breaker tolerance

[hardworker]

I was working on a 120vt circuit. 12awg. It was protected with a 20amp single pole CH breaker. The panel with the breaker was about 130 -150 ft. upstream. I opened an emt box at the 130 -150ft out location to see the connections. As soon as I moved the connections, a hot connector grounded for a mini-second and then stopped. The breaker never tripped.

Do breakers have more tolerance, thus slower to trip, when there is that much distance?

 

[kwired]

I was working on a 120vt circuit. 12awg. It was protected with a 20amp single pole CH breaker. The panel with the breaker was about 130 -150 ft. upstream. I opened an emt box at the 130 -150ft out location to see the connections. As soon as I moved the connections, a hot connector grounded for a mini-second and then stopped. The breaker never tripped.

Do breakers have more tolerance, thus slower to trip, when there is that much distance?

The breaker doesn't change, what does change is the amount of fault current that flows. The longer the run the more "ohm's per foot" are in the path which means more overall resistance. The more resistance the less current will flow. You could have a couple thousand amps flow into a fault near the source, but conductor resistance may limit it to just a few hundred amps at the end of a long run.

It should still trip the breaker but will take longer time to do so at a lower current level, in your case you stopped the fault or it burned itself out before enough time passed to open the breaker.

 

[iwire]

I agree with kwired.

If we made the circuit even longer we could reach a point where the breaker would never trip due to circuit impedance.

 

[hardworker]

Ok, that is what I thought. Thanks

 

[petersonra]

just curious - did you have voltage rated gloves on while you were doing this work?

 

[templdl]

Was it a bolted fault or a mere brief contact? Remember that a breaker only has the ability to respond to current and the time that current is present within its TC trip curve.

 

[brian john]

And lastly the circuit breaker could be defective.

 

[templdl]

And lastly the circuit breaker could be defective.

The only way to solve that question is to replace the breaker. The when it continues to happen is the replacement breaker defective also? The trick is to understand the breaker and then understand what the dynamics of the incident that you are assuming that you think should be tripping the breaker is.

 

[brian john]

The only way to solve that question is to replace the breaker. The when it continues to happen is the replacement breaker defective also? The trick is to understand the breaker and then understand what the dynamics of the incident that you are assuming that you think should be tripping the breaker is.

Actually I'd take the CB and test it. But few electricians have that option.

In a case like this, for the cost of the CB I'd err on replacing. But I would look into all aspects of the wiring involved.

 

[templdl]

Actually I'd take the CB and test it. But few electricians have that option.

In a case like this, for the cost of the CB I'd err on replacing. But I would look into all aspects of the wiring involved.

How do you test your breakers, NEMA AB4? I doubt if you can test the magnetic calibration though. One of my customers called me in as they had received some breakers back from a job that they had at Cape Kennedy. All I had to do is sniff the breaker as they fried the magnetic element when attemptig to test it.
A basic thermal test requires an ammeter and a watch.

 

[brian john]

Circuit breakers are tested per NEMA, ANSI, NETA and the particular manufactures specifications

Breakers are high current tested for long time and instantaneous pick up
Meggered-Insulation resistance test.
Ductored/micro ohmed, DLRO (contact resistance measurement.

 

[templdl]

Circuit breakers are tested per NEMA, ANSI, NETA and the particular manufactures specifications

Breakers are high current tested for long time and instantaneous pick up
Meggered-Insulation resistance test.
Ductored/micro ohmed, DLRO (contact resistance measurement.

Interesting I understand that NETA just uses NEMA/ANSI standards.
Testing breakers is not that easy if you use factory testing techniques. Their equipment cost tens of thousands of dollars. I have the opportunity to test breakers at an actual factory test bench use to test breakers returned under warranty. I find it interesting that this type of equipment can be affordable in the field. NEMA AB4 was the only field testing method that I would accept as a valid field test.
NEMA AB4 is the "Guidelines For Inspection And Preventive Maintenance Of Molded Case Circuit Breakers Used In Commercial And Industrial Applications."
This publication illustrates the test setup.
The only includes a time current trip test is at 300% of rated continuous current of the circuit breaker trip times are as included per table 5-3.
5.6.4.1:For instantaneous a runup and pulse methods can be used. When using the run up method if done incorrectly it can destroy a breaker. As such testing one pole at a time and limiting the run up time between 2-5 sec useing an ammeter.
5.6.4.2:The pulse method uses an oscilloscope where the pulse is 5-10 cycles.
5.7 Covers the hold in test which is a test to varify that the breaker is capable of carrying its rated current.
And last, contact resistance test is irrelevent. Such a test may be used as a way to compare the resistance of the contacts. The are no go or no go values as some would like.
5.4.1 States:The millivolt drop of a circuit breaker pole can vary significantly due to inherent variability in the extremely low resistance of the electrical contacts and connectors. Such variations do not necessarily predict unacceptable performance and SHALL NOT BE USED AS THE SOLE CRITERIA FOR DETERMINAION OF ACCEPTABLILIY.
That's straight from the "NEMA AB4" publication.
I worked as an breaker application engineer for a major manufacturer and have been often amused by the so called field verification test.

 

[Jraef]

Was it a bolted fault or a mere brief contact? Remember that a breaker only has the ability to respond to current and the time that current is present within its TC trip curve.

Yep. The instantaneous magnetic trip would be what should have worked here. Most likely, as was said, the circuit impedance limited the instantaneous fault current to a level below the mag trips on that breaker. I think (strictly from memory) that CH breakers have mag trips set at a range between 11-17X the breaker rating. So on a 20A breaker, the current would have had to jump to over 220A before the breaker would trip instantaneously.

 

[templdl]

Yep. The instantaneous magnetic trip would be what should have worked here. Most likely, as was said, the circuit impedance limited the instantaneous fault current to a level below the mag trips on that breaker. I think (strictly from memory) that CH breakers have mag trips set at a range between 11-17X the breaker rating. So on a 20A breaker, the current would have had to jump to over 220A before the breaker would trip instantaneously.

Right on. The C- H residential breakers are at least 10x no matter what the rating. They do have a low mag option available as I recall which is 5x.
Now SqD is just the opposite where their std is 5x and the they have a high mag 10x as an option.
But when it comes to instantaneous any way that you look at it there are so many variables from the dynamics of an arcing fault to that of a bolted fault and the effect that the impedence has on both.
If the current doesn't rise to a magnetude the the mag element can respond to the breaker will not trip unless the current by chance is high enough and long enough in duration to trip the breaker thermally which in very unlikely. As such there is a distinct possibility the breaker is stuck between a rock and a hard place and will do nothing at all.