| During my 8 years as an electrical inspector and 32 years as an IBEW
| journeyman electrician I have probably looked at over a thousand
| installations where the neutral or grounded conductor is never
| protected for overcurrent.
| This includes plan reviews for the Red Dog $500 million Zinc mine, the
| $500 million Fort Knox Mine, Alaska's North Slope's oil processing
| facilities, and Alaska pipeline pump stations. It is simply not
| done. I am surprised that this question is being asked. It is a no-
In virtually all cases it would not be needed. But I can described some
extreme cases where certain kinds of protection might be needed.
In a three phase system where harmonic currents may develop during adverse
operating conditions, but are unexpected in normal conditions, including
overload current detection capability on the neutral would be essential if
the circuit is not specifically designed for such an overload. It probably
would not be necessary to interrupt the neutral; just interrupt the poles.
Although typical harmonic overloads don't exceed 200%, allowing for double
neutral wiring as a measure to avoid the problems, it is possible to have
more than 200% current on the neutral. In the extreme case where current
flows only on one phase at a time, balanced so each pole is flowing 1/3 of
the time, but flows at a level of current such that each pole averages out
to its full current rating, the neutral will have 3 times the current on a
continual basis. This is so unlikely that the code would not need to deal
with such cases. But if it were known to be possible in some installation,
it would be prudent, possibly mandatory, to protect for it. A four pole
breaker that detects the current on the neutral would be one way to do it.
Just because most electricians will never see it in their life does not mean
it could not exist.
In cases where the small voltage rise on the neutral relative to ground, due
to voltage drop back to the source, poses a safety hazard, disconnecting the
neutral (and by code requirement all the poles not later than the neutral)
could abate the issue. This would not be an issue if the interrupted poles
are the only means for the neutral to have a voltage rise relative to the
point of bonding. However, multiple circuits at a subpanel could still
leave a neutral in a state with voltage relative to ground due to imbalance
on other circuits sharing the same subpanel. The voltage at the subpanel
would be what is relevant; so if the subpanel is close to the main, it would
be a miniscule voltage and generally not of concern. But a subpanel that is
a substantial distance from the main could see a few volts rise under loads
that are out of balance. If it is possible for someone to come into contact
with that neutral, it may be a minor hazard. While I would not be concerned
with such voltages on the neutral in common dry cases like screw in light
bulb sockets, I might be concerned in certain wet situates like a hot tub or
swimming pool. I'd want to have ground fault protection include the neutral
in what is interrupted in these cases. The possibility of an arc between a
risen neutral and true ground igniting fuel is, I suspect, the reason to do
this on fuel pumps. But is there also any such risk of an arc from wiring
damage, between a risen neutral and ground, igniting less combustible stuff
like walls in a house? I don't know about this aspect of risk. Any ideas?
It would certainly depend on how much voltage and current you could get in
some given wiring situation (and still quite rare ... how many people have
a home run from a subpanel that is 100's of feet from a bond?).
A situation with a broken neutral could present a more substantial voltage
on that neutral with respect to earth, potentially as much as a pole voltage
in the worst case. This is not a situation where automatic detection works
very well. But where some portion of a circuit has some reason to expect a
possible failure mode like this, it may be prudent to have the ability to
disconnect the neutral. Normally, one would disconnected the main to shut
off all power until the condition is corrected.
When a utility MV distribution circuit has a broken neutral, and when the
system is out of balance, such as a line breakage elsewhere, the neutral
return currents would flow to ground to reach back to the source. Since
MV neutral and LV neutral on service drops are almost always connected,
those return currents will include the path along the drop to where the
service bonds to ground and is earthed. At that point of earthing, the
voltage difference between the wiring and earth should be miniscule. But
at some distance away from that point, the difference could be substantial,
especially if that distance is nearer to where the MV neutral resumes at
a point of its grounding. This is a big reason I intend to have a way to
interrupt all 3 wires (2 poles and neutral) of my service drop, probably
at some point after the bonding.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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