Do KAIC ratings have a tolerance band?

[jtinge]

Was wondering if the KAIC rating on a breaker is an absolute value or is there some tolerance to the value. The basis for this question being that I ran across a situation where I calculated an available short circuit current of 10.5kA but the existing breaker is rated at 10KAIC. Can the breaker tolerate a some percentage of s.c.c. above the KAIC rating, similar to voltage tolerances on equipment, or does the breaker need to be changed with one having a KAIC rating above 10.5KAIC? If there is a tolerance, what is it and is it defined by UL or the equipment mfrs?

 

[jim dungar]

Was wondering if the KAIC rating on a breaker is an absolute value or is there some tolerance to the value. The basis for this question being that I ran across a situation where I calculated an available short circuit current of 10.5kA but the existing breaker is rated at 10KAIC. Can the breaker tolerate a some percentage of s.c.c. above the KAIC rating, similar to voltage tolerances on equipment, or does the breaker need to be changed with one having a KAIC rating above 10.5KAIC? If there is a tolerance, what is it and is it defined by UL or the equipment mfrs?

There is no 'accepted' tolerance band. It is possible, if not probable, that the breaker has a 'plus tolerance', however the 'listed' value is the one you need to use.
Consider any plus tolerance as a safety margin to cover any difference between your model and the real world (e.g. how accurate is your footage, how did you handle small motor contribution?).

 

[mgookin]

Rating does not mean maximum value before failure. It only means that's what it's tested to.
But I think you're asking what's allowed? The rated load.

 

[jtinge]

What action should one take if they found that the calculated available fault current was 10.5 kA and the circuit breaker interrupting rating was only 10 kAIC? This is 5% over the rated interrupting value. UL 489 indicates testing up to plus 10% of rated value. Would you leave it or replace it? If replace, would you immediately barrier off the panel or put up a sign to warn of an underated device? Would you look to see if a series rating was applicable in lieu of replacing?

 

[JDBrown]

Would you leave it or replace it? If replace, would you immediately barrier off the panel or put up a sign to warn of an underated device? Would you look to see if a series rating was applicable in lieu of replacing?

I think it depends on the situation. If you're at all familiar with series ratings, then you should definitely look into that as a solution. It has the potential to be the least painful of all your options. If you're not familiar/comfortable trying to figure out series ratings to the point that you would need to hire someone to do it for you, then it's probably not worth it. In that case, it may be cheaper to just replace the breaker (plus, that way you know it's "safe").

If I found myself with a panel full of 10kAIC breakers and an available fault current of 10.5kA, I would try every trick in the book to get that fault down below 10kA -- series ratings, make sure your feeder distances include the vertical distances from the panel up into the ceiling and back down to the board it's fed from, etc. If, on the other hand, I found an existing panel full of 22kAIC breakers plus one 10kAIC breaker, I would be much more likely to suggest that the Owner just have that one 10kAIC breaker replaced.

 

[jtinge]

I think it depends on the situation. If you're at all familiar with series ratings, then you should definitely look into that as a solution. It has the potential to be the least painful of all your options. If you're not familiar/comfortable trying to figure out series ratings to the point that you would need to hire someone to do it for you, then it's probably not worth it. In that case, it may be cheaper to just replace the breaker (plus, that way you know it's "safe").

If I found myself with a panel full of 10kAIC breakers and an available fault current of 10.5kA, I would try every trick in the book to get that fault down below 10kA -- series ratings, make sure your feeder distances include the vertical distances from the panel up into the ceiling and back down to the board it's fed from, etc. If, on the other hand, I found an existing panel full of 22kAIC breakers plus one 10kAIC breaker, I would be much more likely to suggest that the Owner just have that one 10kAIC breaker replaced.

Thanks for the feedback. Very common sense approach. If a breaker needed to be changed, what precautions/actions would you suggest be taken until it could be changed? Several safety guys have suggested barriers be erected. Too drastic or reasonable based on perceived risk?

 

[topgone]

Thanks for the feedback. Very common sense approach. If a breaker needed to be changed, what precautions/actions would you suggest be taken until it could be changed? Several safety guys have suggested barriers be erected. Too drastic or reasonable based on perceived risk?

The other poster said "tricks" and the workaround I did (it worked for me) was to loop once, the supply cable to the main of the panel. That tweak inserted some impedance into the circuit and lowered the kAIC
just below the breaker rating.
Hope that helps.

 

[Jraef]

Thanks for the feedback. Very common sense approach. If a breaker needed to be changed, what precautions/actions would you suggest be taken until it could be changed? Several safety guys have suggested barriers be erected. Too drastic or reasonable based on perceived risk?

I don't see the purpose, nor efficacy, of erecting a barrier. Barrier between what and what? The IC rating of a breaker has to do with its ability to survive the electromechanical forces involved in interrupting a fault. The down side risk is that in the attempt the breaker either A) becomes shrapnel, or B) the contacts weld and it fails to open, leading to a fire. I don't see what barriers would accomplish to mitigate those risks.

I've been there before. The right way is to start by double checking the cable distances and remove any under estimations. "That looks like about 25 feet" when in fact it is 30 feet can make a significant difference when you are that close. If that's not enough, then as was mentioned, consider running more cable. You have to weigh the cost of the copper against the cost of changing the panel.

"Series Ratings" are not something you can do on your own, the mfr has to have done them. There will be very specific combinations of devices and panel configurations that have been tested, you can only retrofit to get there if all of those conditions match. If they do though, that can be a very inexpensive way out of this.

 

[jim dungar]

I don't see the purpose, nor efficacy, of erecting a barrier. Barrier between what and what? The IC rating of a breaker has to do with its ability to survive the electromechanical forces involved in interrupting a fault.

Along this line.
The AIC rating is for interrupting a 3-phase bolted fault. These fault only occur as wiring errors. If the wiring in the panel is not being touched, this fault will not occur.
Any bolted fault downstream of this breaker means the breaker probably does not see as much fault current due to the impedance between it and the fault.

By far an away, the most likely fault will be an arcing one L-G. If your breaker is rated for this amount of fault current,you could do a risk assessment and maybe feel comfortable with your installation until the breaker can be replaced, based on no wiring changes being made.

 

[broadgage]

Use of a 10KA device with a potential fault current of 10.5KA is not correct, but in my view is a very minor violation, perhaps in the same league as a continous load of 17 amps on a 20 amp circuit, or a 20 amp load for 183 minutes on a 20 amp circuit.

I feel that a barrier and warning notice would help as a short term measure pending replacement.
If the breaker DID blow up and emit shrapnel, then a barrier that kept persons say 3 feet away would reduce the risk of injury.
A warning notice could reasonably state "underatted breaker(s) in this panel. Not to be reset without isolating the main. No repair or modification permitted in or near panel until breakers replaced"

 

[SparkyHC]

but if someone got hurt and you knew the circuit breaker was underrated you may be liable...