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#40570 - 07/26/04 09:30 PM 3 -phase sub-panel  
Attic Rat  Offline
Joined: Dec 2003
Posts: 524
Bergen Co.,N.J. USA
...Had an emergency call to a diner I do alot of work in,for an intermittently tripping breaker.Went to check it out,and the 20a.Square"D" tandem breaker was HOT to the touch.The breaker governs a refrigerator compressor and drew only 14.3amps.@ 120 volts,...still,the darn thing was hot.The system voltage is 120/208,and the sub-panel feeder is 200amps.I changed the breaker,rearranged the order so that I could put the compressor on it's own 20amp.,HACR rated breaker...Haven't heard from them since,...this was last Saturday.What could've been the problem?? A faulty breaker,??When it did trip,it went to "overload" status,by which,I mean tripped in the middle position,if it had been a short,or a locked rotor,the breaker would've tripped all the way off,
At 14.3 amps,I can't see why it behaved that way...I'm baffled...Could an unbalanced panel load cause this??
I'm all out of theories!! HELP!!

[This message has been edited by Attic Rat (edited 07-26-2004).]

.."if it ain't fixed,don't break a Licensed Electrician"

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#40571 - 07/26/04 09:37 PM Re: 3 -phase sub-panel  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
The most common reason for "hot" breakers is a poor connection. It can be at the buss, the load conductor or even inside of the breaker itself.


#40572 - 07/26/04 09:44 PM Re: 3 -phase sub-panel  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Increased contact resistance in the breaker pole will manifest as excess heat from additional I²r loss. It may be too late to determine if the breaker has been taken out of service, but in-situ AC millivolt-drop tests can sometimes flush out the problem. See

Also, with a little care—similar measurements can be made at the wire/load-side connection and main bus/breaker line-side spring-clip connection.

[This message has been edited by Bjarney (edited 07-26-2004).]

#40573 - 07/26/04 10:48 PM Re: 3 -phase sub-panel  
caselec  Offline
Joined: Apr 2002
Posts: 558
San Jose, CA
I agree with Don and BJ. If the connection to the buss and to the branch wire was good it is probably bad contacts in the breaker. The trip position is the same for both thermal and magnetic.

Curt Swartz

#40574 - 07/27/04 05:30 AM Re: 3 -phase sub-panel  
KevInMA  Offline
Junior Member
Joined: Jul 2004
Posts: 5
Seekonk, MA
Russ, also, short circuit or overload, the breaker always trips to the middle position.

#40575 - 07/27/04 09:53 AM Re: 3 -phase sub-panel  
JBD  Offline
Joined: Jul 2001
Posts: 599
How hot is hot?

The hottest spot of a circuit breaker is the operating handle.

Looking in a 1987 version of NEMA AB-1.
[The] Temperature-rise of various materials and parts;
Main operating means: (b) parts of insulating material - 60C [140F]
Main circuit terminals for external connections - 50C [122F].

Note: the handle rating is based on a temperature rise (increase) above an ambient of 25C (77F).

So even a properly functioning breaker, that is fully loaded, can be too hot to touch.

#40576 - 07/27/04 06:21 PM Re: 3 -phase sub-panel  
aland  Offline
Joined: May 2004
Posts: 186
United Kingdom
Russ, How about position of the breaker in relation to other loaded breakers, are you getting any heat transfer that is causing the tripping problem? You did say that you have rearranged the order, could the new position be cooler? Just a thought.
Good Luck.

#40577 - 07/27/04 07:15 PM Re: 3 -phase sub-panel  
capt al  Offline
Joined: Jun 2004
Posts: 206
Norton, Ma USA
Russ, I agree with BJ and Don. Look at the link BJ provoided. I use the millivolt drop test on breakers all the time. Readings approaching 100 mv toss the breaker and install a new one. This is one of the tests I do when doing Infarred work for my company.

Alan is also correct. Other loads around the tripping breaker can cause overheating.


#40578 - 07/27/04 10:43 PM Re: 3 -phase sub-panel  
Attic Rat  Offline
Joined: Dec 2003
Posts: 524
Bergen Co.,N.J. USA
... Thanx all, usual,you guys were right there for me...JBD,the breaker was HOT,...and I believe that's what contributed to the tripping..Also,the surrounding breaker(s)could've been a heat factor as definetly stopped tripping once I changed the breaker from a tandem to a single pole QO breaker,and relocated it in the panel..The case of the faulty breaker was melted a bit,and the sticker was singed..
I firmly believe that it was the breaker whose contacts may have been carbonized,and contact was poor... Thanx again guys,I'll keep you posted if anything else crops up.. [Linked Image] [Linked Image]

[This message has been edited by Attic Rat (edited 07-27-2004).]

.."if it ain't fixed,don't break a Licensed Electrician"

#40579 - 07/28/04 09:45 AM Re: 3 -phase sub-panel  
JBD  Offline
Joined: Jul 2001
Posts: 599

I agree the breaker probably needed to be replaced. And definitely, the initial panel layout was poor. Remember the ambient air (actually the inside of the panel) around any one breaker needs to be below 25C (77F) or you will reduce it's current carrying capability.

My point was, the temperature of the breaker is not a major factor in troubleshooting a circuit. The behavior of the circuit is more important. If the breaker tripped whenever the compressor kicked in, then it was tripping magnetically. If it tripped after a few minutes of running then it was tripping on overload.

Square D QO breakers only have three positions On, Tripped, and Off. You can not tell what kind of event caused a QO breaker to trip.

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