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

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emahler

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OK, here is the set up...

Siemans Sentron (?) 225A 3PH 480V circuit breaker in a MDP feeding a lighting panel.

Lighting panel also has a second 225A 3PH 480V Sentron Circuit breaker.

This installation has not changed in 3 years. No new load has been added to that panel. Panel is feeding all HID lighting.

About a week ago the 225A breaker in the MDP tripped...It reset, took quick readings on the feeders...A- 183A, B-154A, C-138A...Planned to come back and balance the load a little better.

We haven't gotten back yet...Today it tripped 2x in a span of 45 mins....I got on site and the breaker had reset and was on. I got the same readings on the phases.

Visually checked the connections on both breakers, nothing looked odd...no burning, etc.

Checked the individual cb's...found 5 breakers that had 17-17.5 amps load, and one that had 21.5 amps load. These are all 20A 1P, and all on phase A...

I was able to shut off the circuit with 21.5A and get the loads down to 158ish, 155ish, 140ish....

Now here is the questions...

1) why would the breaker in the MDP trip? instead of the main in the panel? I wasn't able to really open anything (it's a large retail store and they were open) to see if there is an adjustable trip setting on the sentrons...

2) is it possible that the overloaded 1P's have any effect on it?

3) short of just replacing $700 circuit breakers, what should I look for?

thanks

edit to add - it appears that each leg has an adjustable trip setting
 
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A breaker that big at 480, and you just snapped it back on immediately to take some amp readings without checking anything else first? You're out of your gourd. That sort of action is going to get you hurt or some equipment seriously damaged one day.
 
mdshunk said:
A breaker that big at 480, and you just snapped it back on immediately to take some amp readings without checking anything else first? You're out of your gourd. That sort of action is going to get you hurt or some equipment seriously damaged one day.
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sorry, i didn't go into full detail...it wasn't just a snap on situation...don't panic...
 
My first take. For one, has anything else been added recently to the load in question? If this is happening with no additional loads in any sequence, than inrush can't be a conclusion. How about THD? I have to agree with Marc. Throwing the breaker back to take quick readings is scary and obsurd. Please do not take any offense. Can you give us more detail, and go from there?

This doesn't sound like a brain teaser, but who am I? Can you add anymore detail?
 
you can measure the voltage drop across the breaker when its closed to check the internal connections. a bad internal connection causes heat which can cause a breaker to trip. maybe there is a circuit that has a fault and its causing the main to trip instead of the branch circuit ocpd
 
rest assured, when i went out a week ago, it was not simply turned back on..we follow nearly all safety precautions...(we are working on following all, bet heck it's the real world)

the only thing we didn't do was megger the wires...
I honestly didn't think that info was relevant to the question...

that being said...my question really relates to why the MDP breaker and not the panel side main would trip...
 
electricalperson said:
you can measure the voltage drop across the breaker when its closed to check the internal connections. a bad internal connection causes heat which can cause a breaker to trip. maybe there is a circuit that has a fault and its causing the main to trip instead of the branch circuit ocpd
Click to expand...

voltage is constant...no apparent drop...

remember, there are essentially 2 main breakers here...1 in the MDP, and 1 in the Panel itself...
 
emahler said:
rest assured, when i went out a week ago, it was not simply turned back on..we follow nearly all safety precautions...(we are working on following all, bet heck it's the real world)

the only thing we didn't do was megger the wires...
I honestly didn't think that info was relevant to the question...

that being said...my question really relates to why the MDP breaker and not the panel side main would trip...
Click to expand...
a megger might help you in this situation also. i would of meggered the wires first if the amperage was ok. maybe the adjustable trip settings are set too low and a load is coming on causing it to trip occasionally
 
emahler said:
voltage is constant...no apparent drop...

remember, there are essentially 2 main breakers here...1 in the MDP, and 1 in the Panel itself...
Click to expand...
i ment measure the voltage drop across the breaker. put one lead of your meter on the load and the other lead on the line on the same phase. i usually replace anything over 100millivolts
 
electricalperson said:
i ment measure the voltage drop across the breaker. put one lead of your meter on the load and the other lead on the line on the same phase. i usually replace anything over 100millivolts
Click to expand...

i understood what you meant
 
emahler said:
that being said...my question really relates to why the MDP breaker and not the panel side main would trip...
Click to expand...
Sounds like they were having a horserace, and the feeder breaker won. Either that, or the problem exists between the feeder breaker load side and the panel main's line side. Or the feeder breaker in the MDP could simply have issues.
 
With two "identical" breakers in series, it is a toss up as to whcih one will trip if they both see the same fault current.

I would suspect a loose connection in the breaker causing some heating that biases the trip units. (I don't know if the Sentron MCB's have thermal or electronic trips.) But the moderate currents and lack of noticable heating casts some doubt on this theory.

Is there a ground fault trip in the breaker in the MDP?

Get a recording ammeter on the circuit and look for surges.
 
emahler said:
rest assured, when i went out a week ago, it was not simply turned back on..we follow nearly all safety precautions...(we are working on following all, bet heck it's the real world)

the only thing we didn't do was megger the wires...
I honestly didn't think that info was relevant to the question...

that being said...my question really relates to why the MDP breaker and not the panel side main would trip...
Click to expand...


I have yet to see this. It's pretty scary the fact that the fault passes the branch protection and opens in the main. This was discussed here recently.
I am still in wonder. Why does the branch OCPD have a higher time curve than the main feed?
 
i don't believe there was a fault...i believe that there was an overload condition...remember phase A was reading 184A...the rated continuous load for these breakers are 180A...

i just found it interesting that there were multiple branch circuits with continuous loads of over 16A....one of them with a continuous load of 21.5A...on a 20A CB....
 
emahler said:
i just found it interesting that there were multiple branch circuits with continuous loads of over 16A....one of them with a continuous load of 21.5A...on a 20A CB....
Click to expand...

So you watched the meter for 3 hours straight on each of those loads. ;)
 
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