Power Breaker Withstand Ratings

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philly

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I understand that most LV Power Circuit breakers have two ratings which are the interrupting rating, and the withstand rating. For most breakers it is my understanding that the interrupting rating is based off of a 1/2 cycle timeframe while the withstand rating is based off of a 30 cycle timeframe, is this correct?

I was looking at a particular breaker that had a 65kA interrupting rating but only a 10kA withstand or short time rating. I guess my question is when do we need to be concerned with the withstand rating and how it compares to the avaliable fault curent at the given location. Usually for equpment evaluation the lowest device interrupting rating in the equipment is compared to the avalaible fault current to determine the pass/fail of the equipment evaluation. When do we need to check to ensure tha the breakers withstand rating meets the necessary requirements.

The only case I can think of is if the given power breaker is located in the panel and has its instantanous seeting turned off, then the withstand rating has to meet the fault current rating? Or does it depent on what is upstream of the breaker or panel?
 
Power Breaker Withstand Ratings

10kA withstand on a 65kAIC breaker makes no sense. Are you sure it wasn't 100kA? Typically, AIC is the maximum fault a breaker can tolerate and still function afterward (for a certain number of trips), and withstand is the value above which he equipment gets damaged.

Or at least that's what I thought. I'm eager to hear others opinions on the subject.
 
Another way to look at it:

An Interrupt Capacity will INCLUDE the withstand capability in the design criteria, but bit the other way around. Interrupt Capacity applies to devices that will interrupt only. Withstand applies to everything in the circuit.

So yes, a Power Breaker withstand rating is for 1/2 second (30 cycles if it is a 60 cycle system, but 1/2 second regardless). So the current level that is based on will be lower than the Interrupt Capacity rating because it must hold together WITHOUT tripping for that long. The IC rating implies that the breaker is TRIPPING, but certainly if the breaker does not hold together in that time, it fails.
 
Jraef said:
Another way to look at it:

An Interrupt Capacity will INCLUDE the withstand capability in the design criteria, but bit the other way around. Interrupt Capacity applies to devices that will interrupt only. Withstand applies to everything in the circuit.

So yes, a Power Breaker withstand rating is for 1/2 second (30 cycles if it is a 60 cycle system, but 1/2 second regardless). So the current level that is based on will be lower than the Interrupt Capacity rating because it must hold together WITHOUT tripping for that long. The IC rating implies that the breaker is TRIPPING, but certainly if the breaker does not hold together in that time, it fails.
Click to expand...

Let me pose an example so that I make sure I am understanding correctly.

Lets say I have a piece of switchgear named switchgear X which had an avalialbe falut current of 50kA at the switchgear bus. Now lets say the main breaker in this switchgear was a power breaker which has a 65kA interrupting rating and a 10kA withstand rating. Now lets say that the instantaneous setting on the main breaker was turned off for coordinatio purposes, and the short time dealy was set to .5s delay so that the avaliable 50kA fault current would take 30 cycles to clear. Whould this breaker now be considered overdutided?

I know the example shown above may not be realistic in that a breaker has a 65ka interrupting rating with only a 10kA withstand rating (although I thought I saw this once.). I would think a main breaker would have a breaker with a withstand rating that was closer to its interrupting rating in order to be able to turn the INST setting off for coordination purposes.

Would the same concept apply to feeder breakers located in this swtichgear? Most of the time the INST settings on feeder breakers aren't turned off so maybe this wouldn't be an issue with the withstand rating but in the event the INST setting was turned off on a feeder breaker would you then again have to compare the withstand rating to the avalaible fault current which in this case would be inadequate?

Woul
 
What I have understood that a power breaker withstand rating brings to the party is for coordination. If a power breaker has the ability to keep its contacts closed for a given length of time during a short circuit even it with provide just enough of a delay to alow a down streams device to trip because of the down stream devices very limited withstand rated which would be typical of a molded case breaker.
You will notice the adjustable short time delays omnn the power breakers which are much longer than that which may be anailable for a molded case breaker as an option.
 
For UL489 Listed breakers there is no true withstand rating. I believe all of these devices have inherent 'self protection' trip points regardless of the INST function setting.
But breakers built to ANSI C37/UL1066 standards usually have withstand ratings close to their AIC ratings (particularly at 240V).

Sometimes there may be a Close and Latch rating, which is usually lower than the AIC or Withstand, but I have never seen the extreme range mentioned in the OP.
 
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