120/240V versus 240v breakers

[Rick Christopherson]

I was reading another discussion about high-leg delta systems and the topic turned to breaker ratings on the B-phase (high leg). For a 2-pole breaker utilizing the B-phase, it must be rated 240, not the 120/240 slash-rating. I also confirmed this in the Scheider catalog. But that's not my question.

My question is why is this the case? Why should a 2-pole breaker that is intended to see 240 volts between adjacent stabs in the load center need to care about where the grounding point is located? The circuit breaker doesn't know whether its overcurrent trip is due to a short to phase versus a short to ground. It's insulation value still has to be rated for the adjacent buss bar voltage, which is still 240 volts.

Does anyone know enough about the internals of the breaker or UL requirements to explain the difference between a 120/240 breaker versus a 240 breaker?

 

[lowvoltman]

120/240 vs. 240 breakers

120/240 vs. 240 breakers

It's my understanding that a slant rated breaker such as 120/240 is rated at 120v per pole. And a 240v breaker is rated at 240v per pole. with high leg delta you would have 208v so you would need the 240v rated breaker.

 

[jim dungar]

During an arc interruption gases are created internal to the breaker and must be 'vented' to the outside world. These gases may be conductive, so care must be used that they do not create a L-G fault as they exit the breaker, this can be done with additional insulation or alternative venting paths. A slash rated breaker only has insulation of 120V to ground, which could cause problems with the 208V to ground potential of the vented gases. A full 240V to ground breaker is more 'expensive' to build as evidenced by a 120/240 2P is about twice the price of a 120V 1P breaker, but a 240V 3P is more than three times the price.

 

[LarryFine]

Another thought: a line-to-line fault would have two breakers in series with the fault, dividing the voltage between them, while the line-to-ground fault would place the entire voltage across one breaker.

The breaker contacts must be able to open the circuit while subject to the voltage and current without self-destructing, and thus must be rated to withstand the voltage across it at the time.

That said, most 2-pole and all 3-pole breakers I remember checking were not 120/240v slash-rated, but were straight-rated for 240v, the higher voltage.

 

[Jraef]

The "slash rated breaker" issue of higher fault energy has to do with 480/277V rated breakers vs 480 Delta rated. In 240V and below, the construction of the breaker, from a fault energy withstand capability is the same.

My understanding is that the 120/240V breakers are different only for the reason that they cannot physically plug into the B phase, there is a mechanism in the panelboard and/or breaker that prevents it. That way you cannot use them on circuits that may split a 2 pole 240V branch into 2 separate 120V loads, because if you had tapped into the B phase, the line-neutral voltage would not be 120V. So the 120/240V 2 pole breakers can only be plugged into A and C phase circuits.

 

[Rick Christopherson]

Jim, at first blush that made sense, however, if the gasses are conductive and leave the breaker body, the insulation of the breaker can no longer control that conductivity. Moreover, these gasses could just as easily conduct to an adjacent phase, which is closer to the breaker than a ground location.

The only area on a circuit breaker body where it is closer to a ground path than a phase is at the deadfront. However, that is still separated by a significant layer of Bakelite and/or air gap to conductive components. This also doesn't explain the 2x cost.

I'm wondering if the difference isn't more simple, such as the higher cost of the UL listing (more stringent testing = higher cost). For example, the difference between a 1% resistor and a 5% resistor is that they are all made the same, but the 1%'s are filtered out via measurement after the fabrication.

 

[jumper]

The "slash rated breaker" issue of higher fault energy has to do with 480/277V rated breakers vs 480 Delta rated. In 240V and below, the construction of the breaker, from a fault energy withstand capability is the same.

My understanding is that the 120/240V breakers are different only for the reason that they cannot physically plug into the B phase, there is a mechanism in the panelboard and/or breaker that prevents it. That way you cannot use them on circuits that may split a 2 pole 240V branch into 2 separate 120V loads, because if you had tapped into the B phase, the line-neutral voltage would not be 120V. So the 120/240V 2 pole breakers can only be plugged into A and C phase circuits.

From personal experience, I can tell you that single pole or 2 pole slash rated breakers will fit into many B slots in many 240/120 high leg panels.

 

[Rick Christopherson]

Another thought: a line-to-line fault would have two breakers in series with the fault, dividing the voltage between them, while the line-to-ground fault would place the entire voltage across one breaker.

Except the breaker doesn't see any voltage until it opens, and on a phase-to-phase short, if only one breaker trips, the tripped breaker still sees 240 volts. For example, a short within the load center between two different circuits with different breaker sizes. It is likely that only one breaker would trip.

However, maybe that is the answer. Under normal circumstances, when the breaker is open, the air gap between the terminals is shared by both poles, and if the fault is due to a phase-ground short, each terminal is still capable of sustaining that air gap. When the voltage is 208 phase to ground, then only a single contact pair will have to sustain that air gap.

I don't see a voltage interruption rating anywhere, but for some reason I assumed it was much higher than the operating voltage. Standard QO breakers have an Ampere Interruption Rating (AIR) of 10,000 amps, but for some reason I had "AIR" in mind :roll: thinking this was related to an air gap, and therefore, related to voltage interruption capabilities.

 

[jim dungar]

Jim, at first blush that made sense, however, if the gasses are conductive and leave the breaker body, the insulation of the breaker can no longer control that conductivity. Moreover, these gasses could just as easily conduct to an adjacent phase, which is closer to the breaker than a ground location.

The real answer is complicated, of course.

But if the gasses are vented in a different location, when they exit they maybe cooled enough that they are no longer conductive, this can be accomplished by routing the gas through the breaker. Another technique is to actually accelerate the gas so that it quickly disperses (I know of one 480Y/277V breaker that vented the gasses towards its line side plug-on jaws).

Larry also hit on an important area when he mentioned 'voltage across the contacts during operation'.

 

[curt swartz]

That said, most 2-pole and all 3-pole breakers I remember checking were not 120/240v slash-rated, but were straight-rated for 240v, the higher voltage.

Larry, I agree that the 3 pole breakers are 240 volt rated but I have never personally see a common style (QO, BR, CH, THQL, etc.) 2 pole beaker that was 240 rated other than in a catalog.

 

[roger]

From personal experience, I can tell you that single pole or 2 pole slash rated breakers will fit into many B slots in many 240/120 high leg panels.

I agree.

Roger

 

[Jraef]

From personal experience, I can tell you that single pole or 2 pole slash rated breakers will fit into many B slots in many 240/120 high leg panels.

Age old saying:
Just because you can, doesn't mean you should.

120/240V 2 pole breakers are not UL listed to be used in 120/240V 3ph4w panels on the B phase. If you plug / bolt one in, you are violating the UL listing. An AHJ who knows this can red tag it.

 

[jumper]

My understanding is that the 120/240V breakers are different only for the reason that they cannot physically plug into the B phase, there is a mechanism in the panelboard and/or breaker that prevents it. That way you cannot use them on circuits that may split a 2 pole 240V branch into 2 separate 120V loads, because if you had tapped into the B phase, the line-neutral voltage would not be 120V. So the 120/240V 2 pole breakers can only be plugged into A and C phase circuits.

Age old saying:
Just because you can, doesn't mean you should.

120/240V 2 pole breakers are not UL listed to be used in 120/240V 3ph4w panels on the B phase. If you plug / bolt one in, you are violating the UL listing. An AHJ who knows this can red tag it.

Yeah, I know this, I was commenting on your above statement.

 

[Jraef]

Yeah, I know this, I was commenting on your above statement.

Yeah, I understand. When I went back trying to support my statement, I notice that none of the catalogs I have say anything about restrictions, so maybe they did away with the actual barriers to doing it or maybe it was only one or 2 mfrs. that I remembered encountering. I don't deal with that any more, my comment was based on when I did more commercial work, which was ^$# years ago now.