Why straight 240 volt breakers for 240v corner ground delta service?

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I start fires said:
I agree, and have no plans on using the forbidden breakers, just can't figure out how it's constructed differently. I truly believe the only difference is the writing on them.
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I think you have a fair question and I too have always wondered what is the actual difference in, say, a 2 pole 20 amp QO breaker rated 120/240 VS straight 240. I'm not sure any transparency has been extended to the trade from the manufacturers.
 
JFletcher said:
yes, it is possible that there would be a Line to Line fault with one leg coming out of a 120 system, and the other leg off of say a 480 volt Corner grounded Delta. in that case though the other conductor would be protected by a breaker as well. or, the 120 / 240 breaker could eat a lightning strike in the 50,000 kilovolt range. And either case, the breaker may very well fail catastrophically. there is simply no way to design a 4 dollar circuit breaker to withstand every possible fault that may come down wiring or the bus.
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I think it's safe to say that the current isn't going to build up to anywhere near 10,000 amps before the breaker simply clicks off like they always do. Nothing is going to explode because of some crazy magnetic force unless there is a direct lightning hit. If that happens, all bets are off anyway.
 
I start fires said:
So 120/240v breakers are not rated for an asymmetrical fault above 120 volts. Yet I gave a very possible example of an asymmetrical 240v fault that could occur in a 120/240 v single phase system. None of this is adding up.
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OK, sorry, I didn't see that you said the other 240V leg was from a DIFFERENT breaker, but presumably from the same service. It would still be a symmetrical fault because BOTH breakers would be opening the circuit under fault conditions. it is ONLY asymmetrical when something goes to ground, meaning only one pole is having to interrupt the fault current.

Now, as to whether or not the only difference is the wording on the breaker? Maybe, maybe not. I know for a fact that on some large 600V class breakers, there is in fact no difference in construction between one rated 35kAIC and one rated 65kAIC, other than the label and the price. So it's entirely possible that the same holds true for smaller breakers. But that is irrelevant because unless you get that IN WRITING from the specific manufacturer of the breaker, it would be a code violation to use a breaker in a way it is not listed for.
 
I start fires said:
I think it's safe to say that the current isn't going to build up to anywhere near 10,000 amps before the breaker simply clicks off like they always do. Nothing is going to explode because of some crazy magnetic force unless there is a direct lightning hit. If that happens, all bets are off anyway.
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IMO, I don’t think it’s safe to say that at all.
 
I start fires said:
On paper, that sounds like a very well thought out reason, and I would believe that if it wasn't for the fact that most every breaker that I have ever seen is rated for 10,000 amps. A 20 amp 120/240v breaker is rated for 10,000 amps and so is a 60 amp 120/240v breaker even though the parts inside are beefier and need to be able to handle 3 times the load. Plus, I'm not sure I've ever seen a KVA rating on a breaker; I think you just concluded that it would be logical that it would have one based on volts x 10,000 amps. Something's fishy here.
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Higher voltages mean more arc energy and a longer arc as well. 10,000amps at 120 volts is not the same as 10,000amps at 240 volts and so forth. The breaker that is straight 240 volt rated is designed to take that into account.
 
I start fires said:
I think it's safe to say that the current isn't going to build up to anywhere near 10,000 amps before the breaker simply clicks off like they always do. Nothing is going to explode because of some crazy magnetic force unless there is a direct lightning hit. If that happens, all bets are off anyway.
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ptonsparky said:
IMO, I don’t think it’s safe to say that at all.
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Tom's right, it is NOT an assumption anyone can make.

During a fault, ALL available current in the system is going to flow into the fault. The only limiting factor is circuit impedance. While it may be true that by the time a service drop gets to a panel, the transformer size and wire length may reduce the Available Fault Current to something less than 10kA, but the fact remains, the DESIGN STANDARD is 10kA for a reason.
 
mbrooke said:
Higher voltages mean more arc energy and a longer arc as well. 10,000amps at 120 volts is not the same as 10,000amps at 240 volts and so forth. The breaker that is straight 240 volt rated is designed to take that into account.
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I bet it would cost the manufacturer more money to make tooling for both the weaker 120/240v breaker as well as for the beefier straight 240v than to simply make the straight 240 for both and label them differently.
 
I start fires said:
I bet it would cost the manufacturer more money to make tooling for both the weaker 120/240v breaker as well as for the beefier straight 240v than to simply make the straight 240 for both and label them differently.
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They don't necessarily have to, one could have an arc bending plates while the other one does not.
 
I start fires said:
I think it's safe to say that the current isn't going to build up to anywhere near 10,000 amps before the breaker simply clicks off like they always do. Nothing is going to explode because of some crazy magnetic force unless there is a direct lightning hit. If that happens, all bets are off anyway.
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The current that flows is a function of the available fault current and the impedance of the fault path. It will be what ever the physics says it will be. Once the fault current is in the instantaneous trip range of the breaker, more current does not make it trip quicker. A misapplied breaker will blow up.
 
don_resqcapt19 said:
The current that flows is a function of the available fault current and the impedance of the fault path. It will be what ever the physics says it will be. Once the fault current is in the instantaneous trip range of the breaker, more current does not make it trip quicker. A misapplied breaker will blow up.
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Not to throw a curve ball- but doesn't a series rating work on the instantaneous trip range?
 
mbrooke said:
Not to throw a curve ball- but doesn't a series rating work on the instantaneous trip range?
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This pretty much points out the difference between a 120/240 rated breaker and a straight 240 rated one. A 120/240 rated breaker is only expected to successfully interrupt a full-AIC-rating fault at 240V if it's in series with another OCPD (on the other hot leg). A straight 240V rated breaker must interrupt the full AIC rating at 240V all by itself, without another pole in series to help out. It's much easier to interrupt a fault with two poles in series than with just one - which is why series rated systems work. A 120/240V breaker is sort of a self-contained series rated system. A straight 240V breaker does not rely on this.
 
I start fires said:
I bet it would cost the manufacturer more money to make tooling for both the weaker 120/240v breaker as well as for the beefier straight 240v than to simply make the straight 240 for both and label them differently.
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Which may be part of why a straight 240 volt breaker has a higher price tag. They don't sell nearly as many straight 240 breakers, they don't sell nearly as many higher interrupt rated breakers period.

Why are three foot fluorescent tubes two to four times the price of a four foot tube? There is so many more four footers in use that is most of what they make. Though theoretically the three footer should cost less to produce.
 
ptonsparky said:
Apples to apples?
View attachment 20020
Only ones I had that I was willing to destroy. Both are rated 10,000 AIC. Both 20 amp from same mfg. Any guess as to the 240 rated. I did not have a single pole bolt on from this mfg.
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The lower one with the beefier guts and big arc stretcher? :D

FWIW, the bottom one looks almost identical to a 40amp 10kaic 120/240 in terms of beefiness.
 
mpoulton said:
This pretty much points out the difference between a 120/240 rated breaker and a straight 240 rated one. A 120/240 rated breaker is only expected to successfully interrupt a full-AIC-rating fault at 240V if it's in series with another OCPD (on the other hot leg). A straight 240V rated breaker must interrupt the full AIC rating at 240V all by itself, without another pole in series to help out. It's much easier to interrupt a fault with two poles in series than with just one - which is why series rated systems work. A 120/240V breaker is sort of a self-contained series rated system. A straight 240V breaker does not rely on this.
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Yup- spot on! Two poles are literally double the distance and as such more more impedance is presented to the arc circuit as a whole.
 
mbrooke said:
The lower one with the beefier guts and big arc stretcher? :D

FWIW, the bottom one looks almost identical to a 40amp 10kaic 120/240 in terms of beefiness.
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Wow I always wanted to open one of those up thank you for doing it.
 
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