# Megging motor feeders



## SparkyMcSparkerson

Can you meg motor feeders by disconnecting wires at MCC and simply opening the motor disconnect on the load side to eliminate the possibility of megging motor windings? I am obviously testing the wire feeder insulation. Would this practice interfere with a rendering a good ohm reading?


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## joebanana

Sure, unless you want to un-land them. Couldn't hurt to megger the disco at the same time. If you get wonky readings make sure the disco is clean.


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## gpop

unless its on a vfd or soft start most of us meg while its attached to the mcc and disconnect. Only when you see a problem do you start to break the problem into smaller pieces and follow the problem with the meter. 

Are you doing a PM or do you have a problem you are tracing down?


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## 460 Delta

Like gpop says, clip on at the MCC all connected up and crank away, if it reads good you know the system is good to go. If the measurement is wobbly, then start breaking it down until you find the culprit.


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## MDShunk

I do it that way, very often, on purpose. It checks several birds with one stone, so to speak. Same goes for meggering the motor. I'll check it from the load side of the local disconnect and that'll check the conductors in the flex from the disco to the motor. Divide and conquer if you get measurements you don't like. I avoid taking the peckerhead apart at all costs unless it really comes down to no choice.


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## paulengr

You can megger a soft start or VFD if you don't exceed DC bus voltage. The danger particularly with soft starts is there is an effect called self commutation where you can trigger it into conducting from overvoltage. There is overvoltage protection too but then you're not testing insulation. With a long test like PI you can burn it up if you self commutate it.

Don't use hand cranked megger regardless of whether it's a drive or not. No consistency in readings when doing a PM. The instructions for insulation resistance can't be clearer on that. Today's procedure says to apply voltage and take readings after 1 minute. The minute charges the insulation capacitance. In reality I do megger readings all the time but I don't wait 1 minute. I know I'm looking for 5 or 100 megaohms. If it's dead shorted or instantly shoots way above the minimum I'm done. It's only for PM purposes or doing PI that I go the extra distance. I hardly ever have one so close I got to get out the temperature chart.

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## paulengr

And yeah...test at MCC with local disconnects closed. If it passes, done. If it fails disconnect leads or open disconnect. Test both ways at disconnect if it has one to localize the problem(s). Then unwire at peckerheads if it's on motor side and repeat. If it still fails, good motor feed and bad motor. If it passes, it's not insulation resistance. Begin checking other issues.

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## glen1971

Until you get into narrowing down an issue, I wouldn't be disconnecting wires right off.. There has been more than one motor in the industry all of a sudden start going backwards after someone did maintenance in an MCC cubicle or starter.. 
To megger the full length of feeders, you also need to megger from the disconnect to the motor..


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## paulengr

glen1971 said:


> Until you get into narrowing down an issue, I wouldn't be disconnecting wires right off.. There has been more than one motor in the industry all of a sudden start going backwards after someone did maintenance in an MCC cubicle or starter..
> 
> To megger the full length of feeders, you also need to megger from the disconnect to the motor..




This I would have to see. You turn Power off to do insulation resistance (Megger) but you had one start turning from disconnecting powered off wiring? How? Where is the energy coming from?


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## gpop

paulengr said:


> This I would have to see. You turn Power off to do insulation resistance (Megger) but you had one start turning from disconnecting powered off wiring? How? Where is the energy coming from?
> 
> 
> Sent from my iPhone using Tapatalk



I think you missed what he was saying.

If someone disconnects the wires and doesn't mark them for megging then there is a chance they will land them incorrectly so the motor will run backwards.

To the fire call on the next shift "it a motor that suddenly started running backwards"


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## gpop

If im megging from the mcc i tend to use a ohm meter to see if the disconnect is closed as i want to test everything in one go plus im lazy so i only want to test one wire. 

It wouldn't be the first time ive chased a short to find its the plastic things with the clips that join the line and load side on the older disconnects. Load megs good, line megs good, close disconnect and everything megs bad. (no idea what they are called)


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## paulengr

You mean the phase barriers are that contaminated?


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## gpop

paulengr said:


> You mean the phase barriers are that contaminated?
> 
> 
> Sent from my iPhone using Tapatalk


First time trying to post a pic

what do you call the black pieces on the bar?


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## 460 Delta

Paul, you say instructions say to not use a hand crank or MG Megger for insulation, what instructions? I’ve used my hand crank Biddle now for 30 years without a hitch. It’s not lied to me yet.


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## SparkyMcSparkerson

Thank y’all! 
I will mark wires and ensure proper rotation. This is for preventative maintenance if that is what PM stands for?
Thanks again!


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## SparkyMcSparkerson

I always mark & make note of the motors rotation. If there is any uncertainty for any reason I simply bump the effected motor again. If it is something that is submerged in water or difficult to see or check I always pick the brains of competent maintanance workers familiar with their equipment. Many times they have videos and pictures to show as a useful tutorial. Then I become familiar. Been down that road though.
Thanks again.
This is becoming quite a useful website(another useful tool) as I do not have many to ask that know this stuff at my company.


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## Lone Crapshooter

I was cautioned one time that if you are working with a Allen Bradley smart MCC with the solid state overloads you should disconnect the "T" leads before megging the motor.
LC


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## Lone Crapshooter

I was cautioned one time that if you are working with a Allen Bradley smart MCC with the solid state overloads you should disconnect the "T" leads from the overload block before megging the motor.
LC


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## MDShunk

gpop said:


> First time trying to post a pic
> 
> what do you call the black pieces on the bar?


The picture is shadowy, but I'd imagine you're probably asking about arc chutes.


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## glen1971

glen1971 said:


> Until you get into narrowing down an issue, I wouldn't be disconnecting wires right off.. There has been more than one motor in the industry all of a sudden start going backwards after someone did maintenance in an MCC cubicle or starter..
> To megger the full length of feeders, you also need to megger from the disconnect to the motor..





paulengr said:


> This I would have to see. You turn Power off to do insulation resistance (Megger) but you had one start turning from disconnecting powered off wiring? How? Where is the energy coming from?
> 
> 
> Sent from my iPhone using Tapatalk


After "someone" has unhooked the T Leads in the MCC Cubicle or starter in a de-energized cell, meggered them, and not paid attention to where they came from and after re-terminating them, inadvertently reversed the rotation of the motor by landing them on the wrong terminals..

The "energy" you are looking for comes when the motor is put back into service...


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