Think about the current in a lightning discharge, how much higher is it than the current in a typical earth fault. What sort of resistance is there in a typical ground system, assume 1 ohm, and consider the voltage that could develop across it. What sort of voltage does a lightning strike have, even if a fraction of it gets into the wiring system in a building, what are the consequences for equipment connected to it, even if the switches on the wall are off.
Additionally, lightning has high frequency components, so what effect might skin effect and inductance of the earth leads have. How much energy is there in a lightning strike, what sort of heat does that translate into, and what might the consequences of that heat be.
Consider those things, and you have your answer.
I worked on a project where we had an audio modulator for an RF transmitter connected to an unused 33KV high tension transmission line. We were pumping 20KW of audio into the line. A lightning strike estimated to have been 50Km away flashed over in the room where the system was operating, and burned the whole structure to the ground. We had the power system earthed, but we didn't put substantial spark gaps and a separate ground at the points where the cables from the transmission line came into the building!
cheers,
Richard
