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# How an MCB works

## A fuse you can reset

A mechanical (or "miniature") circuit breaker is like a resettable fuse.  It protects a circuit from currents that are too high.

It works by winding the live wire around a piece of iron.  This makes an electromagnet.  The bigger the current in the live the stronger the electromagnet.

The electromagnet is positioned so that it pulls against a switch that can break the circuit.  The switch is held closed by a spring.  It's designed so that when the current is bigger than some preset value, say 20 amps, the switch is pulled open and the circuit is broken.

## It's the volts that jolts but it's the mills that kills

MCBs aren't very good for protecting you against shocks, though.  This is because the current passing through you is typically very small (of the order of milliamps).  This is enough to severely harm you but isn't enough to trip the MCB.

When you get a static shock from touching someone it may be at 5000 V or so.  But it doesn't kill you.  This is because the total energy involved is very small.  There just aren't very many charges with 5000 joules per coulomb.

So the initial current will be exactly the same as if you'd touched a 5000 V power line but it decays to zero almost instantly.

The human body has a resistance of 5000 ohms or so (call it 4800 ohms to make our calculation easier).  So if you touch a live wire at 240 V you can calculate the current that would flow through you using Ohm's law V = IR.

I = V/R = 240/4800 = 0.05 A

0.05 A is 50 milliamps.  This is what we mean by "mills".  It's the current flowing through you that causes the damage but you only get a damaging current with a high voltage.

back to Lesson 10: Domestic Electricity