Misleading fault indications [RE-wrenches]
Christopher Freitas --- OutBack Power
cfreitas at outbackpower.com
Fri Feb 29 09:20:33 PST 2008
William -
Your diagram is correct but I do not see what the "serious implications"
are that you are concerned with - especially on a system which is low
voltage (48vdc or lower). Can you be more specific?
With the diagram you show - the GFP trips and indicates the fault and
stops the PV array from producing power - which is what UL & NEC
requires. Also keep in mind that the GFP requirement is really intended
to handle a ground fault in the PV array wiring - not in the inverter.
It does respond to both of course.
In the situation you show - the entire system effectively switches from
being negative grounded to positive grounded - which is funky but not
necessarily dangerous.
The tripped handle of the GFP clearly indicates a problem has occurred
and battery charging by the PV array is stopped. A qualified service
person (i.e. wrench) should then investigate the source of the fault.
-------------
If you want to talk about a real significant design problem - take a
look at the fuse based GFP used on the Xantrex charge controller - if
you have a low resistance ground fault from the DC negative to the
grounding system it will not detect it - even though the GF fuse
blows... which is not detected by the controller since the DC negative
and ground are still connected. The PV array will still continue to
produce power and the controller does not indicate a fault has occurred.
This does not meet the requirements of the UL1741 standard or the NEC in
my opinion. Check it out and let me know what you think...
With the OutBack type breaker based GFP systems, this situation is not
a problem as the GF breaker will trip - indicating that a ground fault
has occurred and the charging is stopped since the PV array is
disconnected by the other poles of the GFP system.
Also - if(when) the Xantrex charge controller fails - perhaps causing a
ground fault to occur itself due to internal damage (which is common
with electronics) then there is no indication that a ground fault has
occurred since the LCD display will not be operational.
Another issue with having the GF protection built into the charge
controller as done by Xantrex is that if you remove the charge
controller for servicing then the system has lost its negative ground
connection - so the system will operate "un-grounded" until the charge
controller has been replaced - and if a different brand of controller is
installed - then there would not be any GF protection or connection of
the DC negative to ground.
Using a fuse type GF protection system in a battery-less grid-tie
inverter is reasonable to me as these significant issues with the
Xantrex charge controller would not apply since the systems are so
simple - there is just the PV array and the grid-tie inverter involved.
But with a more complex battery system - the ground fault system should
be not be built into an electronic device which will require servicing /
replacement and is subject to failures occasionally.
I think that the ground fault protection system should be located in the
main disconnect / overprotection enclosure and should have a indication
system which is not dependent on the operation of a charge controller's
LCD display or the proper functioning of a inter-unit communication
system.
Just my opinion - all comments appreciated...
Christopher
Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030
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