POE switches and lightning

Larry Sheldon LarrySheldon at cox.net
Fri May 14 19:17:40 CDT 2010


On 5/14/2010 19:00, Seth Mattinen wrote:
> 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.

In my experience, each building has a building ground-point at the
service entrance, as outlined.

I the problem in a campus on some soils is that building grounds might
be several volts apart--except during thunder storms when the voltage
difference might be (it appears) thousands of volts, and with a
lightning strike to one of them many thousands of volts.

That is why I argue for glass only between buildings.  I don't care how
much PoE saves.

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