[spokeshave]

If they are not “live” the EMP affect has no “electromagnetic” activity to disrupt in a damaging way. It’s the same as “do you need a surge protector if your circuit is off/dead”? No. Any “electromagnetic pulse” will not get passed/into the off/dead circuit.

I don't think so

even with everything switched off...miles of high tension wire will have huge induced voltages due to the induced action of the electromagnetic pulse....resulting in flash over at switches and resultant destruction of equipment.

[Boomer]

It’s unclear how much of an effect an EMP will have on electronics not plugged in or powered in any way. Even a desktop computer has a small amount of current flowing through it when unplugged due to the little coin battery to maintain the time. Laptops of course have their onboard battery. It’s thought that the electric cord to devices will actually absorb and relay the pulse to the the inner circuits of devices.

That’s the thing; we really don’t know until it actually happens. It may turn out to only affect unshielded powered on devices.