Out of Tolerance Breaker Test Results

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Sunny_92

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Electrical Engineer
I'm reviewing a report for circuit breaker testing that was performed for one of our customers. Three of the breakers had a result that was out of tolerance, and I'm looking for some input on what recommendations should be made to the customer.

Breaker #1: Square D NW30H w/ Micrologic 6.0A, 3000A
Out of tolerance parameter - ground trip time
Test result = 0.11 sec
Tolerance = 0.22 to 0.32 sec

Breaker #2: Square D MJ600 w/ Micrologic ET 1.0, 600A
Out of tolerance parameter - contact resistance
Test result: A = 82, B = 133, C = 98 (microhms)

Breaker #3: Square D JJ250 w/ TM trip unit, 250A
Out of tolerance parameter - contact resistance
Test result: A = 277, B = 426, C = 345 (microhms)

So we have one breaker whose ground trip operated too quickly, and two breakers with high contact resistance values. Do you think these breakers are out of tolerance enough to warrant recommending breaker replacement? Is there any maintenance that can be performed to correct these results? Or are these results close enough to the tolerances to recommend no action at this point? I believe the breakers are only about 5 years old, so recommending replacement may not be very appealing to the customer.
 
Sunny_92 said:
I'm reviewing a report for circuit breaker testing that was performed for one of our customers. Three of the breakers had a result that was out of tolerance, and I'm looking for some input on what recommendations should be made to the customer.

Breaker #1: Square D NW30H w/ Micrologic 6.0A, 3000A
Out of tolerance parameter - ground trip time
Test result = 0.11 sec
Tolerance = 0.22 to 0.32 sec

Breaker #2: Square D MJ600 w/ Micrologic ET 1.0, 600A
Out of tolerance parameter - contact resistance
Test result: A = 82, B = 133, C = 98 (microhms)

Breaker #3: Square D JJ250 w/ TM trip unit, 250A
Out of tolerance parameter - contact resistance
Test result: A = 277, B = 426, C = 345 (microhms)

So we have one breaker whose ground trip operated too quickly, and two breakers with high contact resistance values. Do you think these breakers are out of tolerance enough to warrant recommending breaker replacement? Is there any maintenance that can be performed to correct these results? Or are these results close enough to the tolerances to recommend no action at this point? I believe the breakers are only about 5 years old, so recommending replacement may not be very appealing to the customer.
Click to expand...

Somewhat interesting. I am not sure why there would be a low end on ground fault trip time, as quicker should be better. I would check with Square D on this one. Regarding the other two. operate the breakers under a reasonable load two or three times and take the resistance readings again. This will very likely blow off any resistive film and seat the contacts. They are actually designed that way.
 
Strathead said:
Somewhat interesting. I am not sure why there would be a low end on ground fault trip time, as quicker should be better. I would check with Square D on this one.
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If the GF pickup is purposefully set with a delay, too fast would be ..... too fast

0.1 seconds is already 1/5th the way up a typical TCC graph
 
ron said:
If the GF pickup is purposefully set with a delay, too fast would be ..... too fast

0.1 seconds is already 1/5th the way up a typical TCC graph
Click to expand...
The only concern that I can see would be if there was a concern about coordination of breakers. Either two with GF trip or whether a downstream low current breaker could clear a ground fault on timed fault current before an upstream breaker tripped on GF.

Sent from my XT1585 using Tapatalk
 
This is right up Brian John's alley, maybe he will see this thread. I know he's on Electrician Talk pretty regularly.
 
Replace? No way, these are minor issues that can be resolved easy. First the contact resistance, the spec is all 3 readings should be within 50% of the lowest phase so yes you are technically out of spec but barely, as mentioned operating the breakers a few times may bring these closer together, if not a simple PM and checking the factory adjustments for contact pressure should resolve them.

As far as the GF delay time, my guess is they didn't dial up the INST or STD setpoint far enough out before performing the GF test, happens all the time. Or the delay setting isn't locked into the setpoint exactly right, which causes it to default to lowest delay time. Could be a few other things but these are easy to fix. Worst case scenario would be replace the trip unit if further investigation shows it did indeed fail.
 
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