Slash rated breaker?

[don_resqcapt19]

Re: Slash rated breaker?

Ron,
On high fault currents, the force of the arc itself is used to open the contacts, the spring just starts the process. Also with a multipole, common trip breaker all of the "cocked springs" release at the same time no matter which pole initiates the trip. Look at the Current Limiting Breaker section of this Siemens document. It talks about how the magnetic force of the fault itself is used to help open the breaker contacts faster. It also talks about using a second set of contacts to increase the current interrupting capacity. These contacts are in series with each other creating a wider total gap between the contacts faster that a single contact can. This is also what happens when you are opening a line to line fault. Two sets of contacts are acting together to open the circuit. In the case of the high leg ground fault, there is only one set of contacts interrupting the current flow, not 2 like in a line to line fault.
don

 

[ronaldrc]

Re: Slash rated breaker?

Don thanks for the help I have followed your post and I know you have a good knowledge of breakers.

I did not no that arc is what cleared the fault on a bolted fault.

Correction it is not the arc that clears the fault it is opposite magnetic forces generated by the fault current in the extra contacts in series with the regular contacts that that force them apart to clear the fault as you said.

I don't think most people are aware of this.
I have learn a lot from this post.

I was wrong

Thanks:Ronald

[ May 18, 2003, 01:57 PM: Message edited by: ronaldrc ]

 

[ronaldrc]

Re: Slash rated breaker?

Hey Don have a question.

I notice on this slash breaker rating discussion that the main breaker feeding the delta high leg system wasn't brought up.

If the main was rated to kick out at the maximun available current wouldn't this protect the 120/240 slash rated breaker downstream?

Isn't this, what really happens and leaves most of us standing there scratching our heads?


I've seen it a thousand times a short on a 30 amp. branch circuit kick a 200 amp. instead of the 30 amp. downstream which was suppose to be protecting the branch circuit.

This was the the high AIC rating of the 200 amp. breaker just doing its job.


Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ron,
It really doesn't have anything to do with the higher AIC rating on the main. It is just that under ground fault and short circuit conditions, the amount of fault current is within the instantaneous trip range of both breakers. The time trip curves for this heavy fault current will over lap each other. It just becomes a race as to which breaker will open first. The trip curves for on brand of breakers shows that for fault currents above 7 times the breaker rating the trip time will be 1 cycle or less. This would mean that if you have a fault current of 1400 amps or more, either the branch breaker or the 200 amp main or both will open to clear the fault. For fault currents less than 1400 amps, the branch breaker should clear the fault first. If the fault is more than 50' or so from the 30 amp branch breaker, the impedance of the wire will often limit the fault to less than 1400 amps.
If you are trying to protect smaller breakers from higher levels of fault currents, you must use a listed series rated system. This overlapping of trip curves may not protect the lower rated breaker. The combination must be tested and listed to be sure that the lower rated breaker is protected.

Don

 

[ronaldrc]

Re: Slash rated breaker?

Don I not getting back to the question of a one pole slash rated breaker verses two poles and its capability of kicking out under a bolted fault again.

I know you are right on that question and the only one on the site that has done enough home work in the breaker area to answer my question and I appreciate that.

I assuming that the main would protect any breaker or bolted fault down stream from doing any permanent damage to the gear.That is my question?

If you say no I will take your word on it,although I do not understand it, but there is a lot in this trade that I do not understand.

Thanks"Ronald

 

[don_resqcapt19]

Re: Slash rated breaker?

Ron,
The main and the gear must be rated for the available fault current on the system. The branch breakers must be rated for the maximum available fault current on the load side of the branch breaker. The only way around this is to use a listed series rated system. In this case the branch breakers may be rated less then the available fault current, but both breakers will open to clear the fault and the branch breaker should not be damaged. On a non-series rated system, the main may not protect the branch breakers.
Don

 

[ronaldrc]

Re: Slash rated breaker?

Don thanks for the replies on this topic I do appreciate the feed back.

I could be wrong but I think most who read my concern about the slash breaker knew what I was referring to and like myself didn't have enough knowledge in this area to make a comment.

I have been in the business or trade for 37 years and I know a lot of top notch electricians and most of them don't study or go into the details of just how a breaker operates mechandically.

I think most on the site are the same I am not degrading them we all can't know everything.

But between all of us as a group we almost do.