Non-tech manufacturer folks on the list [RE-wrenches] Lightning

[Joel Davidson]

A little lightning protection history. Some of the mature (never old) wind power
people learned about the "cone of protection" from Joseph and Marcellus Jacobs and
their protégés. Jacobs use to float the wind system without a ground on the DC
negative. Up until 1984 NEC 690 so did almost all PV techs. Why? Because grounds
attract atmospheric transients. Anyway, we lost that battle so we ground the negative
(unless the PV array is roof mounted and then it needs ground fault protection which
requires floating the system - but that's another story). Jacobs use to bury a ring
of bare copper wire #8 or thicker about 6 to 24 inches in the ground around the wind
charger tower about 10 feet out from the tower legs and attach the ring to the base
of the tower. Although there was no point above the wind machine for an air terminal,
the tower acted as a down conductor to the copper wire ring earth electrode.

If the PV array is ground mounted, really ground the mount it into the ground into
nice moist soil. Then put a few air terminals (aka lightning rods) on the array
highest points. If the PV array is roof mounted in a lightning region and there is a
higher roof point than the top of the PV array, put lightning rods on the structure
and ground the rods separate from the PV array. If the PV array is the highest point
on the structure, then use lightning rods on the array connected to down conductors
and earth electrodes - in addition to your NEC negative ground. You are not required
to connect the lightning ground rods to the electrical system ground rods.



"Michael Welch, Home Power" wrote:

> Hey gang, I am posting this on behalf of Kent, who is undergoing an email glitch.
>
> wrenches, here is my lay understanding of surge arrestors and lightning:
>
> surge arrestors are not going to protect against a direct hit no matter what
> you do. all bets are off regardless of the arrestor. at best they will come
> apart and not take out too much of the stuff they are supposed to protect.
>
> MOV's don't work, they only take a limited amount of energy before they fail
> and short. when they fail, they light on fire real nice and take out
> everything around. they will generally continue to short the array to ground
> until something stops the fire. to protect against this, a fuse must be
> installed in series with the mov, so when the mov shorts the fuse blows.
> typically, there is no indication they have failed, so the next time the
> system is hit there is no surge protection.
>
> SOV's seem to work better. they are designed to take repeated hits without
> failing (or so the mfg says). we have had much better luck with these. at
> one time sandia only recommended sov's. i've heard they have changed their
> tune, but unsure why.
>
> lighning and lightning induced surges are seeking earth ground. as this is
> their electrical return path to complete the circuit (the reason there is
> lightning).
>
> in the event of a near hit, the SA may redirect a large induced dv/dt away
> from the electronics if properly installed. for this to work the SA must be
> grounded, else you will redirect the energy to the other pole. the SA's i've
> seen are just electrodes in sand, spaced an amount to arc when the voltage
> exceeds a threshold. some games may be played by grounding different poles,
> but this will just change the clamping point of the sa. regardless, one of
> the poles must be grounded. if you don't ground the sa i don't see how it
> will work. but, given the nec requirement of pv conductor grounding, you
> will always have a ground referrence through the pv ground. the only way i
> can see connecting these things is either pole-to-pole or
> plus-pole-to-ground with neg-pole-to-groung. either way, the sa is grounded
> if an array conductor is grounded. in terms of a bi-polar array, if you are
> just pole-to-pole connected, the surge will be exaggerated by the negative
> potential of the array, causing even more damage as the surge will more
> readily jump to the electronics as it seeks ground.
>
> in terms of proper grounding, there should be one point of system ground for
> the pv ground fault detection to work properly. multiple pv grounds cause
> circulating ground currents. this is what is referred to in our literature.
> safety grounds (and sa grounds) may be dropped all over the field, but they
> must be bonded together to the single point of system ground. you don't want
> potential differences in your safety grounds.
>
> for what it is worth,
>
> Kent Sheldon
> PV Product Manager
> Distributed Industrial & Utilities Markets
> Xantrex Technology Inc.
> Direct 925 245 5463 - Fax 925 245 1022
> kent.sheldon at xantrex.com  www.xantrex.com
>
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