3-pole breaker vs. 4-pole for a 3-phase wye

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. -N.Morrow
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3-pole 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 protection.
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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
--
Stuart Winsor

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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- brainer!
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wrote:
| 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- | brainer!
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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UK:
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 isolator.
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.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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On Oct 13, 9:49 pm, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

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 overcurrent.
Ref: 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 permitted.
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