Interesting Megger Readings

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What type of environment are you working in? I've seen readings track down the insluation in a soldering room in a factory. The flux and acids on the outside of cables would transmit voltage that you could read with a vom but not a wiggie. A megger is much more sensitive and might show a 0 ohm reading.
 
SEO said:
What type of environment are you working in? I've seen readings track down the insluation in a soldering room in a factory. The flux and acids on the outside of cables would transmit voltage that you could read with a vom but not a wiggie. A megger is much more sensitive and might show a 0 ohm reading.
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The type of enviornment is a cement plant. The installation is fairly new (about 1yr). I'm going to check the cables tomorrom to make sure the outside of the cables are clean and that there is no tracking.

It is possible that the cables are submerged in water in an underground ductbank.
 
mull982 said:
The type of enviornment is a cement plant. The installation is fairly new (about 1yr). I'm going to check the cables tomorrom to make sure the outside of the cables are clean and that there is no tracking.

It is possible that the cables are submerged in water in an underground ductbank.
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I would think that the wires should be dry if thier in a duct bank even if it goes thru water.
 
Kiss

Kiss

Keep It Simple S-----!
Take a DVM and measure ohms phase to phase and phase to ground. If you are reading less than 100 ohms or greater then it is control wiring or maybe a surge suppressor. I agree with the rest of the panel, there is some control circuit or control transformer that you are reading that has not been isolated causing the low megger readings. Otherwise energizing it would cause breakers to trip or burnt cabling/ components.
 
brian john said:
With unexpected readings I

Always question my self and review everything I am doing.

Second question my test equipment and if possible retest and/or use a second piece of test equipment.

3rd I question the equipment I am testing.
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Sound advice Brian.
 
mull982 said:
It is possible that the cables are submerged in water in an underground ductbank.
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That might explain a lot. Still suspect "operator error", but they could be running under water while energized while not tripping you breakers, but still having the insulation damaged.
 
There's still something missing here.
I never saw an answer about the type of megger being used. We have push button unit at work, and I HATE it, I don't trust it, especially on long runs. I'd trade my left pinky for the old hand crank I "grew up on". Does the reading change if the meter is left on over time? (Can you leave it on?) How about readings at 500v? ( We never meg over rated insulation value in house! ) If it's a hand crank does the feel of the crank change? How about DMM readings on the both sides of the cable, checking for voltage drop?

Good Luck,
Doug S.

PS Any one got a hand crank megger they want to get rid of? =)
 
Doug S. said:
There's still something missing here.
I never saw an answer about the type of megger being used. We have push button unit at work, and I HATE it, I don't trust it, especially on long runs. I'd trade my left pinky for the old hand crank I "grew up on". Does the reading change if the meter is left on over time? (Can you leave it on?) How about readings at 500v? ( We never meg over rated insulation value in house! ) If it's a hand crank does the feel of the crank change? How about DMM readings on the both sides of the cable, checking for voltage drop?

Good Luck,
Doug S.

PS Any one got a hand crank megger they want to get rid of? =)
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We have used both the pushbutton digital meggers and a hand crank megger. They left the megger on for about a minute and came up with those readings. Maybe we need to try longer say a couple of minutes to ride through any charging and absorbtion current?

The needle on the hand crank megger initially goes over to infinity but then pings back over to zero when performing the test.

We did not try meggering at 500V but it might be worth a shot. With the lower voltage I would expect the leakage current to be less but still provide the same insulation resistance reading.

I'm not following what you are saying about using a DMM to check for a voltage drop. Are you saying to check the end of the cable with a DMM after the megger voltage is applied to see if there is a voltage drop at the end of the cable?
 
mull982 said:
I'm not following what you are saying about using a DMM to check for a voltage drop. Are you saying to check the end of the cable with a DMM after the megger voltage is applied to see if there is a voltage drop at the end of the cable?
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While megging, see what the voltage is at either end of the wire, just a potential way to confirm megger readings. About DMM, sorry, I guess I said DMM because my Fluke 87 is the first thing I grab, most of the time. Basically is the wire really presenting a "load", or is the megger flakey.
It sounds like you may have something going on with your wire/cable. Have you ran the motor since your readings and clamped things? If it runs, draw readings on both ends would really nail things down.
Turn the breaker on w/ a looong stick, shut your eyes really tight, and wince when as the breaker folds over. Don't hold the stick too tight either. ;)

Regards,
Doug S.
 
mull982 Disconnect both ends of your feeders and isolate conductors on one end. Apply 120 volts to each conductor one at a time and take an amp reading. Use a small in line fuse . If amp readings are satisfactory on individual conductors apply 240 volts across pairs. If you have a bolted fault it will show up. Use small quick blow fuses. Also take voltage reading on insulation with 120 volts applied to conductor. Did you notice any problem with the motor before you did the megger test?
 
bth0mas20 said:
I would think that the wires should be dry if thier in a duct bank even if it goes thru water.
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It would be some what rare to find a dry underground duct bank....a large portion will be water filled.
 
Doug S. said:
Have you ran the motor since your readings and clamped things? If it runs, draw readings on both ends would really nail things down.

Like I mentioned this is an installation that is about a year old and been in service for a year. The motor has been running for a year and we went back in on a down day to get these megger readings, because we are starting to trend these readings.

After seeing these readings we were hesitant to turn the motor back on, but we did with no problem and it has been running since. It is a conveyor that is used to fill a bin, so the motor may be stated and stopped daily depending on how fill the bin gets.

I will take current readings while it is running to see what I get.
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Obviously, the wires to your motor can?t be grounded or you?d have real trouble. Some condition is being overlooked. Get everyone available with any electrical knowledge, even just a little, take them to the location, describe the mystery, and have them brain storm it. Usually the person least expected to, sees something, or asks a question leading to the solution.
 
mull982 said:
Would a long distance cause readings such as these?
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if you have long distances, and large feeders (500 mcm qualifies in my book)
it can take a short while for the megger to indicate true resistance,
especially if it's one of the digital one's like the fluke multimeter/megger
units.

you are charging up the insulation, and i've seen it take up to one
minute with my fluke on 1kv. however, it'll start showing *something*
pretty quickly..... with a 5kv or 50kv such as a utility might use,
the normal test time for a megger is a 5 minute test.

hope that helps.


randy
 
Could induction cause bunk digital megger readings? Say if the cables are runing in tray or conduit with other high current cables nearby?

Is that even possible?
 
K2500 said:
Could induction cause bunk digital megger readings? Say if the cables are runing in tray or conduit with other high current cables nearby?

Is that even possible?
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Sure it can, the capacitive charging current is part of any megger reading that takes a while to dissipate ina cable run, thats why the readings start low and come up after that. Megger test should be done for a minimum of 1 minute (10 for inductive equipment)
 
This motor continues to run and each down day that we have once a week we check the megger readings again and see the same results.

One thing that occured to me today was that if we possibly had an ungrounded source, or ungrounded wye on our distribution transformer then we would not see any fault current for a Line to ground fault. We would still see low megger readings to ground on a particular phase but would not have ground fault current when energized thus not trip any breakers.

However due to the fact that we have multiple phases reading to ground we would then have a L-L fault which would surely appear somewhere.

Just taking a shot in the dark, but would it be possible that the avaliable fault current for this particular area is not enought to be seen by a breaker if we do indeed have a fault?

Another test that I'm going to perform when we re-energize this, is take a voltage reading between the phases and ground. If any of the phases are grounded we should see 0V between the phase and ground similar to what you would see on an ungrounded system.
 
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