RCD
Residual Current Device
A protective device that constantly compares the current flowing out on the line conductor with the current returning on the neutral. Any imbalance means current is escaping the circuit — possibly through a person — and the RCD disconnects the supply within milliseconds. BS 7671 requires RCD additional protection on most socket-outlet circuits in ordinary installations.
Where to look it up: BS 7671 Regulation 411.3.3.
Taught and practised in the Installation section.
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