FOP Testing

[HotConductor]

Is this accurate when you have alot of fluctuating loads? I was FOP testing several points in a main distribution panel today and my readings were consistent between main bus(verticals) and bus stabs(horizontals) and between bus stabs and lugs on the load side of each breaker. But when I read between main bus and the load side of the breakers I was getting readings that kept changing.

FYI, I am troubleshooting an intermittent breaker tripping issue.

Also, I have one pb downstream from the mdp that has 10 amps on the egc going back to the mdp. I'm thinking the best way to find the source of this is to start shutting off breakers one at a time until the current drops off?

 

[ptonsparky]

FOP will change as the load changes so you need to know the current as well. Breakers do fail internaly. Connection at the buss finger can cause grief also.

Shutting down a breaker at a time will work as easy as anything.

 

[HotConductor]

I noticed alot of discoloration on the bus fingers, especially on the bottom half of the mdp. Would it be prudent to shut the system down, remove the breakers and clean everything up?

 

[Cow]

I noticed alot of discoloration on the bus fingers, especially on the bottom half of the mdp. Would it be prudent to shut the system down, remove the breakers and clean everything up?

Are the breaker-buss connections still tight?

Is the breaker that's tripping possibly heating up due to heavily loaded breakers adjacent to it?

Have you megged it?

 

[zog]

I noticed alot of discoloration on the bus fingers, especially on the bottom half of the mdp. Would it be prudent to shut the system down, remove the breakers and clean everything up?

Cleaning everything up won't fix what is causing this issue. You are going to need to do some testing on the breakers and megger of the circuit like Cow mentioned.

 

[gar]

101006-0943 EST


Yesterday I ran a test on a QO15 in a small box to get an estimated internal resistance for another thread.

The load was a 1500 W heater, 12 A, at 120 V. After 15 to 30 seconds for the breaker to warm up the measured voltage drop was about 0.15 V. The calculated resistance is about 0.01 ohm.

The resistance will be approximately constant with varying load current. Not constant because of the change of temperature with current, but close enough for my following comment.

You should expect the voltage across the breaker and its connection to the bus to vary as the load on the circuit varies.

At constant load on this breaker, and after the breaker voltage drop stabilized after a few seconds, I would not expect the voltage to vary more than 0.01 V over short time periods, a minute or so. Actually my measured variation was probably less than half this.

You have to know what is an expected normal voltage drop across a breaker of the rating you are testing at a particular current to judge whether there is a problem in the region of the breaker.

If the load current is constant and the voltage drop across the breaker and its connection to the bus is roughly its expect value, but is jumping around more than 10% of the average reading, then I tentatively would look for problems in this area.

.

 

[wptski]

Here's a PDF on testing breakers that I found: Breaker Testing. Page #31 covers contact resistance.