Are circuit breakers rated to open more than one short circuit?

[rbalex]

I meant after - The NEMA recommended tests include another short-circuit. This is the second fault after the initial one.

NEMA doesn't want you to know that MCCBs are basically disposable – or at best you have absolutely NO idea what the next event will do. A retested MCCB after an initial fault is definitely not “like new.” That is why the NEMA statement is they “…should be able to safely clear two short circuits at its maximum rating.” NEMA has no idea either.

 

[cadpoint]

....
500 times for 25% short circuit values
400 times for 40% short circuit values
100 times for 80% short circuit vslues
15 times for 95% short circuit values
2 times for 100% short circuit values?

Just go replace the Live circuit breakers that turned into construction breakers.

Frankly I find the Demishing Ratio of a circuit breaker a Crazy thought, & ironious ... iIf it faults at less than service size, then its in Fault not a ratio of fault

 

[brian john]

Frankly I find the Demishing Ratio of a circuit breaker a Crazy thought, & ironious ... iIf it faults at less than service size

I think expierence proves this to be true clearing a fault results in damage to the OCP. As I mention I have pics and have been involved in repairs where electricians were injured.

If it faults at less that service size? a 200 amp CB under a bolted fault condition will see a lot more than 200 amps. (I know bolted faults are rare but I have seen bolted faults on distribution parallel feeders for one)

 

[Natfuelbilll]

Do "virgin" MCCB ever fail to perform at less than maximum bolted fault?



BrianJohn
Is there a internet source for these pictures with descriptions?

 

[rbalex]

Do "virgin" MCCB ever fail to perform at less than maximum bolted fault?...

Of course; but it is a very low probability.

 

[templdl]

One thing that I have noted is that the definition of fault is missing. However it does appear that it assumes that there is a bolted fault which is directly related to the available fault current and the breakers interupting rating.
But, bolted faults are rare aren't they? Aren't arcing faults more common? If so a 20at breaker's trip calibration may be 140-200a or even somewhat higher. That same breaker probably has a 10kaic interrupting rating.
There appears to be a big issue made about something that very rarely happens.
You may close the breaker again and again into a faultthat arcs, which is something that you shouldn't do, but it is most likely far from being distructive to the breaker. One should be more concerned with the cause of the arcing fault not damaging the breaker.
On the very rare occasion that breaker trips because of a bolted fault it is extremely dangerous to reclose the breaker back into that fault.

 

[rbalex]

The fact is ALL faults are unlikely. And if the first fault was indeed a bolted fault and its cause was properly repaired, a second fault is highly unlikely. Or maybe it was simply a series of low grade arcing faults.

SO WHAT?

No matter how it is analyzed, after a first fault, there is absolutely no basis for a reasonable expectation of how the MCCB will perform in the future. Neither NEMA nor any other standards organization can tell you. NEMA definitely won?t.

Sure, the odds are in your favor ? the odds are heavily in your favor that you won?t have a fault in the first place. So why don?t we just remove the MCCB altogether? It would save a pile of money. The reason the MCCB was there in the first place is because a new unit does have a reasonable expectation of performance if it is properly applied and there was a flaw in the rest of the circuit.

After a fault ? no matter what level ? the ?moving parts? have moved and no tests have been developed by anyone to say that they can be reset with any level of confidence. The only action beyond "warm-fuzzy" confidence that provides a reasonable basis of future performance is replacement.

If some organization developed tests/procedures to put an MCCB back in service after it cleared a fault and would guarantee a reasonable expectation of future performance ? I would change my position.

 

[brian john]

NAT:

I have 1000's of pictures BUT untill recently I never cataloged them. Recently started saving them in a manner that will let me find them I have to locate the pics will gladly post when located.

went through some I located this one do not have the job or incident cataloged SORRY

 

[templdl]

I have 18 years experience as a sales engineer with breakers for a leading breaker manufacturer, 5 of which were one of two application engineers. I also had the opportunity to work with a return product group manning failure analysis and test bench. I have taken apart breakers from 15a-2000a to view their contacts and arc chutes and to test them to determine a cause for return. Having dealt with 100's of breakers from residential to industrial applications I have never run across a single complaint about a breaker failure as a result of interrupting a fault. As stated before common arcing faults are well within the breaker?s ability to clear them.
Though very rare if a breaker is applies within its rating the breaker will have not problem interrupting a bolted fault.
I tried to post the UL489 Test requirements in a previous post but they appear not very readable. I doubt is a field application would be even close to duplication the UL test requirements.
If a breaker failure of the type as being discussed was an issue I would have ran across at lease one during my tenure of have had discussed it with either our engineering, sales, or return material staff.
Most are not aware of NEMA AB4: Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications.
Individual pole resistance Testing (Millivolt drop test) the results which are addressed in Section 5.4. is commonly misunderstood as being an indicator of the health of a breaker. Contact resistance varies each time a breaker is opened and closed because the moving contact doesn?t simply close against the stationary contact but it rubs against it. Contact resistance when measured with the low voltage of an ohm meter can not be fairly compared to applying 480v across the contacts with 100s of amps. It would be more appropriate to do a millivolt drop test across the contacts while the breaker is in operation. Contact resistance = heat so if there is an appreciable contact resistance there should be heat. Exercising a breaker is a practice that is commonly used to decreases contact resistance.
This may not relate to a breaker?s integrity after clearing a bolted fault but it is important to an overall understanding as to how a breaker functions and how the health of a breaker is determined.
A simple loose line side terminal will cause heating. If great enough that heat can be conducted though the stationary contact to the moving contact and weaken the moving contact spring. The spring can not hold the moving contact firmly against the stationary contact, which increases contact resistance that in turn causes more heating.
Unless a breaker is just plain deficient when manufactured quality breakers are amazingly tough.

 

[LarryFine]

After a fault – no matter what level – the “moving parts” have moved and no tests have been developed by anyone to say that they can be reset with any level of confidence. The only action beyond "warm-fuzzy" confidence that provides a reasonable basis of future performance is replacement.

Then how do you feel about breakers routinely opened and closed under load, especially when used to switch largely-inductive loads such as, let's say, daily fluorescent light switching?

 

[rbalex]

They have documented testing for that function. NEMA only hedges on faults.