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Under normal safe operating conditions, the current flowing into a circuit will be the same as the current flowing out of it. If this is not the case, current must be flowing somewhere else (for example through faulty insulation or even through a human who is being electrocuted). RCD devices detect these current imbalances and turn off the circuit when an imbalance is detected. A typical imbalance is 5 milliamps. They also turn off the balanced circuit if the current exceeds a safe limit such as 30 Amps.
The device in the image above has a test button. When pressed, this deliberately unbalances the current and the device should trip. The RCD is reset by turning the toggle switch on.
Most domestic wiring is protected by RCD devices.
A less safe alternative is a fuse. This only protects against excessive currents.
RCD devices are sometimes tripped by sudden current pulses. These can be caused by lightning strikes.
