# F.O.P. on Main



## A Little Short (Nov 11, 2010)

What is an acceptable F.O.P. on a 225A main breaker?

I ask because I'm no expert on that but I would think very little.

I checked this breaker and had .47V on one leg and 1.50V on the other.

Pretty sure the breaker is going bad (or already there) as it also felt and showed hot on the leg with the higher difference in potential. I shot it with my thermometer and in a very short while it got up to 200° F and the other leg was around 94° F. I had them turn on the HVAC & stove along with a few other loads while I checked.
The highest amp draw that I saw was around 58A so I know it wasn't overloaded. The breaker has tripped a couple of times in the last few days.


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## papaotis (Jun 8, 2013)

have you cheched the connections/ im sure you dont need to be told that, but i sometimes overthink the obvious!


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## Black Dog (Oct 16, 2011)

F.O.P.???!:blink:


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## Vintage Sounds (Oct 23, 2009)

Black Dog said:


> F.O.P.???!:blink:


Fall of potential, aka millivolt drop test, aka, something Bad Electrician turned a lot of people here on to.


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## A Little Short (Nov 11, 2010)

papaotis said:


> have you cheched the connections/ im sure you dont need to be told that, but i sometimes overthink the obvious!


Yes I checked. The "hot" one was a little loose and it did bring the temp down some after I tightened it but it was still hot.




Black Dog said:


> F.O.P.???!:blink:


Fall of Potential

You measure the voltage across the breaker's line and load to see if there is voltage drop or difference in potential.
Maybe "Bad Electrician" will chime in here and explain it better.


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## RIVETER (Sep 26, 2009)

A Little Short said:


> What is an acceptable F.O.P. on a 225A main breaker?
> 
> I ask because I'm no expert on that but I would think very little.
> 
> ...


Good reason to IR shoot it.


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## papaotis (Jun 8, 2013)

sounds like the murray panel i just replaced, but i didnt do any test, i was standing in front of it when i heard the sizzle!


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## telsa (May 22, 2015)

58A x 1.5 V = 87 W

And that's just one pole.

Toasty!


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## Black Dog (Oct 16, 2011)

Vintage Sounds said:


> Fall of potential, aka millivolt drop test, aka, something Bad Electrician turned a lot of people here on to.





A Little Short said:


> Yes I checked. The "hot" one was a little loose and it did bring the temp down some after I tightened it but it was still hot.
> 
> 
> 
> ...


Was It really that hard to spell out?:laughing:


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## A Little Short (Nov 11, 2010)

Black Dog said:


> Was It really that hard to spell out?:laughing:


You have to be in the club to know what it means!:whistling2::laughing:


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## Bogart (Jul 20, 2015)

telsa said:


> 58A x 1.5 V = 87 W
> 
> And that's just one pole.
> 
> Toasty!


I think he meant to say 1.5 mV....1.5 VAC drop across the breaker is a bit more than toasty


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## MDShunk (Jan 7, 2007)

I don't get too wrapped up in the actual number (unless it's "oh ****" high), but worry more about having all the poles within about 10% of each other.


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## Zog (Apr 15, 2009)

MDShunk said:


> I don't get too wrapped up in the actual number (unless it's "oh ****" high), but worry more about having all the poles within about 10% of each other.


Spec for contact resistance is that all 3 poles are within 50% of the lowest reading.


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## A Little Short (Nov 11, 2010)

Bogart said:


> I think he meant to say 1.5 mV....1.5 VAC drop across the breaker is a bit more than toasty


I'm pretty sure is was volts and not millivolts.
And yes, it was hot to the touch and read high on my thermometer.


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## Zog (Apr 15, 2009)

A Little Short said:


> What is an acceptable F.O.P. on a 225A main breaker?
> 
> I ask because I'm no expert on that but I would think very little.
> 
> ...


Expected contact resistances vary on design but you would like to see 100-200 microhms on a 225A breaker, 1.5V/58A= 25.8 milliamps (25,800 microhms), that would just go in my trash bin.


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## telsa (May 22, 2015)

Zog said:


> Expected contact resistances vary on design but you would like to see 100-200 microhms on a 225A breaker, 1.5V/58A= 25.8 milli-ohms), that would just go in my trash bin.


FIFY

1.5V / 58A = 0.025862 Ohms


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## Bogart (Jul 20, 2015)

A Little Short said:


> I'm pretty sure is was volts and not millivolts.
> And yes, it was hot to the touch and read high on my thermometer.


If that is the case then that breaker would be trashed....that is unless you want to keep it as a space heater:thumbup:


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## Bogart (Jul 20, 2015)

MDShunk said:


> I don't get too wrapped up in the actual number (unless it's "oh ****" high), but worry more about having all the poles within about 10% of each other.


+/- 10% of the average is the rule of thumb, that I learned in the Navy Motor Rewind School, I use to determine motor phase winding balance as well. I find that it works rather well on all other balance aspects of the field.

50% of the lowest reading might be the toss out point but a 10% avg imbalance tells you something is wrong and you should start looking at remedying the problem


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## Big John (May 23, 2010)

Nobody publishes values for FOP. For resistance, the NETA rule is anything 50% higher than the lowest value is suspect. This needs to be interpreted carefully: 

If one pole is 12μΩ and the other 18μΩ then there's no problem.

If one pole is 150μΩ and the other 225μΩ then that's cause for concern.

For molded case breakers, I expect to see resistance values in the several hundred μΩ range, you've got values of 8,000 and 25,000 respectively. That's high.

Two things:
1) You're measuring impedance, so it will always be higher than bare bones DC resistance, and for that reason FOP is less accurate. Here's a good paper on why that's true.
2) Because it is a less accurate test and there are no published acceptable values, a good way to get perspective is the lightbulb method: 

Everyone has a good feel for how hot a 100W lightbulb gets, so how many bulbs worth of heat could the breaker be potentially generating?
0.47V @ 225A would be dissipating 105W
1.5V @ 225A would be dissipating 337W

That's damn near like having a 500W halogen lamp inside your circuit breaker. No bueno.


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