another tripping main - parking lot lighting

[emahler]

looking for some thoughts, theories, ideas...

1 PH 200A load center fed from POCO pole top transformer - short circuit rating unknown

mcb is 22k AIC

panel feeds parking lot lighting and site lighting only. Pole top transformer feeds approximately 4 stores in the strip mall.

All site lighting runs through contactors that are controlled by a time clock

all branch circuit breakers are 10k AIC and sized properly for the load.

We've megged all the circuits and have 3 runs that indicate a problem
run 1 - H-G 11.01
run 2 - H-G 0.08
run 3 - H-G 0.78

everything else is over 50, with most being >1000

27 poles - approx 20 of them have in-line fuse in the base (the others were removed at one point or another)

ground rods driven at each pole in the base.

approximately 1.5 yrs ago, mcb tripped 3 times in a 2 week span during a heatwave. no lasting problems...no problems since...

2 weeks ago, mcb tripped again...hasn't since...

have been told that the problem could be a short to ground that carries back to the load center, and trips the main in that it basically makes an 'end run' around the branch circuit breakers.

It was my understanding that the branch breakers should trip 1st due to the lower AIC rating?

also was advised that replacing the 200A MCB with a fused disco with TD fuses would eliminate the main from tripping and cause any problem to trip the branch circuit breaker.

not about to start swapping parts for the heck of it, so, i'm trying to grasp what could possibly cause the 200A MCB to trip vs. the branch circuit breakers....

any thoughts? if any of the mods can explain how to post a PDF, i'll post the megger readings

 

[iwire]

any thoughts?

Sub it out to a qualified electrical contractor? (Joke, we really need better emoticons)






How about adding a number of inline fuses at some key locations to try to isolate it?

 

[jim dungar]

Except for a few breakers with extremely high ratings (i.e. above 50kA), AIC ratings rarely significantly affect tripping times. It is an 'urban myth' that lower rated AIC devices trip faster than higher rated ones.

 

[jim dungar]

have been told that the problem could be a short to ground that carries back to the load center, and trips the main in that it basically makes an 'end run' around the branch circuit breakers.

Not possible. Any current 'leaving' the main breaker must be going through at least one of the branch breakers, unless there is a short in the panel itself.

also was advised that replacing the 200A MCB with a fused disco with TD fuses would eliminate the main from tripping and cause any problem to trip the branch circuit breaker.
not about to start swapping parts for the heck of it, so, i'm trying to grasp what could possibly cause the 200A MCB to trip vs. the branch circuit breakers....

Probably cheaper to pay for a coordination study to be done.

 

[emahler]

Sub it out to a qualified electrical contractor? (Joke, we really need better emoticons)






How about adding a number of inline fuses at some key locations to try to isolate it?

i tried...Peter's phone is disconnected and your office kept hanging up on me as soon as I said I was a friend of yours

what are you thinking? some quick blow fuses?

 

[emahler]

Except for a few breakers with extremely high ratings (i.e. above 50kA), AIC ratings rarely significantly affect tripping times. It is an 'urban myth' that lower rated AIC devices trip faster than higher rated ones.

we inherited this mess...they have plans to replace in the next two years...but...for now, it's triage...

the panel that's there is a siemans 1PH 200A MCB plug in load center...they are willing to swap the panel (which we may do now, since it will remain when the lights get redone)...but I'm not one to just start swapping parts without having an idea as to what the root cause is.

but, I was not aware that the AIC ratings at that level didn't matter...unfortunately, the MCB only comes in a 22k...can't get a higher rating...

 

[emahler]

Not possible. Any current 'leaving' the main breaker must be going through at least one of the branch breakers, unless there is a short in the panel itself.

Probably cheaper to pay for a coordination study to be done.

no short in the panel...how it was explained to me was that the fault came back on the ground and the MCB was the gatekeeper to the utility power...throw in what you said about AIC ratings at that level not mounting to a hill of beans...and who the heck knows...

but no, cheaper to swap the panel and put the fused disco in than trying to get a coordination study done on this one...

 

[iwire]

what are you thinking? some quick blow fuses?

We have used class CC fuses and holders in pole bases to protect the circuits from ballast faults but maybe you could temporarily add some fuse at points in the branch circuits.

 

[Cow]

The very first thing you need to do is unhook the first pole on all three circuits from the timeclock. Putting inline fuses in all the poles isn't going to do anything for you if the bad spot is underground between poles.

Once you have the wires in the first pole on all 3 of the bad circuits isolated, then meg it again. From the first pole back to the timeclock, and from the first pole onto the other poles on the circuit. If all the problems are after the first pole, then putting inline fuses in the poles might be a good idea. However the bad spot may be underground between poles so fusing the poles may not do anything except help prevent future problems.

Megger the crap out of everything. Divide and conquer.

 

[emahler]

we've done that...less the fuses...

we know where there are problems...

what we don't know is if those problem areas are causing the 200A MCB to trip vs. the branch breaker...

what we are trying not to do is spend a ton of time and money changing parts but not solving the issue...

 

[jim dungar]

but no, cheaper to swap the panel and put the fused disco in than trying to get a coordination study done on this one...

There is absolutely no guarantee that the branch breakers will trip before the 'new' main fuse.

This is not rocket science, but it is electrical science. Compare the trip characteristics of your protective devices.

Whoever is telling you about fuses, breaker AIC ratings, and bypass current is sadly ignorant of how things actually work.

 

[cadpoint]

Cow has hit a home run...
I really like the isolation of the home run circuits and go forward!

Four stores, I'll bet there's a common electrical room and everybody's been dancing in there.

Beside the fact that different clients went into the structure at different times with different requirements. My read is that another system smashed the wires, >>Like the landscaper with their sprinkler installer<<<, Or the plumber, or other site services...

Review the site plan and note any pinch points, Ask questions about whom was there last and for what type of construction. Was any other additional client septic work done...? And of course ask about what happened when a client did came in and try to recall any physical changes required on site!

Go forth and meggar

 

[emahler]

no common electrical room - separate drops off the pole to each store - individual meters/panels

we've been there for about 5-6 yrs now, no problems until last year.

original install was bad - EC is out of business and dodging creditors...

no new work underground...

jim- could be right...as you have noticed, not jumping on the proposed solution...

 

[Pierre C Belarge]

Let me see if I understand this installation.

1. Simple single phase, 200A Main CB panel.
2. 20A single phase circuits supplying lighting poles.

Fault conditions in the 20A branch circuits are tripping the 200A Main.

Has anyone thought that maybe the Main CB might be the issue? If the fault condition is passing through the 20A circuit breakers and opening the main before the 20A CBers, there seems to me to be an issue with the coordination or the Main breaker is not functioning properly.
Is anyone aware of any chance lightning strikes in this location?

 

[Electric-Light]

Using a large clamp or clip-on ring, wrap around all three phase conductors.

Hook it up to a meter and monitor. phase-to-phase loads will cancel out and not register and it will only show you the imbalance of phase to neutral load.

If there is a phase-to-ground or phase-to-neutral short, it should register thousands of amps. Use incident capturing data logging or fast peak holding feature on the meter.

The magnitude of peak will tell you if it's dead short to ground.

 

[emahler]

Pierre,

Mcb was replaced after the bout of tripping let year...

Testing is the next step...the infrequency of the problem is the only issue...we could have a monitor on for 6 months with no problems...

Does anyone know a cost effective monitor that we could add permanently?

 

[ptonsparky]

Refer to post 9 & 11.

Replace the branch circuit CBs 1st. I have seen where they just quit tripping after being reset for x amount of times.

Who has access to the panel? Change the lock or install one.

 

[Cow]

I guess I don't understand the thought process here? Everyone is focused on replacing the breakers, that's just treating the symptoms and not the problem.

Why wouldn't you find and fix the pole wiring? It has to be looked into eventually? Coordinating the mains with the branch breakers is a great idea and should be done too, but you'll still end up with tripped branch breakers that need to be fixed, so why not do it now?

Maybe you figure if you coordinate the breakers only the branches will trip. Then you can see which ones do and only fix those? That's a good plan right up until the other circuits with very low readings trip 2 mos. later when the bare wires slap around in the pole or that underground garden box fills up again in a rainstorm....

 

[ptonsparky]

Cow,

I agree with you meg the heck out of things, find the problem first, and isolate.

If you are going to blast parts at the problem start with the cheapest part first, and work your way up. Get to sell more stuff that way. Most likely the trip curves of the CBs, Main and Branch overlap with the main tripping first. This can be exasperated by a Branch CB being wore out due to constant resetting.

 

[emahler]

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...