Nuisance tripping MCC main breaker 1

[splashthepot]

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?

 

[HCBFlash]

20 to 30 years doesn't make the equipment unsafe, I see it as having proved it's soundness. Barring a measurable change in the connected load, I too would suspect the main breaker itself, and probably need not mention the decline in breaker reliability (and repeatability) once trips have occurred. That breaker mechanism can most likely be inspected and cleaned by a skilled tradesman in your facility, or yourself. Replacing it with a new or rebuilt breaker may well be a wise move.

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

 

[tommom]

The only other possibility that occurs to me is if you have a motor which is tripping the main for some reason instead of its own feeder breaker. If the breakers involved are molded-case, they are notorious for not coordinating in the instantaneous range.

Do you notice this tripping in conjunction with any of the loads in particular?

 

[oldfieldguy]

Coordinating instantaneous devices is usually a futile effort.

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