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156-401: An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground , typically the equipments conductive surface, for safety and functional purposes. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow

312-508: A GFCI breaker , for ground fault circuit interrupter , in the United States and Canada. The diagram depicts the internal mechanism of a residual-current device (RCD). The device is designed to be wired in-line in an appliance power cord. It is rated to carry a maximal current of 13   A and is designed to trip on a leakage current of 30   mA. This is an active RCD; that is, it latches electrically and therefore trips on power failure,

468-482: A TT (Latin: terra–terra) earthing system, the protective earth connection for the consumer is provided by a local earth electrode, (sometimes referred to as the terra firma connection) and there is another independently installed at the generator. There is no 'earth wire' between the two. The fault loop impedance is higher, and unless the electrode impedance is very low indeed, a TT installation should always have an RCD (GFCI) as its first isolator. The big advantage of

624-460: A circuit breaker or a residual-current device (RCD) will automatically open the circuit to clear the fault. Functional earthing serves a purpose other than electrical safety. Example purposes include electromagnetic interference (EMI) filtering in an EMI filter, and the use of the Earth as a return path in a single-wire earth return distribution system. In low-voltage networks , which distribute

780-449: A radio mast , is dissipated to the Earth. In the event of a surge, a lightning arrester , a surge arrester or a surge protection device (SPD) will divert the excess current to the Earth before it reaches an appliance. System earthing allows for equipotential bonding to all metal works to prevent potential differences between them. Having Earth as a common reference point keeps the electrical system's potential difference limited to

936-399: A residual-current device is installed to detect the current leaking to ground and interrupt the circuit. International standard IEC 60364 distinguishes three families of earthing arrangements, using the two-letter codes TN , TT , and IT . The first letter indicates the connection between earth and the power-supply equipment (generator or transformer): The second letter indicates

1092-502: A solidly earthed system, earth fault current can be as much as the available short-circuit current. The neutral earthing resistor is monitored to detect an interrupted ground connection and to shut off power if a fault is detected. To avoid accidental shock, current sensing circuits are used at the source to isolate the power when leakage current exceeds a certain limit. Residual-current devices (RCDs, RCCBs or GFCIs) are used for this purpose. Previously, an earth leakage circuit breaker

1248-457: A DC current is present that saturates the core of the detector. The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics. The IEC 61008 and IEC 61009 standards require that RCDs withstand a 200   A "ring wave" impulse. The standards also require RCDs classified as "selective" to withstand a 3000   A impulse surge current of specified waveform. RCDs can be tested with

1404-509: A GFCI (Ground-Fault Circuit Interrupter) breaker in the US and Canada, and as a RCBO (residual-current circuit breaker with over-current protection) in Europe and Australia. They are effectively a combination of a RCD and a MCB . In the US, GFCI breakers are more expensive than GFCI outlets. As well as requiring both live and neutral inputs and outputs (or, full three-phase), many GFCI/RCBO devices require

1560-904: A built-in test button to confirm functionality on a regular basis. RCDs may not operate correctly if wired improperly, so they are generally tested by the installer. By introducing a controlled fault current from live to earth, the operating time and wiring can be tested. Such a test may be performed on installation of the device and at any "downstream" outlet. (Upstream outlets are not protected.) To avoid needless tripping, only one RCD should be installed on any single circuit (excluding corded RCDs, such as bathroom small appliances). A residual-current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detect overload conditions, phase-to-neutral short circuits or phase-to-phase short circuits (see three-phase electric power ). Over-current protection ( fuses or circuit breakers ) must be provided. Circuit breakers that combine

1716-486: A circuit. But it cannot protect against overload or short circuit like a fuse or a miniature circuit breaker (MCB) does (except for the special case of a short circuit from live to ground, not live to neutral). However, an RCD and an MCB often come integrated in the same device, thus being able to detect both supply imbalance and overload current. Such a device is called an RCBO , for residual-current circuit breaker with overcurrent protection , in Europe and Australia, and

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1872-580: A commercial power system in hot climates is the HVAC unit, and ensuring this unit is adequately supplied is an important consideration in commercial power systems. Regulations for commercial establishments place other requirements on commercial systems that are not placed on residential systems. For example, in Australia, commercial systems must comply with AS 2293, the standard for emergency lighting, which requires emergency lighting be maintained for at least 90 minutes in

2028-530: A commercial power system. In 1886, one of Westinghouse's engineers, William Stanley , independently recognized the problem with connecting transformers in series as opposed to parallel and also realized that making the iron core of a transformer a fully enclosed loop would improve the voltage regulation of the secondary winding. Using this knowledge he built a multi-voltage transformer-based alternating-current power system serving multiple homes and businesses at Great Barrington, Massachusetts in 1886. The system

2184-595: A failure of a device to detect the fault will eventually be cleared by a higher-level device, at the cost of interrupting more circuits. IEC Standard 60755 ( General requirements for residual current operated protective devices ) defines three types of RCD depending on the waveforms and frequency of the fault current. The BEAMA RCD Handbook - Guide to the Selection and Application of RCDs summarises this as follows: and notes that these designations have been introduced because some designs of type A and AC RCD can be disabled if

2340-565: A fallen tree or branch. In pre- RCD era, the TT earthing system was unattractive for general use because of the difficulty of arranging reliable automatic disconnection (ADS) in the case of a line-to-PE fault (in comparison with TN systems, where the same breaker or fuse will operate for either L-N or L-PE faults). But as residual current devices mitigate this disadvantage, the TT earthing system has become much more attractive providing that all AC power circuits are RCD-protected. In some countries (such as

2496-438: A far more refined response than circuit-breaker-switched capacitors. Static synchronous compensators take this a step further by achieving reactive power adjustments using only power electronics . Power electronics are semiconductor based devices that are able to switch quantities of power ranging from a few hundred watts to several hundred megawatts. Despite their relatively simple function, their speed of operation (typically in

2652-575: A fault condition occurs. RCDs used on single-phase AC supplies (two current paths), such as domestic power, are usually one- or two-pole designs, also known as single- and double-pole . A single-pole RCD interrupts only the energized conductor, while a double-pole RCD interrupts both the energized and return conductors. (In a single-pole RCD, the return conductor is usually anticipated to be at ground potential at all times and therefore safe on its own). RCDs with three or more poles can be used on three-phase AC supplies (three current paths) or to disconnect

2808-585: A function. These loads range from household appliances to industrial machinery. Most loads expect a certain voltage and, for alternating current devices, a certain frequency and number of phases. The appliances found in residential settings, for example, will typically be single-phase operating at 50 or 60 Hz with a voltage between 110 and 260 volts (depending on national standards). An exception exists for larger centralized air conditioning systems as these are now often three-phase because this allows them to operate more efficiently. All electrical appliances also have

2964-445: A functional earth (FE) connection. This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but live and earth remain. For reasons of space, many devices, especially in DIN rail format, use flying leads rather than screw terminals, especially for the neutral input and FE connections. Additionally, because of the small form factor,

3120-474: A further four years for Westinghouse engineers to develop a workable polyphase motor and transmission system. By 1889, the electric power industry was flourishing, and power companies had built thousands of power systems (both direct and alternating current) in the United States and Europe. These networks were effectively dedicated to providing electric lighting. During this time the rivalry between Thomas Edison and George Westinghouse's companies had grown into

3276-465: A grid through an asynchronous tie such as a HVDC link — these can operate at frequencies independent of the power system frequency. Depending on how the poles are fed, alternating current generators can produce a variable number of phases of power. A higher number of phases leads to more efficient power system operation but also increases the infrastructure requirements of the system. Electricity grid systems connect multiple generators operating at

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3432-493: A grounding conductor. The in-line RCD can also have a lower tripping threshold than the building to further improve safety for a specific electrical device. In North America, GFI receptacles can be used in cases where there is no grounding conductor, but they must be labeled as "no equipment ground". This is referenced in the National Electric Code section 406 (D) 2, however codes change and someone should always consult

3588-481: A high-voltage distribution line. The system lit more than 1000 carbon filament lamps and operated successfully from May until November of that year. Also in 1885 George Westinghouse , an American entrepreneur, obtained the patent rights to the Gaulard-Gibbs transformer and imported a number of them along with a Siemens generator, and set his engineers to experimenting with them in hopes of improving them for use in

3744-459: A licensed professional and their local building and safety departments. The code is An ungrounded GFI receptacle will trip using the built-in "test" button, but will not trip using a GFI test plug, because the plug tests by passing a small current from line to the non-existent ground. It is worth noting that despite this, only one GFCI receptacle at the beginning of each circuit is necessary to protect downstream receptacles. There does not appear to be

3900-410: A means of communication between the various components of the earthing and lightning protection installations (earthing rods, earthing conductors, current leads, busbars , etc.). For high voltage installations, exothermic welding is used for underground connections. Soil resistance is a major aspect in the design and calculation of an earthing system/grounding installation. Its resistance determines

4056-493: A more robust solenoid part as illustrated are now dominant. In the internal mechanism of an RCD, the incoming supply and the neutral conductors are connected to the terminals at (1), and the outgoing load conductors are connected to the terminals at (2). The earth conductor (not shown) is connected through from supply to load uninterrupted. When the reset button (3) is pressed, the contacts ((4) and another, hidden behind (5)) close, allowing current to pass. The solenoid (5) keeps

4212-526: A nuisance, the fault is with the deteriorated element and not the RCD: replacement of the offending element will resolve the problem, but replacing the RCD will not. RCDs are not selective , for example when a ground fault occurs on a circuit protected by a 30 mA I Δn RCD in series with a 300 mA I Δn RCD either or both may trip. Special time-delayed types are available to provide selectivity in such installations. In

4368-438: A particular safety hazard, for example long extension leads, which might be used outdoors, or garden equipment or hair dryers, which may be used near a bath or sink. Occasionally an in-line RCD may be used to serve a similar function to one in a plug. By putting the RCD in the extension lead, protection is provided at whatever outlet is used even if the building has old wiring, such as knob and tube , or wiring that does not contain

4524-408: A person from a phase (live / line / hot) to earth. It cannot protect against electric shock when current flows through a person from phase to neutral or from phase to phase, for example where a finger touches both live and neutral contacts in a light fitting; a device cannot differentiate between current flow through an intended load from flow through a person, though the RCD may still trip if the person

4680-410: A person touching a live component in the attached appliance) causes some of the current to take a different return path, which means that there is an imbalance (difference) in the current in the two conductors (single-phase case), or, more generally, a nonzero sum of currents from among various conductors (for example, three phase conductors and one neutral conductor). This difference causes a current in

4836-400: A power system also limit rushes of current flow, small reactors are therefore almost always installed in series with capacitors to limit the current rush associated with switching in a capacitor. Series reactors can also be used to limit fault currents. Capacitors and reactors are switched by circuit breakers, which results in sizeable step changes of reactive power. A solution to this comes in

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4992-406: A power-distribution system and are almost always of the passive or latched variety, whereas the fourth relates solely to specific appliances and are always of the active or non-latching variety. Active means prevention of any re-activation of the power supply after any inadvertent form of power outage, as soon as the mains supply becomes re-established; latch relates to a switch inside

5148-526: A propaganda campaign over which form of transmission (direct or alternating current) was superior, a series of events known as the " war of the currents ". In 1891, Westinghouse installed the first major power system that was designed to drive a 100 horsepower (75 kW) synchronous electric motor, as well as provide electric lighting, at Telluride, Colorado . On the other side of the Atlantic, Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown , built

5304-402: A range of temporal issues. These include voltage sags, dips and swells, transient overvoltages, flicker, high-frequency noise, phase imbalance and poor power factor. Power quality issues occur when the power supply to a load deviates from the ideal. Power quality issues can be especially important when it comes to specialist industrial machinery or hospital equipment. Conductors carry power from

5460-428: A risk of using multiple GFI receptacles on the same circuit, though it is considered redundant. In Europe, RCDs can fit on the same DIN rail as the miniature circuit breakers ; much like in miniature circuit breakers, the busbar arrangements in consumer units and distribution boards provides protection for anything downstream. A pure RCD will detect imbalance in the currents of the supply and return conductors of

5616-409: A short time delay. They are typically used at the origin of an installation for fire protection to discriminate with 'G' devices at the loads, and in circuits containing surge suppressors. They must not trip at one-half of rated current. They provide at least 130 milliseconds delay of tripping at rated current, 60 milliseconds at twice rated, and 50 milliseconds at five times rated. The maximum break time

5772-461: A single team might be responsible for fault management, augmentation and maintenance. Where as for the electric grid , management is divided amongst several specialised teams. Fault management involves monitoring the behaviour of the power system so as to identify and correct issues that affect the system's reliability. Fault management can be specific and reactive: for example, dispatching a team to restring conductor that has been brought down during

5928-442: A small leakage condition, and another button resets the conductors after a fault condition has been cleared. Some RCDs disconnect both the energized and return conductors upon a fault (double pole), while a single pole RCD only disconnects the energized conductor. If the fault has left the return wire " floating " or not at its expected ground potential for any reason, then a single-pole RCD will leave this conductor still connected to

6084-505: A storm. Or, alternatively, can focus on systemic improvements: such as the installation of reclosers on sections of the system that are subject to frequent temporary disruptions (as might be caused by vegetation, lightning or wildlife). In addition to fault management, power systems may require maintenance or augmentation. As often it is neither economical nor practical for large parts of the system to be offline during this work, power systems are built with many switches. These switches allow

6240-422: A trip if the current on any phase exceeds a certain threshold whereas a set of differential relays might initiate a trip if the sum of currents between them indicates there may be current leaking to earth. The circuit breakers in higher powered applications are different too. Air is typically no longer sufficient to quench the arc that forms when the contacts are forced open so a variety of techniques are used. One of

6396-529: A tripping mechanism). In higher powered applications, the protective relays that detect a fault and initiate a trip are separate from the circuit breaker. Early relays worked based upon electromagnetic principles similar to those mentioned in the previous paragraph, modern relays are application-specific computers that determine whether to trip based upon readings from the power system. Different relays will initiate trips depending upon different protection schemes . For example, an overcurrent relay might initiate

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6552-416: A useful feature for equipment that could be dangerous on unexpected re-energisation . Some early RCDs were entirely electromechanical and relied on finely balanced sprung over-centre mechanisms driven directly from the current transformer. As these are hard to manufacture to the required accuracy and prone to drift in sensitivity both from pivot wear and lubricant dry-out, the electronically-amplified type with

6708-438: A wattage rating, which specifies the amount of power the device consumes. At any one time, the net amount of power consumed by the loads on a power system must equal the net amount of power produced by the supplies less the power lost in transmission. Making sure that the voltage, frequency and amount of power supplied to the loads is in line with expectations is one of the great challenges of power system engineering. However it

6864-438: A wide range of techniques used to spin a turbine's rotor, from steam heated using fossil fuel (including coal, gas and oil) or nuclear energy to falling water ( hydroelectric power ) and wind ( wind power ). The speed at which the rotor spins in combination with the number of generator poles determines the frequency of the alternating current produced by the generator. All generators on a single synchronous system, for example,

7020-456: Is 500   ms at rated current, 200   ms at twice rated, and 150   ms at five times rated. Programmable earth fault relays are available to allow co-ordinated installations to minimise outage. For example, a power distribution system might have a 300   mA, 300   ms device at the service entry of a building, feeding several 100   mA 'S' type at each sub-board, and 30   mA 'G' type for each final circuit. In this way,

7176-411: Is as a safety device to detect small leakage currents (typically 5–30   mA) and disconnecting quickly enough (<30 milliseconds) to prevent device damage or electrocution . They are an essential part of the automatic disconnection of supply (ADS), i.e. to switch off when a fault develops, rather than rely on human intervention, one of the essential tenets of modern electrical practice. To reduce

7332-886: Is based on considerations such as cost, transmission losses and other desirable characteristics of the metal like tensile strength. Copper , with lower resistivity than aluminum , was once the conductor of choice for most power systems. However, aluminum has a lower cost for the same current carrying capacity and is now often the conductor of choice. Overhead line conductors may be reinforced with steel or aluminium alloys. Conductors in exterior power systems may be placed overhead or underground. Overhead conductors are usually air insulated and supported on porcelain, glass or polymer insulators. Cables used for underground transmission or building wiring are insulated with cross-linked polyethylene or other flexible insulation. Conductors are often stranded for to make them more flexible and therefore easier to install. Conductors are typically rated for

7488-410: Is because the power provided by the car's batteries alone is insufficient to provide ignition, air-conditioning, internal lighting, radio and dashboard displays for the life of the car. So the batteries must be recharged while driving—a feat that is typically accomplished using power electronics. Some electric railway systems also use DC power and thus make use of power electronics to feed grid power to

7644-445: Is distant from the load, it is desirable to step-up (increase) the voltage of power at the generation point and then step-down (decrease) the voltage near the load. Secondly, it is often more economical to install turbines that produce higher voltages than would be used by most appliances, so the ability to easily transform voltages means this mismatch between voltages can be easily managed. Solid-state devices , which are products of

7800-907: Is generally unnecessary, provided they have been wired properly. One exception is the case of a TT earthing system , where the earth loop impedance may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100   mA (or greater) trip current time-delayed RCD is installed, covering the whole installation, and then more sensitive RCDs should be installed downstream of it for sockets and other circuits that are considered high-risk. In addition to ground fault circuit interrupters (GFCIs), arc-fault circuit interrupters (AFCI) are important as they offer added protection from potentially hazardous arc faults resulting from damage in branch circuit wiring as well as extensions to branches such as appliances and cord sets. By detecting arc faults and responding by interrupting power, AFCIs help reduce

7956-606: Is grounded (earthed) at each consumer service point thereby effectively bringing the neutral potential difference towards zero along the whole length of LV lines. In the IEC 60364 terminology this is called TN-C-S. In North America, the term "multigrounded neutral" system (MGN) is used. In the UK and some Commonwealth countries, the term "PNE", meaning phase-neutral-earth is used to indicate that three (or more for non-single-phase connections) conductors are used, i.e., PN-S. A resistance earth system

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8112-465: Is in contact with the ground (earth), as some current may still pass through the persons finger and body to earth. Whole installations on a single RCD, common in older installations in the UK, are prone to "nuisance" trips that can cause secondary safety problems with loss of lighting and defrosting of food. Frequently the trips are caused by deteriorating insulation on heater elements, such as water heaters and cooker elements or rings. Although regarded as

8268-517: Is inductive; the current lags behind the voltage. Since the voltage and current are out-of-phase, this leads to the emergence of an "imaginary" form of power known as reactive power . Reactive power does no measurable work but is transmitted back and forth between the reactive power source and load every cycle. This reactive power can be provided by the generators themselves but it is often cheaper to provide it through capacitors, hence capacitors are often placed near inductive loads (i.e. if not on-site at

8424-516: Is mostly installed just as described above, but some wall socket RCDs are available to fit the fourth situation, often by operating a switch on the fascia panel. RCDs for the first and third situation are most commonly rated at 30   mA and 40   ms. For the fourth situation, there is generally a greater choice of ratings available – generally all lower than the other forms, but lower values often result in more nuisance tripping. Sometimes users apply protection in addition to one of

8580-415: Is not the only challenge, in addition to the power used by a load to do useful work (termed real power ) many alternating current devices also use an additional amount of power because they cause the alternating voltage and alternating current to become slightly out-of-sync (termed reactive power ). The reactive power like the real power must balance (that is the reactive power produced on a system must equal

8736-729: Is offset by inherent drawback that the fault location is hard to detect. According to the IEEE standards, grounding rods are made from material such as copper and steel . For choosing a grounding rod there are several selection criteria such as: corrosion resistance, diameter depending on the fault current , conductivity and others. There are several types derived from copper and steel: copper-bonded, stainless-steel, solid copper, galvanized steel ground. In recent decades, there has been developed chemical grounding rods for low impedance ground containing natural electrolytic salts. and Nano-Carbon Fiber Grounding rods. Connectors for earthing installation are

8892-540: Is performed with two, three or four electrodes. The measurement methods are: pole-pole, dipole-dipole, pole-dipole, Wenner method, and the Schlumberger method. Electric power system An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into

9048-443: Is required to produce an AC output but that by its nature produces a DC output. They are therefore used by photovoltaic installations. Power electronics also feature in a wide range of more exotic uses. They are at the heart of all modern electric and hybrid vehicles—where they are used for both motor control and as part of the brushless DC motor . Power electronics are also found in practically all modern petrol-powered vehicles, this

9204-455: Is the fuse. When the current through a fuse exceeds a certain threshold, the fuse element melts, producing an arc across the resulting gap that is then extinguished, interrupting the circuit. Given that fuses can be built as the weak point of a system, fuses are ideal for protecting circuitry from damage. Fuses however have two problems: First, after they have functioned, fuses must be replaced as they cannot be reset. This can prove inconvenient if

9360-454: Is the product of two quantities: current and voltage . These two quantities can vary with respect to time ( AC power ) or can be kept at constant levels ( DC power ). Most refrigerators, air conditioners, pumps and industrial machinery use AC power, whereas most computers and digital equipment use DC power (digital devices plugged into the mains typically have an internal or external power adapter to convert from AC to DC power). AC power has

9516-498: Is typically not a problem in most residential applications where standard wiring provides an active and neutral line for each appliance (that is why your power plugs always have at least two tongs) and the voltages are relatively low however these issues limit the effectiveness of RCDs in other applications such as industry. Even with the installation of an RCD, exposure to electricity can still prove fatal. In large electric power systems, supervisory control and data acquisition (SCADA)

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9672-410: Is used for conductors bonding an Earth/Ground rod, electrode or similar to a service panel. The "local" Earth/Ground electrode provides "system grounding" at each building where it is installed. The "Grounded" current carrying conductor is the system "neutral". Australian and New Zealand standards use a modified protective multiple earthing (PME ) system called multiple earthed neutral (MEN). The neutral

9828-467: Is used for mining in India as per Central Electricity Authority Regulations. Instead of a solid connection of neutral to earth, a neutral grounding resistor (NGR) is used to limit the current to ground to less than 750 mA. Due to the fault current restriction it is safer for gassy mines. Since the earth leakage is restricted, leakage protection devices can be set to less than 750 mA. By comparison, in

9984-403: Is used for tasks such as switching on generators, controlling generator output and switching in or out system elements for maintenance. The first supervisory control systems implemented consisted of a panel of lamps and switches at a central console near the controlled plant. The lamps provided feedback on the state of the plant (the data acquisition function) and the switches allowed adjustments to

10140-415: Is used. In industrial applications, earth leakage relays are used with separate core balanced current transformers. This protection works in the range of milli-Amps and can be set from 30 mA to 3000 mA. A separate pilot wire is run from distribution/ equipment supply system in addition to earth wire, to supervise the continuity of the wire. This is used in the trailing cables of mining machinery. If

10296-546: The Thomson-Houston Electric Company , forming General Electric . In 1895, after a protracted decision-making process, alternating current was chosen as the transmission standard with Westinghouse building the Adams No. 1 generating station at Niagara Falls and General Electric building the three-phase alternating current power system to supply Buffalo at 11 kV. Developments in power systems continued beyond

10452-421: The fuse box and then split into one or more circuits to feed lighting and appliances inside the house. By convention, the lighting and appliance circuits are kept separate so the failure of an appliance does not leave the dwelling's occupants in the dark. All circuits would be fused with an appropriate fuse based upon the wire size used for that circuit. Circuits would have both an active and neutral wire with both

10608-513: The generators that supply the power, the transmission system that carries the power from the generating centers to the load centers , and the distribution system that feeds the power to nearby homes and industries. Smaller power systems are also found in industry, hospitals, commercial buildings, and homes. A single line diagram helps to represent this whole system. The majority of these systems rely upon three-phase AC power —the standard for large-scale power transmission and distribution across

10764-405: The information and communications technology (ICT) field to the power engineering field. For example, the development of computers meant load flow studies could be run more efficiently, allowing for much better planning of power systems. Advances in information technology and telecommunication also allowed for effective remote control of a power system's switchgear and generators. Electric power

10920-525: The national grid , rotate at sub-multiples of the same speed and so generate electric current at the same frequency. If the load on the system increases, the generators will require more torque to spin at that speed and, in a steam power station, more steam must be supplied to the turbines driving them. Thus the steam used and the fuel expended directly relate to the quantity of electrical energy supplied. An exception exists for generators incorporating power electronics such as gearless wind turbines or linked to

11076-400: The semiconductor revolution, make it possible to transform DC power to different voltages , build brushless DC machines and convert between AC and DC power . Nevertheless, devices utilising solid-state technology are often more expensive than their traditional counterparts, so AC power remains in widespread use. All power systems have one or more sources of power. For some power systems,

11232-638: The Earth as one conductor of a circuit, saving the cost of installation of a return wire over a long circuit. Radio antennas may require particular grounding for operation, as well as to control static electricity and provide lightning protection. There are three main purposes for earthing: System earthing serves a purpose of electrical safety throughout the system that is not caused by a short circuit or other electrical fault . It prevents static buildup and protects against power surges caused by nearby lightning strikes or switching. Static buildup, as induced by friction for example, such as when wind blows onto

11388-537: The IEC, thus making it possible to divide RCDs into three groups according to their I Δn value: The 5   mA sensitivity is typical for GFCI outlets. There are two groups of devices. 'G' (general use) 'instantaneous' RCDs have no intentional time delay. They must never trip at one-half of the nominal current rating, but must trip within 200 milliseconds for rated current, and within 40 milliseconds at five times rated current. 'S' (selective) or 'T' (time-delayed) RCDs have

11544-477: The NEC no longer permit this practice. For similar reasons, most countries have now mandated dedicated protective earth connections in consumer wiring that are now almost universal. In the distribution networks, where connections are fewer and less vulnerable, many countries allow the earth and neutral to share a conductor. If the fault path between accidentally energized objects and the supply connection has low impedance,

11700-431: The RCD device has additional overcurrent protection integrated in the same device, it is referred to as RCBO . An earth leakage circuit breaker may be an RCD, although an older type of voltage-operated earth leakage circuit breaker (ELCB) also exists. These devices are designed to quickly interrupt the protected circuit when it detects that the electric current is unbalanced between the supply and return conductors of

11856-463: The RCD is necessary. The difference between the modes of operation of the essentially two different types of RCD functionality is that the operation for power distribution purposes requires the internal latch to remain set within the RCD after any form of power disconnection caused by either the user turning the power off, or after any power outage; such arrangements are particularly applicable for connections to refrigerators and freezers. Situation two

12012-492: The TT earthing system is the reduced conducted interference from other users' connected equipment. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT networks do not pose any serious risks in the case of a broken neutral. In addition, in locations where power is distributed overhead, earth conductors are not at risk of becoming live should any overhead distribution conductor be fractured by, say,

12168-650: The UK) TT is recommended for situations where a low impedance equipotential zone is impractical to maintain by bonding, where there is significant outdoor wiring, such as supplies to mobile homes and some agricultural settings, or where a high fault current could pose other dangers, such as at fuel depots or marinas. The TT earthing system is used throughout Japan, with RCD units in most industrial settings or even at home. This can impose added requirements on variable frequency drives and switched-mode power supplies which often have substantial filters passing high frequency noise to

12324-773: The United Kingdom and Australia both the protective earth and neutral line would be earthed together near the fuse box before the main isolating switch and the neutral earthed once again back at the distribution transformer. There have been a number of minor changes over the years to practice of residential wiring. Some of the most significant ways modern residential power systems in developed countries tend to vary from older ones include: Commercial power systems such as shopping centers or high-rise buildings are larger in scale than residential systems. Electrical designs for larger commercial systems are usually studied for load flow, short-circuit fault levels and voltage drop. The objectives of

12480-495: The advantage of being easy to transform between voltages and is able to be generated and utilised by brushless machinery. DC power remains the only practical choice in digital systems and can be more economical to transmit over long distances at very high voltages (see HVDC ). The ability to easily transform the voltage of AC power is important for two reasons: firstly, power can be transmitted over long distances with less loss at higher voltages. So in power systems where generation

12636-401: The case of RCDs that need a power supply, a dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding live wire remains uninterrupted. The tripping circuit needs power to work and does not trip when the power supply fails. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with

12792-415: The circuit breakers are again closed to reroute power around the isolated area. This allows work to be completed on the isolated area. Beyond fault management and maintenance one of the main difficulties in power systems is that the active power consumed plus losses must equal the active power produced. If load is reduced while generation inputs remain constant the synchronous generators will spin faster and

12948-407: The circuit when it detects the fault. RCDs are designed to disconnect the circuit if there is a leakage current. In their first implementation in the 1950s, power companies used them to prevent electricity theft where consumers grounded returning circuits rather than connecting them to neutral to inhibit electrical meters from registering their power consumption. The most common modern application

13104-582: The circuit. Any difference between the currents in these conductors indicates leakage current , which presents a shock hazard. Alternating 60 Hz current above 20  mA (0.020 amperes) through the human body is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. RCDs are designed to disconnect the conducting wires ("trip") quickly enough to potentially prevent serious injury to humans, and to prevent damage to electrical devices. RCDs are testable and resettable devices—a test button safely creates

13260-482: The connection between earth or network and the electrical device being supplied: In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system. The body of the electrical device is connected with earth via this earth connection at the transformer. This arrangement is a current standard for residential and industrial electric systems particularly in Europe. The conductor that connects

13416-437: The contacts closed when the reset button is released. The sense coil (6) is a differential current transformer which surrounds (but is not electrically connected to) the live and neutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. The currents in the two conductors are therefore equal and opposite and cancel each other out. Any fault to earth (for example caused by

13572-408: The correct operation of the device to be verified by passing a small current through the orange test wire (9). This simulates a fault by creating an imbalance in the sense coil. If the RCD does not trip when this button is pressed, then the device must be replaced. Residual-current and over-current protection may be combined in one device for installation into the service panel; this device is known as

13728-405: The current flowing into the appliance on the active line should equal the current flowing out of the appliance on the neutral line. A residual current device works by monitoring the active and neutral lines and tripping the active line if it notices a difference. Residual current devices require a separate neutral line for each phase and to be able to trip within a time frame before harm occurs. This

13884-482: The device. High-powered power electronics can also be used to convert AC power to DC power for long distance transmission in a system known as HVDC . HVDC is used because it proves to be more economical than similar high voltage AC systems for very long distances (hundreds to thousands of kilometres). HVDC is also desirable for interconnects because it allows frequency independence thus improving system stability. Power electronics are also essential for any power source that

14040-413: The difference between current flowing through the live conductor and that returning through the neutral conductor . If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground or to another circuit), and the device will open its contacts. Operation does not require a fault current to return through the earth wire in the installation; the trip will operate just as well if

14196-423: The earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral. Related to this, a single-pole RCD/RCBO interrupts the energized conductor only, while a double-pole device interrupts both

14352-483: The earth wire is broken, the pilot wire allows a sensing device at the source end to interrupt power to the machine. This type of circuit is a must for portable heavy electric equipment (like LHD (Load, Haul, Dump machine) ) being used in underground mines. In high-voltage networks (above 1 kV), which are far less accessible to the general public, the focus of earthing system design is less on safety and more on reliability of supply, reliability of protection, and impact on

14508-462: The earthing system. There are five types of neutral earthing: In solid or directly earthed neutral, transformer's star point is directly connected to the ground. In this solution, a low-impedance path is provided for the ground fault current to close and, as result, their magnitudes are comparable with three-phase fault currents. Since the neutral remains at the potential close to the ground, voltages in unaffected phases remain at levels similar to

14664-431: The efficiency of the diversion of unwanted currents to zero potential (ground). The resistance of a geological material depends on several components: the presence of metal ores, the temperature of the geological layer, the presence of archeological or structural features, the presence of dissolved salts, and contaminants, porosity and permeability. There are several basic methods for measuring soil resistance. The measurement

14820-420: The electric power to the widest class of end users, the main concern for design of earthing systems is safety of consumers who use the electric appliances and their protection against electric shocks. The earthing system, in combination with protective devices such as fuses and residual current devices, must ultimately ensure that a person does not come into contact with a metallic object whose potential relative to

14976-409: The energized and return conductors. Usually this is a standard and safe practice, since the return conductor is held at ground potential anyway. However, because of its design, a single-pole RCD will not isolate or disconnect all relevant wires in certain uncommon situations, for example where the return conductor is not being held, as expected, at ground potential, or where current leakage occurs between

15132-470: The energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the live wire is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to

15288-437: The equipment in presence of a short circuit. Only the magnitude of phase-to-ground short circuits, which are the most common, is significantly affected with the choice of earthing system, as the current path is mostly closed through the earth. Three-phase HV/MV power transformers , located in distribution substations , are the most common source of supply for distribution networks, and type of grounding of their neutral determines

15444-488: The event of loss of mains supply. In the United States, the National Electrical Code requires commercial systems to be built with at least one 20 A sign outlet in order to light outdoor signage. Building code regulations may place special requirements on the electrical system for emergency lighting, evacuation, emergency power, smoke control and fire protection. Power system management varies depending upon

15600-402: The exposed metallic parts of the consumer's electrical installation is called protective earth ( PE ; see also: Ground ). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral ( N ). Three variants of TN systems are distinguished: It is possible to have both TN-S and TN-C-S supplies taken from

15756-425: The fault current will be so large that the circuit over-current protection device (fuse or circuit breaker) will open to clear the ground fault. Where the earthing system does not provide a low-impedance metallic conductor between equipment enclosures and supply return (such as in a TT separately earthed system), fault currents are smaller, and will not necessarily operate the over-current protection device. In such case

15912-415: The fault current will not be equal to zero: conductors in the circuit — particularly underground cables — have an inherent capacitance towards the earth, which provides a path of relatively high impedance. Systems with isolated neutral may continue operation and provide uninterrupted supply even in presence of a ground fault. However, while the fault is present, the potential of other two phases relative to

16068-459: The fault is cleared. For that reason, they are chiefly limited to underground and submarine networks, and industrial applications, where the reliability need is high and probability of human contact relatively low. In urban distribution networks with multiple underground feeders, the capacitive current may reach several tens of amperes, posing significant risk for the equipment. The benefit of low fault current and continued system operation thereafter

16224-585: The first long-distance (175 kilometers (109 miles)) high-voltage (15 kV, then a record) three-phase transmission line from Lauffen am Neckar to Frankfurt am Main for the Electrical Engineering Exhibition in Frankfurt, where power was used to light lamps and run a water pump. In the United States the AC/DC competition came to an end when Edison General Electric was taken over by their chief AC rival,

16380-409: The form of synchronous condensers , static VAR compensators and static synchronous compensators . Briefly, synchronous condensers are synchronous motors that spin freely to generate or absorb reactive power. Static VAR compensators work by switching in capacitors using thyristors as opposed to circuit breakers allowing capacitors to be switched-in and switched-out within a single cycle. This provides

16536-509: The functions of an RCD with overcurrent protection respond to both types of fault. These are known as RCBOs and are available in 2-, 3- and 4-pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but use a common interrupting mechanism. Some RCBOs have separate levers for residual-current and over-current protection or use a separate indicator for ground faults. An RCD helps to protect against electric shock when current flows through

16692-500: The fuse is at a remote site or a spare fuse is not on hand. And second, fuses are typically inadequate as the sole safety device in most power systems as they allow current flows well in excess of that that would prove lethal to a human or animal. The first problem is resolved by the use of circuit breakers —devices that can be reset after they have broken current flow. In modern systems that use less than about 10 kW, miniature circuit breakers are typically used. These devices combine

16848-432: The generators to the load. In a grid , conductors may be classified as belonging to the transmission system , which carries large amounts of power at high voltages (typically more than 69 kV) from the generating centres to the load centres, or the distribution system , which feeds smaller amounts of power at lower voltages (typically less than 69 kV) from the load centres to nearby homes and industry. Choice of conductors

17004-529: The ground conductor. In an IT network (isolé–terre), the electrical distribution system has no connection to earth at all, or it has only a very high- impedance connection. While the national wiring regulations for buildings of many countries follow the IEC 60364 terminology, in North America (United States and Canada), the term "equipment grounding conductor" refers to equipment grounds and ground wires on branch circuits, and "grounding electrode conductor"

17160-449: The ground reaches 3 {\displaystyle {\sqrt {3}}} of the normal operating voltage, creating additional stress for the insulation ; insulation failures may inflict additional ground faults in the system, now with much higher currents. Presence of uninterrupted ground fault may pose a significant safety risk: if the current exceeds 4 A – 5 A an electric arc develops, which may be sustained even after

17316-476: The higher voltages necessary to minimize power loss during long-distance transmission, so the maximum economic distance between the generators and load was limited to around half a mile (800 m). That same year in London, Lucien Gaulard and John Dixon Gibbs demonstrated the "secondary generator"—the first transformer suitable for use in a real power system. The practical value of Gaulard and Gibbs' transformer

17472-483: The lamps was intermittent and in 1882 Thomas Edison and his company, Edison Electric Light Company, developed the first steam-powered electric power station on Pearl Street in New York City. The Pearl Street Station initially powered around 3,000 lamps for 59 customers. The power station generated direct current and operated at a single voltage. Direct current power could not be transformed easily or efficiently to

17628-406: The leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would probably not increase the total current enough for a fuse or overload circuit breaker to isolate the circuit, and not fast enough to save a life. RCDs operate by measuring the current balance between two conductors using a differential current transformer . This measures

17784-448: The lighting and power sockets being connected in parallel. Sockets would also be provided with a protective earth. This would be made available to appliances to connect to any metallic casing. If this casing were to become live, the theory is the connection to earth would cause an RCD or fuse to trip—thus preventing the future electrocution of an occupant handling the appliance. Earthing systems vary between regions, but in countries such as

17940-515: The likelihood of the home's electrical system being an ignition source of a fire. Dual function AFCI/GFCI devices offer both electrical fire prevention and shock prevention in one device making them a solution for many rooms in the home. Major differences exist regarding the manner in which an RCD unit will act to disconnect the power to a circuit or appliance. There are four situations in which different types of RCD units are used: The first three of those situations relate largely to usage as part of

18096-571: The line. In 1885, Ottó Titusz Bláthy working with Károly Zipernowsky and Miksa Déri perfected the secondary generator of Gaulard and Gibbs, providing it with a closed iron core and its present name: the " transformer ". The three engineers went on to present a power system at the National General Exhibition of Budapest that implemented the parallel AC distribution system proposed by a British scientist in which several power transformers have their primary windings fed in parallel from

18252-434: The locomotives and often for speed control of the locomotive's motor. In the middle twentieth century, rectifier locomotives were popular, these used power electronics to convert AC power from the railway network for use by a DC motor. Today most electric locomotives are supplied with AC power and run using AC motors, but still use power electronics to provide suitable motor control. The use of power electronics to assist with

18408-420: The low voltage distribution lines or cables that run past the dwelling. These operate at voltages of between 110 and 260 volts (phase-to-earth) depending upon national standards. A few decades ago small dwellings would be fed a single phase using a dedicated two-core service cable (one core for the active phase and one core for the neutral return). The active line would then be run through a main isolating switch in

18564-400: The maximum current that they can carry at a given temperature rise over ambient conditions. As current flow increases through a conductor it heats up. For insulated conductors, the rating is determined by the insulation. For bare conductors, the rating is determined by the point at which the sag of the conductors would become unacceptable. The majority of the load in a typical AC power system

18720-455: The mechanism that initiates the trip (by sensing excess current) as well as the mechanism that breaks the current flow in a single unit. Some miniature circuit breakers operate solely on the basis of electromagnetism. In these miniature circuit breakers, the current is run through a solenoid, and, in the event of excess current flow, the magnetic pull of the solenoid is sufficient to force open the circuit breaker's contacts (often indirectly through

18876-613: The mid-1960s generally did not include a ground (earth) pin. In the developing world, local wiring practice may or may not provide a connection to an earth. On low voltage electricity networks with a phase to neutral voltage exceeding 240 V to 690 V, which are mostly used in industry, mining equipment and machines rather than publicly accessible networks, the earthing system design is equally important from safety point of view as for domestic users. From 1947 to 1996 for ranges (including separate cook tops and ovens) and 1953 to 1996 for clothes dryers, US National Electrical Code allowed

19032-519: The modern world. Specialized power systems that do not always rely upon three-phase AC power are found in aircraft, electric rail systems, ocean liners, submarines, and automobiles. In 1881, two electricians built the world's first power system at Godalming in England. It was powered by two water wheels and produced an alternating current that in turn supplied seven Siemens arc lamps at 250 volts and 34 incandescent lamps at 40 volts. However, supply to

19188-448: The most popular techniques is to keep the chamber enclosing the contacts flooded with sulfur hexafluoride (SF 6 )—a non-toxic gas with sound arc-quenching properties. Other techniques are discussed in the reference. The second problem, the inadequacy of fuses to act as the sole safety device in most power systems, is probably best resolved by the use of residual-current devices (RCDs). In any properly functioning electrical appliance,

19344-418: The motor control and with starter circuits, in addition to rectification, is responsible for power electronics appearing in a wide range of industrial machinery. Power electronics even appear in modern residential air conditioners allow are at the heart of the variable speed wind turbine . Power systems contain protective devices to prevent injury or damage during failures. The quintessential protective device

19500-430: The nearest substation) to reduce current demand on the power system (i.e. increase the power factor ). Reactors consume reactive power and are used to regulate voltage on long transmission lines. In light load conditions, where the loading on transmission lines is well below the surge impedance loading , the efficiency of the power system may actually be improved by switching in reactors. Reactors installed in series in

19656-402: The need for security—there have already been reports of cyber-attacks on such systems causing significant disruptions to power systems. Despite their common components, power systems vary widely both with respect to their design and how they operate. This section introduces some common power system types and briefly explains their operation. Residential dwellings almost always take supply from

19812-444: The neutral conductor as well, with four-pole RCDs used to interrupt three-phase and neutral supplies. Specially designed RCDs can also be used with both AC and DC power distribution systems. The following terms are sometimes used to describe the manner in which conductors are connected and disconnected by an RCD: RCD sensitivity is expressed as the rated residual operating current, noted I Δn . Preferred values have been defined by

19968-508: The neutral through a resistance which limits the ground fault current to a value equal to or slightly greater than the capacitive charging current of that system. In unearthed , isolated or floating neutral system, as in the IT system, there is no direct connection of the star point (or any other point in the network) and the ground. As a result, ground fault currents have no path to be closed and thus have negligible magnitudes. However, in practice,

20124-470: The nineteenth century. In 1936 the first experimental high voltage direct current (HVDC) line using mercury arc valves was built between Schenectady and Mechanicville, New York . HVDC had previously been achieved by series-connected direct current generators and motors (the Thury system ) although this suffered from serious reliability issues. The first solid-state metal diode suitable for general power uses

20280-431: The order of nanoseconds ) means they are capable of a wide range of tasks that would be difficult or impossible with conventional technology. The classic function of power electronics is rectification , or the conversion of AC-to-DC power, power electronics are therefore found in almost every digital device that is supplied from an AC source either as an adapter that plugs into the wall (see photo) or as component internal to

20436-415: The other forms, when they wish to override those with a lower rating. It may be wise to have a selection of type four RCDs available, because connections made under damp conditions or using lengthy power cables are more prone to trip-out when any of the lower ratings of RCD are used; ratings as low as 10   mA are available. The number of poles represents the number of conductors that are interrupted when

20592-428: The output cables of some models (Eaton/MEM) are used to form the primary winding of the RCD part, and the outgoing circuit cables must be led through a specially dimensioned terminal tunnel with the current transformer part around it. This can lead to incorrect failed trip results when testing with meter probes from the screw heads of the terminals, rather than from the final circuit wiring. Having one RCD feeding another

20748-428: The part of the system being worked on to be isolated while the rest of the system remains live. At high voltages, there are two switches of note: isolators and circuit breakers . Circuit breakers are load-breaking switches where as operating isolators under load would lead to unacceptable and dangerous arcing . In a typical planned outage, several circuit breakers are tripped to allow the isolators to be switched before

20904-407: The person's potential exceeds a safe threshold, typically set at about 50 V. In most developed countries, 220 V, 230 V, or 240 V sockets with earthed contacts were introduced either just before or soon after World War II, though with considerable national variation. However in the United States and Canada, where the supply voltage is only 120 V power outlets installed before

21060-417: The plant to be made (the supervisory control function). Today, SCADA systems are much more sophisticated and, due to advances in communication systems, the consoles controlling the plant no longer need to be near the plant itself. Instead, it is now common for plants to be controlled with equipment similar (if not identical) to a desktop computer. The ability to control such plants through computers has increased

21216-481: The power system. Residential power systems and even automotive electrical systems are often run-to-fail. In aviation, the power system uses redundancy to ensure availability. On the Boeing 747-400 any of the four engines can provide power and circuit breakers are checked as part of power-up (a tripped circuit breaker indicating a fault). Larger power systems require active management. In industrial plants or mining sites

21372-535: The pre-fault ones; for that reason, this system is regularly used in high-voltage transmission networks , where insulation costs are high. To limit short circuit earth fault an additional neutral earthing resistor (NER) is added between the neutral of transformer's star point and earth. With low resistance fault current limit is relatively high. In India it is restricted for 50 A for open cast mines according to Central Electricity Authority Regulations , CEAR, 2010, rule 100. High resistance grounding system grounds

21528-485: The reactive power consumed) and can be supplied from the generators, however it is often more economical to supply such power from capacitors (see "Capacitors and reactors" below for more details). A final consideration with loads has to do with power quality. In addition to sustained overvoltages and undervoltages (voltage regulation issues) as well as sustained deviations from the system frequency (frequency regulation issues), power system loads can be adversely affected by

21684-520: The recommendations of the International Electrotechnical Commission (IEC). Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants. In addition to electric power systems, other systems may require grounding for safety or function. Tall structures may have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines may use

21840-492: The return path is through plumbing or contact with the ground or anything else. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete. For an RCD used with three-phase power , all three live conductors and the neutral (if fitted) must pass through the current transformer. Electrical plugs with incorporated RCD are sometimes installed on appliances that might be considered to pose

21996-425: The risk of electrocution, RCDs should operate within 25–40 milliseconds with any leakage currents (through a person) of greater than 30   mA, before electric shock can drive the heart into ventricular fibrillation , the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times

22152-424: The same frequency: the most common being three-phase at 50 or 60 Hz. There are a range of design considerations for power supplies. These range from the obvious: How much power should the generator be able to supply? What is an acceptable length of time for starting the generator (some generators can take hours to start)? Is the availability of the power source acceptable (some renewables are only available when

22308-478: The same transformer. For example, the sheaths on some underground cables corrode and stop providing good earth connections, and so homes where high resistance "bad earths" are found may be converted to TN-C-S. This is only possible on a network when the neutral is suitably robust against failure, and conversion is not always possible. The PEN must be suitably reinforced against failure, as an open circuit PEN can impress full phase voltage on any exposed metal connected to

22464-416: The sense coil (6), which is picked up by the sense circuitry (7). The sense circuitry then removes power from the solenoid (5), and the contacts (4) are forced apart by a spring, cutting off the electricity supply to the appliance. A power failure will also remove power from the solenoid and cause the contacts to open, causing the safe trip-on-power-failure behaviour mentioned above. The test button (8) allows

22620-421: The source of power is external to the system but for others, it is part of the system itself—it is these internal power sources that are discussed in the remainder of this section. Direct current power can be supplied by batteries , fuel cells or photovoltaic cells . Alternating current power is typically supplied by a rotor that spins in a magnetic field in a device known as a turbo generator . There have been

22776-458: The standard in HVDC, when GE emerged as one of the top suppliers of thyristor-based HVDC. In 1979, a European consortium including Siemens, Brown Boveri & Cie and AEG realized the record HVDC link from Cabora Bassa to Johannesburg , extending more than 1,420 kilometers (880 miles) that carried 1.9 GW at 533 kV. In recent times, many important developments have come from extending innovations in

22932-416: The studies are to assure proper equipment and conductor sizing, and to coordinate protective devices so that minimal disruption is caused when a fault is cleared. Large commercial installations will have an orderly system of sub-panels, separate from the main distribution board to allow for better system protection and more efficient electrical installation. Typically one of the largest appliances connected to

23088-557: The sun is shining or the wind is blowing)? To the more technical: How should the generator start (some turbines act like a motor to bring themselves up to speed in which case they need an appropriate starting circuit)? What is the mechanical speed of operation for the turbine and consequently what are the number of poles required? What type of generator is suitable ( synchronous or asynchronous ) and what type of rotor (squirrel-cage rotor, wound rotor, salient pole rotor or cylindrical rotor)? Power systems deliver energy to loads that perform

23244-466: The supply neutral wire to be used as the equipment enclosure connection to ground if the circuit originated in the main service panel. This was permitted for plug-in equipment and permanently connected equipment. Normal imbalances in the circuit would create small equipment to ground voltages, a failure of the neutral conductor or connections would allow the equipment to go to full 120 volts to ground, an easily lethal situation. The 1996 and newer editions of

23400-434: The supply voltage. Equipment earthing provides electrical safety during an electrical fault. It prevents equipment damage and electric shock. This type of earthing is not an earth connection, technically speaking. When current flows from a line conductor to an earth wire, as is the case when a line conductor makes contact with an earthed surface in a Class I appliance, an automatic disconnection of supply (ADS) device such as

23556-401: The system earth downstream of the break. The alternative is to provide a local earth and convert to TT. The main attraction of a TN network is the low impedance earth path allows easy automatic disconnection (ADS) on a high current circuit in the case of a line-to-PE fault as the same breaker or fuse will operate for either L-N or L-PE faults, and an RCD is not needed to detect earth faults. In

23712-407: The system frequency will rise. The opposite occurs if load is increased. As such the system frequency must be actively managed primarily through switching on and off dispatchable loads and generation . Making sure the frequency is constant is usually the task of a system operator . Even with frequency maintained, the system operator can be kept occupied ensuring: Residual-current device If

23868-430: The unit housing the RCD that remains as set following any form of power outage, but has to be reset manually after the detection of an error condition. In the fourth situation, it would be deemed to be highly undesirable, and probably very unsafe, for a connected appliance to automatically resume operation after a power disconnection, without having the operator in attendance – as such, manual reactivation of

24024-401: Was demonstrated in 1884 at Turin where the transformer was used to light up 40 kilometers (25 miles) of railway from a single alternating current generator. Despite the success of the system, the pair made some fundamental mistakes. Perhaps the most serious was connecting the primaries of the transformers in series so that active lamps would affect the brightness of other lamps further down

24180-415: Was developed by Ernst Presser at TeKaDe in 1928. It consisted of a layer of selenium applied on an aluminum plate. In 1957, a General Electric research group developed the first thyristor suitable for use in power applications, starting a revolution in power electronics. In that same year, Siemens demonstrated a solid-state rectifier , but it was not until the early 1970s that solid-state devices became

24336-558: Was unreliable and short-lived, though, due primarily to generation issues. However, based on that system, Westinghouse would begin installing AC transformer systems in competition with the Edison Company later that year. In 1888, Westinghouse licensed Nikola Tesla 's patents for a polyphase AC induction motor and transformer designs. Tesla consulted for a year at the Westinghouse Electric & Manufacturing Company but it took

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