MOLDED CASE BREAKER KEEPS TRIPPING AFTER 8 HOURS

TrueRMS

Member
Location
Windsor Colorado
So gents lets see if someone can help with this one.

480v 400 amp molded case breaker (1950's vintage) feeding a motor with a FLA of 219 and operating at a current level of 187 amps steadily has been tripping after 8 or so hours. The contactor has overloads on each phase and are sized appropriately and are not taking the motor out. Motor and cable have been meggered and are good. voltage drop from line side of the breaker and the bottom of the contactor is in the range of .578 volts and contacts have been cleaned and filed. No current difference measured from the same points. Other than heating or a weak breaker I am scratching my head. Using a thermo camera we see a slight temp difference on the center phase in the 50degF range. Breaker holding during starting currents of 1600+ amps with no issue.
 

Strathead

Senior Member
So gents lets see if someone can help with this one.

480v 400 amp molded case breaker (1950's vintage) feeding a motor with a FLA of 219 and operating at a current level of 187 amps steadily has been tripping after 8 or so hours. The contactor has overloads on each phase and are sized appropriately and are not taking the motor out. Motor and cable have been meggered and are good. voltage drop from line side of the breaker and the bottom of the contactor is in the range of .578 volts and contacts have been cleaned and filed. No current difference measured from the same points. Other than heating or a weak breaker I am scratching my head. Using a thermo camera we see a slight temp difference on the center phase in the 50degF range. Breaker holding during starting currents of 1600+ amps with no issue.
I would say it is a no-brainer to replace the breaker. You can offer the customer to have the original breaker tested after replacement or not.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
might be an intermittent short in the motor or wiring. they can sometimes show up when the motor is warmer and not be there when it is cooler.

meggering often shows this kind of thing but not always.

I'd be inclined to get a recording meter of some sort and see if the current shows a spike when the CB trips before I would screw around with a new breaker.
 

JFletcher

Senior Member
Location
Williamsburg, VA
So gents lets see if someone can help with this one.

480v 400 amp molded case breaker (1950's vintage) feeding a motor with a FLA of 219 and operating at a current level of 187 amps steadily has been tripping after 8 or so hours. The contactor has overloads on each phase and are sized appropriately and are not taking the motor out. Motor and cable have been meggered and are good. voltage drop from line side of the breaker and the bottom of the contactor is in the range of .578 volts and contacts have been cleaned and filed. No current difference measured from the same points. Other than heating or a weak breaker I am scratching my head. Using a thermo camera we see a slight temp difference on the center phase in the 50degF range. Breaker holding during starting currents of 1600+ amps with no issue.
Welcome. What is this motor powering? Any chance the load is varying at ~8hrs causing an overload/trip? How long has the problem persisted?

Do you have another breaker to swap out? If there is a 2nd motor with the same breaker, I'd swap breakers to see what happens. Data logger?
 

templdl

Senior Member
Location
Wisconsin
Have you talken a thermal image of the breaker? Loose load connection generate heat, this connection being close to the thermal re elements of the breaker will cause the breaker to derate itself and trip.
Immediately after a trip can the breaker be latch/reset and then closed or do you have to wait before doing do? This is and indicator of a thermal trip.
It is common for a breaker never to be open and closed during its entire life cycle. Exercising a breaker by simply opening and closing it numerous times often reduces the contact resistance. Try to exercise the breaker. In doing do as the moving contact closes against the stationary contact the moving contact actually rubs against the stationary contact.
You can please the contacts if you would like but never file or sand the contacts as it will destroy the surface finish. The only time to do do is in a last resort in an emergency knowing the you will be replacing the breaker immediately.
Yes, while energized and under load by measuring the the line to lo as d voltage across each set of contacts will provide s voltage drop indicator. You then can compare the voltag drops for each pole. The are no bench mark voltage measurements thst I'm aware of but an experience electrician may be able to interpret the voltage drop values to help him troubleshoot. Measuring contact resistance is not a valid way to evaluate contact integrity.
Also, heating of the contact(s) can weaker the moving contact spring causing an increase in contact heating.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Have you talken a thermal image of the breaker? Loose load connection generate heat, this connection being close to the thermal re elements of the breaker will cause the breaker to derate itself and trip.
Immediately after a trip can the breaker be latch/reset and then closed or do you have to wait before doing do? This is and indicator of a thermal trip.
It is common for a breaker never to be open and closed during its entire life cycle. Exercising a breaker by simply opening and closing it numerous times often reduces the contact resistance. Try to exercise the breaker. In doing do as the moving contact closes against the stationary contact the moving contact actually rubs against the stationary contact.
You can please the contacts if you would like but never file or sand the contacts as it will destroy the surface finish. The only time to do do is in a last resort in an emergency knowing the you will be replacing the breaker immediately.
Yes, while energized and under load by measuring the the line to lo as d voltage across each set of contacts will provide s voltage drop indicator. You then can compare the voltag drops for each pole. The are no bench mark voltage measurements thst I'm aware of but an experience electrician may be able to interpret the voltage drop values to help him troubleshoot. Measuring contact resistance is not a valid way to evaluate contact integrity.
Also, heating of the contact(s) can weaker the moving contact spring causing an increase in contact heating.
I agree with all of the above, but just want to finish that thought on the issue of immediate reset or having to wait.

If the breaker DOES allow you to reset immediately after tripping, that then is an indicator that it tripped on an INSTANTANEOUS (magnetic trip) fault. Some of the things that can cause that in something old is a breakdown of insulation somewhere in the system, especially as the conductors heat up. Could be the motor windings, could be the conductors, even a termination. I've had several situations where vibration cause the motor connections to rub against the peckerhead and wear off the insulation, but it only shorts to ground under just the right conditions. I've had a couple where water was getting into the conduit, but only after specific conditions, one of which was a refer compressor and after it had been running for hours, the ice build-up around the coils would start to get so thick that the outer layers would not be cold enough and melt, which ran down into the conduit by wicking action. THAT one was a bear to find, because usually by the time we got from the control building to the refrigeration shed, the external icing was already gone. The point is that if it is NOT a thermal trip situation, you are going to have to start looking at the circuit conditions.

PS: Just re-read your OP and saw "Motor and cable have been meggered and are good." So if it is a mag trip, it's going to be something oddball... A recording meter would be helpful with identifying this because if you know the exact time it trips, you can look for other things that might be contributing.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
FWIW, I have had a 100 well motor that would run fine for hours then Shut down. Everything good in the panel & motor. I was watching the current draw when the lower bearing in the well locked up. Brief but surprising ride as the concrete pad I was on did a dance under my feet.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
FWIW, I have had a 100 well motor that would run fine for hours then Shut down. Everything good in the panel & motor. I was watching the current draw when the lower bearing in the well locked up. Brief but surprising ride as the concrete pad I was on did a dance under my feet.
That'll do it too!

Another one I thought of, a "bouncing" run command signal. It opens, then recloses very fast, which causes a huge current spike when the contactor reconnects into the motor before the magnetic fields have a chance to collapse which gives you the equivalent of connecting two generators out of synch. Had that happen on a 600HP compressor at a glass plant. Looked and looked and looked for the cause for an entire day, then I put a recorder on it and saw the spike, it was exactly 30 minutes after starting, every time. Turned out to be because the electronic compressor control panel had a "Maximum Run Time" in its software that was supposed to prevent it from running unloaded for too long, but the plant was not using it that way and never told the compressor service guy, so it was left like that. So after 30 minutes, the timer would open the run contact, then because the plant control PLC was still wanting it, it immediately reclosed it, but there was just enough time in there to drop out the starter and restart it. That was a fused disconnect on the starter, they probably wasted $10K in fuses before I found that glitch.
 

templdl

Senior Member
Location
Wisconsin
I agree with all of the above, but just want to finish that thought on the issue of immediate reset or having to wait.

If the breaker DOES allow you to reset immediately after tripping, that then is an indicator that it tripped on an INSTANTANEOUS (magnetic trip) fault. Some of the things that can cause that in something old is a breakdown of insulation somewhere in the system, especially as the conductors heat up. Could be the motor windings, could be the conductors, even a termination. I've had several situations where vibration cause the motor connections to rub against the peckerhead and wear off the insulation, but it only shorts to ground under just the right conditions. I've had a couple where water was getting into the conduit, but only after specific conditions, one of which was a refer compressor and after it had been running for hours, the ice build-up around the coils would start to get so thick that the outer layers would not be cold enough and melt, which ran down into the conduit by wicking action. THAT one was a bear to find, because usually by the time we got from the control building to the refrigeration shed, the external icing was already gone. The point is that if it is NOT a thermal trip situation, you are going to have to start looking at the circuit conditions.

PS: Just re-read your OP and saw "Motor and cable have been meggered and are good." So if it is a mag trip, it's going to be something oddball... A recording meter would be helpful with identifying this because if you know the exact time it trips, you can look for other things that might be contributing.
I agree. My post was too darned long and I omitted the short circuit/magnetic/instantaneous aspect because of the time frame given which is usually a thermal trip. Something is causing the .thermal/bimetalic elements to deflect snd eventually activate the trip bar. There are many easons that cause that to happen. I forgot that a high amdient temperature can cause this elements to start to deflect with a load those elements will e loser to the trip bar, close enough that it could trip the breaker. This is often is refered to as derating.
I agree that this probably is not the case here but never the less should be pointed out as quite often the details are left out which leads to guessing.
 

TrueRMS

Member
Location
Windsor Colorado
I agree. My post was too darned long and I omitted the short circuit/magnetic/instantaneous aspect because of the time frame given which is usually a thermal trip. Something is causing the .thermal/bimetalic elements to deflect snd eventually activate the trip bar. There are many easons that cause that to happen. I forgot that a high amdient temperature can cause this elements to start to deflect with a load those elements will e loser to the trip bar, close enough that it could trip the breaker. This is often is refered to as derating.
I agree that this probably is not the case here but never the less should be pointed out as quite often the details are left out which leads to guessing.
We Did use a thermo camera to find heating but the breaker usually trips after hours. Slight evidence of heating in the 50degf range on the line side of the contactor not the breaker itself. Also the current is even across all phases and differential current from line to motor leads is zero. The motor in question is new and is running a river make up water pump. When I said filed the contacts it was with a diamond emery board so the silver is still intact. Changing this breaker would be a nightmare with lock out tag out because of door interlocks. The contactor/breaker setup is in a rather small enclosure with little to no ventilation. Was thinking of adding small axil fans top and bottom. As far as the bearings before I started testing I was able to spin the shaft by hand. The start command is has a seal in contact and a holding relay. All controls have been proved
 

TrueRMS

Member
Location
Windsor Colorado
We Did use a thermo camera to find heating but the breaker usually trips after hours. Slight evidence of heating in the 50degf range on the line side of the contactor not the breaker itself. Also the current is even across all phases and differential current from line to motor leads is zero. The motor in question is new and is running a river make up water pump. When I said filed the contacts it was with a diamond emery board so the silver is still intact. Changing this breaker would be a nightmare with lock out tag out because of door interlocks. The contactor/breaker setup is in a rather small enclosure with little to no ventilation. Was thinking of adding small axil fans top and bottom. As far as the bearings before I started testing I was able to spin the shaft by hand. The start command is has a seal in contact and a holding relay. All controls have been proved
We have never been standing at the breaker when it trips because the location is off site so resetting the breaker has always been done hours later
 

TrueRMS

Member
Location
Windsor Colorado
Welcome. What is this motor powering? Any chance the load is varying at ~8hrs causing an overload/trip? How long has the problem persisted?

Do you have another breaker to swap out? If there is a 2nd motor with the same breaker, I'd swap breakers to see what happens. Data logger?
We have an identical redundant pump set up with all the exact same controls. Same pit with common header and check valves. Check valves are functional. Pump impellers are guarded with multiple screens before the pit so its unlikely that any debris is locking up the impeller.
 

templdl

Senior Member
Location
Wisconsin
We have never been standing at the breaker when it trips because the location is off site so resetting the breaker has always been done hours later
Yes, how is going to stand around waiting for hours for a trip unless you were working in the area or someone else was who could be a reliable observer that would follow your directives in the proper set an closing of a breaker and it's response, such as could it be reset immediately after the trip or not.
I had and unusual situation where a 400a breaker in sewerage lift station was tripping frequently at 5:00am. The breaker had an old technology peak sensing electronic trip unit which can respond to a voltage a spike. A voltage spike results in a current spike high enough to trip the breaker instantaneously. I could have put a recorder on the line as a monitor but instead of going through that trouble I concluded that there must be a power factor correction capacitor switching by the POCO which was causing a voltage spike. . I gave my customer an basic 400A nonelectronic TM breaker as a replacement and problem solved.
In your case it is important to determine if the breaker is tripping, thermal or magnetically. If thermally is the heat being generated in or around the breaker causing it to derate or because of the load by which you attention is directed to the motor as pointed out by the other posters. You certainly don't want to assume that it is a motor (load) issue until you know how/why the breaker is tripping first.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
... I concluded that there must be a power factor correction capacitor switching by the POCO which was causing a voltage spike. ...
And let me guess, they denied it repeatedly, right?

I once had to investigate a chemical plant outside of Las Vegas that was losing Toshiba VFDs at an alarming rate. Toshiba is not known for making junk, their drives are as good as any on the market, so there had to be something else. My Dranetz showed a lot of periodic high voltage transients, consistent with capacitor switching, but this site had no caps. I asked the PoCo, they denied using caps to boost their line voltage. Deny, deny, deny. Finally on one trip to the site, I took notice of the power lines feeding it, followed them as best I could and lo and behold, about 1/2 mile from the plant was a huge bank of caps on the PoCo lines, sitting on a platform supported by their tower high in the air. I asked them again, got the denial, then showed them the photo and asked what these were and where they were located. They claimed, later, that they must have been something left behind from a previous end user near by, and there were no records of them in their files. But a month later I went out again and looked at them, "someone" had added a set of air core reactors to them. It did fix the spike problem but if the PoCo didn't own them, why wouldn't they just remove them instead of fix it, or why would "someone" fix PFC caps they had abandoned??
 

templdl

Senior Member
Location
Wisconsin
And let me guess, they denied it repeatedly, right?

I once had to investigate a chemical plant outside of Las Vegas that was losing Toshiba VFDs at an alarming rate. Toshiba is not known for making junk, their drives are as good as any on the market, so there had to be something else. My Dranetz showed a lot of periodic high voltage transients, consistent with capacitor switching, but this site had no caps. I asked the PoCo, they denied using caps to boost their line voltage. Deny, deny, deny. Finally on one trip to the site, I took notice of the power lines feeding it, followed them as best I could and lo and behold, about 1/2 mile from the plant was a huge bank of caps on the PoCo lines, sitting on a platform supported by their tower high in the air. I asked them again, got the denial, then showed them the photo and asked what these were and where they were located. They claimed, later, that they must have been something left behind from a previous end user near by, and there were no records of them in their files. But a month later I went out again and looked at them, "someone" had added a set of air core reactors to them. It did fix the spike problem but if the PoCo didn't own them, why wouldn't they just remove them instead of fix it, or why would "someone" fix PFC caps they had abandoned??
Go figure. Yours was a bit more difficult to resolve than my problem as got me it was a calculated guess and how the electronic trp breaker was responding to it.. Simply changed the breaker to a basic TM breaker.
With your application those VFDs were another issue. ITT goes to show that a knowledge of a distribution system and how it all works together pays off when troubleshooting.
How many would suspect a voltage transient and go out as you did and trace the power lines for a PVC bank? For more me it was the consistency of the time of day the breaker tripped.
 
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