# Fall of Potential



## Going_Commando (Oct 1, 2011)

So I recall Brian Johns post about doing a fall of potential test on breakers and starters to see if there are problems with the internals. Lets say I have a starter (Siemens/Furnas Nema size 2 with solid state overloads) running a 15 hp 480v motor. The motor is running a conveyor, and sits happily drawing 10ish amps. The motor fla is 21a, so thats what the overloads are set to. The damned thing trips the overloads about every 30-60 minutes, the current draw doesnt change, and the conveyor isnt getti g jammed up. I do a FOP test across the starter contacts and get 280 mV on pole 1, 20mV on pole 2, and 30 mV on pole 3. Is this indicative of high resistance of pole 1 causing heating after running the motor for a while causing the overloads to trip? The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.


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## splatz (May 23, 2015)

Wait a second. Are you saying you had a problem, followed a troubleshooting procedure, identified a possible cause, rectified that issue, and it's running right now? 

HOLY **** 

Go home right now, it isn't going to get any better than this.


If this ever happens to me - I can possibly afford to retire right then, I will. Go out on top, end on a high note kind of thing.


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## AK_sparky (Aug 13, 2013)

Going_Commando said:


> So I recall Brian Johns post about doing a fall of potential test on breakers and starters to see if there are problems with the internals. Lets say I have a starter (Siemens/Furnas Nema size 2 with solid state overloads) running a 15 hp 480v motor. The motor is running a conveyor, and sits happily drawing 10ish amps. The motor fla is 21a, so thats what the overloads are set to. The damned thing trips the overloads about every 30-60 minutes, the current draw doesnt change, and the conveyor isnt getti g jammed up. I do a FOP test across the starter contacts and get 280 mV on pole 1, 20mV on pole 2, and 30 mV on pole 3. Is this indicative of high resistance of pole 1 causing heating after running the motor for a while causing the overloads to trip? *The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.*


I think you answered your own question. A high resistance would cause the higher FOP you are seeing, and would cause more heat in the contactor, and that will (depending on proximity and type of OL) cause the OL to trip. What tpe of OL are they, and where are they mounted in relation to the contactor contacts?

Pole 1 is creating about 10 times more heat than the other poles.


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## emtnut (Mar 1, 2015)

Going_Commando said:


> So I recall Brian Johns post about doing a fall of potential test on breakers and starters to see if there are problems with the internals. Lets say I have a starter (Siemens/Furnas Nema size 2 with solid state overloads) running a 15 hp 480v motor. The motor is running a conveyor, and sits happily drawing 10ish amps. The motor fla is 21a, so thats what the overloads are set to. The damned thing trips the overloads about every 30-60 minutes, the current draw doesnt change, and the conveyor isnt getti g jammed up. I do a FOP test across the starter contacts and get 280 mV on pole 1, 20mV on pole 2, and 30 mV on pole 3. Is this indicative of high resistance of pole 1 causing heating after running the motor for a while causing the overloads to trip? The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.


I'd edit that  ... When Brian sees it, he's gonna want a commission :whistling2:

That's why we call him a Bad Electrician !


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## Going_Commando (Oct 1, 2011)

AK_sparky said:


> Going_Commando said:
> 
> 
> > So I recall Brian Johns post about doing a fall of potential test on breakers and starters to see if there are problems with the internals. Lets say I have a starter (Siemens/Furnas Nema size 2 with solid state overloads) running a 15 hp 480v motor. The motor is running a conveyor, and sits happily drawing 10ish amps. The motor fla is 21a, so thats what the overloads are set to. The damned thing trips the overloads about every 30-60 minutes, the current draw doesnt change, and the conveyor isnt getti g jammed up. I do a FOP test across the starter contacts and get 280 mV on pole 1, 20mV on pole 2, and 30 mV on pole 3. Is this indicative of high resistance of pole 1 causing heating after running the motor for a while causing the overloads to trip? *The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.*
> ...


Cool. Thats about what I figured but wanted to double check if the reading I got was a legit problem like I thought it was.


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## RePhase277 (Feb 5, 2008)

Pole 1 is dissipating 14 times the heat of pole 2 and over 9 times the heat of pole 3. I'd say that's a problem.


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## just the cowboy (Sep 4, 2013)

*Whats the current doing*

What’s the current doing on that phase? Even though you have that much difference I don't see that as your problem. A FOP test is checking for a voltage drop that may affect the voltage to the motor. 200mv is not that much just a contact that may be dirty. what did the contacts look like?
The coil may have been breaking down after heating up causing it to chatter quite common, or there was a bad connection. The SS overload works on current not like the old metal alloy ones that heat could travel down and affect it.


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## JRaef (Mar 23, 2009)

That SSOL will detect the current imbalance and bias the tripping point of the OL relay downward so that it trips even though the load is light. In addition, if the current on ony one leg falls below 20% of rating for 3 seconds or longer, it will trip (phase loss). So your running condition is already at 50%, it would not take much more to drop it enough to trigger than.


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## Going_Commando (Oct 1, 2011)

JRaef said:


> That SSOL will detect the current imbalance and bias the tripping point of the OL relay downward so that it trips even though the load is light. In addition, if the current on ony one leg falls below 20% of rating for 3 seconds or longer, it will trip (phase loss). So your running condition is already at 50%, it would not take much more to drop it enough to trigger than.


Awesome! The current on pole 1 was running about .4-.5 amps lower than the other 2 legs. 

I appreciate all the help guys. I was pretty sure that was the problem, but I just wanted to double check with guys with much more experience than me to make sure I wasn't on a wild goose chase. I've learned a ton on here, and it sure is nice to apply what I've learned on a problem that isn't staring me in the face (like a failed motor winding or whatever). The conveyor ran fine after I reset the overloads the first time when I was tweaking the programming on a VFD, everything was running great for 45 minutes while I was there, and 2 minutes after my tail-lights faded the fricken conveyor starter tripped again. :laughing:


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## Big John (May 23, 2010)

If that was your cause, that's a new one to me: 

280 mV @ 10 A = 0.028 Ω impedance.

That's nothing, it's certainly not gonna cause a severe enough current imbalance to trip the OL, especially since most of those seem to look for something like 25% or greater differential. You only saw a 5% imbalance.


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## Going_Commando (Oct 1, 2011)

Big John said:


> If that was your cause, that's a new one to me:
> 
> 280 mV @ 10 A = 0.028 Ω impedance.
> 
> That's nothing, it's certainly not gonna cause a severe enough current imbalance to trip the OL, especially since most of those seem to look for something like 25% or greater differential. You only saw a 5% imbalance.


Anything else I can test on that starter to find the problem? The conveyor ran on a different starter for the rest of the day with no problems, with nothing else changed including material and feed rates on the conveyor.


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## frenchelectrican (Mar 15, 2007)

Going_Commando said:


> Anything else I can test on that starter to find the problem? The conveyor ran on a different starter for the rest of the day with no problems, with nothing else changed including material and feed rates on the conveyor.


the other test you can do is manually close that starter and try to read the ohms to see how far close you are and it will be wise to take a apart to see how far the concants was pitted out. 

sometime put in new concants then that starter will work just fine.


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## frenchelectrican (Mar 15, 2007)

I have done few FOP testing on starter ., switches ., Breakers even bad fuse too.

I just got in not too long ago got a service call related to semi flicking lights on three way switch set up. 
Their own handydolt replace the switch and ballast but could not slove the semi flicking lights so I ran the FOP test on the three way switch to make sure and it was instering I catch couple voltage drop then rise back to normal on one side of switch while inverted the postion of switch and reran the test.,, it came out good.

Did more investation and somehow I did memeorized the shade of conductor color and look at the junction box and there were no splices but differnt shade of Blue so I knew something was up .,, I told that dolt that I am replacing the conductor and he was balking at first but he relized I make a good point and yank that bad one and found it was a hidden splice in the conduit.,,, :blink: 
after ran new conductor in., it do work better.


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## sbrn33 (Mar 15, 2007)

Going_Commando said:


> The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.


All this tells me is that you wasted a ****load of money and time running this needless test. 
You cost your company time, you cost your customer downtime when you knew the $350 starter was bad the whole time.


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## Big John (May 23, 2010)

Sounds like the problem was in the starter, not with your motor, so I don't think there's anything else you can do to prove it at this point.

Seems to me like either the OL was bad or else you had an intermittent control or single-phase condition. Intermittent single-phasing seems unlikely, because they probably would've run into difficulty restarting, and that didn't happen.

Only thing I might've tried is if you've got the OL relay it in your shop, daisy chain all three poles together L1, T1-L2, T2-L3, T3 and then plug in a heater or something so you run ~10A through it and see if it trips prematurely.


sbrn33 said:


> All this tells me is that you wasted a ****load of money and time running this needless test.
> You cost your company time, you cost your customer downtime when you knew the $350 starter was bad the whole time.


 What? 

It literally takes 30 seconds to perform a millivolt drop test. 

All he knows for sure is he threw parts at it and now it works, which is unfortunately, just luck. But still, he used the tools that he had available to try and narrow it down, which is all we can do.

It's one thing to not be able to find a problem, but throwing parts at stuff because you think troubleshooting is just a "needless test" is a fantastic way to get callbacks on your own dime and to piss off your customer.


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## varmit (Apr 19, 2009)

I would bet on the electronic overload relay being the problem. I replace bunches of them each year.

On a size 1 or 2 starter and overload, usually any FOP reading greater than 50 mv indicates a problem.


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## sparkiez (Aug 1, 2015)

I have not really seen anyone bring up taking an amp draw. Comparing the amp draw to each leg can give you a very quick clue as to what is happening.

Good job on troubleshooting.


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## RePhase277 (Feb 5, 2008)

Big John said:


> If that was your cause, that's a new one to me:
> 
> 280 mV @ 10 A = 0.028 Ω impedance.
> 
> That's nothing, it's certainly not gonna cause a severe enough current imbalance to trip the OL, especially since most of those seem to look for something like 25% or greater differential. You only saw a 5% imbalance.


But that resistance is going to dissipate 2.8 watts across it, compared to 0.3 watts for the next highest resistance. That doesn't sound like a lot, but it is going to get much hotter than the other poles. Will that not be a problem?


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## Big John (May 23, 2010)

RePhase277 said:


> But that resistance is going to dissipate 2.8 watts across it, compared to 0.3 watts for the next highest resistance. That doesn't sound like a lot, but it is going to get much hotter than the other poles. Will that not be a problem?


 Not in my opinion. A) That's not a lot of heat. I often find equipment in service dissipating dozens of watts, or in the event of a critical problem, hundreds of watts.

B) There don't seem to be any thermal-sensitive components here, so even if it is producing more heat, what's that heat affecting?


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## micromind (Aug 11, 2007)

varmit said:


> I would bet on the electronic overload relay being the problem. I replace bunches of them each year.
> 
> On a size 1 or 2 starter and overload, usually any FOP reading greater than 50 mv indicates a problem.


I agree, I've seen a bunch of electronic O/Ls fail over the years. I've replaced more than a dozen of them with the heater type and in every case, the problem was solved. 

Some of these replacements were more than 5 years ago and the heater type does exactly what it's supposed to do. A few have tripped on an actual overload or single phasing but every one has protected its motor just as it was designed to. 

Can't say the same for the electronic ones.......


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## MechanicalDVR (Dec 29, 2007)

micromind said:


> I agree, I've seen a bunch of electronic O/Ls fail over the years. I've replaced more than a dozen of them with the heater type and in every case, the problem was solved.
> 
> Some of these replacements were more than 5 years ago and the heater type does exactly what it's supposed to do. A few have tripped on an actual overload or single phasing but every one has protected its motor just as it was designed to.
> 
> Can't say the same for the electronic ones.......


Absolutely! I prefer the heater type O/L myself and always have.


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## Going_Commando (Oct 1, 2011)

sbrn33 said:


> Going_Commando said:
> 
> 
> > The motor is now running on a different starter happy as can be, that averages 30mV per pole for FOP.
> ...


Lol. You dont even know what I did for a test to realize I was doing it when the line was running, so the only downtime was swapping wires over to another starter. How do you figure I knew what the problem was the whole time? 

If you can get NEMA size 2 starters brand new for $350, I am all ears. i am looking at $625ish with aux contacts.


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