Nuisance tripping MCC main breaker
Nuisance tripping MCC main breaker
(OP)
Hi all, have a MCC tripping the 800a main feeding it,{three times this week}, after isolating the feeders and branch circuits this weekend, megohmeter readings were less than what Im used to seeing. The 500mcm cables all megged fine to ground, 28megohms and better, the MCC however was much worse,A phase to ground was 2.5Mohms, B phase, 3Mohm,and C phase 3Mohm. Gear is at least 20 years old and in poor shape. Ive suggested to plant management that this gear is unsafe and should be replaced. What is acceptable minimum reading? Any test standards that could back me up?





RE: Nuisance tripping MCC main breaker
It's tough to analyze the megger data because we don't know what you included in the circuit under test.
Unless you're seeing some other evidence of fault current in the MCC, I'd be more inclined to suspect the main breaker itself. I'd suggest having it tested.
RE: Nuisance tripping MCC main breaker
Just random trips starting this week, no obvious arcs,or flash burns nothing, no load fed from MCC over 5 hp. Overloading is not the problem.
RE: Nuisance tripping MCC main breaker
The megger readings aren't scaring me too much. Anything over 1 Meg can be considered acceptable below 1000V. What voltage did you megger at?
You could try a polarization index test to see how the meggered impedance changes between 1 minute and 10 minutes.
Based on your theory, the breaker is tripping on an intermittent fault in the MCC. This would require maybe 5000-8000 A. This amount of fault current at 480 V should leave some trace.
I'm **sure** this has already been done, but you can try setting the instantaneous trip elements in the breaker to Maximum.
RE: Nuisance tripping MCC main breaker
I'd put an event recorder on it to see if in fact you are getting an event that is tripping the breaker, or if the breaker mechanism is just letting go under slight stress. Once a breaker starts tripping numerous times, the mechanism parts can start getting sloppy.
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RE: Nuisance tripping MCC main breaker
Try taking the cover off the breaker and cleaning the trip mechanism. When the toggles are sticky you sometimes have to try several times to get the breaker to latch when resetting after a trip.
respectfully
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
Wait a minute, you dont just crank up a setting if a breaker is tripping. When was the last time the breaker was tested? What style trip unit do you have? If you have the old electro mechanical style with OD's they tend to fail on the fast side (Tripping too soon due to oil leaks, etc). Have the breaker tested with primary injection by a qualified company. www.netaworld.org has a search for companies in your area that can do this for you.
RE: Nuisance tripping MCC main breaker
I'm talking about the instantaneous trip pickup, not the long-time pickup.
If you read my earlier post, I did recommend that the breaker be tested.
I don't know what industry you work in, but in my experience, about 90% of all Molded case circuit breakers in service for more than five years have the instantaneous units set at maximum.
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
You recommended a PI test, not a primary injection trip test. Just because you see something all of the time does not make it right, inst settings need to be coordinated to prevent motor damage.
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
In my first post, I recommended having the breaker tested - I did not specify a particular test. The OP seemed hung up on the megger readings of the **bus**, so I suggested a PI.
I'm very well acquainted with issues related to motor protection and protective device settings. The fact that you believe you know the right way to do things does not mean that these procedures are being followed everywhere.
So lighten up - you're not the only engineer who supports proper testing of equipment.
RE: Nuisance tripping MCC main breaker
I know that you know what the right thing to do is, it is the person looking for advice I am worried about. I dont want him to think "Oh, I just need to crank up the setting, thats easy!"
And I am lightning up, 7 Lbs since 1/1/07
RE: Nuisance tripping MCC main breaker
I seem to have taken them off you!
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Sometimes I only open my mouth to swap feet...
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
If your having problems with grounds, I found this website helpful in explaining nuicense? (bad speller) trips if caused by grounds and bad grounding.
http://ecmweb.com/grounding/
I guess the other issue is ground wire carrying current? Which is a no no.
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
RE: Nuisance tripping MCC main breaker
I myself would be certain to investigate connected load changes. Obviously 20 to 30 years has provided you with plenty of change in the characteristics of your connected load.
Please let us all know how this works out for you! and don't forget FAQ731-376
RE: Nuisance tripping MCC main breaker
Do you notice this tripping in conjunction with any of the loads in particular?
RE: Nuisance tripping MCC main breaker
Several devices may see the fault, i.e., the main, set to trip at 3000 amps, the feeder, set to trip at 1000 amps, and the branch breaker, set to trip at 400 amps, all see a dead short on the branch which amounts to 4000 amps. Which breaker is going to trip? The lowest setting? Nope! Instantaneous action has been initiated on ALL these breakers. The FASTEST will trip. If that happens to be the main, well, there goes the whole facility. I have personally seen this happen.
On another note, I was called in to investigate the tripping of a certain lighting panel once. I asked if there was any new load added, and got "no" for an answer. I then pointed out that my measurements showed that the total measured load for the panel was 150% above the main breaker trip, and that it was also much higher than the design's load calculation. The problem only surfaced on cold winter mornings. The problem was that every desk in the admin building had a 1500-watt space heater plugged in to keep the ladies' legs warm in a building where the central heat was set low for "energy saving". I suggested that they raise the master thermostat and ban the portable heaters...
old field guy