800A MCB tripping?

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kvkkva

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I have an 800A MCB that trips occasionally and usually takes 10 -15 minutes to reset. The breaker has been replaced twice. Once with a spare and once with a new breaker and still trips trip units turned all the way up. It has gone for six months without tripping and has done it three times in one day. All the equipment fed from the panel has been running when the breaker trips so i dont believe it is caused by a machine starting The Electrical service consists of 5 800A MCB I line panelboards fed from a 1500kva transformer 480/277 secondary. I currently have an analyzer on the panel but have not found the problem any help or suggestions would be appreciated.

Load on panel 380A - 410A with unbalance of 15- 25A

Voltage phase to phase AB 476 AC 477 CB 468

Voltage Phase to Nuetral A 278 B 277 C 270

PF .89
 
It is possible you have a 'high resistance ground fault'. That could account for the time frame of the tripping and then the 15 min reset.

Adjusting breakers out in the field without knowing exactly what you are setting, can be a problem.
 
Pierre C Belarge said:
It is possible you have a 'high resistance ground fault'. That could account for the time frame of the tripping and then the 15 min reset.

Adjusting breakers out in the field without knowing exactly what you are setting, can be a problem.
Click to expand...
I am thinking same thing There has been no mention of gf settings and or if the gf equipment was checked for propper installation. It is also most likely the culprit. Many never get set on original install and the switchgear cos claimmthey are shipped at the lowest settings for safety.
 
Panel has been in service for 20 years our problem started a year ago no new loads have been installed on the panel and we actually shed about 150A from it
breaker square D MAP36800
 
Did not search the CB type but assume it is thermal magnetic, but either way.

Don't guess find out.

1. Micro ohm the CB.
2. High Current test.
3. Megger.
4. High speed disturbance analyzer.

Do you have the old CB's test them.

This is for starters and if this doesn't work there are other options to check.


My philosophy has always been if you have not found the issue you obviously over looked something. This ain't magic.
 
kvkkva said:
Panel has been in service for 20 years our problem started a year ago no new loads have been installed on the panel and we actually shed about 150A from it
breaker square D MAP36800
Click to expand...

This is a thermal magnetic breaker. The P in the catalog number says that there are lugs on the "top" of the breaker and bus bar connection on the "bottom". If you decide to replace this breaker it is perfectly "legal" to get an MAL36800 and simply remove the lugs yourself.
 
I would recommend checking the contacts. and mv drop across the contacts..

20 years is definitely a good working life..having said that,can you check whether it has developed any loose connections at the terminals?

Cheers.
 
Is anything warm to the touch when it trips? Any visible heat damage anywhere? Tried an IR viewer?
 
Warning, this is just a guess!!!!!

1. This is the third breaker installed, and it is doing the same thing.
IMO, It's not the breaker.

2. Amps are at 420a and 150a have been removed, for 20 years it had 570a

of motor load and motor starting load. 2nd c.b. didn't last long before trip.
IMO, It's not the breaker.

3. Installed brand new breaker, lasted 6 months, tripped 3 times today had to

wait 10-15 min. to reset. In other words, 10-15 min. for it to cool down

enough to be reset.
IMO, It's not the breaker.

My mind says high resistance connection, and I'll guess it is right behind the

breaker where it gets supplied.
 
did a thermal image of the panel and was ok no hot spots also checked connections in utility connection cabinet cleaned bus connections in panel before new breaker was installed sent old breaker out to testing lab and checked out fine
 
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