Residual-current device (Sưu tầm)

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Residual-current device

A Residual Current Device is a generic term covering both RCCBs and RCBOs.

A Residual-Current Circuit Breaker (RCCB) is an electrical wiring device that disconnects a circuit whenever it detectsthat the electric current is not balanced between the energized conductor and the return neutral conductor. Such animbalance may indicate current leakage through the body of a person who is grounded and accidentally touching theenergized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quicklyenough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or short-circuit conditions.

In the United States and Canada, a residual current device is most commonly known as a ground fault circuitinterrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). InAustralia they are sometimes known as "safety switches" or simply "RCD" and in the United Kingdom, along with circuitbreakers, they can be referred to as "trips" or "trip switches".

A Residual Current Circuit Breaker with Overload protection (RCBO) combines the functions of overcurrent protectionand leakage detection. An earth leakage circuit breaker (ELCB) may be a residual-current device, although an older typeof voltage-operated earth leakage circuit breaker exists.

Purpose and operation

RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30milliamperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes). RCDs areintended to operate within 25-40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, themost common cause of death through electric shock. In Europe, the commonly used RCDs have trip currents of 10–300 mA.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. Thismeasures the difference between the current flowing out the live conductor and that returning through the neutralconductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to anothercircuit), and the device will open its contacts.

Residual current detection is complementary to over-current detection. Residual current detection cannot provide protectionfor overload or short-circuit currents, except for the special case of a short circuit from live to ground (not live to neutral).

For a RCD used with three-phase power, all live conductors and the neutral must pass through the current transformer.

Typical features

The photograph depicts the internal mechanism of a residual current device (RCD). The device pictured is designed to bewired in-line in an appliance power cord. It is rated to carry a maximum current of 13 amperes and is designed to trip on aleakage current of 30 mA. This is an active RCD; that is, it latches mechanically and therefore trips on power failure, a usefulfeature for equipment that could be dangerous on unexpected re-energisation.

The incoming supply and the neutral conductors are connected to the terminals at (1) and the outgoing load conductors areconnected 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 hidden behind (5)) close, allowing current to pass. Thesolenoid (5) keeps 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 andneutral conductors. In normal operation, all the current down the live conductor returns up the neutral conductor. Thecurrents in the two conductors are therefore equal and opposite and cancel each other out.

Any fault to earth (for example caused by a person touching a live component in the attached appliance) causes some of thecurrent to take a different return path which means 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 phaseconductors and one neutral conductor).

This difference causes a current in the sense coil (6) which is picked up by the sense circuitry (7). The sense circuitry thenremoves power from the solenoid (5) and the contacts (4) are forced apart by a spring, cutting off the electricity supply tothe appliance.

The device is designed so that the current is interrupted in milliseconds, greatly reducing the chances of a dangerouselectric shock being received.

The test button (8) allows the correct operation of the device to be verified by passing a small current through the orangetest wire (9). This simulates a fault by creating an imbalance in the sense coil. If the RCD does not trip when this button ispressed then the device must be replaced.

Form factors

A Ground Fault Circuit Interrupter (GFCI in USA) and Residual Current Breaker with Overload (RCBO in Europe) are deviceswhich combines Residual Current Device (RCD) with a Circuit Breaker or miniature circuit breaker (MCB) which bothdetects supply imbalance and limits the current that may supplied.

In Europe RCDs can fit on the same DIN rail as the MCBs, however the busbar arrangements in consumer units anddistribution boards can make it awkward to use them in this way. If it is desired to protect an individual circuit an RCBO (Residual-current Circuit Breaker with Overcurrent protection) can be used. This incorporates an RCD and a miniaturecircuit breaker in one device.

Electrical plugs which incorporate an RCD are sometimes installed on appliances which might be considered to pose aparticular safety hazard, for example long extension leads which might be used outdoors or garden equipment or hair dryerswhich may be used near a tub 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.

Electrical sockets with included RCDs are becoming common.

Combined with over current devices

Residual current and overcurrent protection may be combined in one device for installation into the service panel; this deviceis known as a GFCI breaker (Ground Fault Circuit Interrupter) in USA/Canada and as an RCBO (Residual current circuitbreaker with overload protection) in Europe. In the US, RCBOs are more expensive than RCD outlets.

As well as requiring both line and neutral (or 3-phase) input and output, GFCI/RCBO devices require a functional earth (FE) connection. For reasons of space some devices use flying leads rather than screw terminals, especially for the neutral inputand FE connections.

More than one RCD feeding another is unnecessary, provided they have been wired properly. One exception is the case of aTT earthing system where the earth loop impedance may be high, meaning that a ground fault might not cause sufficientcurrent to trip an ordinary circuit breaker or fuse. In this case a special 100 mA (or greater) trip current time-delayed RCD isinstalled covering the whole installation and then more sensitive RCDs should be installed downstream of it for sockets andother circuits which are considered high risk.

Testing

RCDs can be tested with the built-in test button to confirm functionality on a regular basis. RCDs if wired improperly may notoperate correctly and are generally tested by the installer to verify correct operation. Use of a solenoid voltmeter from liveto earth provides an external path and can test the wiring to the RCD. Such a test may be performed on installation of thedevice and at any "downstream" outlet.

Limitations

A residual current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detectoverload 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 the functions of an RCDwith overcurrent protection respond to both types of fault. These are known as RCBOs, and are available in 1, 2, 3 and 4pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current butwill have a common interrupting mechanism.

An RCD will help to protect against electric shock where current flows through a person from a phase (live / line / hot) toearth. It cannot protect against electric shock where current flows through a person from phase to neutral or phase to phase, for example where a finger touches both live and neutral contacts in a light fitting; a device can not differentiate betweencurrent flow through an intended load from flow through a person.

Whole installations on a single RCD, common in the UK, are prone to nuisance trips that can cause safety problems with lossof lighting and defrosting of food. RCDs also cause nuisance trips with appliances where earth leakage is common and not acause of injury or mortality, such as water heaters.

A dangerous condition can arise if the neutral wire is broken or switched off before the RCD while its live wire is notinterrupted. In this situation the tripping circuitry of the RCD that needs power to be supplied will cease to work. The circuitwill look like it is switched off, but if someone touches the live wire thinking that it is de-energized, the RCD will not trip. Forthis reason circuit breakers must be installed in a way that ensures that the neutral wire is turned off only at the momentwhen the live wire is also turned off. Separate single-pole circuit breakers must never be used for live and neutral, only twoor four pole breakers must be used in cases there is a need for switching off the neutral wire.

Technical characteristics

Number of poles

RCDs may comprise two poles for use on single phase supplies (two current paths), three poles for use on three phasesupplies (three current paths) or four poles for use on three phase & neutral supplies (four current paths).

Rated current

The rated current of an RCD is chosen according to the maximum sustained load current it will carry (if the RCD is connectedin series with, and downstream of a circuit-breaker, the rated current of both items shall be the same).

Sensitivity

RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by theIEC, thus making it possible to divide RCDs into three groups according to their IΔn value.

• • High sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection)

  • Medium sensitivity (MS): 100 – 300 – 500 – 1000 mA (for fire protection)

  • Low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine)

Type

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCDdepending on the characteristics of the fault current.

• • Type AC: RCD for which tripping is ensured for residual sinusoidal alternating currents

  • Type A: RCD for which tripping is ensured

    • • for residual sinusoidal alternating currents

      • for residual pulsating direct currents

      • for residual pulsating direct currents superimposed by a smooth direct current of 0.006 A, with or without phase-angle control, independent of the polarity

  • Type B: RCD for which tripping is ensured

    • • as for type A

      • for residual sinusoidal currents up to 1000 Hz

      • for residual sinusoidal currents superposed by a pure direct current

      • for pulsating direct currents superposed by a pure direct current

      • for residual currents which may result from rectifying circuits

        • • three pulse star connection or six pulse bridge connection

          • two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of thepolarity

Break time

There are two groups of devices:

• • G (general use) for instantaneous RCDs (i.e. without a time delay)

    • • Minimum break time: immediate

      • Maximum break time: 200 ms for 1x IΔn, 150 ms for 2x IΔn, and 40 ms for 5x IΔn

  • S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surgesuppressors)

    • • Minimum break time: 130 ms for 1x IΔn, 60 ms for 2x IΔn, and 50 ms for 5x IΔn

      • Maximum break time: 500 ms for 1x IΔn, 200 ms for 2x IΔn, and 150 ms for 5x IΔn

Surge current resistance

The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics ( an 8/20 µs impulse, named after the time constants of the rise and fall of current).

The IEC 61008 and IEC 61009 standards impose the use of a 0.5 µs/ 100 kHz damped oscillator wave (ring wave) to test theability of residual current protection devices to withstand operational discharges with a peak current equal to 200 A. Withregard to atmospheric discharges, IEC 61008 and 61009 standards establish the 8/20 µs surge current test with 3000 Apeak current but limit the requirement to RCDs classified as Selective.

History and nomenclature

The world’s first high-sensitivity earth leakage protection system (i.e. a system capable of protecting people from thehazards of direct contact between a live conductor and earth), was a second-harmonic magnetic amplifier core-balancesystem, known as the magamp, developed in South Africa by Henri Rubin. Electrical hazards were of great concern inSouth African gold mines, and Rubin, an engineer at the company F.W.J. Electrical Industries, initially developed a cold-cathode system in 1955 which operated at 525 V and had a tripping sensitivity of 250 mA. Prior to this, core balance earthleakage protection systems operated at sensitivities of about 10 A.

The cold cathode system was installed in a number of gold mines and worked reliably. However, Rubin began working on acompletely novel system with greatly improved sensitivity, and by early 1956, he had produced a prototype second-harmonicmagnetic amplifier-type core balance system (South African Patent No. 2268/56 and Australian Patent No. 218360). Theprototype magamp was rated at 220V 60A and had an internally adjustable tripping sensitivity of 12.5 to 17.5 mA. Veryrapid tripping times were achieved through a novel design, and this combined with the high sensitivity was well within thesafe current-time envelope for ventricular fibrillation determined by Charles Dalziel of the University of California, Berkeley, USA, who had estimated electrical shock hazards in humans. This system, with its associated circuit breaker, included overcurrent and short-circuit protection. In addition, the original prototype was able to trip at a lower sensitivity inthe presence of an interrupted neutral, thus protecting against an important cause of electrical fire.

Following the accidental electrocution of a woman in a domestic accident at the Stilfontein gold mining village nearJohannesburg, a few hundred F.W.J. 20 mA magamp earth leakage protection units were installed in the homes of themining village during 1957 and 1958. F.W.J. Electrical Industries, which later changed its name to FW Electrical Industries, continued to manufacture 20 mA single phase and three phase magamp units.

At the time that he worked on the magamp, Rubin also considered using transistors in this application, but concluded thatthe early transistors then available were too unreliable. However, with the advent of improved transistors, the company thathe worked for and other companies later produced transistorized versions of earth leakage protection.

In 1961, Charles F. Dalziel, working with Rucker Manufacturing Co., developed a transistorized device for earth leakageprotection which became known as a Ground Fault Circuit Interrupter (GFCI), sometimes colloquially shortened to GroundFault Interrupter (GFI). This name for high-sensitivity earth leakage protection is still in common use in the U.S.A.[1][2][3][4][5]

In the early 1970s most GFCI devices were of the circuit breaker type. However the most commonly used in the USA sincethe early 1980s are built into outlet receptacles. The problem with those of the circuit breaker type was that of many falsetrips due to the poor alternating current characteristics of 120 volt insulations, especially in circuits having longer cablelengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightestincrease of current imbalance.

Regulation and adoption

Rules and regulations, as well as adoption and specific application, differ widely from country to country. In most countries, not all circuits in a home are protected by RCDs. If a single RCD is installed for an entire electrical installation, any fault willcut all power to the premises.

Australia

In Australia, they have been mandatory in all new houses since 1991 on all power and lighting circuits.[6]

Europe

Germany

Germany requires the use of RCDs on sockets up to 20A which are for general use. This rule was introduced in June 2007 (DIN VDE 0100-410 Nr. 411.3.3).

Norway

In Norway, it has been required in all new homes since 2002, and on all new sockets since 2006.

United Kingdom

The UK has only mandated the use of RCDs in new installations since July 2008. In the 16th Edition of the IEE ElectricalWiring Regulations, they were used to add extra fault protection to socket outlets. The current edition (17th) of theregulations state that all new installations, as well as a change of distribution board or the installation of new circuits in aproperty wired to any previous installation, must have a split load distribution board with two RCDs covering the installation, with upstairs and downstairs lighting and power circuits spread across both RCDs in case of a fault on one RCD, thereforeleaving power to at least one lighting and power circuit.

Normal practice in domestic installations in the UK[citation needed] was to use a single RCD for all RCD protected circuits butto have some circuits that are not protected at all (sockets usually are on the RCD; lamp holders usually aren't; other circuitsvary by who installed the system). Regulation introduced in 2008 dictate that on all new electrical installations in the UK, allcircuits must be protected by an RCD[citation needed] however, this does not affect existing installations.

It is common to install an RCD in a consumer unit in what is known as a split load configuration where one group ofcircuits is just on the main switch (or time delay RCD in the case of a TT earth) and another group is on the RCD.

North America

In North America, RCD (“GFCI”) sockets are usually of the Decora form (which harmonizes outlets and switches, so thatthere is no difference between an outlet cover and a switch cover). For example, using the decora size outlets, RCD outletscan be mixed with regular outlets or with switches in a multigang box with a standard cover plate.

In Canada and the United States, two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect againstelectrocution, and a grounding wire does not need to be supplied to that GFCI. The outlet must be labeled as such. The GFCImanufacturers provide tags for the appropriate installation description.

United States

GFI receptacles in the USA have connections to protect downstream receptacles so that all outlets on a circuit may beprotected by one GFI outlet.

In the United States, the National Electrical Code requires GFCI devices intended to protect people to interrupt the circuitif the leakage current exceeds a range of 4–6 mA of current (the trip setting is typically 5 mA) within 25 ms. A GFCI devicewhich protects equipment (not people) is allowed to trip as high as 30 mA of current; this is known as an EquipmentProtective Device (EPD). "RCDs" with trip currents as high as 500 mA are sometimes deployed in environments (such ascomputing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDsserve for equipment and fire protection instead of protection against the risks of electrical shocks.

GFCI outlets are required by law in wet areas (See National Electrical Code (US) for details.)

In the U.S., the National Electrical Code requires GFCIs for underwater swimming pool lights (1968); construction sites (1974); bathrooms and outdoor areas (1975); garages (1978); near hot tubs or spas (1981); hotel bathrooms (1984); kitchen counter receptacles (1987, revised 1996 and specifically excluding the refrigerator outlet, which is usually on adedicated circuit); crawl spaces and unfinished basements (1990); wet bar sinks (1993); laundry sinks (2005).[7]

Related technologies

A related electrical safety device is the arc-fault circuit interrupter (AFCI) which detects electrical arcing due to loose orcorroded wiring connections, which can cause building fires.

See also

• Earth leakage circuit breaker

  • Arc-fault circuit interrupter — protects against electrical arcing due to bad wiring

• Insulation monitoring device

  • Circuit breaker — protects against overload and short-circuit conditions

  • Fuse (electrical) — protects against overload and short-circuit conditions

  • Domestic AC power plugs and sockets

References

• 1. ^ Charles F. Dalziel, Transistorized ground-fault interrupter reduces shock hazard, IEEE Spectrum, January 1970

  2. ^ The Professional Engineer, Official Journal of the Federation of Societies of Professional Engineers of South Africa, pp67, Vol 6(2) 1977

  3. ^ Earl W. Roberts, Overcurrents and Undercurrents – All about GFCIs: Electrical Safety Advances through Electronics, Mystic Publications, Mystic CT, 1996

  4. ^ Edward L. Owen, Power System Grounding Part II: RCD & GFCI, IEEE Industry Applications Magazine, July/August1996

  5. ^ Forging ahead: South Africa’s Pioneering Engineers, G R Bozzoli, Witwatersrand University Press, 1997

  6. ^ SAA Wiring Rules AS/NZS 3000:2000, SAI Global Limited

  7. ^ "GFCIs Fact Sheet". US Consumer Product Safety Commission. http://www.cpsc.gov/cpscpub/pubs/099.pdf. Retrieved 2009-06-28.

External links

• GFCIs Fact Sheet (Consumer Product Safety Commission)

Categories:

• Electrical wiring

• Electrical engineering

  • Safety switches

  • Electrical safety

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