The place I work at makes electronic housings that have to be protected from EMP and Chemicals...
You're probably thinking of EMC which isn't quite the same thing..
EMP or "Electro Magnetic Pulse" Is basically a very strong, fast moving magnetic field. When a changing magnetic field travels past a conductor (wire) a voltage is generated. This effect is used in things like generators and transformers.
The faster the change - the bigger the voltage induced
the more length of wire in the magnetic field - the bigger the voltage induced
The stronger the magnetic field - the bigger the voltage induced..
Now if you can produce a very strong, very fast moving magnetic field (like on the leading edge of a nuclear blast wave, for example) Then even a couple of feet of wire - like you have in your car wiring loom - can generate voltages large enough to damage whatever is hanging on the end of that wire - like your car's engine management unit.