nirvic23

i have a 3 phase motor,need to check insulation resistance.can you please tell me how to connect probes and what reading should i get if the motor is good?

p_logix

A megger is used to read from phase (winding) to ground. This checks for breakdown of insulation of motor windings. The rule of thumb is 1000ohms per volt. Then .5 or half a meg to ground would be acceptable for a 480 volt motor.

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p_logix
"Meggers Don't Lie, Electricians Lie". Go LAKERS!!!!

Zog

Be careful of "Rules of thumb", they are usually wrong.

Here is the NEMA spec, all valkues need to be temperature corrected to 40 degrees C (Very important, otherwise your readings are meaningless)

Minimum IR readings:
· IR 1 min = 100MW for DC armature and AC winding built after 1970
· IR 1 min = 5MW for most machines and random-wound stator coils and form wound coils rated below 1kV
· IR 1 min = kV+1 for machines made before 1970, all field windings, and others not listed above

As you can see 0.5M would never be considered acceptable. You also need to calulate the DAR or PI ratios, depending on the size of the motor.

wptski

Nice breakdown! How about minimums for cables below 600V like NM or even simple cords? I've searched and never found anything for cables.

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Bill

Zog

600V rated cables should be tested at 1000VDC and be at least 100M corrected to 20 degrees C.

Control wiring IR tests performed at 500V for 300V rated wires and 1000V for 600V rated wires. Minimum spec is 2M

p_logix

For the OP who is asking how do i connect the probes on a megger its safe to say follow the 1000ohms per volt. A "Rule of Thumb" should give him a good understanding of what to look for when he's meggering. A three phase 480 volt motor at .5 meg should be acceptable when trouble shooting a Faulty circuit such as a VFD/mag/motor problem. If I read more than .5 I am looking somewhere else on the circuit. It is a judgement call and I don't pull motors based on the no less than "One Megohm Rule". Assuming he is not installing new cables or working on switch gear if he were than he must follow specs for workmanship/Safety reasons/liability. I work mostly with 480 volt AC induction motors where we run till fail unless we have some downtime. Usually the VFD will pick up the Faulty motor before a megger will. Personally I think "Rule of Thumbs" are great because they will lead you in the right direction as opposed to a wild goose chase when troubleshooting. When a system is down and thousands of dollars per minute is drainning away I have no time to look at specs, or "google". I have to use the tools I've been taught and make the judgement call myself.

A megger can also be used for trending purposes if you have a decent preventitive maintenance program. When you install you megger the motor and from there you continue to monitor the results regularly. Then you can get a idea when a breakdown is going to occur.

Thanks for the list!

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p_logix
"Meggers Don't Lie, Electricians Lie". Go LAKERS!!!!

Zog

It is not that hard to carry the specs with you. As a certified NETA test tech I am required to follow these specs. Anything less and I need the customer to sign a waiver to energize or I could lose my certification. Only takes a few seconds to look it up. I also carry all the specs in PDF format on my smartphone.

Excellent point! I would even go as far as saying trending is the most valuable part of megger testing, even more so that the actual value itself. Knowing when something is going to be below the spec in the future allows for planning a replacement or repair during the next shutdown and having the right equipment ready for it. We call that predictive maintenence.

TheRick

That's fine, but as a maintenance electrician in an industrial facility, I am required to keep production rolling. To that end, the bottom line is that the 1K ohm per volt rule WORKS.

I could not agree more! Many do not understand that there are 3 differerent types of maintenance! Preventative, Predictive, and Corrective. The fact is corrective is always the more expensive way to go. Regular testing, and the trending of those tests is very valuable information that can definitely save lots of $$ in lost production time.

However, at 2:00am if the megger reads .5M ohm or better, I am moving on in my troubleshooting, because the motor is not my problem, and I need to get prodcution going again!

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Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"