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A Residual Current Device (RCD) is a circuit protection device which disconnects the load ("trips") if the current flowing out in the live wire exceeds the current returning in the neutral by a given amount. (Typically 30mA, 100mA, 300mA, depending on the device). 30mA = 0.03A

This imbalance in the return current (the residual current) is leaking to earth, which could indicate a fault in the circuit or a person receiving an electric shock (to earth).

If an RCD is being used to provide additional protection against electric shock, the Wiring Regulations specify that a type with no time delay & 30mA sensitivity be used. In most cases a shock of under 30mA or one for the brief period before the RCD trips will not be fatal.

Where possible a 30mA RCD should therefore be provided on final circuits. To allow diversity in a power distribution system, upstream RCDs can be of time delayed type and higher rating. It's important to note that a 30mA RCD will not necessarily trip before a 300mA RCD for a specific fault, rating alone should not therefore be relied on to provide diversity.

RCDs should be regularly tested by the user (at least once a quarter) by pressing the Test button (usually marked "T"), which should cause the device to operate and disconnect the load. The test button establishes that the mechanical parts have not ceased and the electronics are still functioning however more in depth testing by specialist equipment may be required as part of an installation's test & inspection schedule to ensure it is within specification.

RCDs are not as reliable as other forms of protection and this is particularly the case with those that are not tripped (either by a fault or using the test button) at regular intervals. See the Electrical Safety Council website for more information.

Not to be confused with a Miniature Circuit Breaker (MCB), which provides circuit protection against overloads. A combined RCD & MCB is known as an RCBO.

RCD Types

• Plug type, 30mA

• Socket type, 30mA

• Consumer unit / distribution type, 30mA