DC-GFP/2 protection [RE-wrenches]

Bill Brooks billb at endecon.com
Fri Aug 23 13:37:06 PDT 2002


Hey Steve,

One point for needing to detect ground faults (most often in wiring) is that
as soon as you have a ground fault, you no longer have an ungrounded system.
Whatever protections you were counting on as ungrounded, no longer exist.
One benefit to an ungrounded system is that you must have two ground faults
to provide a current path. That is less likely, but if you have one due to a
wire being mishandled, a second is probably not far behind.

Another one of the issues with an ungrounded system is that it still must
have a method to reference it to ground potential. You can't have a module
operating at 5000 Volts sitting next to a module frame at ground potential.
It will flashover and cause a big mess as well as potentially energize the
array frames if the ground gets loose. Somehow you must limit the
differential from an electrical insulation point of view.

The bottom line is safety, and I believe a well-designed safe system will
also be the least likely to die in a lightning strike. Ultimately, it is up
to God whether your equipment survives a lightning strike.

Bill.




-----Original Message-----
From: Steve Bell, Sunwize Technologies [mailto:sebpv at stelle.net]
Sent: Friday, August 23, 2002 1:20 PM
To: RE-wrenches at topica.com
Subject: Re: DC-GFP/2 protection [RE-wrenches]


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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