another tripping main - parking lot lighting

[jim dungar]

Mostly, I'm trying to find what would cause the 200a mcb to trip - so we can resolve the problem...

It does depend on what equipment you have, your available fault current, and the location of the fault.

A Siemens QJ 200a breaker will trip on a short circuit of about 1300A. If your available fault current is 6,000A, then even with 100' of #10 the main could trip before a BL branch.

 

[zog]

Just to clarify:

Not looking to change CB's at this point until we know it is a solution
We will be repairing the problem wiring

Mostly, I'm trying to find what would cause the 200a mcb to trip - so we can resolve the problem...

Has a coordination study ever been done? What are the breaker types you have? What are the settings on any with adjustable trips? Could be one of the feeders tripped in the past and someone "fixed" it by dialing the settings to max, then when the real problem occurs again the main trips first. See that all the time, guys cranking up settings is good for business

 

[Pierre C Belarge]

Is it understood that it may be a fault condition? Is there a chance that there may be an overload that is just high enough that the main trips over a long time period?

Changing equipment is definitely not the answer, but without some more info here, we are all reaching in the dark.

I don't see how it would be all that hard to locate a fault condition in this situation, unless I am not understanding the info presented.

 

[kwired]

Is it understood that it may be a fault condition? Is there a chance that there may be an overload that is just high enough that the main trips over a long time period?

Changing equipment is definitely not the answer, but without some more info here, we are all reaching in the dark.

I don't see how it would be all that hard to locate a fault condition in this situation, unless I am not understanding the info presented.

I was thinking along the same lines - We have been told 200 amp main, 20 amp branches, 27 light poles but no mention of what actual load is.

Another possibility is overheating breaker from resistance at a terminal. Simple IR thermometer may reveal this, or a FOP test across a loaded breaker.

 

[emahler]

OK, lets see if this helps - not trying to hide info, just don't think of everything

No adjustable trip settings....
200a 1Ph mcb - load is 150A on phase a and 143 on phase b
Light string 1 - 2p 40a cb- load of 29a
Light string 2 - 2p 40 a cb - load of 25 a
String 3 _ 2p 40a cb- load of 26 a
String 4 - 2p 30a - load of 15 a
String 5 - 2p 20a - load of 10 a

Some misc 1p 20a feeds of little load

No thermal has been done
No coordination study
Distance from transformer to mcb is 60 feet as crow flies, 100 foot wire run

 

[wirenut1980]

I agree with Jim Dungar. There is most likely not coordination between the main and the branch breaker, and that is why the main is tripping first. Don't manufacturers publish their TCC curves? Could you download and compare the 200 Amp TCC vs. the 20 Amp TCC?

It sounds like you have already identified the problem circuits with low ohm readings. You just need to find where in the wiring the fault is. Either replace all the wiring on the problem circuit, or isolate with inline fuses to find the bad spot. Seems to me that replacing the wiring would be cheaper.

Edit to add: If you are still unsure where the fault is, try monitoring current on the hot wires of the branch breakers.