Re: Long cat5 run question

"w_tom" <w_tom1@xxxxxxx> wrote in message
news:8a5dea7f-db01-48b0-89bd-d3653ad98e72@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Read both Electrical Engineering Times articles entitled
"Protecting Electrical Devices from Lightning Transients" to
appreciate which type of transient does damage. Destructive surges
seek earth ground. Never forget a damning question: where is surge
energy dissipated? An MOS does not absorb that energy. Review its
spec numbers. It cannot. Where is that energy dissipated? That EE
Times article describes what an effective MOS device does.

it is a long time since i had to worry about serious lightning protection
(on chemical plants at the time) - but here goes.

So - ignoring the argument about what type / how surge suppressors work on
an actual strike, the previous paragraph assumes you can send the spike into
a perfect ground.

the original question was about whether to use Cat 5 between buildings.

What happens in practice is that if the surge is shunted to ground (by any
means - lightning rod for example), you have 10k Amps radiating out from the
ground point.

This raises the local ground voltage momentarily relative to other places
away from the strike - how much depends on ground type, resistance, how wet
it is and so on. The effective voltage is increased since the ground isnt a
pure resistive load, and the current is fast attack spike.

A lot of lightning damage can be put down to cables linking this ground,
that may change voitage by 10k volts to somewhere much less affected by the
spike.

this is more than enough to defeat the insulation rating on an 100 Base-Tx
etc isolation transformer, and flash over into the equipment.

however - these are secondary effects to the actual strike, and much less
current is involved, so a protection device which shunts the spike may be
enough to protect the equipment.

<SNIP>
--
Regards

stephen_hope@xxxxxxxxxxxx - replace xyz with ntl


.

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