800 Amp Bucket Tripping

Status
Not open for further replies.
D

Drake5265

Member
I have an 800 amp Circuit Breaker tripping in the building that I operate and maintain. The equipment supplied by this breaker are 4 rooftop ac units 1 passenger elevator and a transformer that feeds some 120/208v panels. The 800 amp circuit breaker is 480 volts. This circuit breaker doesn't trip all the time though. Only when the weather has been really bad though for the most part. Sometimes all 3 indicators are popped out when it trips (Overload, Short Circuit, and Ground Fault). I am still waiting on getting a megger to Meg out equipment circuits. Does anyone have a suggestion in the meantime?
 
D

Drake5265

Member
Basically it's been Raining just about every time it's tripped. The wiring and equipment all seems to be dry though.
 
billsnuff

billsnuff

Senior Member
Location
East of 480sparky
Rain would indicate maybe a cold front and temp drop. Do the RTUs have heating ?

Don't know your location, but here in Iowa it's been 50 one day and 90 the next, and when it rain about 55 to 65. Spring in Iowa, gotta love it.

On the other hand, should the RTU trip before the main? coordination thingy.
 
D

Drake5265

Member
I'm in Massachusetts lately it's been a very rainy spring the roof units are all on vfd's so one would think that it would trip the branch circuit ocpd but it doesn't I'm thinking a short circuit could be the problem because the rms on the branch ocpd is higher than on the 800 amp CB which is as high as it can go which is 12000 amps and the rms on the branch ocpd is at 22000. That's been the only thought lately is the short circuit isn't strong enough to trip the banch ocpd.
 
C

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I've seen too many blowups now to reset big breakers like this just willy nilly. Some people just don't realize the potential....

Ignorance is bliss, I guess.
 
zog

zog

Senior Member
Location
Charlotte, NC
Drake5265 said:
Sometimes all 3 indicators are popped out when it trips (Overload, Short Circuit, and Ground Fault).
Click to expand...
That is not possible, or I should say should not ever happen. If you are getting all 3 targets you likely have a bad trip unit. Have you tested the breaker?

Drake5265 said:
I am still waiting on getting a megger to Meg out equipment circuits. Does anyone have a suggestion in the meantime?
Click to expand...
Have them rush delivery you that megger. Or call a testing company that can test the breaker and megger the loads.
 
D

Drake5265

Member
The trip unit on that breaker tests fine everytime I test it with the actual testing unit that was left to me from the previous tenants of the building.
 
zog

zog

Senior Member
Location
Charlotte, NC
Drake5265 said:
The trip unit on that breaker tests fine everytime I test it with the actual testing unit that was left to me from the previous tenants of the building.
Click to expand...

All that tests is the trip unit circuitry, not considered a test of the breaker by any standards. A proper test would be via primary current injection. What type of trip unit is it?
 
D

Drake5265

Member
It is an 800 amp RMS-9 Versatrip Circuit Breaker. This breaker that is in there now is a replacement for one that was in there before we sent the old one out for testing and servicing and was clean this one had the same thing done last year and nothing was wrong with this one either.
 
zog

zog

Senior Member
Location
Charlotte, NC
Drake5265 said:
It is an 800 amp RMS-9 Versatrip Circuit Breaker. This breaker that is in there now is a replacement for one that was in there before we sent the old one out for testing and servicing and was clean this one had the same thing done last year and nothing was wrong with this one either.
Click to expand...

Talked to my trip unit guru. We see that same problem with RMS-9 units occasionally. Probably one of the Op-Amp IC?s has failed. They still test good when you test them.
 
zog

zog

Senior Member
Location
Charlotte, NC
There is also a NRC part 21 issued on these, 2 of them actually. The one that could be a concern for you is false tripping from transients as short as 100mS.
 
D

Drake5265

Member
We may have Isolated the problem it may be with an air handler possibly between the branch ocpd and the line side of the motor drive.
 
D

Drake5265

Member
I got my Megger finally yesterday The first unit that I tested came up at 4100 Megaohms the second unit tested 0 megaohms the third unit also tested 0 megaohms. Isn't the higher resistance a good thing when testing from phase to ground?
 
D

Drake5265

Member
The first string of megger tests I did was from the the branch circuit terminating point and now the second test is down at the breaker and my phase to ground megaohms are all relatively low the Highest reading was 90 megaohms and the lowest was 11 would the unit reading the 11 megaohms potentially be my problem?
 
zog

zog

Senior Member
Location
Charlotte, NC
Drake5265 said:
The first string of megger tests I did was from the the branch circuit terminating point and now the second test is down at the breaker and my phase to ground megaohms are all relatively low the Highest reading was 90 megaohms and the lowest was 11 would the unit reading the 11 megaohms potentially be my problem?
Click to expand...

Yep, and also explains the breaker tripping when it rains, if you meggered it after a rain I bet it is really low. It also explains the breaker tripping on GF but not all 3 targets, that is still a mystery. Possible there are 2 bad units running on different phases? 2 grounds make a short :)
 
Status
Not open for further replies.
Top