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