On 5/14/2010 16:42, Ingo Flaschberger wrote:
We had a lightning strike nearby yesterday that looks to have come inside our facility via a feeder circuit that goes outdoors underground to our facility's gate.
Perhaps there was a "move" of the earth-level relative to the neutral line. I have no idea how neutral-line to earth potential is handled in us, but here in austria we use a so called "nullung". That means that the earth-ground potential line of the building (which includes also the lightning conductor) is connected to the neutral power line where it enters the building, keeping this potential-difference low.
In the US neutral and earth ground are supposed to be bonded only once at the service entrance. A separate ground from the neutral conductor is carried to sub-panels where is it not bonded. Additional bonding can cause weirdness and will turn the ground into a current carrying conductor. However, an older building I used to be in (built 1978) only gave me a neutral with bonded subs, so you'll run into all kinds of stuff depending on the age of the building. Working at a university was particularly interesting with of the vast range of building ages. ~Seth