DC GFCI [RE-wrenches]

John Berdner jberdner at sma-america.com
Fri Jan 11 16:06:35 PST 2002


Drake:

If you take the array positive to ground with a nail you will likely
trip over current device between the nail and the battery.
Since we have to size all the array wiring and O.C. protection at 1.56 *
Isc it is unlikely the O.C. device between the Array and the nail will
trip.
If the nail hits the negative conductor then the O.C. device in the
positive will not trip but the current in the negative could start
flowing through the nail instead.
The nail could overheat and cause a fire but none of the O.C. devices
would ever trip.

This is the reason for the DC.
The imbalanced current in the ground will cause the GFDI to open
interrupting the current flow to ground.

I think I understand what I just wrote but I am not sure if anyone else
will be able to make heads or tails of my mutterings.

If you have any questions or I can be of any further assistance please
do not hesitate to contact me.

Best regards,

John Berdner

SMA-America, Inc.
20830 Red Dog Road
Grass Valley, CA  95945
Tel: 530.273.4895
Fax: 530.274.7271


-----Original Message-----
From: Drake Chamberlin - Electrical Energy
[mailto:solar at eagle-access.net]
Sent: Friday, January 11, 2002 3:30 PM
To: RE-wrenches at topica.com
Subject: RE: DC GFCI [RE-wrenches]


The ground fault detector is near the batteries not the array.  In the
event of a nail through a cable, shorting positive to ground, wouldn't
the
circuit from the array continue to short even if the device were
tripped?

Drake

At 02:58 PM 01/11/2002 -0800, you wrote:
>Travis,
>
>On a line-to-ground fault, the grounding path is removed and therefore
the
>return path for the fault current is broken. Again, it doesn't solve
all
>faults, just those that are more likely to happen. Historically, ground
>faults have been the source of failures of older arrays and various
wiring
>mistakes.
>
>If you bolt the positive and negative together (e.g. with a metal
staple
>through both--good reason never to use metal staples), the GFP may not
>detect this fault, but the inverter likely will because most inverters
>detect array shorts. If it is a battery-based system, the reverse
current
>from the battery will normally trip the main breaker.
>
>Can you think of a fault that isn't covered? I'm sure there are some
high
>impedance line-to-line faults that might go undetected, but they will
likely
>deteriorate into one of these other faults and cause a trip.
All-in-all, the
>GFP fills a major gap in detection of faults that normal overcurrent
>protection is likely to miss.
>
>Bill.
>
>
>
>-----Original Message-----
>From: Travis Creswell, Ozark Solar [mailto:ozsolar at ipa.net]
>Sent: Friday, January 11, 2002 12:33 PM
>To: RE-wrenches at topica.com
>Subject: Re: DC GFCI [RE-wrenches]
>
>
>I never did understand what it's protecting.  I don't remember anything
>about a Canadian lab questioning them.  That does sound an awful lot
like
>what I posted, though.
>
>The main problem I saw, and still do see with them was as such;
>
>A fault happens somewhere in between the inverter and the roof mounted
>panels.  The GFP detects this fault and opens the circuit.   Since the
>panels are still producing power the fault is still merrily smoldering
away
>on the roof or in the attic until sunset.  Hopefully the customer
notices
>the breaker is tripped before the house catches fire.
>
>Wow, now that's helpful a device.
>
>Travis Creswell
>Ozark Energy Services, Inc.
>----------
> >From: Drake Chamberlin - Electrical Energy <solar at eagle-access.net>
> >To: RE-wrenches at topica.com
> >Subject: DC GFCI [RE-wrenches]
> >Date: Fri, Jan 11, 2002, 1:08 PM
> >
>
> >Long ago, someone on the list mentioned that personnel in a Canadian
Lab
> >felt that the DC GFCI, mounted near the batteries, for roof mounted
systems
> >wasn't an effective safety device.
> >
> >Does anyone remember the reference on that?
> >
> >Thanks,
> >
> >Drake
> >
> >
>
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