Gridtie, breaking grounded conductor [RE-wrenches]

Jason Fisher jfisher at nahbrc.org
Wed Feb 25 10:23:22 PST 2004


John,

You make an important clarification about 690.13 overriding other
sections of the NEC, and I appreciate the opportunity to now etch into
my mind that in most PV systems breaking the grounded conductor in a
disconnect is not allowed. I would add though, that 404.2(B)-Exception
does allow this for many other applications. Granted, the situations
where this is really desirable are limited but it still is allowed. I
would encourage anyone who needs to address this issue with their
inspectors to familiarize yourself with this reference since it is
probably what the inspector is thinking of.

If you think about it, breaking the grounded conductor simultaneously
with ungrounded conductors is exactly what happens anytime a cord and
plug connected AC device is disconnected from a receptacle.

Hope this helps,

Jason Fisher
Aurora Energy
Annapolis, MD

------------

From: John Berdner [mailto:jberdner at sma-america.com] 
Sent: Tuesday, February 24, 2004 5:57 PM
To: RE-wrenches at topica.com
Subject: [SPAM] - Re: Gridtie, breaking grounded conductor [RE-wrenches]
- Email found in subject

Wrenches:
 
I happen to agree with the inspector that the ungrounded conductor
should be opened for servicing.
Unfortunately this is not Code at the moment and inspector confusion
over this issue is a common problem.
The NEC is quite clear on the issue.
Article 690.3 says Articles 690 and 705 over rule all other sections of
the code in the event of a conflict.
This is one of the places there are conflicts.
 
Article 690.13 specifically says that you may NOT have a switch,
breaker, or fuse in any grounded conductor UNLESS that device is part of
a Listed GFDI required by 690.5.
The FPN following 690.13 allows the grounded conductor to be interrupted
by a bolted connection to allow maintenance or troubleshooting by
qualified personnel.  The removable GFDI fuse in the Sunny Boys is a
bolted connection for this purpose.  
 
Article 690.5 outlines the requirements for the GFDI.
UL, ETL, or CSA tests the GFDI and certifies that the Listed GFDI device
meets the requirements of the Code.
The location of a Listed GFDI (inside or outside the inverter) is
irrelevant.
(The note following 690.5 in the 2002 NEC Handbook specifically points
out that the normally grounded conductor may be at any voltage up to Voc
below ground, e.g. -400 Vdc when the GFDI has interrupted the bond)
 
Article 690.17 gives the requirements for switches and circuit breakers
used as disconnecting means for UNGROUNDED conductors.  690.17 follows
the NEC's long tradition of not allowing switches in grounded
conductors. 
 
The shock the inspector experienced form the negative conductor is a
capacitive discharge due to the small capacitive coupling between the
array and earth (as Bill Brooks correctly pointed out).  
You can verify this with a DVM measurement from PV + to ground and PV -
to ground with the GFDI fuse removed.
Unless you have a ground fault the system is ungrounded at that point
and you should see about 0.5 * Voc on each measurement.
This voltage should decline towards zero as the capacitive coupling is
discharged through the DVM.
A fixed voltage that does not decline normally indicates a ground fault.
The value of the fixed voltage compared to Voc even tells you about
where the ground fault is!
 
In a  Sunny Boy the bonding of the PV negative to earth is not "on the
AC side" as the inspector indicated.   
The dc system is bonded through the Listed GFDI device that complies
with the requirements of 690.5 as noted above.
There still may be some valid debate about the routing of the equipment
grounding conductor compared to the DC bonding jumper.
Although these conductors are redundant (going from the same place to
the same place) they have differing requirements for the size and
routing of the conductor.
The latest opinion I have on the subject from J. Wiles is that a single
grounding conductor meeting the more restrictive of the two requirements
might be enough.
 
Best Regards,
 
John Berdner
 
 

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