Residual Current Device

Residual Current Device

A graphic illustrating a Residual Current Device, or RCD. A simplified circuit representing how an RCD works is shown. A difference of just 30mA between the live and neutral currents will trip the RCD and isolate the appliance in under 40ms.

An RCD, or residual current device, is a life-saving device which is designed to prevent you from getting a fatal electric shock if you touch something live, such as a bare wire. It can also provide some protection against electrical fires. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers cannot provide.

n RCD constantly monitors the electric current flowing through one or more circuits it is used to protect. If it detects electricity flowing down an unintended path, such as through a person who has touched a live part, the RCD will switch the circuit off very quickly, significantly reducing the risk of death or serious injury.

A residual-current device (RCD), or residual-current circuit breaker (RCCB), is a device to quickly disconnect current to prevent serious harm from an ongoing electric shock. Injury may still occur in some cases, for example if a person falls after receiving a shock. In the United States and Canada, the device is more commonly known as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI) or an appliance leakage current interrupter (ALCI). In the United Kingdom, these are better known by their initials RCD, and a combined RCD+MCB is known as a RCBO (residual-current circuit breaker with overcurrent protection). In Australia, they are sometimes known as safety switches or a RCD. An earth leakage circuit breaker (ELCB) may be a residual-current device, although an older type of voltage-operated earth leakage circuit breaker also exists. In German speaking countries the device is sometimes known as FI where the F stands for fault (Fehler) and I for the symbol that represents Electric current.

These electrical wiring devices disconnect a circuit when it detects that the electric current is not balanced between the energized (line) conductor(s) and the return (neutral) conductor. In normal circumstances, these two wires are expected to carry matching currents, and any difference usually indicates that a short circuit or other electrical anomaly is present. Even a small leakage current can mean a risk of harm or death due to electric shock if the leaking electric current passes through a human; a current of around 30 mA (0.030 amperes) is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. RCDs/RCCBs are designed to disconnect the conducting wires quickly enough to prevent serious injury from such shocks. (This is commonly described as the RCD/RCCB being "tripped".)

A RCD does not provide protection against unexpected or dangerously high current when current is flowing in the usual wires in the circuit, therefore it cannot replace a fuse or protect against overheating or fire risk due to overcurrent (overload) or short circuits if the fault does not lead to current leakage. Therefore RCDs are often used or integrated as a single product along with some kind of circuit breaker, such as a fuse or miniature circuit breaker (MCB), which adds protection in the event of excessive current in the circuit. RCDs also cannot detect the situation where a human accidentally touches both conductors at the same time, since the flow of current through an expected device, an unexpected route, or a human, are indistinguishable if the current returns through the expected conductor.

RCDs are usually testable and resettable devices. Commonly they include a button that when pressed, safely creates a small leakage condition, and a switch that reconnects the conductors when a fault condition has been cleared. Depending upon their design, some RCDs disconnect both the energized and return conductors upon a fault, while others only disconnect the energized conductor and rely upon the return conductor being at ground (earth) potential. The former are commonly known as "double-pole" designs; the latter as "single-pole" designs. 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.

What are the main types of RCD?

RCDs can help protect you from electric shock in potentially dangerous areas like bathrooms and gardens, and there are various types of RCDs that can be used to make sure you are always as safe as possible.

Fixed RCDs

These are installed in the consumer unit (fusebox) and can provide protection to individual or groups of circuits. A fixed RCD provides the highest level of protection as it protects all the wiring and the sockets on a circuit, and any connected appliances.

Socket-Outlet RCDs

These are special socket-outlets with an RCD built into them which can be used in place of a standard socket-outlet. This type of RCD provides protection only to the person in contact with equipment, including its lead, plugged into the special socket-outlet.

Portable RCDs

These plug into any standard socket-outlet. An appliance can then be plugged into the RCD. They are useful when neither fixed nor socket-outlet RCDs are available but, as with socket-outlet RCDs, they provide protection only to the person in contact with the equipment, including its lead, plugged into the portable RCD.

Are RCDs reliable?

We’ve found that fixed RCDs are about 97% reliable. This improves if they are tested regularly. If you have fixed RCD protection, it will reduce the risk of electric shock to you and your family. It can also protect your home against the risk of fire caused by faulty wiring or appliances.

Remember – Although RCD protection reduces the risk of death or injury from electric shock it does not reduce the need to be careful. Have your wiring checked at least once every 10 years to ensure the safety of you, your family and your home. If you find a fault with your wiring, or an appliance, stop using it immediately and contact a registered electrician.

Don’t forget to test – You should test all fixed and socket RCDs about every three months. Manufacturers recommend that portable RCDs are tested every time you use them.

Beware – If you hold the test button in for a long time and the RCD does not switch off the electricity supply, then get advice from a  registered electrician.

 

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