Metal Roofing Grounding [RE-wrenches]

[Jason Fisher, Aurora Energy]

Christopher Freitas wrote> The DC-GFP is intended to prevent fires due
to 
> the hazard of arcs and overheated conduction paths (that might not
have
> been designed to actually carry power - such as metal roofing or other

> wiring).

Anybody have any knowledge of what the requirements are, and what the
practicalities are for grounding of metal roofs? If arrays are mounted
to existing metal roofs (not anybody's favorite mount I would imagine,
except UniSolar) what would be the best method for grounding the pans if
required?
If the pans were not grounded and a hot wire was in contact with the
roof, how would the GFP trip?

Jason Fisher
Aurora Energy

-----Original Message-----
From: Christopher Freitas - OutBack Power
[mailto:cfreitas at outbackpower.com] 
Sent: Friday, January 11, 2002 6:20 PM
To: RE-wrenches at topica.com
Subject: RE: DC GFCI [RE-wrenches]

The DC-GFP is intended to prevent fires due to the hazard of arcs and 
overheated conduction paths (that might not have been designed to 
actually carry power - such as metal roofing or other wiring).  

The real hazard from a ground fault is not necessarily from the PV 
panels but from the stored energy in the battery and/or capacitors in 
the inverter.  The DC-GFP system will limit the amount of fault current 
that flows from the battery / inverter when the ground fault occurs. 

In most DC-GFP systems the PV array is disconnected in case the fault 
was caused by a problem at the PV array.  The DC-GFP will actually trip 
due to faults anywhere in the system - and will respond to both DC and 
AC ground faults as well. By switching the system from a low impendance 
to high impedance negative to grounding system connection,  the fault 
current is reduced to essentially zero.  

On a system with two negative to ground connections the DC-GFP would 
trip once the system is fired up (no pun intended).  This is probably 
the best reason for including a GFP - it tells you when you finished the

installation that there is not a ground fault.  I was surprised when we 
first started shipping GFPs how many calls we got from people saying 
"the GFP must be broken - it won't stay on" which of course were 
actually caused by having swapped wires or multiple bonding locations on

the DC side.    

I think since the cost is so low now ($129 retail for two 60 amp PV 
arrays at 12, 24 or 48 vdc) that it is more than reasonable to include a

DC-GFP in nearly all systems,  roof mounted or not.  The added benifit 
of knowing when something is not right is worth it in my opinion.  

BTW - I have seen on more than one occasion installations where the DC 
system was carrying AC current due to ground faults from bad 
transformers inside an inverter or incorrect system design.  The DC-GFP 
would have warned them of this very dangerous situation.  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com   www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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