Tripping OCP on MDP without obvious problem

[MDECONTRACTOR]

Called to site on intermittent tripping of 175A OCP on mulch coloring machine; MDP is adjacent with 300A main and several ancillary loads other than the 175A feed for the entire machine. Problem described as an "instant trip, like a dead short" only occasionally and with no apparent rhyme or reason. Observation of equipment and several motor starts were flawless; could not repeat fault condition while onsite. The 175A feed runs several things on the machine: VFD for small compressor; conveyor motor; water pump; auger motor. The only time the 175A OCP trips is when the main motor is started...and very infrequently at that. I thought perhaps there was a mechanical issue causing locked rotor, but you would think it would not cause immediate facilitation of the OCP and perhaps the overloads to trip; furthermore, there would be some obvious mechanical noise associated with this "binding" of which the operator did not speak. As well, there is a motor starter that is not tripping at any time. Now, the nameplate data is a 75Hp motor @ 480V which 430-250 indicates is 96 FLA and could have a higher rated OCP for starting; however, this equipment worked all last year with the same OCP and no problems. Now, this is my question to all you tech minded folks: Does this sound like an internal motor issue? Perhaps a faulty armature? I took phase current readings and all were balanced and within normal range. I am not certain if a 3 phase AC motor would or could have the same consequence of a simple dc starter motor on a car having a bad armature and not being able to turn until past that spot. Sorry for the ignorance here, just looking for some thoughts and throwing out some insight.

 

[brian john]

Did you megger anything/everything?

Does the CB have adjustable instantaneous?

Can you test the CB?

Can you monitor the CB to capture current at time of trip?

The motor has an across the line starter?

What operates the starter, did you check all controls for possible multiple start stops almost simultaneously?

Did you check FOP during run?

Check all connections?

Can you did you perform IR during run?

 

[quogueelectric]

After that I have NOTHING to add that was awesome!!!!

 

[MDECONTRACTOR]

Did you megger anything/everything?

Does the CB have adjustable instantaneous?

Can you test the CB?

Can you monitor the CB to capture current at time of trip?

The motor has an across the line starter?

What operates the starter, did you check all controls for possible multiple start stops almost simultaneously?

Did you check FOP during run?

Check all connections?

Can you did you perform IR during run?

CB is not adjustable

Not sure on the testing procedure for a CB

There is no phase monitoring and I am looking into access to a data logger for 3phase...one of my first thoughts

Motor is line starting and pulled 200A on starting...FLA on nomenclature is 88A so I feel this value is fine

PLC controls process; however, the machine is brought online manually and set to auto. All ancillary loads were at rated speed when main motor was started (at time of fault condition and when I was present)

Megger testing was not an option at time of service as the equipment was not "down" and was running great. Couldn't shut down production to test..so no.

As with Megger testing, inspection of motor connections was not possible. all visible connections were integrous (i.e. panelboard/cb, motor starter, etc)

IR/FOP not familiar with the acronyms...Explain? New to the site

 

[brian john]

Not sure on the testing procedure for a CB

Requires speciality equipment and removal of CB for testing, depending on the price of the CB testing can be more expensive than replacement. In our case you would hire us or remove the CB bring it to us we test itt you return and install the CB my cost typically 300-500 dollars plus you labor.

There is no phase monitoring and I am looking into access to a data logger for 3phase...one of my first thoughts

I meant data logger

Megger testing was not an option at time of service as the equipment was not "down" and was running great. Couldn't shut down production to test..so no.

Need to arrange a shut down but if production is full time necessity back to data logger idea

IR/FOP not familiar with the acronyms...Explain? New to the site

IR, thermal imaging infrared

FOP, Fall of Potential test, millivolt drop across connections and CB, under load.

 

[sgunsel]

Did you say there are 4 other motors on the same 175A breaker?

 

[MDECONTRACTOR]

Did you say there are 4 other motors on the same 175A breaker?

Yes, there are several other loads on the breaker; I thought that the OCP was too small for the combined loads initially (if you do the calculations it makes sense) but the code states you utilize the smallest OCP that allows starting. This CB worked fine for the last 3yrs and only recently began tripping. I need to add this as well, it trips very rarely and at no particular time (i.e. only on first start of day; only after running over time; only under load, Etc.) Is that where you were going, or did you have another thought?

 

[templdl]

You have provided very little incormation about the distribution system.
It appears as though the main is tripping intantaneously either by instantaneous or GF pick up.
What is the the voltage if the ditribution system?
What is the main breaker rating? What if the intantaneous set at?
Does the main breaker have ground fault protection? If so what is it'sm setting.

The down steam device may not be coordinated with the main breaker if it is even able to be.
Often time a down up stream OCPD can no coordinate with a down stream device should a shrt circuit current be great enough to come with in the trip pick up of both devices. So it wouold be a race which on will actually trip.
Ifi the main has GFt is not uncommon to trip a main should there be a GF as the GF current often is not great enough to trip the down steam OCPD on instantaneous but high enough to trip the main GF,

 

[MDECONTRACTOR]

Requires speciality equipment and removal of CB for testing, depending on the price of the CB testing can be more expensive than replacement. In our case you would hire us or remove the CB bring it to us we test itt you return and install the CB my cost typically 300-500 dollars plus you labor.



I meant data logger




Need to arrange a shut down but if production is full time necessity back to data logger idea





IR, thermal imaging infrared

FOP, Fall of Potential test, millivolt drop across connections and CB, under load.

I have not done an FOP or IR scan. I would assume the results of the FOP across connections and CB should be negligable? What would you see as a typical resultant?

In an armature with an open coil, would you typically see a locked rotor condition as the motor would not have enough impetus to start?

 

[brian john]

I have not done an FOP or IR scan. I would assume the results of the FOP across connections and CB should be negligable? What would you see as a typical resultant?

In an armature with an open coil, would you typically see a locked rotor condition as the motor would not have enough impetus to start?

With a 3 phase load you should have similar readings pole to pole 35-85 millivolts (MAYBE)

If you can FOP all connections then IR is not as important.

 

[MDECONTRACTOR]

Thanks for your input, Brian. It has been helpful. I have yet to go back onsite, but will check this and do a megger test when able.

Any thoughts on the armature question I posed?

 

[AV ELECTRIC]

You must eliminate the possibility of the breaker malfunctioning as the other post said you can get it tested get a new or refurbished breaker or maybe you have one of equal size in the panel that you can swap temporarily . Pull the breaker out visually inspect smell for any burnt odors . Check to see if its getting extremely hot , burnt odor and arcing sound under load.

 

[MDECONTRACTOR]

You must eliminate the possibility of the breaker malfunctioning as the other post said you can get it tested get a new or refurbished breaker or maybe you have one of equal size in the panel that you can swap temporarily . Pull the breaker out visually inspect smell for any burnt odors . Check to see if its getting extremely hot , burnt odor and arcing sound under load.

As with the megger test I have not been able to take out of service.

 

[ssynoground]

I have a similar issue with a 150A / 3-pole / SQD / I-line / breaker, tripping whenever I'm not around. It feeds parking lot pole lights. I could not find any smoking evidence but, when the lights were on for about an hour, I checked the breaker and it was warm. I decided to turn it off but, the breaker moved to the "trip" position and would not return to the "off" position. I noticed the instantaneous setting was at the halfway point so, I turned it to max setting. Once the breaker was off for an hour, I turned it back on and waited for the temp to go back up. It never did reach the same temp. I wonder if the adjustment caused this effect? I checked amps on all 3 poles and none of them were over 65% or 90 Amps. One more thing, the breaker began tripping more frequently once they added two more lighting loads. Finally, the 150A breaker that is tripping, is located above the MDP's 800A Main Breaker and, right below a 200A / 3-pole / breaker. I want to move the breaker to another spot in the panel and possibly replace the breaker altogether. Am I barking up the right tree?

 

[brian john]

I have a similar issue with a 150A / 3-pole / SQD / I-line / breaker, tripping whenever I'm not around. It feeds parking lot pole lights. I could not find any smoking evidence but, when the lights were on for about an hour, I checked the breaker and it was warm. I decided to turn it off but, the breaker moved to the "trip" position and would not return to the "off" position. I noticed the instantaneous setting was at the halfway point so, I turned it to max setting. Once the breaker was off for an hour, I turned it back on and waited for the temp to go back up. It never did reach the same temp. I wonder if the adjustment caused this effect? I checked amps on all 3 poles and none of them were over 65% or 90 Amps. One more thing, the breaker began tripping more frequently once they added two more lighting loads. Finally, the 150A breaker that is tripping, is located above the MDP's 800A Main Breaker and, right below a 200A / 3-pole / breaker. I want to move the breaker to another spot in the panel and possibly replace the breaker altogether. Am I barking up the right tree?

It would seem like your issue is related to the long time operation and the adjustments you made are related to the instantaneous operation. One has nothing to do with the other. In addition if the CB setting was mid range it would seem the instantaneous MAY HAVE been set there for a reason possible a coordination study was completed and your adjusting a predetermined setting is not a wise move.

It is possible that adding the other CB's may have minimized heat dissipation from the existing CB, which led to the thermal trip; additionally you may not have waited long enough with your touch test.

Had you performed a VD/FOP, Fall of Potential test you may have been able to isolate the problem.

 

[zog]

It would seem like your issue is related to the long time operation and the adjustments you made are related to the instantaneous operation. One has nothing to do with the other. In addition if the CB setting was mid range it would seem the instantaneous MAY HAVE been set there for a reason possible a coordination study was completed and your adjusting a predetermined setting is not a wise move.

I love the "It was not cranked all the way up" theory of troubleshooting

 

[Jraef]

Thanks for your input, Brian. It has been helpful. I have yet to go back onsite, but will check this and do a megger test when able.

Any thoughts on the armature question I posed?

There is no "armature" on a 3 phase motor. There is a rotor, but no direct electrical connections to it. The windings in an AC squirrel cage motor are only in the stator (non-moving part) of the AC motor.

You may have a bad winding in the stator. The intermittent tripping may be related to the phase angle of the power at any given start moment causing the short circuit to have no other impedance from adjacent phases. In other words, there is s very small window of time in which all the phase angle conditions are wrong for your failing motor, and every now and then you hit that window when a start command is given. Your megger test will likely find it if that's the case. You may also have a nick or pin hole in a wire and only when there is some condensation in a conduit does it find a path to ground, the megger would find that too.

If the fault is a short circuit of some sort, a motor starter overload would NOT be the device to pick that up by the way. That is what the instantaneous trips on your circuit breakers are for. What happens a lot is that the MCP breaker (magnetic only) in the motor starter has adjustable instantaneous trips and when commissioning was done, someone turned them up high enough to avoid the nuisance tripping. On a motor with 88FLA, that could be as high as 1144A. If you then have non-adjustable mag trips on the 175A breaker, AND they are factory set for 1000% the rating, that is 1750A so theoretically no problem. But some thermal-mag breakers have factory mag trips settings as low as 400% of rated, so it might be as low as 700A. If you have a local starter MCP set at 1144A and a main in the MDP factory set at 700A, guess which one is going to trip first?

 

[ssynoground]

There have been multiple service techs to the store troubleshooting the problem mentioned by my post. We have narrowed it down to: (1) a possible breaker replacement; (2) megg out every conductor for a possible leak.

 

[skeshesh]

Jraef brings up a good point. Try to get a hold of the coordination study report if they've had one done for the facility. If not right down the product identification for the 175 breaker and the breaker upstream from it. Either dig up the trip curves online or contact manufacturer for the info. and compare the information. A complete coordination study for a facility is tricky but should be easy enough to compare at least those two breakers. Unwanted tripping could even occur due to miscoordinations with breakers other than one immediately upstream of the 175A but your best guess is the closest one and it should take too much time to check. Good luck.