Consider the following:
A 3-phase wye source is used to power several single phase loads in the same
cabinet. Each load would be wired phase-to-neutral. It is desired to install
a single circuit breaker in the cabinet to protect the incoming line. Would
it be necessary for the breaker to be 4-pole (to protect the 3 phases plus
the neutral) or would a 3-pole be satisfactory? Would the answer be
different for North America versus the UK?
I realize that other issues come up such as phase balancing, but the
immediate question is how many poles should the breaker contain.
Don't know about the UK, but here in the US we generally never install
overcurrent protection for the grounded conductor or neutral.
Here's the2005 NEC rule on this:
240.22 Grounded Conductor. No overcurrent device shall
be connected in series with any conductor that is intentionally
grounded, unless one of the following two conditions is met:
(1) The overcurrent device opens all conductors of the circuit,
including the grounded conductor, and is designed
so that no pole can operate independently.
(2) Where required by 430.36 or 430.37 for motor overload
Well, I'm not an electrical engineer but I have worked on plenty of
enclosures with a three phase supply. My gut feeling would be a 4pole
breaker but overload sensing would only be on the phases. This would
comply with your note (1) quoted
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-
All 4 conductors must be switched, except for a neutral conductor
which is considered to be connected with earth by a suitably low
resistance in the supply network (e.g. TN-S or TN-C-S systems).
Such a neutral can be switched if you want to, except a PEN
(Protective Earth and Neutral) conductor must never be switched,
but TN-C systems (where PEN conductors are found) are not used
in the UK AFAIK except for things like train rail supplies.
There may be some special locations where isolation of the supply
for maintenance requires breaking the neutral too, in which case
a 3PSN (3-pole and Switched Neutral) breaker could be used to
meet the isolation for maintenance requirement avoiding a separate
Note that a breaker at the load end of the supply cable can
only be used to protect the cable against overload, and not
fault currents, so you will also need fault current protection
at the supply end of the cable. A breaker at the supply end can
provide both overload protection and fault current protection.
A breaker at the load end may well be used to protect the load
against some failure mode (e.g. seized rotor on a motor), but
that's not what you indicated it was for.
British Petroleum in Alaska apparently doesn't follow the UK standards
because they don't use a four pole breaker on a 3-phase 4-wire solidly
grounded system, ever.
About the only place a neutral is switched is for gasoline pumps at
fuel dispensing stations, but it still isn't protected for
514.11 Circuit Disconnects.
(A) General. Each circuit leading to or through dispensing
equipment, including equipment for remote pumping systems,
shall be provided with a clearly identified and readily
accessible switch or other acceptable means, located remote
from the dispensing devices, to disconnect simultaneously
from the source of supply, all conductors of the circuits,
including the grounded conductor, if any.
Single-pole breakers utilizing handle ties shall not be
| 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.