POE switches and lightning

Seth Mattinen sethm at rollernet.us
Fri May 14 19:00:44 CDT 2010


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




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