DC-GFP/2 protection [RE-wrenches]

Steve Bell, Sunwize Technologies sebpv at stelle.net
Fri Aug 23 13:20:23 PDT 2002


Hi Bill,
Perhaps I am just confused and my brain is working slowly because it is
Friday, but exactly what sort of ground fault would we be trying to detect
and protect against, when there would be no grounding whatsoever on the DC
wiring? Are we trying to detect a fault in the chassis/array
mounting-structure ground? It seem to me that a floating, ungrounded system
would not need to protect against a fault to ground when the grounding
system  has no electrical commonality with the floating DC system (except
through the SOV arrestors). Exactly what are we protecting against?
Electrical shock? Roof fires?

What is the technical reason for requiring PVGFP on ungrounded systems? How
does Europe manage to safely operate ungrounded DC systems?

And, yes I agree that most of my positive experience with lightning
protection has probably been because of the SOVs, particularly on the AC
lines because that is where the vast majority the lightning problems get
into most systems. Good chassis grounds help prevent most of the possible
lightning problems on the DC side.

Have a great weekend.
Sincerely,
Steve Bell
sebpv at stelle.net


----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Friday, August 23, 2002 11:34 AM
Subject: Re: DC-GFP/2 protection [RE-wrenches]


> Steve:
>
> I wholeheartedly agree that we need to have provisions for ungrounded PV
> arrays in the NEC, but that does not get us out of the Ground Fault
> Protection issue. On the contrary, we have to be even more diligent about
> sensing Ground Faults in a floated system. Fortunately, it is easier to
> sense ground faults in a floated system. The key issue is equivalent
> electrical safety to grounded systems. Grounded systems definitely provide
a
> level of safety by referencing the system to ground potential. All the
> safety benefits we get from a grounded system have to be found in an
> ungrounded system. Both types of systems can be safe if designed
> properly--however that is a big IF. Just ungrounding systems is not the
> answer. The Europeans have been doing it safely for a very long time so it
> is not like we are charting new ground here. We just need a very well
> written and well substantiated section to address the issues of ungrounded
> systems. It is very likely that provisions for systems above 300Vdc will
> precede other ungrounded provisions.
>
> I would be willing to bet that the SOVs have been the reason you have done
> so well with your system--not the fact that you have ungrounded the
system.
> I really don't think you have created an unsafe situation, but it does
> operate above 50 Volts, so it should either be grounded or have equivalent
> grounding means.
>
> Bill.

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