Neutral Overcurrent Protection of a 4P Breaker

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ish1284

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Hi all, I have a hypothetical 4P Schneider Masterpact NW breaker with neutral protection available on the neutral. It has 3 settings available: 100% of phase protection, 50% of phase protection, and disabled.

I am wondering when/if it's appropriate to ever turn it completely off. Are there certain topologies that deem neutral protection not necessary? Are there other topologies that deem neutral protection absolutely necessary?

Any help would be much appreciated!
 
As far as I know, nothing in the NEC requires neutral over current protection at all. The only place I see it used is in equipment designed by people in other countries who have different rules, but don't understand that it is not necessary here. So if they have a specific reason for requiring it, you would have to ask them. The industry that tends to do this is the semiconductor fabrication machinery manufacturers. 99% of their machines end up overseas, but when they occasionally build something here in the US, they use the same designs requiring 4p breakers. Every time I've asked this question, the EEs for these companies have no clue as to why, they just want it done because that's what their standard is.
 
Jraef said:
As far as I know, nothing in the NEC requires neutral over current protection at all. The only place I see it used is in equipment designed by people in other countries who have different rules, but don't understand that it is not necessary here.
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I'd argue it can be necessary in theory, not just because of harmonics but the fact the neutral conductor is current carrying and will become live is inadvertently disconnected or a terminal overheats/loosens up.


So if they have a specific reason for requiring it, you would have to ask them. The industry that tends to do this is the semiconductor fabrication machinery manufacturers. 99% of their machines end up overseas, but when they occasionally build something here in the US, they use the same designs requiring 4p breakers. Every time I've asked this question, the EEs for these companies have no clue as to why, they just want it done because that's what their standard is.
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Two reasons:

1) In a TT supply there is no main bonding jumper, any voltage at the service will not be eliminated relative to all grounded and bonded objects within the building like as with an NEC TN-C-S supply. This especially holds true where a TT supply is chosen as means to comply with and manage over voltages during an MV to LV fault as required by 442.2.3 in IEC 60364-4-44 Protection for safety - Protection against voltage disturbances and electromagnetic disturbances .



2) The neutral is a current carrying conductor. If it becomes disconnected it will typically rise over 50 volts to anything grounded within the building and remote earth making it technically a live conductor and should be treated as such.
 
mbrooke said:
I've said it before, but I personally think neutral conductors should be treated like ungrounded conductors and grounding conductors like grounded conductors...
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A neutral carries the unbalanced current in a system, that's all! A ground conductor will only carry current when there's a fault! That should be clear enough on how to treat those conductors. A grounded neutral will always create a spark when they break, the spark depends on the amount of current it carries. Fuel dispensers in gas stations do not ground neutrals
 
topgone said:
A neutral carries the unbalanced current in a system, that's all! A ground conductor will only carry current when there's a fault! That should be clear enough on how to treat those conductors. A grounded neutral will always create a spark when they break, the spark depends on the amount of current it carries. Fuel dispensers in gas stations do not ground neutrals
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Right, but remember the endings ed vs ing.
 
mbrooke said:
2) The neutral is a current carrying conductor. If it becomes disconnected it will typically rise over 50 volts to anything grounded within the building and remote earth making it technically a live conductor and should be treated as such.
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Sorry, I don't see any connection between "treating the neutral as a live conductor" and placing overcurrent protection on the neutral conductor.

Opening the neutrals connection back to the ground only makes it more likely for it to become energized.

You certainly wouldn't want to fuse the neutral where the neutral fuse could blow while the line fuses are still energized.
 
ish1284 said:
Hi all, I have a hypothetical 4P Schneider Masterpact NW breaker with neutral protection available on the neutral. It has 3 settings available: 100% of phase protection, 50% of phase protection, and disabled.

I am wondering when/if it's appropriate to ever turn it completely off. Are there certain topologies that deem neutral protection not necessary? Are there other topologies that deem neutral protection absolutely necessary?

Any help would be much appreciated!
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If a feeder served mostly Line-Line loads, you might have a neutral wire that is smaller than the phase wires. Again, not typically required, but some cases like this, the 50% setting might provide a little more protection.
 
steve66 said:
Sorry, I don't see any connection between "treating the neutral as a live conductor" and placing overcurrent protection on the neutral conductor.

Opening the neutrals connection back to the ground only makes it more likely for it to become energized.

You certainly wouldn't want to fuse the neutral where the neutral fuse could blow while the line fuses are still energized.
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You'd never fuse the neutral, any over current protection will simultaneously open all 2 or 4 poles.

Harmonics can over load a neutral.
 
IIRC the maximum possible neutral current is 173% of phase current. Given that possibility it doesn’t sound silly to have a four pole breaker. IMHO it would only be needed on a 120/208 system in the USA. And then likely only for high percentage personal computer loads.
 
mbrooke said:
You'd never fuse the neutral, any over current protection will simultaneously open all 2 or 4 poles.

Harmonics can over load a neutral.
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Do you have any statistics on the # of fires started from harmonics on neutrals, cost of property destroyed, or lives lost due to harmonics on the neutral?

I know you are somewhat against AFCI's because the statistics don't really justify them. And I tend to agree with you. The manufacturers have too much clout to push products that make them more profit.

So I would be a little surprised if you are advocating for a new code requirement if there isn't some hard data to support that harmonics are a real problem, and not just another way to push more product by people like the Copper.org.

IMO, harmonics can be safely limited by proper design.
 
steve66 said:
Do you have any statistics on the # of fires started from harmonics on neutrals, cost of property destroyed, or lives lost due to harmonics on the neutral?

I know you are somewhat against AFCI's because the statistics don't really justify them. And I tend to agree with you. The manufacturers have too much clout to push products that make them more profit.

So I would be a little surprised if you are advocating for a new code requirement if there isn't some hard data to support that harmonics are a real problem, and not just another way to push more product by people like the Copper.org.

IMO, harmonics can be safely limited by proper design.
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Code change is debatable, espeiclaly with a full size neutral.

Me personally I only advocate from a theory perspective. Theory also says AFCIs will not prevent a single fire.

Its been enough of a statistic that codes elsewhere require it. But then again TT earthing and reduced size neutrals are much more common.

So going back it is debatable.
 
mbrooke said:
You'd never fuse the neutral, any over current protection will simultaneously open all 2 or 4 poles.

Harmonics can over load a neutral.
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Hmmm! How does a 4-pole breaker work? The opening of the ground pole works the same as "fusing" the neutral!
 
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