3-pole vs. 4-pole LV circuit breaker

[iwire]

I believe the main intention for disconnecting the neutral has more to do with the hazardous location and the spark producing potential that may be there if there is even just a few volts to ground on the neutral conductor,

I believe water is wet.

 

[mbrooke]

That is to many ifs for me to be a practical worry.

But depending on the application some see it as a legit concern. But as they say to each his own

 

[Besoeker]

I believe water is wet.

And your wit is dry?

 

[kingpb]

But depending on the application some see it as a legit concern. But as they say to each his own

Exactly how many contingencies are you supposed to plan for? Typically, single contingency is all that is legit, the rest is speculation.

 

[mbrooke]

Exactly how many contingencies are you supposed to plan for? Typically, single contingency is all that is legit, the rest is speculation.

An open or loose neutral is a single contingency. The NEC has a 2 strikes rule, ie for an appliance to become energized an egc must be open and fault develop. However plenty of areas could use some improvement. A good example is AFCIs. Regardless whether we like them or not (yes many including me at times believe they are unnecessary), have caught a lot of bad electricians over the years doing their function detecting wiring errors. Those single contingencies such as over driven staples are now eliminated.

 

[GoldDigger]

Do you really think that an AFCI is enough more likely to detect an overdriven staple than the simpler more reliable GFCI.

 

[zxfabb]

I know But if one wanted to, the IEC in by itself will allow a TN-S system to have a 4 pole breaker? I know for a fact in France (though the go by a different "local" of the IEC) its very common for breakers to switch the neutral even at branch circuits breakers. In the UK (BS7671) I have seen main incomer switches break both Live and neutral in consumer units even for TN-S supply networks. Hence my wondering if the IEC in by itself allows it for this in TN-S, or its the local regulations.


Sorry for the confusion :ashamed1:

I believe IEC allows by saying "need not". It is only a matter of cost to use 4-pole breaker in TN-S. : )

 

[mbrooke]

Do you really think that an AFCI is enough more likely to detect an overdriven staple than the simpler more reliable GFCI.

Not really, but because with most AFCIs comes 30ma GFCI protection. IMO, CMP should have just required equipment ground fault protection where AFCIs are required. However, I could certainly see people saying "why do we need EGFPs, its not proven we need them" But upon their inception the 30ma protection has already shown some installations are not code compliant to begin with.



While we can do without AFCIs and GFCIs in branch circuits I most cases, contingencies do happen that can be a safety issue where these devices take care of it.

 

[mbrooke]

I believe IEC allows by saying "need not". It is only a matter of cost to use 4-pole breaker in TN-S. : )

Thanks I guess Im not used terms ("need not"), more used to the NEC definitions.

 

[iceworm]

Other than the cases already listed, I know of two other design issues where a 4P CB could be used (typically are not - but could be). They are a bit out of the ordinary, but I have seen them often enough - cause this is the kind of stuff I work on.

Solidly grounded 480/277W, fed with multiple paralleled generators:
The issue is if a generator has to be taken off line for maintenance, one does not want the neutral still connected to system neutral. If the system develops a ground fault, the neutral can be elevated to line potential until the faulted circuit CB trips. And this really irritates the tech that has her hands in the alternator - possibly only for fractions of a second, but still not good.

The solution is usually to mount a disconnect on the gen to isolate the neutral fromn the system neutral. N-G bonds are in the switchgear. Often a standard 3 pole disconnect used with one pole used as an auxilary to inhibit the gen start when open.

High Impedance grounded 480V, paralleled transfromers:
If one transfromer is to be pulled off-line for service, the transformer neutral must be disconnected from the grounding resistor. A system ground fault will elevate the transformer neutral and leave it there until the fault is removed. That really really iritates the techs.

Again, a disconnect is used in the neutral to grounding couductor connection, with one pole used to trip the primary CB if the disconnect is opened.

These are design issues, not NEC issues.

ice

 

[mbrooke]

Other than the cases already listed, I know of two other design issues where a 4P CB could be used (typically are not - but could be). They are a bit out of the ordinary, but I have seen them often enough - cause this is the kind of stuff I work on.

Solidly grounded 480/277W, fed with multiple paralleled generators:
The issue is if a generator has to be taken off line for maintenance, one does not want the neutral still connected to system neutral. If the system develops a ground fault, the neutral can be elevated to line potential until the faulted circuit CB trips. And this really irritates the tech that has her hands in the alternator - possibly only for fractions of a second, but still not good.

The solution is usually to mount a disconnect on the gen to isolate the neutral fromn the system neutral. N-G bonds are in the switchgear. Often a standard 3 pole disconnect used with one pole used as an auxilary to inhibit the gen start when open.

High Impedance grounded 480V, paralleled transfromers:
If one transfromer is to be pulled off-line for service, the transformer neutral must be disconnected from the grounding resistor. A system ground fault will elevate the transformer neutral and leave it there until the fault is removed. That really really iritates the techs.

Again, a disconnect is used in the neutral to grounding couductor connection, with one pole used to trip the primary CB if the disconnect is opened.

These are design issues, not NEC issues.

ice

Great contribution to the discussion.


Out of curiosity, in regards to the use in data centers (415Y/240 applications), is it done since the neutral shift becomes greater in long runs?

 

[Tony S]

The times I?ve used 4P switches/breakers they have been 3P+N, where the neutral is late break, early make. Normally they would be 3P with a removable neutral link.

 

[GoldDigger]

...where the neutral is late break, early make.

Sounds like a great idea whenever there are line to neutral loads. There are transfer switches built the same way.

 

[Sahib]

Life safety mattered in the earlier posts as regards provision of 4P breaker in the American electrical systems. Equipment safety is also there:when a nearby POCO overhead phase line is grounded in such a way that the resulting high ground leakage current flowing and seeking better ground back to source through the unprotected neutral conductor in a building could damage its insulation.





But that would happen only if there were already a ground fault in the grounded wire inside the building.