Residual-current device

From Academic Kids

In electrical installations, residual current devices (RCD) or residual current circuit breakers (RCCB) are circuit breakers that operate to disconnect their circuit whenever they detect that current leaking out of the circuit (such as current leaking to earth through a ground fault) exceeds safety limits. These devices can be tested to see if they are operational and/or they have been wired correctly.

The terms ground fault circuit interrupter (GFCI) or ground fault interrupter (GFI) are used in the United States and Canada, but these terms are not, strictly speaking, correct (or at least complete), because a RCD will trip if current leaks anywhere, not just to ground. 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, this 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.

A Residual Current Breaker with Overload (RCBO) is a combination of a RCD and a MCB



Single phase RCDs operate by measuring the balance between in-bound and out-bound currents using a differential current transformer, and operating the circuit breaker if there is a balance fault (i.e. a difference in current between the "hot" and "neutral"). 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.).

RCDs are designed to attempt to prevent electrocution or fire by detecting the leakage current, which can be far smaller (typically milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. RCDs are intended to operate quickly, 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.


Missing image
Internal mechanism of RCD

The photograph depicts the internal mechanism of a 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: hot) and 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.

However 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.


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). US law (the National Electrical Code) requires RCDs 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.

In many countries it is now required that a hair dryer have an RCD built into its plug, because hair dryers pose an especially severe electrocution hazard.

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. A more common practice is to link all the outlets in a room to a single RCD. More than one RCD on a single circuit 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.

It is a good idea to check RCDs regularly. One way to test a 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).


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-phase short circuits. Some sort of over-current protection (fuse or circuit breaker must be employed to guard against these occurrences.

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).


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

In North America RCD 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, ethernet jacks, etc., in a multigang box. Many companies such as Hubbell, provide multigang boxes that facilitate the mixing of RCD sockets, switches, ethernet jacks, phone jacks, video connectors, and the like, in various permutations, that can be accept a standardized cover plate solution. However care should be taken to ensure that regulations involving separation of mains and signal wiring are followed in such installations.

See also

External links

es:Interruptor diferencial he:ממסר פחת it:Interruttore differenziale nl:Aardlekschakelaar


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