A residual current device (RCD) or a residual current circuit breaker (RCCB) commonly know as a safety switch is designed to isolate power when it detects an in-balance commonly of 30 mA (milliamps) between the active and neutral conductor.
These in-balances are commonly caused by current leakage through earth. This leakage can be caused via a faulty appliance or through the body of a person accidentally touching an energized piece of equipment.
RCD/RCCB's are designed to disconnect power to the circuit when a difference of 30mA is detected. This disconnection or tripping of these breakers should happen without due delay, if the RCD takes too long to trip the results could be fatal.
Testing RCD's using the correct equipment ensures that the protective device will do its job within the correct allowable time frame, if it is required to do so.