POE switches and lightning

Pete Carah pete at altadena.net
Thu May 13 11:39:10 CDT 2010


On 05/13/2010 12:19 PM, Larry Sheldon wrote:
> On 5/13/2010 10:36, Caleb Tennis 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.  
>>
>> What's interesting is that various POE switches throughout the entire building seemed to be affected in that some of their ports they just shut down/off.  Rebooting these switches brought everything back to life.  It didn't impact anything non-POE, and even then, only impacted some devices.  But it was spread across the whole building, across multiple switches.
>>
>> I was just curious if anyone had seen anything similar to this before?  Our incoming electrical power has surge suppression, and the power to the switches is all through double conversion UPS, so I'm not quite sure why any of them would have been impacted at all.  I'm guessing that the strike had some impact on the electrical ground, but I don't know what we can do to prevent future strikes from causing the same issues.  Thoughts?
>>     
>
> I don't know how to account for this in a PoE world, but when I last
> managed a campus network, we had major issues (particularly in an
> active-thunder-storm environment) of severe difference in
> ground-potential between buildings.
>   
Cat 5 has isolation transformers in or just behind each jack.  However,
in most equipment the grounds aren't really isolated, and in the case of
POE they (mostly) aren't at all.

Lightning likes to do "interesting" things.  It can induce a 20kv per
few feet gradient (or more) across the ground mesh of a power substation
(like 4/0 wire in a mesh of 4 foot squares or so; normally more
complicated than that since it has to clear equipment etc...).  It likes
to eat power supplies in well-grounded equipment and leave cheaper stuff
alone.  It can hit an antenna, leave the receiver completely intact, and
fry the power supply of the next box over.  We tended to lose either
fluorescent ballasts or the thermostat transformer in our furnace when I
lived in an active ham's house in Alabama, the radios tended to live. 
(you should have seen his coax entry panel (1/4 inch copper sheet,
grounded outside)), and stuff got manually disconnected from both
antennas and power when a storm was expected (every afternoon :-).

It wouldn't surprise me if the first answer was right and either the
ground pulse or EMP reset the safety switches in the POE feeders.

-- Pete





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