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88-447: If 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

176-515: 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,

264-457: 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

352-514: 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

440-904: 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

528-416: A circuit will generally create a sag in the voltage level. If the ratio of voltage to current measured at the relay terminals, which equates to an impedance, lands within a predetermined level the circuit breaker will operate. This is useful for reasonably long lines, lines longer than 10 miles, because their operating characteristics are based on the line characteristics. This means that when a fault appears on

616-486: 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

704-599: 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

792-575: 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

880-410: A fault occurs in an overlap region and the two zones which encompass the fault are isolated, the sector of the power system which is lost from service is still small despite two zones being isolated. Disturbance-monitoring equipment (DME) monitors and records system data pertaining to a fault . DME accomplish three main purposes: DME devices include: Protection engineers define dependability as

968-448: 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,

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1056-503: 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

1144-478: A junction point between the wiring system proper, and the more flexible cloth-clad wiring found in light fixtures or other permanent, hard-wired devices. When a generic power outlet was desired, the wiring could run directly into the junction box through a tube of protective loom and a ceramic bushing. Wiring devices such as light switches, receptacle outlets, and lamp sockets were either surface-mounted, suspended, or flush-mounted within walls and ceilings. Only in

1232-458: 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

1320-470: A minor branch distribution line may have very simple low-cost protection. Protection of the transmission and distribution system serves two functions: protection of the plant and protection of the public (including employees). At a basic level, protection disconnects equipment that experiences an overload or a short to earth. Some items in substations such as transformers might require additional protection based on temperature or gas pressure, among others. In

1408-493: 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

1496-483: 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

1584-438: 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

1672-408: 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

1760-410: 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

1848-412: A power plant, the protective relays are intended to prevent damage to alternators or to the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relays have to operate very rarely. If a protective relay fails to detect a fault, the resulting damage to the alternator or to

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1936-406: 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

2024-642: A protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible in operation. The devices that are used to protect the power systems from faults are called protection devices . Protection systems usually comprise five components For parts of a distribution system, fuses are capable of both sensing and disconnecting faults . Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with

2112-428: 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

2200-409: 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

2288-643: 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 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

2376-412: A switched-on light bulb, for example). Modern electrical codes generally do not require a neutral fuse. Instead, they explicitly forbid configurations that might break continuity of the neutral conductor, unless all associated hot conductors are also simultaneously disconnected (for example, by using ganged or "tied" circuit breakers). In retrofit situations electricians may place a higher value fuse on

2464-574: A three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance. If this imbalance exceeds a pre-determined value, a circuit breaker should operate. Restricted earth fault protection is a type of earth fault protection which looks for earth fault between two sets of current transformers (hence restricted to that zone). Distance protection detects both voltage and current. A fault on

2552-416: 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

2640-479: A wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible cloth insulating sleeving called loom. The first insulation was asphalt-saturated cotton cloth, then rubber became common. Wire splices in such installations were twisted together for good mechanical strength , then soldered and wrapped with rubber insulating tape and friction tape (asphalt saturated cloth), or made inside metal junction boxes. Knob and tube wiring

2728-456: 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,

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2816-477: Is an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1930s. It consisted of single-insulated copper conductors run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes , and supported along their length on nailed-down porcelain knob insulators. Where conductors entered

2904-409: 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

2992-408: Is desired today. While these installations were adequate for the electrical loads at the time of installation, modern households use a range and intensity of electrical equipment unforeseen at the time. Household power use increased dramatically following World War II, due to the wide availability of new electrical appliances and devices. Modern home buyers often find that existing K&T systems lack

3080-907: 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

3168-513: 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

3256-516: 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

3344-416: Is needed for other electricians to understand multiple interwoven circuits, especially if the wiring is not fully visible throughout its length. By contrast, modern electrical codes now require that all residential wiring connections be made only inside protective enclosures, such as junction boxes, and that all connections must remain accessible for inspection, troubleshooting, repair, or modification. Under

3432-423: Is to minimize an outage to the greatest extent possible. Historically, protective device coordination was done on translucent log–log paper. Modern methods normally include detailed computer based analysis and reporting. Protection coordination is also handled through dividing the power system into protective zones. If a fault were to occur in a given zone, necessary actions will be executed to isolate that zone from

3520-540: 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

3608-466: The NEC to conditionally allow insulation around K&T. They did not find a single fire that was attributed to K&T, and permit insulation provided the home first passes inspection by an electrician. As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased risk. Several companies will not write new homeowners policies at all unless all K&T wiring

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3696-463: 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

3784-408: The US electrical code, Carter system wiring layouts have now been banned, even for permissible new installations of K&T wiring. However, electricians must be aware of this older system, which is still present in many existing older electrical installations. Another practice that was common (or even originally required) in some older K&T designs was the installation of separate fuses in both

3872-538: The United States National Electrical Code forbids the use of loose, blown-in, or expanding foam insulation over K&T wiring. This is because K&T is designed to let heat dissipate to the surrounding air. As a result, energy efficiency upgrades that involve insulating previously uninsulated walls usually also require replacement of the wiring in affected homes. However, California, Washington, Nebraska, and Oregon have modified

3960-512: The aims of protection of assets and ensuring continued supply of energy. Switchgear is a combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switches are safe to open under normal load current (some switches are not safe to operate under normal or abnormal conditions), while protective devices are safe to open under fault current. Very important equipment may have completely redundant and independent protective systems, while

4048-440: The best choice of protection is current differential protection. The objective of protection is to remove only the affected portion of plant and nothing else. A circuit breaker or protection relay may fail to operate. In important systems, a failure of primary protection will usually result in the operation of back-up protection. Remote back-up protection will generally remove both the affected and unaffected items of plant to clear

4136-407: The best decisions on improving a protection system, managing dependability versus security tradeoffs, and getting the best results for the least money. A quantitative understanding is essential in the competitive utility industry. Reliability: Dependability vs Security There are two aspects of reliable operation of protection systems: dependability and security. Dependability is the ability of

4224-592: The capacity for today's levels of power use. First-generation wiring systems became susceptible to abuse by homeowners who would replace blown fuses with fuses rated for higher current. This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage or fire. Knob-and-tube wiring may also be damaged by building renovations. Its cloth and rubber insulation can dry out and turn brittle. It may also be damaged by rodents and careless activities such as hanging objects from wiring running in accessible areas like basements or attics. Currently,

4312-401: 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

4400-416: 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 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

4488-437: 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

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4576-405: The control systems in the grids. These control systems are connected to the internet and makes it easier for hackers to attack them. These attacks can cause damage to equipment and limit the utility professionals ability to control the system. Protective device coordination is the process of determining the "best fit" timing of current interruption when abnormal electrical conditions occur. The goal

4664-408: 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

4752-477: The current exceeds a predetermined level for the circuit breaker to operate. Time overcurrent protection operates based on a current vs time curve. Based on this curve, if the measured current exceeds a given level for the preset amount of time, the circuit breaker or fuse will operate. The function of both types is explained in "Non-Directional Overcurrent Protection" on YouTube . Earth fault protection also requires current transformers and senses an imbalance in

4840-413: 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

4928-423: 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

5016-409: 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

5104-470: 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

5192-598: The entire system. Zone definitions account for generators , buses, transformers , transmission and distribution lines , and motors . Additionally, zones possess the following features: zones overlap, overlap regions denote circuit breakers, and all circuit breakers in a given zone with a fault will open in order to isolate the fault. Overlapped regions are created by two sets of instrument transformers and relays for each circuit breaker. They are designed for redundancy to eliminate unprotected areas; however, overlapped regions are devised to remain as small as possible such that when

5280-464: The fault. Local back-up protection will remove the affected items of the plant to clear the fault. The low-voltage network generally relies upon fuses or low-voltage circuit breakers to remove both overload and earth faults. The bulk system which is a large interconnected electrical system including transmission and control system is experiencing new cybersecurity threats every day. (“Electric Grid Cybersecurity,” 2019). Most of these attacks are aiming

5368-509: 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

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5456-511: The hot wire and the neutral (return) wire of an electrical circuit. The failure of a neutral fuse would cut off power flow through the affected circuit, but the hot conductor could still remain hot relative to ground , an unexpected and potentially hazardous situation. Because of the presence of a neutral fuse, and in the event that it blew, the neutral conductor could not be relied on to remain near ground potential; and, in fact, could be at full line potential (via transmission of voltage through

5544-433: The last case were metal boxes always used to enclose the wiring and device. In many older K&T installations, the supply and return wires were routed separately from each other, rather than being located parallel to and near each other. This direct routing method has the advantage of reduced cost by allowing use of the shortest possible lengths of wire, but the major disadvantage is that a detailed building wiring diagram

5632-406: 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

5720-515: 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

5808-413: The line the impedance setting in the relay is compared to the apparent impedance of the line from the relay terminals to the fault. If the relay setting is determined to be below the apparent impedance it is determined that the fault is within the zone of protection. When the transmission line length is too short, less than 10 miles, distance protection becomes more difficult to coordinate. In these instances

5896-444: 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

5984-455: The neutral, so that fuse blows last. In the early 1900s, K&T wiring was less expensive to install than other wiring methods. For several decades, electricians could choose between K&T wiring, conduit, armored cable, and metal junction boxes. The conduit methods were known to be of better quality, but cost significantly more than K&T. In 1909, flexible armored cable cost about twice as much as K&T, and conduit cost about three times

6072-415: 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

6160-428: 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

6248-500: The price of K&T. Knob and tube wiring persisted since it allowed owners to wire a building for electricity at lower cost. Modern wiring methods assume that two or more load-carrying conductors will lie very near each other, as for instance in standard NM-2 cable. When installed correctly, the K&;T wires are held away from the structural materials by ceramic insulators. Over the K&T era multiple wire types evolved. Early wiring

6336-485: The price of electrician labor grew faster than the cost of materials. This removed the price advantage of K&T methods, especially since they required time-consuming skillful soldering of in-line splices and junctions, and careful hand-wrapping of connections in layers of insulating tape. Knob-and-tube wiring can be made with high current carrying capacity. However, most existing residential knob-and-tube installations, dating to before 1940, have fewer branch circuits than

6424-468: The protection system to operate when called upon to remove a faulted element from the power system. Security is the ability of the protection system to restrain itself from operating during an external fault. Choosing the appropriate balance between security and dependability in designing the protection system requires engineering judgement and varies on a case-by-case basis. Knob-and-tube wiring Knob-and-tube wiring (sometimes abbreviated K&T )

6512-428: The return and earth conductors. In these cases, a double-pole RCD will offer protection, since the return conductor would also be disconnected. Overcurrent protection Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the disconnection of faulted parts from the rest of the electrical network . The objective of

6600-492: 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

6688-424: 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

6776-416: 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

6864-407: The supply and return conductors of 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

6952-403: The tendency of the protection system to operate correctly for in-zone faults. They define security as the tendency not to operate for out-of-zone faults. Both dependability and security are reliability issues. Fault tree analysis is one tool with which a protection engineer can compare the relative reliability of proposed protection schemes. Quantifying protection reliability is important for making

7040-428: The transformer might require costly equipment repairs or replacement, as well as income loss from the inability to produce and sell energy. Overload protection requires a current transformer which simply measures the current in a circuit and compares it to the predetermined value. There are two types of overload protection: instantaneous overcurrent (IOC) and time overcurrent (TOC). Instantaneous overcurrent requires that

7128-430: 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

7216-433: The wall studs or floor joists. Most had a circular groove running around their circumference, although some were constructed in two pieces with pass-through grooves on each side of the nail in the middle. A leather washer often cushioned the ceramic, to reduce breakage during installation. By wrapping electrical wires around the knob, and securing them with tie wires, the knob could be used to securely and permanently anchor

7304-419: The wire. The knobs separated the wire from potentially combustible framework, facilitated changes in direction, and ensured that wires were not subject to excessive tension. Because the wires were suspended in air, they could dissipate heat well. Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them. This kept the wires from coming into contact with

7392-407: The wood framing members and from being compressed by the wood as the house settled. Ceramic tubes were sometimes also used when wires crossed over each other, for protection in case the upper wire were to break and fall on the lower conductor. Ceramic cleats, which were block-shaped pieces, served a purpose similar to that of the knobs except that cleats were generally used in places where the wiring

7480-413: Was eventually displaced from interior wiring systems because of the high cost of installation compared with use of power cables, which combined both power conductors of a circuit in one run (and which later included grounding conductors). At present, new concealed knob and tube installations are permitted in the U.S. by special permission. Ceramic knobs were cylindrical and generally nailed directly into

7568-443: Was installed correctly, and not damaged or incorrectly modified since then, is fairly safe when used within the original current-carrying limits, typically about ten amperes per circuit. Historically, wiring installation requirements were less demanding in the age of knob-and-tube wiring than today. Compared to modern electrical wiring standards, these are the main technical shortcomings of knob-and-tube wiring methods: Over time,

7656-492: Was insulated with cotton cloth and soft rubber, while later wiring was much more robust. Although the actual wire covering may have degraded over the decades, the porcelain standoffs have a nearly unlimited lifespan and will keep any bare wires safely insulated. Today, porcelain standoffs are still commonly used with bare-wire electric fencing for livestock, and such porcelain standoffs carry far higher voltage surges without risk of shorting to ground. In summary, K&T wiring that

7744-474: Was surface mounted. Not all knob and tube installations utilized cleats. Ceramic bushings protected each wire entering a metal device box, when such an enclosure was used. Loom, a woven flexible insulating sleeve, was slipped over insulated wire to provide additional protection whenever a wire passed over or under another wire, when a wire entered a metal device enclosure, and in other situations prescribed by code. Other ceramic pieces would typically be used as

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