Substituting a 3 Phase breaker. Necessary to occupy all 3 poles?

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Fourbin

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I'm sure many others in this forum can relate to the squeeze that the parts shortage has been putting on many of us.

I'm faced with a situation where a 2 pole din rail mount breaker we have been using has become very hard to come by. I need several of these breakers for an application where we use 2 poles of a 3 phase 480v system to power a single phase device. Our supplier can get the 3 pole version of this breaker, which is physically the same dimensions. The only apparent difference is that unlike the 2 pole where the center position is empty, the 3 pole version allows connections to all 3 positions.

A much more experienced engineer that I work with, says that this can be done, however we should send one phase through the breaker twice (go through once, loop back up to the top of the breaker, and go through a second time on another pole) so that all 3 poles are occupied. I could see this maybe being done on a contactor where you are switching large inductive loads and want some additional protection from contacts welding, or an arc jumping the gap, but I don't see the benefit in our situation.

Is there any benefit to looping around to use all parts of the breaker? We already have several of the 2 pole breakers out in the field, and I would much rather wire the 3 pole breakers straight through so that our wiring is consistent throughout the project.

NEC and UL compliance is necessary.

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Fourbin said:
I'm sure many others in this forum can relate to the squeeze that the parts shortage has been putting on many of us.

I'm faced with a situation where a 2 pole din rail mount breaker we have been using has become very hard to come by. I need several of these breakers for an application where we use 2 poles of a 3 phase 480v system to power a single phase device. Our supplier can get the 3 pole version of this breaker, which is physically the same dimensions. The only apparent difference is that unlike the 2 pole where the center position is empty, the 3 pole version allows connections to all 3 positions.

A much more experienced engineer that I work with, says that this can be done, however we should send one phase through the breaker twice (go through once, loop back up to the top of the breaker, and go through a second time on another pole) so that all 3 poles are occupied. I could see this maybe being done on a contactor where you are switching large inductive loads and want some additional protection from contacts welding, or an arc jumping the gap, but I don't see the benefit in our situation.

Is there any benefit to looping around to use all parts of the breaker? We already have several of the 2 pole breakers out in the field, and I would much rather wire the 3 pole breakers straight through so that our wiring is consistent throughout the project.

NEC and UL compliance is necessary.

View attachment 2559757
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I don't see any benefit to that.
 
There may be benefit in terms of reduced contact wear if there are many operations, but I doubt there is a requirement to wire in this fashion.

The definitive answer will be in the breaker manufacturer instructions. I bet you will find a requirement to wire this way for DC applications, but not for single phase AC applications.

Jon
 
winnie said:
There may be benefit in terms of reduced contact wear if there are many operations, but I doubt there is a requirement to wire in this fashion.

The definitive answer will be in the breaker manufacturer instructions. I bet you will find a requirement to wire this way for DC applications, but not for single phase AC applications.

Jon
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I have seen DC wring through multiple poles of disconnects to multiply the gap created by the switch in order to quench arcing when the switch is opened, but I don't see the need to treat breakers that way.
 
Fourbin said:
I'm sure many others in this forum can relate to the squeeze that the parts shortage has been putting on many of us.

I'm faced with a situation where a 2 pole din rail mount breaker we have been using has become very hard to come by. I need several of these breakers for an application where we use 2 poles of a 3 phase 480v system to power a single phase device. Our supplier can get the 3 pole version of this breaker, which is physically the same dimensions. The only apparent difference is that unlike the 2 pole where the center position is empty, the 3 pole version allows connections to all 3 positions.

A much more experienced engineer that I work with, says that this can be done, however we should send one phase through the breaker twice (go through once, loop back up to the top of the breaker, and go through a second time on another pole) so that all 3 poles are occupied. I could see this maybe being done on a contactor where you are switching large inductive loads and want some additional protection from contacts welding, or an arc jumping the gap, but I don't see the benefit in our situation.

Is there any benefit to looping around to use all parts of the breaker? We already have several of the 2 pole breakers out in the field, and I would much rather wire the 3 pole breakers straight through so that our wiring is consistent throughout the project.

NEC and UL compliance is necessary.

View attachment 2559757
Click to expand...
The only reason you would have to do something like that generally deals with things like motor overload relays that also have phase loss detection incorporated in them but you are using them on a single phase load.

Wear and tear on the contacts? They are all carrying the same current if middle and right pole in drawing do not make/break simultaneously the one that makes last will wear faster during closing operations and the one that breaks first will wear faster during opening operations.

The left one may or may not open/close in absolute unison either so really on a single phase load you can include it in what I said above as well.
 
I have wired battery chargers that instruct you to use all three poles on their input breakers when wiring for 1ph.
 
LarryFine said:
I have wired battery chargers that instruct you to use all three poles on their input breakers when wiring for 1ph.
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On the AC side or the DC side? On the AC side I don't see the point.
 
ggunn said:
I have seen DC wring through multiple poles of disconnects to multiply the gap created by the switch in order to quench arcing when the switch is opened, but I don't see the need to treat breakers that way.
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Breakers are often used as switches.

In any case, the OPs situation is AC, the only reason that I mentioned the DC connection is because it might show up in the breaker data sheet as a requirement for DC, and thus explain the nugget of truth that the other engineer stretched a bit too far.

Jon
 
ggunn said:
On the AC side or the DC side? On the AC side I don't see the point.
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Would the magnetic tripping force of a thermal-mag breaker be stronger in a short circuit if the amps go through three poles instead of 2, providing faster clearing? Just asking, I'm no engineer. I have used 2 legs of 3p loadcenter breakers many times and they can't be wired like this anyway!
 
PlasticFarmer said:
Would the magnetic tripping force of a thermal-mag breaker be stronger in a short circuit if the amps go through three poles instead of 2, providing faster clearing? Just asking, I'm no engineer. I have used 2 legs of 3p loadcenter breakers many times and they can't be wired like this anyway!
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All breakers listed to UL 489 are tested using combinations of their poles. These breakers will provide proper branch and feeder protection even if all poles are not used.

As others have noted, special applications often gave specific requirements.
 
I've had to do this now and then because two pole breakers have always been harder to get than three pole. Unless it is DC I don't believe there's any special wiring requirements.
 
ggunn said:
On the AC side or the DC side? On the AC side I don't see the point.
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It was the incoming AC switch/breaker. I'm guessing because the current is higher when the three transformers are re-wired for 1ph, and they are a highly-inductive load. (I think it was a breaker.)
 
PlasticFarmer said:
Would the magnetic tripping force of a thermal-mag breaker be stronger in a short circuit if the amps go through three poles instead of 2, providing faster clearing? Just asking, I'm no engineer. I have used 2 legs of 3p loadcenter breakers many times and they can't be wired like this anyway!
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All three poles should have same magnetic trip mechanism and should respond very similarly to same level of fault current. The common trip mechanism trips all the poles even if only one of them has fault current on it.

Whether or not line to line fault or line to neutral fault is capable of delivering higher fault current depends on source impedance, size, type and length of conductors. Most the time line to line fault current is higher, but with some short conductor length situations line to neutral fault current can be higher than line to line.

AFAIK instantaneous trip setting is the trigger in all cases, how long until the circuit is opened is dependent on mechanical properties of the components that need to function and open the circuit once that trip setting has been reached.
 
LarryFine said:
It was the incoming AC switch/breaker. I'm guessing because the current is higher when the three transformers are re-wired for 1ph, and they are a highly-inductive load. (I think it was a breaker.)
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As long as a breaker is correctly rated for OCP and with high enough kAIC to handle the available fault current I do not see any necessity for looping through multiple poles.
 
Just to add some detail, the breaker in question is an Allen Bradley 140UT style breaker.

Well things have gotten much more involved that I expected. I was able to find two articles in Rockwell's knowledgebase that supported using 1 or 2 poles of the 3 pole140U breakers and they did not mention anything about a jumper (article numbers QA12069 & QA4995). The 140UT breaker we use supersedes the 140U in the articles so just to play it safe, I shot an email over to our Rockwell rep, and asked if they could confirm that the 140UT could also be used in the same way that the articles indicated the 140U could be used. I intentionally did not bring up anything about the jumper that had been brought up by my coworker.

An email chain was quickly formed that included a few additional guys from Rockwell. One guy said that all phases had to be occupied for the breaker to work properly, and another guy chimed in with the following picture showing how this could be done...

140M loop1.PNG

I'm sure you will notice how the picture on the right resembles the drawing that my coworker made. I did some searching and found where the picture came from. It's from a different knowledgebase (QB QA18361) article that addresses the 140M overloads which externally look the same as the 140U breakers. My understanding is that the 140M overloads can sense phase imbalance, so I can see why this would need to be done.

In the end, the consensus from the Rockwell guys was that the jumper was indeed necessary even when using a circuit breaker rather than an overload. Their reasoning was that breakers would trip magnetically without issue, however the breakers thermal overload protection would not. I pullzed over this for a while, and couldn't work out in my head why that would be, but I think it boils down to how the breakers are constructed.

This is just me speaking, none of what I'm saying has been confirmed or corroborated by any other engineer or Rockwell representative. If the breaker is looking at the heat being generated in each phase separately, then 10 amps on a wire is 10 amps on a wire. The same amount of heat will be generated per phase no matter how amny phases. If however, the thermal overload is based on how much heat is generated overal, then I can understand why the jumper would be needed. A 2 pole breaker with 10 amps on each phase will generate X amount of heat per phase, so the total heat in the breaker will be 2x. A 3 pole breaker with the same 10 amps on each phase will make 3X heat.

So if you measure the current on each phase going through a breaker, then add that number together, a 3 pole breaker should theoretically be able to stand a higher total amperage. So this is my best guess as to why they are saying the jumper is needed. It seems to be that it makes much more sense to keep each phases tripping mechanisms totally separate, but the thermal trip mechanism must be effected by all phases. I guess there is no way to keep the heat generated on one phase from effecting another phase during a slow heating process. Even ambient temperature is going to effect how much current a breaker will stand before heating up enough to trip.

So, does this make sense? Am I way out in left field? This is the only explanation I can come up with for the answer they gave.

I don't think I'm going to restrain myself from taking one of these apart.
 
At this point you have spent a lot of time on the question. If the tech support guys are telling you to wire it through all the terminals than do so.

It is a little frustrating to have to dig out the details from people posing questions here. A motor protection circuit breaker is not the same thing as a standard UL489 circuit breaker, although both are listed to UL489. If you had mentioned what you really had up front a lot of this back and forth could have been eliminated.
 
Call me surprised. As I said, I expected this for DC applications but not for AC.

Remember that an AC breaker has to be able to trip even if only a single phase faults to ground or overloads. Three phase breakers are often used for MWBCs, for example, which could have only a single phase overload.

But what it boils down to is 'follow the manufacturer instructions'...and they might not actually have a good reason.

-Jon
 
Fourbin said:
Just to add some detail, the breaker in question is an Allen Bradley 140UT style breaker.

In the end, the consensus from the Rockwell guys was that the jumper was indeed necessary even when using a circuit breaker rather than an overload. Their reasoning was that breakers would trip magnetically without issue, however the breakers thermal overload protection would not. I pullzed over this for a while, and couldn't work out in my head why that would be, but I think it boils down to how the breakers are constructed.
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These are not molded case breakers listed under UL489 for branch circuit protection. There are designed to meet IEC motor protection requirements. As such the contain an amount of phase loss protection as part of the overload mechanism. This phase loss 'sensitivity' causes the device to trip somewhat early if there is a current imbalance.

The jumper makes sure that all of the phases see an equal amount of current therefore preventing the phase loss early trip.
 
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