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# How an RCD works

## The basic principle

A residual current device is designed to protect you from shocks.

It uses the idea that the current in the live and the neutral must be exactly the same.  If it isn't then some current, even a very small one, must be leaking to ground somewhere, perhaps through you.  A current leaking to ground always indicates a fault so the RCD breaks the circuit.

So what an RCD does is compare the current in the live and the neutral.  If they're not identical it trips out.

## Equal currents; equal and opposite magnetic fields

All electric currents produce magnetic fields.  If the electric current changes the whole time, like it does with AC, then the magnetic field changes the whole time.

If we wind the live and the neutral wires around the same iron ring then they produce changing magnetic fields in the ring that exactly cancel each other out.

Now we wind a third coil around the iron ring and connect it to an electromagnet.  If the circuit is working normally there is no magnetic field in this third coil.

However if there is a fault and some current flows to earth then the currents in live and neutral aren't equal, the magnetic fields don't cancel and there's a residual changing magnetic field running through the third coil.

This changing magnetic field induces a current in the third coil.  This current also flows in the electromagnet so it 'turns on'.  The electromagnet pulls the switch open and the live is isolated.

## An RCD isn't a fuse

A fuse is designed to blow if the current in the circuit gets too big.  The type of fault that causes a big current may also cause an RCD to trip out.  For example if the live comes loose and touches the metal case of a toaster the current would be big enough to blow the fuse and it would also cause an RCD to trip out because some current escapes through the earth.

However if the current gets very big, for example because too many appliances are connected, this may still produce a dangerous amount of heating in the wires but wouldn't cause an RCD to trip out because the currents in the live and neutral are always equal.

back to Lesson 10: Domestic Electricity