Fall of Potential Testing

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Cow

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Electrician
I was on a service call today to check a 200 amp MB panel in a residence with a hot main breaker. The renter had shut it off in the morning so it wasn't hot when I showed up. What I found was the 2p 90 amp furnace breaker right below the main had a bad connection and had gotten really hot. Replaced that breaker and the breaker that sat opposite it, and then turned everything back on. I read 75 amps on A & B phase when I took my readings across the main. They read about even at 89 millivolts from line side breaker lug to load side breaker lug. Does this sound about right?

For fun:) I checked another 2p 30 amp breaker in the panel that seemed to make a very light buzzing sound. It read 18 amps and around 75 millivolts.

These are all molded case Siemens breakers I was testing. Is there a rule of thumb or some kind of chart saying this many amps should equal this many millivolts or less, etc? Anything above this many millivolts at a given current should be replaced? I've read several of Brian John's posts on the subject, I just haven't seen any hard and fast numbers to go by.

Thanks...
 
I look for differences in the fop across each pole of the breaker. 85 MB on one side and 90 on the other would be OK. 85 & 175 would.be flag if the current was equal. I would think that a large breaker with corresponding large current flow could exhibit the same fop as a smaller breaker with less current. I have checked fop across different fuse mfgs and there is a reason some of them are made with fiberglass bodies. They get substantially warmer.
 
100209-2124 EST

Cow:

75 A at 0.089 V is 6.68 W for 1/2 of the breaker. For the entire breaker this has to be doubled, or 13.4 W a fair amount of heat. Extrapolate this to 200 A, (200/75)^2 = 7.11. The total power in the breaker will be about 13.4*7.11 = 95.3 W. Likely to get quite hot. I do not know what you should expect, but this might be high. There is probably a lot of force required to trip the latch and maybe this much power (heat) is required.

Also would be a good idea to measure wire to wire voltage as well as lug to lug voltage. In this case it is probably buss bar to wire.

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I read a Application Note at the Fluke site that said anything over 100mv voltage drop needs to be replaced.

I was looking into a 2P 30A breaker on a hot water heater. It was drawing 18.3A, dropping 120mv on one side and 135mv on the other. I replaced the breaker, it dropped 30mv and 60mv then.

I actually did a before/after with a IR imager which showed a reduction in heat also with the new breaker.
 
The amount of voltage drop accross a breaker is a funtion of the amount of current and the resistance of the connections and contacts within the breaker, while it can show if there is a signifecant loss accross a breaker only a IR imaging camera will allow you to see where the hot spot is if there is one. ohms law will tell how much loss there realy is for the amount of current and voltage that is applied. 100mv drop with a 240 volt load at 75 amps is not much, the buss bars in the panel alone will show that much or more.

What I didn't see is in the OP the heating breaker was found bad and the breaker stright accross from it, which to me would sound like a bad buss stab connection. (very common with aluminin buss ITE) was the condition of the stabs checked and or dressed up and cleaned? I have seen many times someone would just slap another breaker in without checking this and the new breaker just fails in a short time, also this would explain the heat felt on the main through the heat transfer through the buss to the main.
 
Wptski, thanks for the heads up on that Fluke note.

hurk27 said:
What I didn't see is in the OP the heating breaker was found bad and the breaker stright accross from it, which to me would sound like a bad buss stab connection. (very common with aluminin buss ITE) was the condition of the stabs checked and or dressed up and cleaned? I have seen many times someone would just slap another breaker in without checking this and the new breaker just fails in a short time, also this would explain the heat felt on the main through the heat transfer through the buss to the main.
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That's exactly it, bad buss finger on one side. The furnace breaker was relocated, the breaker across from it was replaced just to be on the safe side. I'm figuring just like you were that the heat generated tripped the main, but I did the FOP test across the main just to satisfy my own curiousity. I just didn't know what kind of MV reading was good/bad.
 
I checked a ordinary single pole light switch on two 100W bulb fixture that seem to function that dropped >300mv. I think that a 100W bulb draws about .5A. After repalcing the switch the voltage drop was about 30mv, IIRC.
 
Cow said:
For fun:) I checked another 2p 30 amp breaker in the panel that seemed to make a very light buzzing sound. It read 18 amps and around 75 millivolts.
Click to expand...

Forgot to post a followup, new breaker is around 60 mV. I would of thought it'd been less:confused: But it works.....:)

I do have a new problem in an existing AB MCC. Three breakers(HMCP) are tripping intermittently. The MCC has stickers on it leading me to believe it was installed new around '99. I have three 3 amp starter buckets running 1-1.5 HP motors, I believe. Megged everything, it all checked out. They said the problem seemed to happen the most in the morning when the production lines were first started. Breakers would trip a bit AFTER they had been running already. Of course they told me bearings were OK, belts were tracking right and the headrollers/tailstocks weren't rubbing.:grin:

All three buckets measure about 490-510 mV drop across them measured from breaker lug-breaker lug with only .9-2 amps running. This seems a little excessive to me? I checked another 3 amp bucket that wasn't having any issues and it also measured about 490 mV too.

Right now I'm trying to get them to order a spare bucket with at least a 7 amp breaker in it(we seem to have too much trouble with 3's) and then I will just swap it out and see if the problem goes away. Thanks.
 
ptonsparky said:
I look for differences in the fop across each pole of the breaker. 85 MB on one side and 90 on the other would be OK. 85 & 175 would.be flag if the current was equal. I would think that a large breaker with corresponding large current flow could exhibit the same fop as a smaller breaker with less current. .
Click to expand...

Right, there are specs for contact resitance for all breakers but those are hard to come by. The best rule, which ASNI and NETA use is no phase shall be more than 50% higher than the lowest phase.
 
wptski said:
I read a Application Note at the Fluke site that said anything over 100mv voltage drop needs to be replaced.

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Click to expand...

So I have a 4000A breaker with a 50 microhms (Thats about right, 0.00005 Ohms) resistance from line to load so a FOP test would show 200mV. According to fluke I should replace this $30,000 breaker?????
 
100218-1943 EST

Cow:

I need some education.

What is an MCC? I did look it up, but it would be useful if you gave me a more complete description of this box, what is in it, and how would it relate to the problem?

What is an HCMP breaker so I do not I have to go find some literature relating to it? Current rating? Kind --- thermal, magnetic, combined, or electronic? What does a trip time curve look like?

Buzzing breaker sounds like it could be magnetic. Is it?

Your original 2 pole 30 A breaker had 75 MV drop with 18 A. This is 1.35 W. Not large, but might increase to about 3.75 W at 30 A. Not unreasonable. Did both sides have 18 A and read 75 MV?

Your new breaker at 60 MV, if at the same current level, has about 64% of the power dissipation of the original one.


Your HCMP breakers are I assume in your MCC box. Is that so?

What is a 3 A starter bucket? How are these three starter buckets connected relative to each other and where are they located? How do these relate to said 2 pole 30 A breakers? Or are they unrelated?

What voltage are you operating at? Does a starter bucket have a mechanical contactor and are the contacts of it what you are measuring the 400 to 500 MV drop across?

If you have about a 1.5 HP motor on something with a 3 A rating, then I assume you are at 480 V. A 3 A rating seems marginal, but would be dependent upon the trip time curve, and other criteria.

Apparently you have breakers in these starter buckets, and at 0.9 to 2 A the voltage drop across the breaker is about 0.5 V. Is it correct that a starter bucket contains something like a motor starter contactor with heater coils? If so what are the resistance specifications for these heaters? If these exist they may be your major voltage drop. Is there a breaker plus heater coils in this starter bucket?

So what is tripping out? The overload device in the starter bucket, or your 30 A breaker?

Do you have a recording ammeter that you can monitor the current to a motor and see what the current level is when tripping occurs? Do this on the one most likely to trip.

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zog said:
So I have a 4000A breaker with a 50 microhms (Thats about right, 0.00005 Ohms) resistance from line to load so a FOP test would show 200mV. According to fluke I should replace this $30,000 breaker?????
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I believe this thread and that Fluke Application Note which I'll try to find is refering to the type of breakers that can be purchased at Home Depot or Lowe's!:roll:
 
zog said:
So I have a 4000A breaker with a 50 microhms (Thats about right, 0.00005 Ohms) resistance from line to load so a FOP test would show 200mV. According to fluke I should replace this $30,000 breaker?????
Click to expand...
The Application Note was indeed about typical service panel breakers. It was the last paragraph in the article.

Circuit breakers
A lot of people don?t think of breakers as having finite lifetimes. In reality, contacts and springs wear out. Measurements of circuit breaker voltage drop can help us determine the condition of the breaker. Measure across the line-to-load side of the branch breaker. If the voltage drop exceeds 100 mV, the breaker should be replaced. In the 35 to 100 mV range, readings should be documented and trended. In summary, the service panel is the crossroad of the building?s electrical system and the place where an experienced electrical troubleshooter can start down the right path to locate and fix any problems.

Fluke would like to thank Steve Uhrich of Valley Electric, Mt. Vernon, WA, for sharing his experience and insight in the writing of this article.
 
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