Gridtie, breaking grounded conductor [RE-wrenches]

mlafferty at universalenergies.com mlafferty at universalenergies.com
Tue Feb 24 14:07:47 PST 2004


Bob M asked (Paraphrased by myself - Hope I got the message and context
correct):

Oregon now has a subsidy.  Multiple inspectors are involved.  One
inspector said he got shocked from a negative conductor in the DC
Disconnect of a Battery-Less PV System.  Now they are saying that the
Positive and Negative DC Conductors should be disconnected and the DC
System Ground needs to occur at the array.  Others are saying that a DC
Disconnect must be installed at the array for a rooftop mounted system.
Any help / ideas?

Nick N replied:

>  The Sunny Boy's negative/ground bond is not bonded when the GFCI Fuse
is removed.  Therefore, some shock hazard is present.

Bob / All:

Please pass along my condolences to all Good Oregon PV'ers for the
existence of a subsidy program.  It is a mixed bag of blessings and
curses, let me assure you.  My best advice is to be proactive in the
area of "Quality Assurance of System Installations" and make sure the
systems that get approved / subsidized are good examples... No matter
how "underfunded" it may be, snakes crawl from beneath their rocks
whenever "free money" appears.  Money disappears fast when snakes suck
it up!  Perhaps (I hope) the "underfunded" element may be somewhat
self-limiting in this regard and quality, integrity-based systems will
proliferate.  Two Cents.

On the technical / advisory side:  My perspective is that DC Disconnects
on the roof serve no significant, valuable purpose, other than Fire
Department Comfort, if the conductors are properly installed in conduit
external to the building envelope, as long as the DC Disco is mounted in
a "readily accessible" location.  Arguments relating to whether or not a
NEMA 3 DC Disco lying on its back at roof-slope is installed in
compliance with its enclosure rating will arise, as well.  The
difference between a NEMA 3 & NEMA 4 Disco is significant,
dollar-wise... I wish there were a simple answer for BIPV products or
even conventional rack-mounted arrays that have conductors run within
the building envelope, but don't know of one that will stand up to NEC
requirements under scrutiny.... Glass Manus:  Are you listening????  

Nick N is right regarding the SB's, with the exception that it is NOT A
GFCI.... It IS A GFP... Two different applications and two different
purposes.  No mention is made in Bob's original post regarding the
inverter / application that indicates this is an SB system.  I've taken
a bit of grief over my view on this, but strongly belieive it has merit.
I DO NOT RECOMMEND termination of the Array DC Negative within the DC
Disco on an SB System.  (Negative Home Run passes through the Disco
without landing on anything, including under a wirenut)  This minimizes
/ eliminates the potential for shock hazard out there in the real world,
with unknown "fingers in the disconnect".  If the GFP fuse is removed
(As it should be any time the system is inspected or tested!), the
Positive doesn't know the "Ground" exists unless there is a
ground-fault!.... Always measure with a clamp-on to verify!  

There are numerous scenarios that could be present with the description
provided... A likely one is what I refer to as "static".... Array Glass
and Racking builds up a static charge much like the provervbial "socks
on carpet / light switch" effect.  This is an indeterminable "floating"
voltage affected by numerous factors like humidity, wind speed &
direction, physical square footage of array, etc., (Discharged without
damage or personal injury if no fault is present) and virtually
independent of whether or not it is grounded.  This condition changes
"relative" polarity and voltage instantaneously with changes in things
like wind direction, etc.  I hope we all have seen, are aware of, and
tested this condition!

I am left to wonder:  What the heck was the "Inspector" doing actually
touching the Negative Conductor in the first place?  Perhaps the first
order of business would be to invest in some Practical Safety Training
for the Inspectors?????  (PV Systems should have all DC system ground
references removed prior to handling or testing... Just the opposite of
traditional AC practice!)

Disconnecting the Array Negative Conductor defeats the GFP operation
with the disconnect in the open position.  Modern Grid-Tied Only
inverters will not properly operate with solid grounding of the array
negative if a GFP device is incorporated, creating a major conflict
between various sections of the NEC, UL, and IEEE standards.  (Defer to
individual manufacturers for specifics...)

All this raises the larger question of whether or not we should be using
a Grounded Electrical System in the first place.  This isn't an easily
displaced issue...  On one hand, a floating system ground is contrary to
virtually everything else in the NEC except cranes, elevators, and
hoistways... On the other hand, an argument can be made that this scheme
offers greater personnel safety under certain conditions.  I would hold
that, considering all the related factors and applications, it is easier
/ better to make sure our stuff complies with the "norm" in lieu of
trying to "blaze our own trail" on a broad-based level.  Disconnecting
DC Negative Conductors in a Disconnect is traditionally applied to DC
Loads like DC Motors (except for the exceptions noted above), not
"generators"... Convention holds that "measuring voltage to ground"
determines potential exposure to hazard.  If "excessive" voltage is
measured, the next step should be to measure the ampacity capacity of
the "circuit".  (In a properly installed system, the amperage will be
nil and immediately discharged when shorted... Fact.)

If either of these steps are sidestepped, expect a shock.  Plain and
simple.  Go home, take your shoes off, walk across the carpet and over
to the hallway light switch and flip it.  If you get a shock, then come
back and talk to me.  Meanwhile, pay attention.  If you're gonna inspect
any other electric circuits, do you "touch" any conductors without
testing their voltage and potential capacity?  Geez.   You're still
alive?  If you are, I would argue that you should immediately go home,
call your Electrical Service Provider (Utility) and have your home
electrical system disconnected, along with the any and all electrical
system grounding references to make sure you're safe!  If you've never
experienced the "socks on carpet / light switch" effect, you need to get
out more often!

Bottom line:  Know what you're touching.  Measure all potential voltage
relationships and UNDERSTAND THEM prior to touching them!
(Positive-Negative, Positive-Ground, Negative-Ground)  When voltage is
measured, discharge it with a shorting conductor and measure the
ampacity of the short.  If the short instantaneously drops to ZERO, it
is JUST STATIC!

I would love to help sincere Oregon and other entities understand "what
it is we do" wherever possible.  If I can be of any futher assistance on
this or related topics, just let me know.

Pray for Sun! 

Matt Lafferty
Universal Energies Institute
mlafferty at universalenergies.com
(916) 422-9772
(916) 628-7694 Cell
(916) 914-2247 Fax
www.universalenergies.com

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