Molded Case Breakers, how many trips are too many? Jim D?

[iwire]

Does anyone know the design life of a typical circuit breaker is?

How many faults before it is considered done?

 

[renosteinke]

The UL testing standard requires the breaker to 'trip' and reset 50 times without failure.

 

[raider1]

Bob, I believe that it depends on the level of the fault current that the breaker clears. A breaker that clears a fault at or near its AIC rating may be damaged and need to be replaced.

I will see if I can get more information on this.

Chris

 

[iwire]

Thanks. :grin:

I should have said this before but I am talking about breakers tripping in the instantaneous range not the thermal trip from an overload.

I think what Reno pointed out applies to overloaded breakers.

 

[jim dungar]

All I have in front of me is NEMA AB-1 from 1986.

Switching Endurance test at 100% current
<100A @ 6 per minute: 6,000 with current, 4,000 without current = 10,000 total
101-225A @ 5 per minute: 4,000 with current, 4,000 without current = 8,000 total
226-600A @ 4 per minute: 1,000 with current, 5,000 without current = 6,000 total
601-800A @ 1 per minute: 500 with current, 3,000 without current = 3,500 total
801-2500A @ 1 per minute: 500 with current, 2,000 without current = 2,500 total
2501-6000A @ 1 per minute: 400 with current, 1,100 without current = 1,500 total

Fault Interrupting test
One break operation, followed by a cool down period, followed by a "making and breaking" operation.
(effectively it allows one reset of a breaker that has tripped at full fault current rating)

 

[iwire]

Interrupting test
One break operation, followed by a cool down period, followed by a "making and breaking" operation.
(effectively it allows one rest of a breaker that has tripped at full fault current rating)

Thanks, that was what I was looking for.

I have to admit I really don't understand what it is saying.

Is it saying, 'one fault, rest, close, another fault, junk?

 

[weressl]

Thanks, that was what I was looking for.

I have to admit I really don't understand what it is saying.

Is it saying, 'one fault, rest, close, another fault, junk?

To be more accurate: the breaker is required to interrupt a fault at its full fault rating, 20-30-45kA, whatever it maybe. After that the breaker can be reclosed at normal nameplate rating and subsequently opened, but its fault interrupting capability is UNTESTED regardless of the magnitude of the fault. Therefore after a breaker after a full fault interruption is nothing more - or even less - than a molded case switch, as far as the testing agency is concerned.

 

[G._S._Ohm]

The rest and reclose seems to be testing for welded contacts.

Some companies must repeatedly test at least a sample of breakers at full fault current until the breaker stops functioning, catches fire, explodes, or fails in some obvious way. Otherwise I don't see how they can be assured of the certainty of withstanding just one fault trip.

 

[renosteinke]

The following pdf explains:

http://static.schneider-electric.us...nsulated Case Circuit Breakers/0613DB9902.pdf

I was referring to the requirement to trip 50 times at 600% of the rated current. I think that it's fair to say that is a wee bit more than just a mild overload. OK, maybe not anywhere near the AIC - but I'd bet it covers the vast majority of trips- even for motors.

 

[jim dungar]

The rest and reclose seems to be testing for welded contacts.

No, it simply acknowledges that after a trip, the breaker needs a cool down period before it can be reset. The standard is more like; 2 sec minimum up to 1 hour maximum cool down period.

 

[jim dungar]

Is it saying, 'one fault, rest, close, another fault, junk?

Yes,

After a full 'AIC' rated fault, the breaker needs to be replaced.

It does not become a molded case switch, rather its ability to operate safely is undefined.

 

[iwire]

The following pdf explains:

http://static.schneider-electric.us...nsulated Case Circuit Breakers/0613DB9902.pdf

Thanks, I saved that on my PC.

 

[iwire]

Yes,

After a full 'AIC' rated fault, the breaker needs to be replaced.

Thanks.

 

[weressl]

The rest and reclose seems to be testing for welded contacts.

Some companies must repeatedly test at least a sample of breakers at full fault current until the breaker stops functioning, catches fire, explodes, or fails in some obvious way. Otherwise I don't see how they can be assured of the certainty of withstanding just one fault trip.

Actually, the UL short circuit interrupting capability testing continues until the breaker is destroyed, they just won't include the data - eg. how many times did it subsequently interrupt the AIC current - in the listing parameters.

 

[zog]

Molded case low voltage circuit breakers are typically tested to UL standard 489. UL uses the following test goals to determine if a breaker is considered to be safe (incompliance with their safety standard):
?The breaker must interrupt the maximum short circuit current two times.
?The breaker must protect itself and the connected conductor and the equipment it is installed in.
?After having been tested the breaker must be fully functional and pass a thermal calibration trip test at 250% of its rated ampacity; and pass a dielectric withstand test at two times its rated voltage or a minimum of 900 volts.
?The tested breaker must also operate properly and have continuity in all of its poles.

So how do you know if the breaker tripped on overload (Thermal) or short circuit (Magnetic)? Unless you have a trip unit with indicators that can be difficult. But depending on the system you may be able to figure it out. You can also test the breaker to ensure it still trips per the published TCC's, but for smaller breakers replacement may be more economical.

 

[zog]

Poorly worded, the guys having this discussion can usually recognize a short circuit, but the average Joe may not know, they often just reset over and over.

 

[zog]

After a full 'AIC' rated fault, the breaker needs to be replaced.

Jim, I read the UL test criteria as two AIC rated faults, how do you get one? Not saying you are wrong, maybe I am understanding it incorrectly.

 

[jim dungar]

Jim, I read the UL test criteria as two AIC rated faults, how do you get one? Not saying you are wrong, maybe I am understanding it incorrectly.

I have previously stated the NEMA test is Trip - Wait (2 sec to 1 hour)- Reset- Close &Trip.

So yes, while the breaker can experience two full AIC trips, it can only be rest once. Therefore the breaker would not need to be replaced until you wanted to reset it a second time.

What really confuses the issue is how many >600% FLA and less than full AIC trips can a breaker tolerate?

 

[zog]

I have previously stated the NEMA test is Trip - Wait (2 sec to 1 hour)- Reset- Close &Trip.

So yes, while the breaker can experience two full AIC trips, it can only be rest once. Therefore the breaker would not need to be replaced until you wanted to reset it a second time.

That makes sense. How about this? 1st AIC trip, order spare. 2nd AIC trip, replace

What really confuses the issue is how many >600% FLA and less than full AIC trips can a breaker tolerate?

That is a huge difference in operations. They should make MCCB's with see through windows to allow for contact inspections.

 

[rbalex]

Does anyone know the design life of a typical circuit breaker is?

How many faults before it is considered done?

[FONT=Helvetica-Bold+2]Of course, this is strictly personal opinion, but if there is any reason to believe a molded case circuit breaker, has interrupted a fault, it should be replaced. This is especially true for the instantaneous only type commonly used in combination starters.[/FONT]


[FONT=Helvetica-Bold+2]From IEEE 1015[/FONT]

[FONT=Helvetica-Bold+2][FONT=Helvetica-Bold+2]3.35 Circuit breaker useful life[/FONT]
[FONT=Times-Roman+2]It is prudent to replace any MCCB that has interrupted, at most, two faults at rated maximum current. This is so because the MCCB short-circuit proof test consists of an ?O-t-CO? sequence, which means that in proof testing of the circuit-breaker design and in periodic follow-up testing thereafter, the circuit breaker is required to open a fault from an initially closed position (corresponding to the ?O? operation), then after a period of time (t) to reset is allowed, to be closed into a maximum fault and trip open for a second time (corresponding to the ?CO? operation). This demonstrates a circuit breaker?s ability to perform at least two maximum level fault interruptions with the second at least a little worse than the first. No maintenance of the circuit breaker on test is permitted between interruptions.[/FONT]

[FONT=Times-Roman+2]The problem, of course, is that fault-current levels are not usually monitored. It is difficult and expensive to tell if a fault was a maximum fault and in general, low-voltage system faults tend to be less than maximum. Therefore, circuit-breaker inspections should be performed according to a plan developed to suit the application. NEMA AB 4-1996 should be referenced for MMCB and ICCB field inspection and maintenance.[/FONT]
[/FONT]

Interestingly enough, NEMA AB 4-1996 (GUIDELINES FOR INSPECTION AND PREVENTIVE MAINTENANCE OF MOLDED CASE CIRCUIT BREAKERS USED IN COMMERCIAL AND INDUSTRIAL APPLICATIONS) recommended a short circuit test as part of the inspection procedures ? this was the MCCB?s second fault!!! I pointed this out during one of the IEEE/PCIC Tech Committee meetings I was working on. The MCCB reps choked. (Several users did too) Since then, NEMA AB 4 has been rewritten slightly. It is available for free at NEMAs website.


From NEMA AB 4-2009

Introduction​

?
?and two interrupting tests at maximum short-circuit-current rating. Thus circuit breakers have an extensive but finite interrupting capability, and breakers that experience multiple high short-circuit-current faults should receive a thorough inspection with replacement if necessary.
?
It is not intended that molded case circuit breakers be disassembled for inspection. Rather, NEMA AB 4 should be referenced during periodic maintenance or during specific inspection following a high short circuit- current fault. This document is intended to ensure that molded case circuit breakers are well maintained, and provides guidelines for circuit breaker replacement.
?
In summary, following an automatic overcurrent interruption at or near its interrupting rating, the condition of any protective device should be checked prior to circuit re‑energization. For molded case circuit breakers, the condition of the circuit breaker is assessed without opening or disassembling the breaker. Circuit breakers that have experienced multiple high short-circuit-current faults, as evidenced by conditions at the source of the faults, should receive a thorough inspection per the guidelines of NEMA AB 4. This document should also be used for recommended, periodic, preventive maintenance.

I recommend getting a copy of NEMA AB 4 and reading Clauses 4 through 7, especially sub‑clause 6.6.