An RCD monitors the algebraic sum of the currents in the line and neutral conductors (the residual current) at the point at which it is installed in a circuit, and mechanically interrupts the circuit when this sum attains a predetermined value (the rated residual operating current ).
The following definition is given in Part 2 of BS 7671:
‘Residual current device (RCD).A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions.’
‘Residual current device’ (RCD) is the generic name for a range of devices including:
|•||RCCB (residual current operated circuit-breaker without integral overcurrent protection)|
|•||RCBO ( residual current operated circuit-breaker with integral overcurrent protection)|
|•||SRCD (socket-outlet incorporating an RCD)|
|•||An RCD formed of a suitable association of devices including current transformers, a residual current sensor, a test button and a circuit-breaker.|