Slash rated breaker?

[ronaldrc]

I do not have the latest code book. I?ll get one soon.

I need to know if a slash rated 120/240 2 pole breaker can be used in a 120/240 volt
service with a high leg as long as you don't put the breaker on the high leg?

In other words the high leg should be B phase could I drop down from the top of the
panel and insert a 120/240 rated breaker 2 pole and install in slots 5 and 7 and be legal? 5
would be C phase which would be 120 volts to ground and 7 would be A phase 120 volts
to ground or can I not use this breaker at all in this panel?

According to Mike Holts video on this art. 240-85A he say it can't be used on a high leg
system?

Here is a shortcut to the video very informative you'll enjoy it.

http://www.mikeholt.com/videodisplay.php?id=17

Ronald

[ May 02, 2003, 09:32 AM: Message edited by: ronaldrc ]

 

[ronaldrc]

Re: Slash rated breaker?

After studing this subject a little I am a little confused.

How is the 208 voltage across the one single breaker to ground from the high leg or C phase more damaging than the voltage the 240 to line A or C from this same breaker?

Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ronald,
The amount of current required to be interrupted by a single contact in the breaker is greater when there is a 208 high leg fault to ground. In the case of a 240 volt fault, that would be a line to line fault and both breaker contacts would be acting at the same time to clear the fault.
The code rule doesn't say that you can't use the 120/240 volt breaker on a "high leg" system. It says you can't use it on a circuit where the voltage to ground exceeds 120 volts. I don't think it would be a code violation to use it on a circuit connected to A and C phases, but it is more practical to use multipole breakers on the high leg to make the most efficient use of the panel space.

 

[ronaldrc]

Re: Slash rated breaker?

Don, I did consider the single breaker not having enough mechanical power to trip the two
before it destroyed itself if this is true answer.I know the two breakers with 240 volts
across them would have more mechanical power than the single.Therefore maybe being able to kick before they destory there self.

Do you know for a fact this is the answer or are you just like me assuming that?Wouldn't hurt to hear this from a the manufacturer.

And you know the definition of assume.

And in Mikes? video he states in a high leg system,I would read this as not being able to
use a 120/240 volt breaker in this panel at all?

Ronald

 

[ronaldrc]

Re: Slash rated breaker?

I hate to bring this back up but shown here is a single pole Square D 20 amp. breaker.

It is a slash 120/240 volt rated breaker.

Question how can this breaker withstand a fault current of 240 volts and clear under a fault condition and not a fault current of 208 volts in a delta high leg system to ground.

The only differents is the voltage is lower and going to ground instead of line 1 or line 3.

I alway assumed voltage is voltage and had the same energy no matter what the route it takes?

The only logical answer I can come up with is maybe the 208 volts across the breaker is not enough voltage to kick the breaker and clear the fault before the breaker destroys itself.

And if this is the case how would 120 volts be enough.

 

[ronaldrc]

Re: Slash rated breaker?

Maybe I should ask this question in a different way.

Could someone please give me a technical reason of why we can't use a 120/240 volt
breaker on a 208 high leg?

If the breaker is rated at 240 volt to another breaker on line 1 or 3 assuming the high leg is
line 2 then why would a 208 volt fault to ground be a hazard?

The only difference between the breaker having a fault to ground is that the fault would
be a lower voltage of 208 instead of 240 and it is a single breaker in the fault path instead
of two.

I could understand if someone would come back and give me this as an answer and the litature to back it.

This answer would make sense.

"The combined mechanical power of the two breakers coupled together with a common
enternal trip is enough to trip the breaker upon a line to line fault where as the single
breaker has not got enough mechanical power to trip and clear itself."

Doesn't hurt to know the underlying reasons a code is inforced does this one have any
substances? This may be one reason we have codes that make no sense.


Did some one just happen to look at a breaker one day and say to someone else, Hey this
breaker is just rated or marked 120 volts should it be used in a 208 volt system and this is
where this idea come from?

I think this is a legitimate question if I'm missing something here please point it out.

Ronald

 

[gwz2]

Re: Slash rated breaker?

For the 120/240V CB, there is two single pole breakers seeing the 240v line to line short circuit, whereas each of the two single pole breakers are actually seeing 120v to the ground or the grounded conductor.

On a 3? 240 corner grounded system, the slash CB's can not be used. They must be 240V CB's.

Your example of the 208V of a (high leg ) to ground, the 120/240 CB is actually seeing 208 volts to ground or the grounded conductor, thus violating the 120V rating of the slash CB.

[ May 17, 2003, 12:38 PM: Message edited by: gwz2 ]

 

[ronaldrc]

Re: Slash rated breaker?

Are you saying you have to have a neutral with a 120/240 volt breaker?

That makes no sense.
If I use a 120/240 rated breaker in a 120/240 volt single phase panel without a neutral to feed a 240 volt load that doesn't have a neutral doesn't the breaker see 240 volts across its contact in this configuration?


Unless the NEC outlaws a 120/240 from being use for a 240 circuit altogether.This ruling makes no sense.

Shouldn't they make it a code that 120/240 breaker can not be used on a 240 circuit without a neutral?

Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ron,
The 120/240 volt breaker does not have to have a grounded conductor on its circuit, but it can only be used on a system that has 120 volts or less to ground. In the case of a ground fault, the current flowing though the one pole of the breaker is based on the impedance of the fault and 120 volts. If this same exact fault occurs on the 208 volt high leg the fault current will increase by a factor of 1.73. This higher current may exceed the breker rating.
Don

 

[ronaldrc]

Re: Slash rated breaker?

Don I understand what both of you are getting at.

But if we use a 120/240 volt breaker on a 240 load without a grounded conductor or neutral it has 240 volts flowing across the contacts of both breakers, is this not true?

If for some reason you had a bolted short or fault in your load you would have a 240 volt fault across both breakers and the breakers do not present that much impedance they are just contacts two in series in this case.

This 240 volt fault would be more of a hazard than the 208 volt.

Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ron,

This 240 volt fault would be more of a hazard than the 208 volt.

In the case of a 240 vlot fault, the fault current is flowing through 3 contacts, not just one as in the 208 volt fault.
Look at this Bussmann article. It covers the same issue with 277/480 and 480 volt breakers. Note that the interrupting capacity of the breaker with a single pole fault is a much less than the 3 pole interrupting capacity.
Don

 

[stamcon]

Re: Slash rated breaker?

Ron, with a 240v circuit, each breaker is providing 120v. With a fault to ground, there will be 120v from each(or either) breaker to the ground fault.

 

[ronaldrc]

Re: Slash rated breaker?

We could hash this around forever and argue that maybe the circuit impedance would be higher because it makes a longer trip to load and back and therefore the AIC would be a lot lower.

I know the AIC or avaible instantaneous current would be higher just from the breaker terminal to the panel can but I think this is grasping at straws to sell a higher price breaker.

Ronald

 

[ronaldrc]

Re: Slash rated breaker?

Stamcon
That is what is throwing most off on this subject everyone is seeing 120 volts if you use it as a feeder for 240 volts it has to withstand 240 volts across it.

Ronald

 

[ronaldrc]

Re: Slash rated breaker?

Don
I did go over that Copper Bussman PDA you pointed out.

And according to what it says if you are feeding a 240 volt load I don't care what system it is on it will have to be a fully rated 240 volt breaker.

Here is a direct quote

" Where it is possible for full phase-to phase voltage to appear across only one pole, a fully rated breaker must be utilized."

That make no sense its just a twisted use of words. I will admit it crosses two breakers but whats that got to do with the price of eggs in China?

Any time you use a 2 pole breaker to feed a 240 volt load 240 appears across both theres no way you can get it to appear across just one contact and get 240 volts.

Ronald :roll:

 

[gwz2]

Re: Slash rated breaker?

You keep noting Neutral, but as has been noted before, the 120V is to the EGC which must be with all circuits.

See 250.134(B).

240.85 is of the Slash rating.

 

[gwz2]

Re: Slash rated breaker?

I believe ED had one of his excellant drawings which showed the circuit we are discussing.

I do not remember the title subject though.

 

[ronaldrc]

Re: Slash rated breaker?

GW. I don't no what you mean by a circuit?

Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ron,

" Where it is possible for full phase-to phase voltage to appear across only one pole, a fully rated breaker must be utilized."

The Bussmann paper is talking about corner grounded delta system. In this case with a ground fault on one of the ungrounded conductors, you do in fact have a phase to phase fault on only one pole of the breaker. It is never possible to have a line to line fault on one pole of a 120/240 single phase systems or on one pole of a 208Y/120 volt system, but a ground fault on the high leg of a 120/240 4 wire delta results in a fault on one pole that is 1.73 times as great as a ground fault on either of the other two phases.
don

 

[ronaldrc]

Re: Slash rated breaker?

Don

I know we where talking about a corner ground.

This is my argument.

If they have tested these breakers and applied a bolted fault current and the double pole kicks out and saves the breaker and under the same bolted fault conditions the single pole did not, I do not think it being a single contact had anything to do with it.

I believe the reason is because you have two springs cocked and have double the kicking power to trip the breaker.

I think they are giving the wrong reason the single pole will not clear the fault.

I think the wording here where the is the only place we differ.

You said one contact isn't enough to clear and two is.

That is confusing since the contacts do open the circuit to clear the fault but the springs in the breaker are what provide the power to open the circuit.

We have no disagreement, but are these the facts?

Ronald

[ May 18, 2003, 09:59 AM: Message edited by: ronaldrc ]