of European housing stock (60%) is already over 30 years old. Unless these buildings are properly adapted, maintained and renovated, their technical installations become progressively less suited to the higher standards of functionality, security and safety required by today's society.

    dwellings. The consequences of electric shock range from transitory discomfort to death, depending on the severity, timing and duration of the shock. Typically, the internal resistance between two limbs is a few hundred ohms. Current paths between both arms (the most likely path) and between a leg and an arm are the most hazardous because the current will pass through the area of the heart as well as through the muscles that control breathing. The physiological effects of a 50 Hz current vary with current magnitude, timing and duration and with the contact points on the body. The heart is particularly sensitive to shock during the period of recovery from excitation during each heartbeat. This means that the response to short shocks is unpredictable - a person may survive several severe short shocks by chance, so gaining a false sense of security, and yet be killed by a similar shock in the future.

    Installation standards require that electrical installations are designed to ensure that the users of the installation are not exposed to dangerous shocks. Designing for electrical safety is a technical issue for which regulations, standards and technical solutions exist. At the end of the 19th or the beginning of the 20th century, when public use of electricity started, standardisation bodies emerged in most European countries and in USA. Their establishment was largely stimulated by insurance companies whose interest was to reduce their fire losses but was also necessary because of the potential hazards of electric shock.

    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.

    Protecting electrical equipment and installations generally use the following protective measures:

    • Adequate earthing: This enables a fault current to flow safely and directly to the earth. Following the low resistance earthing path, a fault current will be sufficiently large to operate over-current protective devices within the required time.
    • Adequate protection against earth leakage: Today, an increasing number of electrical appliances permit a small current to 'leak' into the protective conductor of the main wiring system of a home. This 'earth leakage' currentis, for each individual device, quite small but, when there are many items of equipment, the sum of the currents is large enough to be potentially dangerous. To avoid this danger, various design features (such as RCDs) should be installed in every domestic installation.
    • Over-current protection: Circuits are designed to carry the expected load and are fitted with a protective device, i.e. a fuse or a circuit breaker. When the demand for current exceeds the rating of the protective device the circuit is disconnected.
    • Correct sizing of wires: Inadequate conductor size can cause overheating. It should be stressed that wires that were acceptable some 25 or 30 years ago, when electricity demand was lower, are often inadequate today. In theory, assuming the over-current protection devices are correct, those older wires will not cause safety problems.
    • Over-voltage protection: As electrical equipment in the home is becoming more sophisticated and more expensive (audio-visual entertainment, information technology equipment, white goods, etc.), the potential material loss due to over-voltage is increasing.
    The use of electricity out of doors increases the risk of electric shocks or electric burns. Important attention areas include:
    • use of weather-proof equipment and cables
    • consistent use of 3-pin plugs (grounded plugs)
    • avoidance of product or cable misuse
    • avoidance of over-long cables (long cables have large resistance, causing more voltage loss on equipment use, and in ground fault case slower protection fuse operation and posbily higher fault voltages before fuse burns)
    • unwinding the cable completely before using a retractable cable on a reel (to avoid cable oerheating)
    • use of a local RCD

    Ensuring electrical safety requires that:

    • Design and installation work is carried out in accordance with the standards in force at the time, using appropriate materials, techniques and equipment.
    • The installation is properly maintained and, if necessary, modified to match the new needs and new safety standards.

    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 fromits 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 currentthrough 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 previouslyoccured that caused another part of the circuit to be shorted to groundthen a return path will already exist, you will complete the circuit and acurrent 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 ofthe 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 wiring method that uses ungrounded power supply, it is called TT wiring. In TT wirign system you have to fit analarm 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 toinsulation failure, cable damage, water ingress, breakdown of capacitorsetc etc, all of which are particularly likely in a mobile, temporary installation with a large amount of equipment and huge quantities ofcabling 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, asnot being life threatening. This is always indicative of a fault situation, butthe 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 woul dbe morally, legally and financially liable. Operator Exposure safety details. Operators shall not be exposed to:

    • Energy levels of 240 VA or more.
    • Stored energy levels of 20 J or more.
    • Potentials in excess of 42.4 V peak (30 VRMS) or 60 VDC in dry areas.
    • Potentials in excess of 10 VAC or DC in wet areas.
    The operator(s) shall be protected from electrical and mechanical hazards by one or more of the following:
    • Enclosures, shields, and covers that require a tool to open.
    • Interlock switches on doors, shields, and ovens.
    • Grounded or insulated handles, levers, knobs, shields and covers that are touched held or actuated in normal use.
    Service personnel shall not be exposed to inadvertent contact with hazardous potentials or energy levels. All areas not defined as operator access areas that skilled service personnel must gain access to service or maintain the equipment. Here are some tips for good electrical safety:
    • All systems shall be installed as intended by manufacturer and according local electrical codes.
    • Any electrical installation, materials, equipment or apparatus within a workplace must be so designed, constructed, installed, protected, maintained and tested as to minimise the risk of electrical shock or fire.
    • Electrical wall outlets should be free of cracks, breaks, or other obvious damage. Damaged outlets should be immediatly repaired.
    • Personnel should conduct periodic inspections of all equipment to ensure that all cords are free of wear and splices, and that the casing or insulating covering is free of cracks, holes, or other damage.
    • Any electrical equipment that is damaged, malfunctioning or shows signs of unusual, excessive heating or producing "burning" odors, should be pulled from service and submitted for repair by qualified personnel.
    • If equipment produces shock, no matter how small, it should be removed from service and immediately repaired by a qualified electrician before returning to service.
    • Avoid excess bending, stretching and kinking of electrical supply cords.
    • Overloading electrical circuits is extremely dangerous and should not be permitted at any time. Significant amounts of heat can be generated by electrical leads which may lead to fires; especially if the current rating for the lead is exceeded.
    • Ensure that the wire sizes of extension cords are capable of handling the load without heating. When using extension leads ensure that they are fully extended, not covered by mats, and not placed where they could be a tripping hazard. If extension cord is coiled or covered with mat, it's safe current carrying capacity can be seriously reduced.
    • All electrically operated appliances that are designed to be grounded shall be effectively grounded. Tools or appliances protected by an approved system of double insulation, or its equivalent, need not be grounded
    • All electrical equipment should bear the label of a nationally recognized testing laboratory to guarantee that they are constructed safely.
    • If the competent person decides the equipment is not safe to use, they must attach a durable tag warning not to use the equipment; the equipment must also be immediately withdrawn from use.
    • Properly installed residual current device / earth leakage protection increases safety in dangerous locations. Residual current device should be periodically tested. Correct selection of the type of earth leakage protection is also important to avoid an unacceptable level of circuit tripping by the devices.
    • It is important to ensure that all electrical extension leads are in good condition before they are used.
    • The risk associated with electrical installations in hazardous atmospheres created by flammable gases, vapours from flammable liquids or combustible dusts should be carefully considered. Electrical appliances should either be specially designed equipment or be excluded from hazardous locations.
    • Special circuit protection such as residual current devices (RCDs) or isolation transformers are required for specified electrical equipment in workshops, laboratories, construction sites and other outdoor areas.
    • Patient treatment areas such as medical and dental surgeries have particular requirements on electrical safety.
    • Electrical heating appliances are a common cause of fires. Where possible appliances should have thermostat control and thermal overload protection.
    • Where electrical installations, equipment or extension leads are liable to damage from vehicles, other machinery or heavy people traffic, they should be protected from physical damage by appropriate covers or barriers.
    • The use of multi-outlet power boards or cords can be potentially unsafe because of the potential for overloading, and inadequate protection of circuits. In hazardous or wet areas multi-outlet power boards should be secured in a safe position.
    • All flexible cords will have two layers of insulation throughout their length, and will show no signs of excessive ware or physical damage.
    • Cord sets intended to be permanently attached to an item of equipment will be securely clamped to that equipment (internally or externally).
    European standards are different from US standards because they areintended for use in different overall regimes. Often the concepts forsafety 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 distributionschemes etc. are simply different, and are not under the control of one single oganizations who writes standards (there are many organizations, both national and international on this field).

    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.

    There eelectrical equipment are generally classified to several classes:

    • 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.

    Safety Alert: As with any electrical project, make sure that the power to the circuit where you are working is turned off at the breaker box. Test the wires with a tester to make absolutely certain that the power is off. It's advisable to place a note reading "electrical work in progress" on the breaker box while you are working to make sure that someone else doesn't unknowingly turn the power back on while you are working.

    The circuits which you are making connection must not be energized when you work with them. So you need to turn off the power before working with the wiring. If you are working on the wiring of a circuit (not just connecting device to outlet or similar), 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 aproper lock device with a key that only you have, lock it and tag it withyour name, date and reason why it is locked out. No one can randomly turnit 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 re-energize that circuit. You are then the one responsible for whether or not the circuit is safe to reenergize.

    Safety Alert:If you feel uncomfortable or unqualified to do electrical work yourself, then you should consider hiring a licensed electrician to do the work.

    Human sensitivity to electricity

    A number of years ago an awareness and concern about the effectsof power frequency EMFs arose. For a number of years it hadbeen known that electrical workers that were in close proximity to very strong magnetic or electric fields sometimes "saw" flashing lights or patternsthat 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 somedirect 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 ofthese 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 leastsome 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.

    Mains wiring

    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.

      General

        Protection devices

        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 melt?ng 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).

        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 componentslike wires, transformers electronics circuit modules against overload.The general idea of the fuse is that it "burns fuse link" when currentgets 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 itwill trip. The first is the rated current, and thesecond is the class of breaker. The breaker will hold the rated current forever. The class of breaker determines how large a surge current the breakerwill allow without tripping. Breakers are defined into classes by EN 60898. Type B and C breakers are likely to be suitable for generaluse (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' whateverthe 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 seethe 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.

        When applying any overload protection device, it is important to know that the available short circuit fault current at the device is not in excess of that which can safely be interrupted. Available short circuit current is the maximum RMS current which would flow if all active conductors were solidly bolted together at the point of fault protected by the device. In reality, actual fault current is much less than available fault current. The primary factors that determine the available fault current are supply transformer size, the impedance of the cable or wire and that of the connections. These factors, in addition to, the fault resistance, determine the actual fault current. As a rule of thumb, the available fault current from a 60Hz transformer is usually about 20 times the full load current, while a 400Hz transformer can produce about 12.5 times the full load current. It is determined by the supply voltage and percent impedance. The percent impedance is basically a statement of the internal impedance of the transformer and is available on the nameplate or from the manufacturer. The percent impedance for 60Hz transformers is approximately four to seven percent for average size transformers. Although a transformer can provide a severe limiting effect on fault current, wire and connector, resistance becomes very significant as distance increases. The resistance of a few yards of cable can reduce the fault current considerably. The effective current capacity of a line can be computed roughly by a simple differential measurement, i.e., the output voltage difference of the line from no load to full load. For example, if a 120 volt line supplying 30 amperes has a 6 volt drop. The total impedance back to the original generator is R = 6/30 = 0.2 ohms and the short-circuit current is 600 amperes until something lets go. The short circuit current should be in such range that it is so much higher than maximum allowed circuit load that the fuse/breaker with disconnect short circuit quicly and still lower than the maximum braking current of the fuse/breaker (if this is exceeded that protector is not capable of cutting the short circuit current flow properly). Because of the increasing capacity of power systems sometimes it is possible to have short-circuit current high enough to seriously damage conductor insulation even at short time. A short-time temperature limit of 150 degrees C is typical for thermoplastic insulation. Paper, rubber and varnished cloth insulation has a slightly higher short-circuit capability based on a short-time 200 degrees C temperature limit.

        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/electromechanicalprotection device that cuts off the fault circuit immediately on theoccasion 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 currentexceeds a preset value (30mA is common, but 5 mA, 10mA, 30mA, 100mA, 300mA and 3A types are available). Trip times are usually specified as less than 30ms, but delay types (to provide sub circuit discrimination) are available. Ground-fault current interrupt sockets or breakers with 5 mA trip current are commonly used in damp environments in USA. This kind of GFCIs are not to trip at 3 mA (more than 40 kohms leakage resistance, hot to ground, 120V AC), may trip at 4 mA (30 kohms), should trip at 5 mA (25 kohms), and must trip at 6 mA (20 kohms). In european installations the outlets or a group of outlets are typically protected with 30 mA ground fault breaker (7 kohm leakage resistance at 230V AC).

        RCDs are implemented usually in such way that phase (live) and neutral wirespass 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 voltageis 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 throughthe 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 foruse 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 in European electrical installation. 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).

        Earth leakage protection is an important part of the electrical safety scheme for any system, although it may present some implementation difficulties where there are conflicts between the safety ideal and the operational reality. In the best of all possible worlds every electrical circuit would be protected by an earth leakage device, but of course there is a down side to their use: firstly they're not cheap, but then again safety rarely is; more importantly they're subject to nuisance tripping. It seems that there are a number of circumstances where there may be an imbalance between the active and neutral currents without any safety implications. For example some discharge lamp ballasts, light dimmers and computer devices quite commonly have a small and often variable amount of earth leakage; put several of them on the same circuit, protected by the same earth leakage device, and you have an unscheduled blackout just waiting for the most inconvenient moment. Also 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. The ideal way to avoid a major production catastrophe is to place each device in your electrical system on its own rather costly earth leakage protected circuit ensuring that only the faulty device will be disconnected. A more cost-effective alternative may be to place devices with known leakage on to a circuit which has an earth leakage protection device with higher activation threshold (if electrical code allows) or one designed specifically to work with the ballasts of discharge light sources. Some devices may actually be so reduced in operational effectiveness/reliabity that they are better installed on circuits not fitted with an earth leakage device (for example refridgerators etc.). These variations are often only possible in fixed installations and must always be done in consultation with an electrician or electrical engineer who is fully cognisant with the safety and operational requirements of the situation.

        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 bybad contacts and terminal connections, particularly relevantfor 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.

        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."

        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.

        Grounding and earthing

        We all know that grounding (or "earthing" as the Europeans call it) is a necessity. It's required by electrical codes; it's required by equipment manufacturers; and we all know it would be "good practice" even if it wasn't required.

        In any real-worldsituation where there is any chance of an insulation fault, or any otherresistive, capacitive or inductive path to local 'ground', proper safety earth bonding is desirable.

        Grounding is often misunderstood because there are so many different reasons for doing it, each with its own set of concerns, considerations and installation methods. It's also misunderstood because the problems that can occur when it's done wrong are essentially invisible, difficult to comprehend, often without a good explanation and hard to track down when they happen.

        Most professionals deal with only one or two types of grounding in their careers. The majority don't necessarily know that the communications industry has its own set of requirements, and don't realize that, while there are similarities, what is fine in one field doesn't always do the job in another.

        Let's identify some of these most commonly seen grounding specialties:

        • Electrical safety grounds: Probably the most fundamental of all grounds, these are required by code to protect people from injury in the event of a short or "fault" that puts current onto an equipment housing. That's why the "U-ground" pin or other form of grounding connection is found on lots of appliance connections. One of the power wires, called the neutral (white conductor), is also normally grounded, but if something goes amiss with it, the "U-ground" keeps you safe. The building power ground goes to an "earth terminal," is bonded to building steel and is also carried to every electrical panel in the building. Code requires a building safety ground to have a ground resistance of 25 Ohms or less in USA.
        • Lightning grounds: These are designed to conduct lightning strikes directly to ground so they don't damage the building or its electrical systems, or injure people. The intent is to carry the lightning strike to earth through the building steel or through wires run down the outside of the structure to rods driven into the ground. These ground rods are also bonded to the main electrical ground, as is the building steel.Lightning, by its nature, includes a large high frequency component, and should have very direct route to ground (doesn't bend corners very well)
        • RF shielding and grounding: Radio frequencies are very high, and therefore have very short wavelengths. RF tends to find its way into everything, especially where it is not wanted. The only way to stop RFI (radio frequency interference) is with a virtually continuous grounded shield aroud the signal carryign wires and sentive electronics. Commonly seen in broadcasting, this type of grounding is achieved by making sure all metal parts are solidly bonded together -- essentially grounded everywhere. An RF shielded cabinet typically have doors that close against hundreds of small, spring bronze fingers or against some sort of metallic braid that forms a continuous electrical connection around the entire door edge.
        • Electro-static grounds: After the mandatory electrical safety ground, this is what we want in electronics manufacturing factories, repair shops and in data centers. It's the reason we wear (or should wear) wrist straps when we work on micro-electronics and why we use anti-static floor tiles in data centers instead of carpet. Static discharge is just a personal lightning bolt, similar to lighting strike but lower in energy. The discharge may find its ground path right through our sensitive and expensive hardware, causing operation errors (data changes, computer "crashing" etc.) and possibly component damage. The smaller and faster our hardware becomes, the more vulnerable it is to static problems. We want to draw those electrons away from anything important and get them to ground as quickly and as completely as we can. If everything is well bonded to a robust and virtually omnipotent grounding system, that's the path any static discharges are going to take if the system leads back to the main building ground through a very low impedance path.

        Today electrical installation standards are highly developed and cover all major aspectsfor a safe installation. Standard maker have paid particular attention to the measuresto be implemented to guarantee protection of of the personnel and property. This concern has resulterd in standardization of three Earthign Systems.

        • TN-system: The mains distribution transformer is neutral earthed and electrical load frames are connected ot neutral. An insulation fault on a phase becomes a short-circuit and the faulty part is disconnected by a Short-Circuit Protection Device (SCPD). There are three variations of this: TN-C (same conductor acts as neutral and a protective conductor), TN-S (neutral and protective conductors are separate) and TN-C-S (a combination of TN-C and TN-S).
        • TT-system: The mains distribution transformer is neutral earthed and electrical load frames are also earthed. The current of an insulation fault is limited by earth connnection impedance. Protection is provided by Residual Current Devices (RCD) which disconnects the faulty load when too much leakage current to ground exists.
        • IT-system: The transformer neutral is not earthed (theoretically unearthed, practically connection to earth exist through stray capacitances in cables or voluntarily by high impedance of around 1500 ohms). The electrical load frames are earthed. If a single insulation fault occurs, a low fault current develops. The contact voltage developed in the frame (no mode than few volts) is not dangrous. The SCPDs and/or the RCDs provide the necessary protection against two simulaneous faults.

        Many mains distribution networks between buildings use Multiple earth grounding principle. In multiple earth groundingthere is a ground rod at the service entrance panel of the buildingand there are other ground rods connected at other buildings to theneutral in a similar manor.

        One grounding approach used in sensitive electrical installationis "single point grounding", which is tying allof 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 smallenough, 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 byhi-voltage systems is to protect the low voltage 'stuff' from an accidental high voltage. Transformer insulation breakdown, a high-voltage line fallingacross 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-voltageline 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 toavoid burning down houses.

        As to running a grounding wire to every outlet, the idea there is if thecasing 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) willshort to the grounding conductor and back to the service entrance panel,tripping the breaker. The voltage developed on the case will be mild andunless you're grounded by a separate circuit, you won't be seriouslyinjured. Double insulated tools/appliances are excused from third prongIIRC 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 shockif 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.

        Circulating ground currents create their own electrical noise, so are to be avoided. In principle, they're easy to stop. Just keep everything at the same electrical potential or voltage. Current will only flow between two points that have a difference of potential. f we ground everything together with heavy wires, then everything should be at "equal potential" and no current will flow. Not surprisingly, this is called an "equal potential ground" and is exactly what J-STD-607-A is trying to achieve. The difficulty is doing it in a practical way. It's unrealistic to weld everything in the building or even in just the data center, together with heavy copper bars.

        "isolated grounds" have no place in the properly designed data center. The minute a metal chassis is screwed into a metal cabinet, another ground path is established -- and not a very good one either. Each piece of equipment does the same thing, until there are multiple ground paths, none of them very low-impedance, all running through small-gauge wires and ending up at the building ground via different paths of all different lengths. The result is a poor static ground and loads of circulating currents due to the many different electrical levels that result. It's a waste of money on something that will be counter-productive in the end.

        Grounding is a safety issue, absolutely required by code. A good telecommunications ground can be built as a "separate system" all the way to the electrical vault, although it should really be bonded to building steel and local electrical panels at various places along the way. It can even have its own set of ground rods if that becomes necessary to approach the lower 5-Ohm ground resistance recommended for telecommunications services. But these ground rods had better be bonded to the main electrical ground for the building. If you have a vendor who tells you they require a "separate ground" connected only to its own ground rods, tell them to consult a qualified engineer or code authority. God forbid there should ever be something called a "ground fault" in your incoming, high-voltage, building electrical service. The soil resistance between the separated grounds will result in a huge voltage difference if a "fault" occurs, and the resulting current will instantly boil the earth causing a sort of explosion. Also the resulting power surge on your "separate ground" could fry everything, and everybody, that's in contact with a grounded device. In short, usign separate grounding systems that are not properly interconnected is not a wise approach.

        The "ultimate" in telecommunications grounding practice is called the "PANI" ground. This approach actually divides the ground bar into four sectors identified as "producers," "surge arrestors," "non-isolated" and "isolated" ground networks (PANI). Those sectors are wired in such wayt that ground currents flow within the ground bar in a way that further avoids ground current interaction. PANI grounds are used in major telecommunications carrier installations and are often required by the military.

        Good data center or telecommunications utility grounding requires understanding, careful planning, proper execution and good supervision. It is not inexpensive. In normal residential and commercial building the grounding need are not as trict, but still doing those applications adequately well requires understanding, careful planning, proper execution and good supervision.

        Mains power connectors

        Different countries and different environments use differnt mains connectors. In USA NEMA Plug & Receptacle configuarations a