How is a Residual Current Device activated within a circuit?
Now sit back, this may take a while. The live and neutral currents create magnetic fields, but as they are flowing in opposing directions these magnetic fields cancel each other out, leaving a net result of no magnetic field. When the RCD sees this everything is sweetness and light and it is quite happy, therefore doesn’t trip. If the returning neutral current is less than the outgoing live current it suggests that it is flowing somewhere else, usually to earth. As this is then considered an earth leakage current the RCD will trip when the leakage value exceeds the rated tripping current of the RCD, usually 30mA. The RCD works by having a toroidal coil that will have no overall magnetic field under normal operation, but a small one under leakage conditions. This is sensed by a further coil wrapped around the toroid, which trips when its sensed current exceeds a limit. It is basically a very simple, very low current transformer, working by the principle of mutual induction. Clear as mud
