RESIDUAL-CURRENT DEVICE

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').

Contents

Purpose and operation

Example

Use and placement

Testing

Limitations

History and nomenclature

Types

Active/Passive Latching/non-Latching

Main characteristics

Number of poles [2P, 3P or 4P]

Rated current [in A]

Sensitivity [in mA]

Type [AC or A or B]

Break time [in ms]

Surge current resistance [in A]

Important note regarding TN-C earthing system

See also

References

External links

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 "live" (phase) conductor plus that in the "neutral" conductor must equal zero (within some small tolerance), otherwise there is a leakage of current to somewhere else (to earth/ground, or to another circuit, etc.). In the United States, the National Electrical Code, requires GFCI devices intended to protect people to interrupt the circuit if the leakage current exceeds a range of 4–6 mA of current (the exact trip setting can be chosen by the manufacturer of the device and is typically 5 mA) within 25 milliseconds. GFCI devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCDs have trip currents of 10–300 mA.
RCDs are designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5–30 mA milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. RCDs are intended to operate within 25–40 milliseconds, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.
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.
RCDs with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry an unacceptable risk of accidental trips. These high-current RCDs serve more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks.
In some countries, 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 appropriate installation description).

Example

The photograph depicts the internal mechanism of an RCD. The device pictured is designed to be wired in-line in an appliance flex. It is rated to carry a maximum current of 13 amperes and is designed to trip on a leakage current of 30 mA. This is an active RCD, that is it doesn't latch mechanically and therefore trips out on power failure, a useful feature for equipment that could be dangerous on unexpected re-energisation.
The incoming supply live (US: ''phase'' or ''hot'' ) and the grounded neutral conductors are connected to the terminals at (1) and the outgoing load conductors are connected to the terminals at (2). The earth (US: ''ground'') 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. The solenoid (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 and neutral conductors. In normal operation, all the current flowing 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 a person touching a live component in the attached appliance, causes some of the current to take a different return path which means there is an imbalance (difference) in the current flowing 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 to flow in 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.
The device is designed so that the current is interrupted in a fraction of a second, greatly reducing the chances of a dangerous electric shock being received.
The test button (8) allows 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 device does not trip when this button is pressed then a fault has developed and the device must be replaced.

Use and placement

In most homes, 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) and exterior areas. In the U.S., 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.

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.

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).

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 imbalance.
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.

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 at 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.

Active/Passive Latching/non-Latching

RCDs may be obtained that have different behaviours if the circuit they are protecting is de-energised. 13A RCD ACCESSORIES

★ One type will trip on power failure and not re-make the circuit when the circuit is re-energised. This type is know as ''non-latching'' or ''active'' MEMSTYLE 13A RCD Protected Socket. X10 – Electrical Safety for Outside Appliances .

★ Another type will re-make the circuit when the circuit is re-energised. This type is know as ''latching'' or ''passive''.
The first type are used when the power-drawing equipment is regarded as a safety hazard if it is unexpectedly re-energised after a power failure e.g. lawn-mowers and hedge trimmers.
The second type may be used on equipment where unexpected re-energisation after a power failure is not a hazard. An example may be the use of an RCD on a circuit providing power to a food freezer, where having to reset an RCD after a power failure may be inconvenient.

Main characteristics

The following key parameters determine the RCD:

★ Number of poles [2P or 3P or 4P]

★ Rated current [in A]

★ Sensitivity [in mA]

★ Type [AC or A or B]

★ Break time [in ms]

★ Surge current resistance [in A]

Number of poles [2P, 3P or 4P]

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

Rated current [in A]

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

Sensitivity [in mA]

RCD sensitivity is expressed as the rated residual operating current, noted 'IΔn'. Preferred values have been defined by the IEC,
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 machines).

Type [AC or A or B]

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending 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 apure direct current,

★
★ for residual currents which may result from rectifying circuits, i.e.:

★
★
★ three pulse star connection or six pulse bridge connection,

★
★
★ two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity.

Break time [in ms]

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: 200ms for 1x IΔn, 150ms for 2x IΔn, and 40ms for 5x IΔn;

★ 'S' (selective) for RCDs with a short time 'delay' (typically used in circuits containing surge suppressors).

★
★ 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 [in A]

Peak current an RCD is designed to withstand (8/20 μs impulse).
The IEC 61008 and IEC 61009 standards impose the use of a 0.5 μs/ 100 kHz damped oscillator wave (ring wave) to test the ability of residual current protection devices to withstand operational discharges with a peak current equal to '200 A'. With regard to atmospheric discharges, IEC 61008 and 61009 standards establish the 8/20 μs surge current test with '3000 A' peak current but limit the requirement to RCDs classified as Selective.

Important note regarding TN-C earthing system

According to IEC 60364:

★ RCD should not be used in a TN-C system (per definition, RCD would not provide necessary protection in circuits with L-PEN wiring)!

★ When an RCD is used in a TN-C-S system, a PEN conductor should never be used downstream (instead, separate PE and N wiring is required).

See also

★ Domestic AC power plugs and sockets

★ Arc-fault circuit interrupter

★ Insulation Monitoring Device

References

External links

★ More detail on RCDs from Electricians Toolbox

★ Example Electrical Safety Policy (University of Edinburgh)

★ Troubleshooting US/Canadian GFCI/GFI devices

★ What is a GFCI outlet? – short video

★ Understanding RCDs by John Ware, IET Wiring Matters, Summer 2006