3-pole vs. 4-pole LV circuit breaker

[zxfabb]

In some countries, 4-pole LV circuit breakers are often used to interrupt the neutral conductor in case of equipment maintenance for personnel safety.


But I rarely see 4-pole breakers in switchgear or MCC in North America. Why? Is there any code or standard saying about this?


Be grateful if you have any ideas.

 

[iwire]

There really is no reason to break the grounded conductor.

There is one application that the NEC requires all conductors to be opened and that is gasoline dispensers.

 

[zxfabb]

There really is no reason to break the grounded conductor.

There is one application that the NEC requires all conductors to be opened and that is gasoline dispensers.

But in a switchgear usually there shall have a disconnet link on the neutral, it is to break the neutral, isn't it?

 

[iwire]

But in a switchgear usually there shall have a disconnet link on the neutral, it is to break the neutral, isn't it?

Sure but that is not personal safety, that is used if you want to unground the neutral for testing.

 

[kingpb]

Outside USA, quite common to see built-in earthing switches, which replaces need for maintenance personnel to apply their own earthing conductors, but US has not really adopted this practice yet.

 

[zog]

Seeing it more often these days, some new data centers in my area use 4 pole breakers.

 

[ron]

Seeing it more often these days, some new data centers in my area use 4 pole breakers.

It is used a bit more in data centers I design when we are distributing 3Ph, 4W circuits out at the UPS level. For a very long time we tried to stay 3Ph 3W and derive the neutral as close to the IT equipment as possible, but the concept of distributing 415/240V, and utilizing the 240V at the rack, meaning we have to transport and transfer the grounded conductor more than ever, so now we consider 4 pole breakers more often.

 

[iwire]

Outside USA, quite common to see built-in earthing switches, which replaces need for maintenance personnel to apply their own earthing conductors, but US has not really adopted this practice yet.

What voltage are you talking about? We don't install earthing conductors to do repairs.

 

[Besoeker]

In some countries, 4-pole LV circuit breakers are often used to interrupt the neutral conductor in case of equipment maintenance for personnel safety.


But I rarely see 4-pole breakers in switchgear or MCC in North America. Why? Is there any code or standard saying about this?


Be grateful if you have any ideas.

We build variable speed drive panels. Often they end up overseas (i.e. not UK).
Quite often the main contractor is British and we may not know who, or where, the end user is.

Usually the datasheets provided with the request to tender (RFQ) will spell out the requirements.
This is an excerpt from a the data sheet for a current project:

"
Phases/Wire : 3 phase, 4 wires.
Frequency : 60 Hz ? 2.5%
Cable Entry : Bottom.
Rated Input Voltage : 480V
Incoming Isolators : 4 pole isolator for 3 phase + N supply."

In this particular case the destination is the Middle East.

 

[mbrooke]

In some countries, 4-pole LV circuit breakers are often used to interrupt the neutral conductor in case of equipment maintenance for personnel safety.


But I rarely see 4-pole breakers in switchgear or MCC in North America. Why? Is there any code or standard saying about this?


Be grateful if you have any ideas.

NEC does not require it, but the NEC certainly does allows it as long as the ungrounded conductor(s) open at the same time as the neutral conductor. Just the way things are, their is no specific reason as to why its not done. Price could be a concern, but keep in mind that North America is decades behind in electrical safety. So it shouldn't be to surprising.


Just of note, in a TN-C system you never break the neutral since its also functioning as a grounding (protective earth) wire. However, once the system becomes TN-S, its a good idea to switch the neutral along with the hots.

There really is no reason to break the grounded conductor.

There is one application that the NEC requires all conductors to be opened and that is gasoline dispensers.

It would appear to be that way. But what happens should it become disconnected upstream? It will no longer be at ground potential; up to 120 (230 EU) or 277 volts as load back feeds it. Being able to disconnect the neutral along with the phase conductor certainly eliminates that danger.

 

[zxfabb]

NEC does not require it, but the NEC certainly does allows it as long as the ungrounded conductor(s) open at the same time as the neutral conductor. Just the way things are, their is no specific reason as to why its not done. Price could be a concern, but keep in mind that North America is decades behind in electrical safety. So it shouldn't be to surprising.


Just of note, in a TN-C system you never break the neutral since its also functioning as a grounding (protective earth) wire. However, once the system becomes TN-S, its a good idea to switch the neutral along with the hots.






It would appear to be that way. But what happens should it become disconnected upstream? It will no longer be at ground potential; up to 120 (230 EU) or 277 volts as load back feeds it. Being able to disconnect the neutral along with the phase conductor certainly eliminates that danger.

In IEC 60364-5-53, it states:
536.1.2 (461.2) In TN-C systems, the PEN conductor shall not be isolated or switched. In TN-S systems, the neutral conductor need not be isolated or switched.

In system other than TN-S, the neutral need to be isolated or switched.

 

[iwire]

It would appear to be that way. But what happens should it become disconnected upstream? It will no longer be at ground potential; up to 120 (230 EU) or 277 volts as load back feeds it. Being able to disconnect the neutral along with the phase conductor certainly eliminates that danger.

How would you get a backfeed? The hots are all opened.

 

[mbrooke]

How would you get a backfeed? The hots are all opened.

All the hots to the entire system back to the source? Then your correct. However, if you trip a breaker in a panel board that feeds a subpanel, that panel board still has other loads which feed into the same neutral buss as your sub panel. Same with branch circuits in any panel, if you trip a breaker for a single circuit you don't kill the whole panel board. Even though circuits 1,3,5 are tripped, should the feeder neutral to the panel become loose or compromised, all other 39 circuits with 120 or 277 volt loads will back feed into the neutral bar. That would mean that the neutral in your branch circuit assumed de-energized will be well over several volts to ground. Same logic can apply to feeders from switchgear serving panel boards.

 

[mbrooke]

In IEC 60364-5-53, it states:
536.1.2 (461.2) In TN-C systems, the PEN conductor shall not be isolated or switched. In TN-S systems, the neutral conductor need not be isolated or switched.

In system other than TN-S, the neutral need to be isolated or switched.

My take is that if you have a TN-S system, you can switch the neutral but your not required? And in TT its a requirement?

 

[zxfabb]

My take is that if you have a TN-S system, you can switch the neutral but your not required? And in TT its a requirement?

In TT systems, the incoming neutral conductor cannot reliably be regarded as being at earth potential. This means that for TT supplies, a multi-pole switching device which disconnects the phase and neutral conductors must be used as the means of isolation. For similar reasons, in IT systems all poles of the supply must be disconnected.

 

[zxfabb]

My take is that if you have a TN-S system, you can switch the neutral but your not required? And in TT its a requirement?

536.2.1.1 (462.1) Every circuit shall be capable of being isolated from each of the live
supply conductors, except as detailed in 536.1.2 above.

The above implies that the neutral in TT and IT system shall be isolated as a live conductor.

 

[kwired]

Originally Posted by iwire
There really is no reason to break the grounded conductor.

There is one application that the NEC requires all conductors to be opened and that is gasoline dispensers.


It would appear to be that way. But what happens should it become disconnected upstream? It will no longer be at ground potential; up to 120 (230 EU) or 277 volts as load back feeds it. Being able to disconnect the neutral along with the phase conductor certainly eliminates that danger.

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, disconnect that neutral (or any grounded conductor) with the other circuit conductors and you reduce the potential to whatever capacitive coupling may be available, which is probably low enough to not be a threat to the hazardous location.

 

[mbrooke]

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, disconnect that neutral (or any grounded conductor) with the other circuit conductors and you reduce the potential to whatever capacitive coupling may be available, which is probably low enough to not be a threat to the hazardous location.

I agree, that is another good reason why the neutral may be disconnected with the phases. But also realize that the only thing that keeps the neutral near ground potential is being fully connected (unbroken) all the way back to the source and that the source is bonded to the equipment grounding system. If the neutral breaks at any point a conductor that is normally assumed to be at ground potential now carries significant risk. Disconnecting it removes that risk, providing an extra layer of safety.


A possible driving factor for the difference may be that from a safety standpoint we lucked out with 120/208, where the risk is less than that of 230/400 volt systems should something go wrong becoming energized.

 

[mbrooke]

536.2.1.1 (462.1) Every circuit shall be capable of being isolated from each of the live
supply conductors, except as detailed in 536.1.2 above.

The above implies that the neutral in TT and IT system shall be isolated as a live conductor.

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:

 

[iwire]

All the hots to the entire system back to the source? Then your correct. However, if you trip a breaker in a panel board that feeds a subpanel, that panel board still has other loads which feed into the same neutral buss as your sub panel. Same with branch circuits in any panel, if you trip a breaker for a single circuit you don't kill the whole panel board. Even though circuits 1,3,5 are tripped, should the feeder neutral to the panel become loose or compromised, all other 39 circuits with 120 or 277 volt loads will back feed into the neutral bar. That would mean that the neutral in your branch circuit assumed de-energized will be well over several volts to ground. Same logic can apply to feeders from switchgear serving panel boards.

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