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?

 

[dpc]

Does this main breaker have ground fault protection? Is is tripping when a motor is started, or at random times? Is it an electronic trip unit?

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.

 

[splashthepot]

Old FA style I-line no gf protection. I isolated the gear after determining the cables were good. So essentially megged the buss and its connection points. With buckets removed readings were as high as 5 Mohms but degraded as buckets were stabbed back in. Have installed new MCC in adjacent plant, it megs at over 100Mohm{just buss}.
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.

 

[dpc]

I'd be suspicious of the breaker until proven wrong. Do you have another 800 A breaker you can swap it with?

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.

 

[jraef]

The decreased readings when the buckets are plugged in would just indicate that the stab isolation is lower than the bus isolators, not uncommon because the bus isolators are typically glastic and the stab isolators are probably some sort of thermoplastic. Bus stab units are also great dust collectors.

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.

JRaef.com
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[waross]

I have also seen tight breaker mechanisms. I have had old breakers in which the toggles were crudded up. A little corrosion, a little oil misting, and a little dust clinging to the oil film. After a test trip or in some cases after just switching the breaker off, the toggles do not always return completely to the proper position. Even physical vibration may cause tripping when the toggles are not fully set.
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

 

[Scramode]

Do you have access to a FLIR Camera?

 

[Zogzog]

"I'm **sure** this has already been done, but you can try setting the instantaneous trip elements in the breaker to Maximum."

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.

http://www.netaworld.org

has a search for companies in your area that can do this for you.

 

[dpc]

Zogzog,

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.

 

[wbd]

I have also seen MCC buckets that did not line up with the stabs correctly after being re-inserted after a change out or PM. This led to overheating. Was the bucket removed prior to this sporadic tripping starting?

 

[Zogzog]

DPC,

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.

 

[splashthepot]

Took IR camera readings on the gear yesterday, inconclusive, no hot spots, and have hooked up a power analysis meter to record for a week. Hopefully something will shake out. Also , this gear has been operating for 20-30 years without an equipment ground, all motors have been grounded to the frame of the MCC, but the MCC is isolated. This seems to be prevalent throughout the plant{no grounds}, and probably partially explains the dumpster full of motors and drives they go through in a month. What a hornets nest. Thanks for all the input.

 

[wbd]

Please post the results of your power analysis meter finding after you've analyzed the readings.

 

[dpc]

Zogzog,

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.

 

[Zogzog]

dpc,

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

 

[ScottyUK]

Hey zogzog,

I seem to have taken them off you!



----------------------------------

Sometimes I only open my mouth to swap feet...

 

[dpc]

I'd like to lose 7 pounds. I've been working on that for about five years.....

 

[controlsdude]

"This seems to be prevalent throughout the plant{no grounds}, and probably partially explains the dumpster full of motors and drives they go through in a month"

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.

 

[dpc]

But the breaker has no ground fault protection.

 

[shreesathe]

Just my 2cents worth. Do you have some form of recording instrument that you can plug in to monitor the system? RPM and or Fluke 43b would be ideal. Leave it on for say a week and see what you get.