Circuit Breaker tripping...not sure why...

Status
Not open for further replies.
"Then, after a bi-metal thermal element has tripped once it takes incrementally less heating to make it trip again and gets worse and worse with each successive trip. The phenomenon is called "thermal memory". UL testing and calibration specs for a thermal trip element in a breaker only call for it to trip, reset and trip once more. After that, all bets are off."

Yep, thats what i was thinking too, a primarary injection test would verify this. but if you dont have the equipment to do this test replaceing the breaker in this case (For such a small breaker) makes sense.
 
iwire said:
Yeah, but which part?

The fact I don't follow sports

The fact it has been in the news a lot

Or that I live in Teddy's State. :D
Click to expand...

you can always tell someone from mass.....you just can't tell them much:D
 
zog said:
"Then, after a bi-metal thermal element has tripped once it takes incrementally less heating to make it trip again and gets worse and worse with each successive trip. The phenomenon is called "thermal memory". UL testing and calibration specs for a thermal trip element in a breaker only call for it to trip, reset and trip once more. After that, all bets are off."

Yep, thats what i was thinking too, a primarary injection test would verify this. but if you dont have the equipment to do this test replaceing the breaker in this case (For such a small breaker) makes sense.
Click to expand...

Thermal memory only exists wihile the breaker element is warm. After the breaker has returned to ambient temperature its memory is gone. Bimetal trip elements do not move enough to cause permanent distortion and loss of calibration (if they did a house themostat would be useless after a few days).
 
Jeff,
Just to be clear, all breakers are sized to deliver 100% of rated current only for short periods, defined as under 3 hours and only under ideal conditions (40deg C, free air flow, properly sized conductors etc.). They are also designed to carry 80% continuously under those same conditions (often referred to as the "80% rule").
Click to expand...
I don't agree. They are all designed to carry their full ampacity forever under the test conditions. That is a single breaker in a large enclosure. The only reason for the 80% rule is to account for the heat from other breakers the panel.
Don
 
jim dungar said:
Thermal memory only exists wihile the breaker element is warm. After the breaker has returned to ambient temperature its memory is gone. Bimetal trip elements do not move enough to cause permanent distortion and loss of calibration (if they did a house themostat would be useless after a few days).
Click to expand...


Thermal memory is programmed into most electronic trip units, I dont think this breaker has bimetal strips.
 
I've metered QO120s that were loaded over 20A (maybe 24A?) very slowly and didn't trip. I've never performed a slow overload of a MB. With shorts I've had mains trip and the circuit breaker didn't. It's SOP to replace a breaker after a few trips, which I'm sure you would have done by now except...what if you replace it & the problem remains? Tell them you need to add more circuits to bring the loads below 16A, then that expensive breaker won't seem so much.

Dave
 
zog said:
Thermal memory is programmed into most electronic trip units, I dont think this breaker has bimetal strips.
Click to expand...

I was quoting your post in specific reference to the bi-metal. But even electronic thermal memory "goes away" after a period of time.
 
You quoted me quoting about Bimetals which I missed until I looked at it again today, my bad. I need to read qoutes better, however after a breaker has interupted a fault (Not overload, fault, thats where I meant to go with this) all bets are off.
 
zog said:
. . . after a breaker has interupted a fault (Not overload, fault, thats where I meant to go with this) all bets are off.
Click to expand...
That makes breakers no better than fuses.
 
zog said:
You quoted me quoting about Bimetals which I missed until I looked at it again today, my bad. I need to read qoutes better, however after a breaker has interupted a fault (Not overload, fault, thats where I meant to go with this) all bets are off.
Click to expand...

Only if it is a full AIC rated fault, must the breaker be replaced.
 
I had a similar situation at a Co-Gen facility at the Brooklyn Navy Yard; turned out to be an adjustable trip breaker. The facility added a fan motor off a spare bucket in the MCC and the breaker needed tweaking as it was set to its low point out of the box.
 
jim dungar said:
Only if it is a full AIC rated fault, must the breaker be replaced.
Click to expand...

True, but how do you know? Thats the advantage of indicators on modern trip units, if you had an INST trip, breaker should be replaced or reconditioned/tested.
 
zog said:
True, but how do you know? Thats the advantage of indicators on modern trip units, if you had an INST trip, breaker should be replaced or reconditioned/tested.
Click to expand...

If the fault current of the circuit can not approach the fault current rating (AIR) of the breaker why should it be replaced even if it tripped on a bolted fault?

If the breaker trips based on its INST setting (i.e. on a motor starting current) why should it need to be replaced?

I am not arguing against replacing breakers. I encourage testing. I am absolutely not condoning simply resetting a breaker without investigating the circumstances of the trip. But, I am against blanket statements that fly in the face of 80+ years of real life breaker installations.
 
Look at the maintenance manual for any power circuit breaker, the manufacturer makes this statement. They will list a number of normal operations (Usually a few thousand) and number of fault current interuptions (Usually 1-3) the breaker is rated for before it should be overhauled or replaced.

What do you mean by "Statements that fly in the face of 80+ years of real life breaker installations"? You have been installing breakers for 80+ years??
 
zog said:
Look at the maintenance manual for any power circuit breaker, the manufacturer makes this statement. They will list a number of normal operations (Usually a few thousand) and number of fault current interuptions (Usually 1-3) the breaker is rated for before it should be overhauled or replaced.

What do you mean by "Statements that fly in the face of 80+ years of real life breaker installations"? You have been installing breakers for 80+ years??
Click to expand...

An INST trip is not always a fault condition but you said that after one the breaker must be replaced or at least inspected. Molded case breakers have different maintenace procedures than do power circuit breakers but you lump all breakers into a single statement.

Since their introduction to the electrical industry more than 80 years ago, most molded case and power circuit breakers have not needed to be replaced after successfully clearing low level faults in their INST region.
 
"An INST trip is not always a fault condition but you said that after one the breaker must be replaced or at least inspected."

When would a INST trip not be a fault? OSHA 1910.342 requires an inspection after a fault interuption.

"Molded case breakers have different maintenace procedures than do power circuit breakers but you lump all breakers into a single statement."

I agree they are different but how did I lump them all into one statement?

"Since their introduction to the electrical industry more than 80 years ago, most molded case and power circuit breakers have not needed to be replaced after successfully clearing low level faults in their INST region."

No not replaced but they need to be maintained, as I have done on thousands of power breakers. IEEE Red Book states that 79% of all power circuit breakers (Happy?!? I didnt "Lump them all into one ststement") that have not been maintained (Per manufactures recommendation) in the past 24 months will fail (By this I assume they mean wont trip in the T/C band).
 
zog said:
When would a INST trip not be a fault? OSHA 1910.342 requires an inspection after a fault interuption.
Click to expand...

Do you feel that a 1200AF power circuit breaker rated for 65kAIC at 480V, applied on a circuit with 25kSCA, with a 400AT sensor, and an INST setting of 3x (1200A) needs to be inspected if it tripped once while trying to energize a 150kVA transformer with an inrush current of 1600A?

NFPA 70E 225.3 says "Circuit breakers that interrupt faults approaching their ratings shall be inspected and tested in accordance with the manufacturer's instructions". I can not find anything on OSHA 1910.342.

And let me emphasize - misapplied, poorly maintained, and abused protective devices can lead to property damaged at the least and deaths at the worst. Protective devices should never be returned to service unless the reason for their trip is understood and their readiness for service confirmed.
 
"Do you feel that a 1200AF power circuit breaker rated for 65kAIC at 480V, applied on a circuit with 25kSCA, with a 400AT sensor, and an INST setting of 3x (1200A) needs to be inspected if it tripped once while trying to energize a 150kVA transformer with an inrush current of 1600A?"

Nope, I think the person who did the PDC study (Or who didnt do a study and set the setings wherever they felt like) should be to blame. your example is an overload, not a fault, but may be misinterpreted as a fault with the wrong settings, after all you would never know what the actual fault level was for most devices (Unless you had something like an AC PRO that logged the fault value).

"NFPA 70E 225.3 says "Circuit breakers that interrupt faults approaching their ratings shall be inspected and tested in accordance with the manufacturer's instructions". I can not find anything on OSHA 1910.342."

Yep, OSHA (I think it is section (b), I will look that up tommorow) says something like when a protective device has interupted a fault, the device and circuit shal be tested to ensure it is safe to energize. I will get the exact wording tommorow for you.


"And let me emphasize - misapplied, poorly maintained, and abused protective devices can lead to property damaged at the least and deaths at the worst. Protective devices should never be returned to service unless the reason for their trip is understood and their readiness for service confirmed."

Couldnt agree more
 
this is great...2 engineers in a pissing contest:D

an update...remember that 20A 1P cb that had a load of 21.5A? well, with that circuit off, the 225A held...we were able to adjust the trip settings on both 225A breakers the other day...they were all on "Low", we just bumped them up to "2"...moved the 21.5A load to the phase that was drawing 136ish...had all 3 phases drawing approx 155A...about 1 hour later, the 225A breaker in the MDP tripped...

it ran for 3 days with no problems when the 21.5A circuit was off...60 mins after turning the 21.5A circuit on, it trips...shut that circuit down...

been working fine for the last 24ish hours...we are shutting them down on Friday to replace the 225A in the MDP, we'll also trace out the 21.5A lighting circuit and see if that in fact is the problem...

i'll keep you posted
 
emahler said:
it ran for 3 days with no problems when the 21.5A circuit was off...60 mins after turning the 21.5A circuit on, it trips...shut that circuit down...

been working fine for the last 24ish hours...we are shutting them down on Friday to replace the 225A in the MDP, we'll also trace out the 21.5A lighting circuit and see if that in fact is the problem...

i'll keep you posted
Click to expand...

Why are you replacing the 225A breaker if it only trips when the 21.5A lighting circuit is on? Is it because it's cheaper than having it tested?
 
Status
Not open for further replies.
Top