Residual-current device

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A residual current device (RCD)

A residual current device (RCD), or residual current circuit breaker (RCCB), is an electrical wiring device that disconnects a circuit whenever it detects that the flow of current is not balanced between the phase ("hot") conductor and the neutral conductor. The presumption is that such an imbalance may represent current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A shock, possibly lethal, is likely to result from these conditions; RCDs are designed to disconnect quickly enough to prevent such shocks.

In the United States and Canada, a residual current device is also known as a Ground Fault Circuit Interrupter (GFCI) or an Appliance Leakage Current Interrupter (ALCI).

1 Purpose and operation
2 Use and placement
3 Testing
4 Limitations
5 History and nomenclature
6 Types
7 See also
8 External links

[edit] Purpose and operation

RCDs operate by measuring the current balance between two conductors using a differential current transformer, and opening the device's contacts if there is a balance fault (i.e. a difference in current between the phase conductor and the neutral conductor). More generally (single phase, three phase, etc.) RCDs operate by detecting a nonzero sum of currents, i.e. the current in the "hot" or "hots" plus that in the "neutral" must equal zero (within some small tolerance), otherwise there is a leakage of current to somewhere else (to ground, or to another circuit, etc.). The National Electrical Code, which is the enforcable code in most of the United States, requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4 to 6 milliamps of current (the exact trip setting can be chosen by the manufacturer of the device and is typically 5 milliamps) within 25 milliseconds. GFCI devices which protect equipment (not people) are allowed to trip as high as 30 milliamps of current.

RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5- 6 milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. RCDs are intended to operate within 25 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

These values were set by tests at Underwriters Laboratories during which volunteers holding cups of rice were subjected to shocks of known amperage and voltage. Initially, the GFCI was developed using pigs and hogs in swimming pools, because their skin is like that of humans.

Residual current detection is complementary to, rather than a replacement for, conventional over-current detection, as residual current detection cannot provide protection for faults which do not involve an external leakage current, for example faults that pass the current directly from one side of the circuit through the victim to the other. Notably, RCDs do not provide protection against overloads or short circuits between phase (live, hot, line) and neutral or phase to phase.

Two-wire (ungrounded) outlets may be replaced with three-wire GFCIs to protect against electrocution, and a grounding wire does not need to be supplied to that GFCI, but it must be tagged as such (the GFCI manufacturers provide tags for the approprate installation description).

[edit] Use and placement

In most houses, only some (if any) circuits are protected by RCDs. German law, for example, requires the installation of RCDs only for circuits leading to bathrooms (due to the highly increased danger of leakage currents when operating electrical devices in a wet environment; a hair dryer falling into a bathtub might otherwise be fatal). U.S. law (the National Electrical Code) requires GFCIs in bathrooms, kitchens, garages, exterior areas, crawl spaces, unfinished basements, near wet bars, swimming pools, and spas. Additionally, it might be a good idea to protect circuits leading to outlets in reach of children, or outlets that are indoors but near a door (where people are likely to plug something in while working outside) by RCDs.

Most manufacturers of utilization devices to be used in wet environments (for example, hair dryers and hydrotherapy devices for use in bathtubs) now build in RCDs. In many countries this is now required.

Sometimes a single RCD is installed covering the entire electrical installation in a property. However this is considered bad practice by some because any fault will cause all power to be cut to the premises including to devices such as freezers, fire alarms, etc. and injury may be caused by occupants being suddenly plunged into darkness. Normal practice in domestic installations in the UK is to use a single RCD for all RCD protected circuits but to have some circuits that are not protected at all (sockets usually are on the RCD, lights usually aren't other circuits vary by who installed the system). This practice is widely regarded as far from ideal but cost considerations make it by far the most common. GFI outlets in the USA have connections to allow further outlets to be protected by the RCD; a very common practice is to connect the other outlets in a room "downstream" of a single GFI outlet so that they are also protected. For example, this is very common if a house has multiple bathrooms. RCD protection is also available in combination with an overcurrent breaker for fitting in a consumer unit/distribution board/breaker panel (known as a GFCI breaker in the US and as a RCBO in europe). In the US, this has become less common because RCBOs are much more expensive than RCD outlets.

More than one RCD feeding another is 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 flow to trip an ordinary circuit breaker or fuse. In this case a special 100mA (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 which are considered high risk.

[edit] Testing

RCDs can be tested to see if they are operational and/or they have been wired correctly.

It is a good idea to check RCDs monthly. One way to test an RCD is to press the button labelled "Test" or "T" on the RCD unit (which will simulate a ground fault by bypassing some current) and see if the RCD reacts by correctly opening the circuit. If it does not trip, the RCD should be replaced. Unfortunately, the test button is a fairly crude test and it is quite possible (though rare) for an RCD to trip on the pressing of the test button even when it would not pass a proper test involving passing known leakage currents and measuring the resulting trip time (and comparing those values to the requirements given in a standards document such as BS 7671). For example, an incorrectly wired RCD may still trip when the test button is pressed even though a real ground fault may not cause it to trip. Use of a solenoid voltmeter from live to earth may provide a more effective test of the RCD; such a test should be performed at least once upon installation of the device. The test should be repeated at every outlet "downstream" of the RCD to ensure that the downstream outlets are also wired correctly.

[edit] Limitations

A residual current circuit breaker can improve the safety of an electrical system but cannot remove all risk of electric shock or fire. In particular, an RCD will not detect overload conditions, phase to neutral short circuits or phase-to-phase short circuits. Some sort of over-current protection (fuse or circuit breaker) must be employed to guard against these occurrences. Combined RCD/circuit breaker units are available, and these combine the functions of an RCD with those of a conventional circuit breaker, responding appropriately to fault currents and overload conditions. These are known as RCBOs, and are available in 1, 2, 3 and 4 pole configurations. RCBOs will typically have separate circuits for detecting current imbalance (RCD function) and for detecting overload current (circuit breaker function); however the device for interrupting the flow of current will be common to both functions.

An RCD will help to protect against electric shock where current flows through a person from a phase (live / line / hot) to earth. 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. It is virtually impossible to provide electrical protection against such shocks as there is no way for a device to differentiate between current flow causing an electrical shock to a person and normal current flow through an appliance. Protection against electrical shock of this nature must be through mechanical means (guards or covers to protect against accidental contact) and procedure (e.g. switching off power before undertaking maintenance).

[edit] History and nomenclature

In the early 1970s most GFCI devices were of the circuit breaker type. However the most commonly used GFCIs since the early 1980s are built into outlet receptacles. The problem with those of the circuit breaker type was that of many false trips due to the poor alternating current characteristics of 120 volt insulations, especially in circuits having longer cable lengths. So much current leaked along the length of the conductors' insulation that the breaker might trip with the slightest increase of current unbalance.

One might more properly call the device a Balance Fault Interrupter (BFI), rather than GFI, because it will trip if current, for example, leaks to or from another circuit such as either the "hot" or "cold" side of a nearby 12 volt DC renewable energy system, or a nearby ethernet jack, etc. The device will trip on any balance fault, not just a balance fault to ground. However, the term "Balance Fault Interrupter" is rarely used in practice.

The term earth leakage circuit breaker (ELCB) is also (incorrectly) used, though strictly speaking this refers to a different type of device.

[edit] Types

A Residual Current Breaker with Overload (RCBO) is a combination of an RCD and a miniature circuit breaker (MCB).

In Europe RCDs can fit on the same DIN rail as the MCBs, however the busbar arrangements in consumer units and distribution 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 miniature circuit breaker in one device.

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

Electrical plugs which incorporate an RCD are sometimes installed on appliances which might be considered to pose 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 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 you provide protection whatever outlet is used even if the building has old wiring.

Electrical sockets with included RCDs are becoming common. In the U.S. these are required by law in wet areas (See National Electrical Code (US) for details.)

In North America, RCD ("GFCI") sockets are usually of the decora size (a size that harmonizes outlets and switches, so that there is no difference in size between an outlet cover and a switch cover). For example, using the decora size outlets, RCD outlets can be mixed with regular outlets or with switches in a multigang box with a standard cover plate.