IN THIS ARTICLE we discuss Residual Current Device (RCD) operation, which can occur due to electrical faults both downstream and upstream of the device. An RCD, when correctly selected and installed, can provide protection against electric shock due to indirect contact. RCDs are also used to provide supplementary protection against direct contact and protection against thermal effects. Operation of an RCD can occur due to a downstream fault such as a crossed neutral on a split-load board, high protective current or an upstream effect such as mains-borne disturbances.