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

[I start fires]

Can someone please give me a scenario where a standard 120/240v double pole breaker will fail or be overloaded If used on a 240v corner ground delta service? I know it’s not code and you’re supposed to use straight 240v breakers because phase to ground is 240v in a delta and not 120v. But the only difference I can see is that the plastic housing of the breaker itself being in contact with the metal cover of the panel has a higher voltage potential phase to ground in a corner ground than in a 120/240v single phase service. Am I really supposed to believe that the plastic that the 120/240v is made out of can’t handle the higher voltage potential between the phases and the panel cover but CAN handle the ever present 240v phase to phase voltage potential within the breaker itself? I call BS!

 

[kwired]

Can someone please give me a scenario where a standard 120/240v double pole breaker will fail or be overloaded If used on a 240v corner ground delta service? I know it’s not code and you’re supposed to use straight 240v breakers because phase to ground is 240v in a delta and not 120v. But the only difference I can see is that the plastic housing of the breaker itself being in contact with the metal cover of the panel has a higher voltage potential phase to ground in a corner ground than in a 120/240v single phase service. Am I really supposed to believe that the plastic that the 120/240v is made out of can’t handle the higher voltage potential between the phases and the panel cover but CAN handle the ever present 240v phase to phase voltage potential within the breaker itself? I call BS!

I think the plastic case of the breaker is about the same thing in either unit. Three pole breakers are straight 240 volt rated - and on the outside don't really appear any different either other then there is a third pole.

 

[ptonsparky]

BS is not allowed. Never happens here.

I can’t give you an example of failure, but most likely everyone on here can point to existing installations that have worked successfully for years. I also rode in a car for years with no seatbelt.

 

[kwired]

BS is not allowed. Never happens here.

I can’t give you an example of failure, but most likely everyone on here can point to existing installations that have worked successfully for years. I also rode in a car for years with no seatbelt.

I can agree with that also.

I don't know exactly what the differences are but don't think it is the plastic case.

Only difference I am aware of is catalog numbers and price.:blink:

 

[Jraef]

Under normal operating conditions there is no noticeable difference, it has no effect on trip times etc. this is ONLY about the interrupting capacity of the breakers.

In a slash rated breaker, the maximum voltage to ground is the lower value, therefore during an asymmetrical fault (meaning line to ground), the maximum physical stress possible is based upon that lower voltage, so less kVA in the same amount of fault current. So when a 120/240V rated breaker rated for 10kAIC sees a ground fault in a system rated for 10kA, that is 10kA at never more than 120V, so 1200kVA of fault power.

But if used in a delta system, the maximum voltage to ground is 240V. So in an asymmetric fault, the power in the fault is 10kA x 240V, so 2400kVA, twice as much power, twice the physical stress. If you use a slash rated breaker rated for 10kAIC, it might blow up. In theory if you used a slash rated breaker rated for 22kA, it would be fine, and in reality, that’s exactly what a “straight” 240V rated 2 pole breaker is, and why it costs more.

 

[I start fires]

Under normal operating conditions there is no noticeable difference, it has no effect on trip times etc. this is ONLY about the interrupting capacity of the breakers.

In a slash rated breaker, the maximum voltage to ground is the lower value, therefore during an asymmetrical fault (meaning line to ground), the maximum physical stress possible is based upon that lower voltage, so less kVA in the same amount of fault current. So when a 120/240V rated breaker rated for 10kAIC sees a ground fault in a system rated for 10kA, that is 10kA at never more than 120V, so 1200kVA of fault power.

But if used in a delta system, the maximum voltage to ground is 240V. So in an asymmetric fault, the power in the fault is 10kA x 240V, so 2400kVA, twice as much power, twice the physical stress. If you use a slash rated breaker rated for 10kAIC, it might blow up. In theory if you used a slash rated breaker rated for 22kA, it would be fine, and in reality, that’s exactly what a “straight” 240V rated 2 pole breaker is, and why it costs more.

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.

 

[ptonsparky]

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.

It may be fishy, but you got the answer.

 

[I start fires]

I think what's going on here is that the OCPD manufacturer decided that it wasn't good practice to have 120/240v written on their breaker if the minimum voltage available is 240, so they just repackaged and relabeled these breakers as straight 240. That also allowed them to charge a little more for it because it's a specialty item.

 

[ptonsparky]

"You have the option of charging more for that breaker and IIRC 35% of $120 is more than 35% of $12." Basically what our CEU instructor told us this weekend when someone complained about added costs of NEC changes.

 

[JFletcher]

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.

Yes, a 60 amp breaker has to carry three times the amount of current that a 20 amp breaker does, however you need to look at it from the other end. Both are rated 10000 AIC, and 10000 amps is a hell of a lot more than either 20 or 60 it's not what the breaker is rated for it's what it's rated to carry without exploding

 

[I start fires]

Yes, a 60 amp breaker has to carry three times the amount of current that a 20 amp breaker does, however you need to look at it from the other end. Both are rated 10000 AIC, and 10000 amps is a hell of a lot more than either 20 or 60 it's not what the breaker is rated for it's what it's rated to carry without exploding

The reason they are all rated for 10,000 regardless of size is probably limited to that sketchy spring loaded clip that connects them to the main bus. It is equally as flimsy on a 20A and a 100A breaker.

 

[I start fires]

In a slash rated breaker, the maximum voltage to ground is the lower value, therefore during an asymmetrical fault (meaning line to ground), the maximum physical stress possible is based upon that lower voltage, so less kVA in the same amount of fault current. So when a 120/240V rated breaker rated for 10kAIC sees a ground fault in a system rated for 10kA, that is 10kA at never more than 120V, so 1200kVA of fault power.

Okay, I have got to ask why I am guaranteed that an asymmetrical fault in a 120/240v single phase system is going to be 120 volts (phase to ground)? Why can't one leg coming from a 120/240v breaker come in contact with the opposite hot leg from a larger circuit in the same system? That would give me an asymmetrical fault of 240volts on my breaker. Are you telling me a 120/240v breaker in a single phase panel isn't rated to handle this type of fault? I hope that's not true because this can absolutely happen. And if that breaker can handle this fault safely in a single phase panel, what secret law of physics would not allow a 120/240v breaker to work safely in a 240v CG delta service?

 

[JFletcher]

The reason they are all rated for 10,000 regardless of size is probably limited to that sketchy spring loaded clip that connects them to the main bus. It is equally as flimsy on a 20A and a 100A breaker.

Like you, I rarely see Breakers rated for more than 10,000 AIC. I can tell you though that a Cutler-Hammer ch breaker rated for 22000 AIC has the exact same physical dimensions as the one rated for 10,000.

When you start getting into Breakers that can interrupt substantially more fault current without failing, like 65,000A you run into the larger frame breakers that cost big money.

A 100-amp Square D three phase breaker for a qo panel might cost $100. A 100-amp three phase breaker for a floor-to-ceiling switchgear like an I-line or something would cost 10 times that much. Much physically larger, and built to withstand fault currents approaching 200,000 AIC vs 22k AIC.

The bottom line is that you cannot use a slash rated breaker for 120 / 240 volts on a corner grounded Delta. Even if there is absolutely zero chance of it failing, it is not listed for it, and I'm sure that if there were any other type of electrical injury or accident involving that circuit, that their lawyers would have a field day with you for putting in the wrong type of breaker, assuming the inspector doesn't reject it and make you take it out.

 

[I start fires]

The bottom line is that you cannot use a slash rated breaker for 120 / 240 volts on a corner grounded Delta. Even if there is absolutely zero chance of it failing, it is not listed for it, and I'm sure that if there were any other type of electrical injury or accident involving that circuit, that their lawyers would have a field day with you for putting in the wrong type of breaker, assuming the inspector doesn't reject it and make you take it out.

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.

 

[ptonsparky]

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.

Break one each apart and let us know. I’ve got used ones, I’ll do it tomorrow. IIR.

 

[electrofelon]

Another question along the same lines: is a three pole miniature CB just made of 3 single poles riveted together? IIRC all three pole breakers are fully rated....any actual construction difference?

 

[I start fires]

Another question along the same lines: is a three pole miniature CB just made of 3 single poles riveted together? IIRC all three pole breakers are fully rated....any actual construction difference?

I suppose it's possible that if you allowed fully mechanical 120/240v breakers (even though they would work flawlessly) on a CG delta, you would also then need to explain to dimwits why you couldn't use 120/240v GFCI breakers which are full of electronics. I guess forbidding all single phase components is just a way of protecting certain people who shouldn't be working with electricity.

 

[Jraef]

Okay, I have got to ask why I am guaranteed that an asymmetrical fault in a 120/240v single phase system is going to be 120 volts (phase to ground)? Why can't one leg coming from a 120/240v breaker come in contact with the opposite hot leg from a larger circuit in the same system? That would give me an asymmetrical fault of 240volts on my breaker. Are you telling me a 120/240v breaker in a single phase panel isn't rated to handle this type of fault? I hope that's not true because this can absolutely happen. And if that breaker can handle this fault safely in a single phase panel, what secret law of physics would not allow a 120/240v breaker to work safely in a 240v CG delta service?

A line-to-line fault on a 240V system is a symmetrical fault and both poles will open, so each pole has to interrupt only half of the energy in the fault. On multi-pole breakers there are always two fault ratings, asymmetrical and symmetrical, but because the symmetrical value is always higher anyway, the number we work from is only the asymmetrical value, because that's the worst case scenario.

You cannot equivocate normal actions in a breaker with what happens during a fault. In a fault, ALL of the current carrying components are stressed, not just the contacts, clips, springs etc. The magnetic fields around all of the current carrying components increase exponentially with the current that is flowing, even if just for a fraction of a second and are, depending on their orientation with one another inside of the breaker, attracting and repelling each other with extreme force. So virtually EVERYTHING is trying to move away from everything else inside of that breaker until the instant that the current stops flowing. Interrupting capacity is all about making sure these parts don't become shrapnel.

 

[I start fires]

A line-to-line fault on a 240V system is a symmetrical fault and both poles will open, so each pole has to interrupt only half of the energy in the fault. On multi-pole breakers there are always two fault ratings, asymmetrical and symmetrical, but because the symmetrical value is always higher anyway, the number we work from is only the asymmetrical value, because that's the worst case scenario.

You cannot equivocate normal actions in a breaker with what happens during a fault. In a fault, ALL of the current carrying components are stressed, not just the contacts, clips, springs etc. The magnetic fields around all of the current carrying components increase exponentially with the current that is flowing, even if just for a fraction of a second and are, depending on their orientation with one another inside of the breaker, attracting and repelling each other with extreme force. So virtually EVERYTHING is trying to move away from everything else inside of that breaker until the instant that the current stops flowing. Interrupting capacity is all about making sure these parts don't become shrapnel.

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.

 

[JFletcher]

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.

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.