Fall of Potential Test Results

[A-1Sparky]

I recently performed FOP testing on a breaker to determine if the breaker was faulty. The milivolt readings that I got were 30-40 times higher than those of other breakers with similar loads. I'm thinking that this verifies that the breaker is bad, but I don't have much experience with FOP testing, so I want to make sure before I spend $500 on a new breaker. Brian John, Zog? What do you guys say?

 

[bob]

I've never heard of using a FOP test on a breaker. Usually done to test grounding system impedance. What is the problem with the breaker? Did you put a megger test on it?

 

[A-1Sparky]

I actually learned the method from a response that Brian John had posted quite a while back. This test was done because it did not involve having to turn power off, disconnection of cables, etc. Originally, I was told that they had lost power to one leg. But, of course, when I got there everything was working fine with no voltage anomalies. So, I figured I would try the FOP method I had read about and see what I got for readings. Like I had posted earlier, it seems that the breaker may very well be no good, but I was hoping for confirmation from someone more experienced with this than I am. Also, these cables are spliced in a manhole, so I'm a little reluctant to megger them without knowing the DC voltage rating of the splices.

 

[templdl]

I recently performed FOP testing on a breaker to determine if the breaker was faulty. The milivolt readings that I got were 30-40 times higher than those of other breakers with similar loads. I'm thinking that this verifies that the breaker is bad, but I don't have much experience with FOP testing, so I want to make sure before I spend $500 on a new breaker. Brian John, Zog? What do you guys say?

"Breaker". Medium voltage, low voltage, Vacuum, power, air, insulated case, molded case? What type?
I will assume it?s a LV molded case breaker. Exercise the breaker turning it on and off and the measured values will change and will with almost all certainty decrease. When a breaker is exercised the contacts actually rub together when they close.
Also, remember that and resistance will equate to a voltage drop across the contacts ending up in heating. But I highly doubt that there will be any heating at all when the breaker is in use and no voltage drop across the contacts. Should the contact surfaces degrade to the point of heating it goes down hill from there. The heating with weaken the springs which hold the contacts together escalating contact failure. After sell thousands of breakers over the years I have found this type of failure to be rare.
Measuring contact resistance is not a valid way to determine the condition of breaker contacts. When using an ohm meter measures resistance using a very low voltage. Even using a higher voltage a very small current is used to measure resistance. Under normal use breakers commonly are applied at least 120v if not up to 480 volts with voltages and currents much higher than those that are applied when measuring resistance with an ohm meter.
A much better way would be to measure the voltage drop across the line and load terminals of a pole knowing the applied voltage and the current using ohms law to compute what the contact resistance if.

 

[A-1Sparky]

200 amp 240 volt molded case CB. Tested this breaker and two other breakers directly above it in the same gear and got the aforementioned results. Same source voltage and similar amperage, but vastly different milivolt readings across the different breakers. Any explanation for this? Also, the A phase reading was approximately 500 mv...about 3X the reading across C phase. And A phase apparently was the leg that was lost.

 

[rcwilson]

I think you are doing a millivolt drop or voltage drop test across the breaker contacts - measuring the millivolts at a fixed amps. A breaker contact with high contact resistance will have a higher voltage drop at the same current.

The "Fall of Potential" test usually refers to a ground rod or grid resistance test.

If the measured current is about the same in the breakers under test, a reading 30-40 times higher indicates a loose joint or bad contacts in the breaker. An old NETA standard says a variation in excess of 25% from similar breakers is bad.

A high resistance joint will cause heating. Check for hot spots on the breaker.

 

[A-1Sparky]

I realize that FOP is usually reserved for ground resistance testing, but a voltage gradient would be a difference in potential too, no?

 

[A-1Sparky]

I think you are doing a millivolt drop or voltage drop test across the breaker contacts - measuring the millivolts at a fixed amps. A breaker contact with high contact resistance will have a higher voltage drop at the same current.

The "Fall of Potential" test usually refers to a ground rod or grid resistance test.

If the measured current is about the same in the breakers under test, a reading 30-40 times higher indicates a loose joint or bad contacts in the breaker. An old NETA standard says a variation in excess of 25% from similar breakers is bad.

A high resistance joint will cause heating. Check for hot spots on the breaker.

So, maybe an IR scan after the full load has been up and running for a while?

 

[zog]

I think you are doing a millivolt drop or voltage drop test across the breaker contacts - measuring the millivolts at a fixed amps. A breaker contact with high contact resistance will have a higher voltage drop at the same current

The "Fall of Potential" test usually refers to a ground rod or grid resistance test..

Correct, mV drop is the correct term, Brian always calls it FOP

If the measured current is about the same in the breakers under test, a reading 30-40 times higher indicates a loose joint or bad contacts in the breaker. An old NETA standard says a variation in excess of 25% from similar breakers is bad.

A high resistance joint will cause heating. Check for hot spots on the breaker.

NETA says >50% of the lowest value when comparing phases.