[RE-wrenches] Generator bonding discussion

[Allan Sindelar]

The problem of the bonded chassis in most generators has been with us for
years, and there is no real solution. With most cheap (and ubiquitous)
portables, the AC neutral is internally bonded to the chassis and can't be
easily undone.  When I wrote the HP generator article I attempted to address
this issue without getting too technical. I ran this specific issue by John
Wiles for his advice, and the final wording reflected his position that the
AC ground loop - that is, maintaining the neutral-ground bond in both the
generator and the inverter system and accepting that the ground conductor
would carry a portion of any current otherwise being carried on the neutral
- was the best of the options. 

 

.         The final wording as submitted to HP was: "Quality generators
allow the neutral conductor to either connect to the generator chassis (for
prime power application) or remain separate from it. Portable generators are
seldom properly grounded, so manufacturers ground the neutral output
conductor to the chassis. When connected into a grounded power system,
however, this presents a safety hazard, as the neutral is now bonded to the
ground at two places-one in the power system AC and one in the generator-and
the safety ground wire becomes current-carrying. Some units also include AC
ground-fault protection, which is incompatible with connection to a grounded
power system.  There is no simple, Code-compliant solution to this. The
safest noncompliant approach is to bundle an insulated green ground
conductor with the power conductors between the generator and the main
ground buss, to both ground the chassis and minimize shock potential."

 

Inherent in this is that that grounding conductor between gennie and ground
buss be bundled in conduit and boxes, rather than being left bare or
exposed, as would otherwise be OK with any grounding conductor.

 

 

Allan Sindelar

Allan at positiveenergysolar.com

NABCEP Certified Photovoltaic Installer

EE98J Journeyman Electrician

Positive Energy, Inc.

3201 Calle Marie

Santa Fe, New Mexico 87507

505 424-1112

www.PositiveEnergySolar.com <http://www.positiveenergysolar.com/> 

 

Larry,
Sounds like there are two neutral ground bonds: one at the generator and one
at the house.  This causes part of the neutral current to flow on the ground
- a guaranteed way to trip the GFI.  This is the catch 22 of connecting a
portable generator, or one with an outlet, to a house system.  The house
panel has to have a neutral ground bond.  The generator also has to have a
neutral ground bond (Although the small Honda and Yamaha inverter generators
don't. How do they get away with that? - No exposed metal to touch I guess.)
or the GFI won't work and ground faults won't trip the generator output
breakers.

Solutions (unfortunately none of these may be suitable and they all have
potential problems):

Permanently wire the generator.  On the generator, remove the neutral/ground
bond and remove all of the electric outlets.  In some locations, you aren't
allowed to permanently wire a generator unless it is U/L listed.  And most
(maybe all) portable generators aren't U/L listed.

Replace the GFI outlet with a non-GFI outlet (so it will be like the old
Homelight generator).  Part of the neutral current will flow on the ground
wire.  That's a code violation, but for 20 and 30-amp outlets the ground
wire has adequate ampacity so nothing is going to overheat.  In fact, even 6
AWG SO cord has a ground wire the same size as the conductors.

Use a 2-prong cheater.  This will prevent the GFI from tripping.  It also
prevents any ground fault past the cheater from returning to the generator.
It's probably best to locate the cheater at the load end of the cord; that
way if the cord is damaged the GFI will still trip.  If there is ground
fault past the cheater, there will be one or probably more than one
locations where there is shock hazard.  If you touch the generator frame,
it'll tingle; if the ground is wet, it might be severe.

Use a mobile-style inverter that will switch the neutral/ground bond.  Most
of these are not U/L for house wiring.  I think this is the best solution
but it doesn't cover the circumstance of a grid backup inverter connected to
a portable generator.

See. I don't have any good solutions.

Kent Osterberg
Blue Mountain Solar

  
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