Main Breaker tripping

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crod4x4

Member
Location
Texas
I have a main breaker feeding a subpanel approx 90' away in EMT conduit. Main breaker is a 250amp Square D with electronic trip unit Micrologic 6.0A. The load is 10 heaters in VAV boxes that are used very little.

The problem is that the breaker trips once in a blue moon mostly at night when no one is around. I have meggered the wire and get readings to eachother and ground over 16G Ohms at 500V @ 5min time. None of the branch circuit breakers has ever tripped. I have rasised the trip ratings to max and it still trips the breaker, installed new breaker and still tripped.

Any sugestions?? Thanks in advance!
 
brian john

brian john

Senior Member
Location
Leesburg, VA
Full load Current
Rating of CB
Settings available on the CB - LT, ST, INST, GFP?
What are the settings?
Have you checked each VAV for possible shorts.
Have you meggered the cables and the VAV's
Any transformers on the load of the CB tripping
 
zog

zog

Senior Member
Location
Charlotte, NC
crod4x4 said:
I have a main breaker feeding a subpanel approx 90' away in EMT conduit. Main breaker is a 250amp Square D with electronic trip unit Micrologic 6.0A. The load is 10 heaters in VAV boxes that are used very little.
Click to expand...

My money is on a phase imbalance, the GF function of the micrologic 6.0A actually compares the 3 phase currents for GF detection. Have you set up any monitors?

crod4x4 said:
None of the branch circuit breakers has ever tripped.
Click to expand...

I am guessing they don't have a GF function and your main does, correct me if I am wrong

crod4x4 said:
I have rasised the trip ratings to max and it still trips the breaker, installed new breaker and still tripped.
Click to expand...

This method never ceases to amaze me, how much that breaker cost? Any testing done to the breaker before buying a new one?
 
C

crod4x4

Member
Location
Texas
Thanks guys,

Balanced loads within 10% Max amp draw About 170amps.
Instant feature turned off.
Breaker is 250amp
lr & lsd set to max with .4 seconds
gf set to h with .4 seconds.
No transformers.
Main breaker is electronic and branch are regular.
Replaced breaker with a spare that was oredered with switchboard.
 
steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
My bet is that one of the VAV's has a problem, but only turns on once in a while. Try forcing each VAV to run. It could be a Ground fault in a VAV, which along with Zog's theory, explains why a branch breaker doesn't trip first.

Steve
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
It's behind the second breaker! Or did you meggar only the branch circuit or all the wires behind the breaker that's not tripping?
I have meggered the wire and get readings to each other and ground over 16G Ohms at 500V @ 5min time.
Click to expand...

As said, you'll have to QA and ID those circuit services, I've seen this before with open short's bring up a building, it's an open short.

Not that The VAV couldn't be spiking, how smart are they. They been QA lately?
How about cycling all of them and see if there's a measureable difference?
See if theres a fault to ground ...
 
C

crod4x4

Member
Location
Texas
I have only meggered the main feeder.
We can have the vavs turning on all day, with all of them with simular amp readings and the breaker will trip at night.
Have only visually inspected vavs.
Did also make sure all terminations are tight.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
steve66 said:
My bet is that one of the VAV's has a problem, but only turns on once in a while. Try forcing each VAV to run. It could be a Ground fault in a VAV, which along with Zog's theory, explains why a branch breaker doesn't trip first.

Steve
Click to expand...

Why force them the CORRECT way is to test them
 
brian john

brian john

Senior Member
Location
Leesburg, VA
So the CB has not aux trips UV or OV relays? These could be built in the CB check the CB for any aux devices.

Is there a neutral CT?

I would check each VAV
I would test the CB primary and secondary injection (we have had some issues with the trip devices in these CBs)
I would verify the neutral CT polarity and all connections including testing the CT as part of the primary injection testing.

I would connect a monitor that captures/records at a minimum 4 current readings or better 5 current readings.
 
brian john

brian john

Senior Member
Location
Leesburg, VA
zog said:
My money is on a phase imbalance, the GF function of the micrologic 6.0A actually compares the 3 phase currents for GF detection. Have you set up any monitors?
Click to expand...
But if there is a neutral sensor/CT (properly installed) this should not be an issue.


This method never ceases to amaze me, how much that breaker cost? Any testing done to the breaker before buying a new one?
Click to expand...

I think I may have caused a few heart attacks when quoting a new CB tested and installed.
 
zog

zog

Senior Member
Location
Charlotte, NC
brian john said:
But if there is a neutral sensor/CT (properly installed) this should not be an issue.
Click to expand...

Yep, I was assumuing a 3 wire system, just don't see many neutral CT's these days.

brian john said:
I think I may have caused a few heart attacks when quoting a new CB tested and installed.
Click to expand...

And I have made many smile when I can fix thiers for a fraction of the cost of replacement. I have yet to see a breaker we cannot repair.
 
E

electrics

Senior Member
zog said:
My money is on a phase imbalance, the GF function of the micrologic 6.0A actually compares the 3 phase currents for GF detection. Have you set up any monitors?



I am guessing they don't have a GF function and your main does, correct me if I am wrong



This method never ceases to amaze me, how much that breaker cost? Any testing done to the breaker before buying a new one?
Click to expand...

can a relay compare just three phase currents and dedect ground fault by only phase imbalance without neutral current data?
 
E

electrics

Senior Member
"My money is on a phase imbalance, the GF function of the micrologic 6.0A actually compares the 3 phase currents for GF detection. Have you set up any monitors?"
he says so above, but an imbalance doesnt mean a ground fault, i am really confused now.how a relay can determine if it is gf or not just evaluating the three line current??
 
zog

zog

Senior Member
Location
Charlotte, NC
electrics said:
"My money is on a phase imbalance, the GF function of the micrologic 6.0A actually compares the 3 phase currents for GF detection. Have you set up any monitors?"
he says so above, but an imbalance doesnt mean a ground fault, i am really confused now.how a relay can determine if it is gf or not just evaluating the three line current??
Click to expand...

In protective relaying when the neutral is grounded and ground fault current can flow in the line conductors, ground fault protection is usually provided by a ground fault relay (51N device). This relay is a normal overcurrent relay with tap values that allow lower pickup settings than those of the phase overcurrent relays. This relay is usually connected to the common lead of the secondary of three ?Wye? connected current transformers. Fault current that occurs due to a ground fault will be seen as unbalanced current on the neutral. The relay can be set to detect this low-level current and initiate tripping of a circuit breaker when a fault occurs.

The same concept is used in many modern electronic trip units on LVCB's but it is done electronically with a comparitive circuit.
 
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