Electrician Talk banner
1 - 7 of 7 Posts

·
fertilizer distrubuter
Joined
·
1,250 Posts
Discussion Starter · #1 ·
What would you guys consider a failing fop test on different breaker types?

I am wondering because I just took a reading of .4 volts on a bad breaker at 155 amps. Would I be correct in thinking the breaker must than be dissipating 62 watts?
If any of you want to lay out a rant on how to properly check breakers I would be all ears. There may even be a previous thread I haven't seen that explains this.
 

·
Registered
Joined
·
5,448 Posts
What would you guys consider a failing fop test on different breaker types?

I am wondering because I just took a reading of .4 volts on a bad breaker at 155 amps. Would I be correct in thinking the breaker must than be dissipating 62 watts?
If any of you want to lay out a rant on how to properly check breakers I would be all ears. There may even be a previous thread I haven't seen that explains this.
There are not any real pass/fail criteria for a FOP test, it is just a quick easy test that may lead to to believing there is a problem. Contact resistance should be checked with a microhmeter and compared to manufactures published values, also an insulation resistance test. But neither of those tell you if the breaker is good, the only way to know that is to do a primary injection test and compare trip times the the TCC's for that specific breaker.
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
In the absence of manufacturer's data, the NETA standard for contact resistance is any pole that is more than 50% higher than the lowest neighbor.

If all three poles are 0.4V drop and even better, if other breakers of the same make, carrying similar currents, are 0.4V drop, then it's reasonable to assume you're good.

But millivolt drop can also be misleading because of a naturally high AC impedance. This article explains it well.
 

·
Registered
Joined
·
32,071 Posts
But one has to be careful with the 50% Standard. On large frame circuit breakers and bolted pressure switches the micro-ohm readings very low so if the readings are 2-5 micro-ohm 50% would be 3-7.5 and that could still be acceptable
 

·
Registered
Joined
·
32,071 Posts
Additionally 99% of electrical contractors do to have access to Ductors/micro-ohm meters/DLRO. And the FOP is the go to test for many.

Safety should always be number one
Current measurements are important are widely varying current will affect the Voltage drop readings
and common sense and experience

But is 6 20 amp SP CBs are reading 25mv at 12 amps and the another is reading 120 MV and 12 amps you have issues.

You can apply ohms law and get a idea of the resistance.
 
1 - 7 of 7 Posts
Top