What is a short circuit?

What happens

The current in a circuit is limited by whatever is using it. A lamp, a machine, a charger: each has resistance, and that resistance sets how much flows.

When live and neutral meet directly, that resistance is gone. What remains is the resistance of the cable itself, which is very small.

By Ohm's law that means an enormous current: hundreds of amps instead of a few. A cable designed for 16 A suddenly carries 300 A.

Why it is dangerous

The cable does not warm up gradually, it heats instantly. Insulation melts in a fraction of a second, and at the point of contact an arc forms at several thousand degrees.

That is why a breaker has to disconnect in milliseconds rather than seconds. The magnetic trip inside it does that job.

What causes it

A cable crushed by a screw or a nail driven into a wall. Water reaching a joint. Insulation that has given way on old cable, usually after long overheating. A device wired incorrectly, or conductors swapped during a repair. Rodents, in cellars and lofts, more often than people expect.

How to recognise it

The breaker trips immediately, not after minutes of running. There is often a bang and a smell. If you reset it and it trips again at once with everything unplugged, the fault is in the fixed wiring.

At that point you stop. Locating a short inside a wall is done by measurement, splitting the circuit into sections until the fault is narrowed to a few metres. That is fault diagnostics, not chasing it with a chisel.

Not the same as overload

Overload means too many appliances on one circuit. The current is above what is allowed but still of a normal order, and the breaker disconnects after some time, through its thermal part.

In a short circuit the current is many times greater and the disconnection is instant. Same device, two different mechanisms inside it.