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#210629 07/16/13 07:05 PM
Joined: Jul 2013
Posts: 44
Member
A dealership I work at occasionaly has an old GE split panel. Some of the Branch breakers are adj-trip. Yesterday and today they were having a problem with one of them tripping. This only happens when it is really hot outside like it has been here in the NE.
The breaker feeds linear and non-linear loads.
I reset the breaker and measured all three phases on that breaker. Pretty even at about 147a the breaker would trip. (200a breaker)
I noticed it had adj trip dials and they were set in between L and H. I turned them to about 3/4, reset the breaker and measured loads again. load went to about 150 and stayed there for the time I was there. (1hr)
I am concerned however because in some of the reading on these breakers I have done is #1 the settings are determined by an engineer based on the application. This application is definitely not original. Other thing I have read is a study should be performed to determine correct settings. I feel the breaker should be simply replaced with non adj trip. I am concerned about a fire. Opinions please!

Joined: Nov 2005
Posts: 826
J
Member
What you want to do requires some study, time/current curves for every fuse or breaker you're dealing with, and log-log graph paper. Fuses and many breakers aren't adjustable. Long time, short time, and instantaneous trip settings work together to create a taylored, adjustable time/current curve. If co-ordinated, you should be able to plot the curves of the various fuses and breakers, without the curves intersecting. The curves might get close in places, but crossing is not a good sign. That indicates a time and current combination which might cause an upstream breaker to trip first or concurrently. But even if you put your thinking cap on and soak it all in, you still need an appropriate test set to verify trip unit dial settings.
Joe

Joined: Apr 2002
Posts: 7,288
Likes: 4
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Joe:
Reading the OP, how do you feel about the ambient temperature?

Also, the word 'old' in describing the CBs, and it reads like this has happened previously in 'hot' weather.

Forecast is for 90+ until the late weekend here.


John
Joined: Jul 2001
Posts: 599
J
JBD Offline
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There are almost no adjustable electronic trip elements which are sensitive to ambient temperatures.

There are almost no thermal magnetic breakers with 'dial adjustable' long time (nominal amp rating) elements. The majority of thermal magnetic molded case breaker adjustments are for the magnetic (instantaneous)portion of their trip curve.

The best answer probably depends on the actual installed breaker.
Old dusty dirty thermal magnetic breakers in poorly ventilated locations/equipment are likely to be affected by high ambient. for about the past 50 years, the target ambient temperature for molded case breakers is 40C (104F). Breaker manufacturers usually offer information on determining the breaker's performance in non-standard ambients.

Joined: Mar 2004
Posts: 947
T
twh Offline
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If the conductors are sized properly, I wouldn't be worried about a fire at 147 amps.

You could put a harmonics anlayzer on the circuit to see if you have something else tripping the breaker.

Joined: Apr 2002
Posts: 7,288
Likes: 4
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JBD:
Thanks for the response.
I remember some 'older' GE breakers that had trip issues in the summer.
Ambient was over 90 degrees; loads were under 60% of the CBs rating (ie: 12 amps on 20 amp, lighting load) harmonics were a non-issue. Mains for the panels (200-225) from an MDP had similar issues.

We replaced with new CBs & all was well.



John
Joined: Jul 2013
Posts: 44
Member
Originally Posted by JBD
for about the past 50 years, the target ambient temperature for molded case breakers is 40C (104F). Breaker manufacturers usually offer information on determining the breaker's performance in non-standard ambients.


I called my suppler today and he said basically what you are saying. The breaker in question is rated to work optimally up to 104 deg F.
Thanks for the discussion folks.

Joined: Nov 2005
Posts: 826
J
Member
John,
I was thinking that some long time trip circuits actually try to factor in conductor heating and ambient temperature. I was also thinking that the distributed loads could be higher on hot days. Finally, I was thinking that you can't over think everything and might just need to go in and tighten every lug in sight. That's what it was the last time I visited a passenger station where the Reliable Main was nuisance tripping.
Joe

Joined: Apr 2002
Posts: 7,288
Likes: 4
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Thanks Joe!!


John

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