# Acceptable Phase to ground resistance



## telsa

wb6sub said:


> What would be the point where you would say that the insulation is bad on the leads going to a motor. Motor tested fine with PDMA tester but I would still consider 300 Mega ohms ok..please any feedback is requested


Please translate.

Thanks.


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## chicken steve

wb6sub said:


> What would be the point where you would say that the insulation is bad on the leads going to a motor. Motor tested fine with PDMA tester but I would still consider 300 Mega ohms ok..please any feedback is requested



here's a paper on pdma testing that might help

~CS~


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## micromind

I think the standard depends on who you're talking to.........

I've heard 1 meg per volt (500 megs for a 460 volt motor) , I've heard anything less than a zillion megs will cause an immediate burn up and untold billions of $$$ in damage.......lol.

Personally, I've seen motors that megged less than 50K last for years. I've also seen ones that sat unused for a while and meg 10K burn up in a couple of hours. 

I really don't have a hard and fast answer.


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## Jhellwig

It depends on what your notion of bad is. From a pm perspective below a trend down to 50 meg might be the cutoff but from an operational standpoint as long as it isn't blowing the fuses run it. If everything about the motors operation is in spec then there would be little chance of it going bad if it is staying at a steady reading over time regardless of the actual reading.


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## Zog

It depends on the type of motor, this is from ANSI/NETA MTS and IEEE std 43

The dielectric absorption ratio or polarization shall be compared to previously obtained results and should not be less than 1.0. The recommended minimum insulation resistance (IR 1 min) test results in megohms should be corrected to 40° C and read as follows: 
 
1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not described in 2.2 and 2.3. (kV is the rated machine terminal-to-terminal voltage in rms kV) 
 
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (form-wound coils). 
 
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and form-wound coils rated below 1 kV.


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## JRaef

micromind said:


> ...
> 
> Personally, I've seen motors that megged less than 50K last for years. I've also seen ones that sat unused for a while and meg 10K burn up in a couple of hours.
> 
> I really don't have a hard and fast answer.


I have a little 208V 1/2HP 3 phase motor that I picked up (literally) from the lot of a flea market after everyone had left. It megs out at under 50k, but it runs just fine on my drill press from a VFD. Now mind you, I get a "tingle" when I touch the case while it's running, so it's not really "fine" from a safety standpoint, but I just made a mental note not to touch it...


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## Big John

We follow the same standard Zog posted: Bare minimum is 1 meg per 1000V plus 1.

So for a 480V motor, you're looking at 1.5 megaohm minimum. That said, I would be very concerned about a motor that actually megged that low.

The screwball side of that is I've had old hygroscopic windings test so bad that they read as an absolute dead short, like to the point where we had several _amps _of leakage current to ground. Customer still opted to energize them and I'll be damned if they didn't go on and stay online, and once they heated and dried they megged at a much more acceptable level.

That said, these were DC systems that were much more tolerant of ground-fault leakage. I've had AC motors meg like that and blow right the hell up after we told them not to put it back online.


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## BrettC

Zog said:


> It depends on the type of motor,


Does location play a factor? I had always heard relative humidity, ambient temperature, etc, played a factor in the life cycle of windings but I have no practical experience on this front.


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## Big John

BrettC said:


> Does location play a factor? I had always heard relative humidity, ambient temperature, etc, played a factor in the life cycle of windings but I have no practical experience on this front.


 Definitely. Humidity or temperature increase and insulation-resistance goes down, and depending on how severe an increase the IR can drop like a rock.

That's why if you're trying to trend you've gotta correct for temperature and compare values for similar humidity conditions, because that's the only way to accurately determine if your insulation is deteriorating with time.


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## Flyingsod

Fwiw EASA standard is 1 megohm + 1meg per kilovolt for motors manufactured previous to 1974.
For motors 1974 and onward its a straight 5 megs.

If you think its too low just apply ohms law and do the calcs. Although personally I'm uncomfortable at anything less than 100 ohms. Theres no reason forme to have this gut reaction, Ive never seen any motor ground out that had more than 1 megohm resistance


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## Zog

BrettC said:


> Does location play a factor? I had always heard relative humidity, ambient temperature, etc, played a factor in the life cycle of windings but I have no practical experience on this front.


Temp makes a huge difference as Big John stated, you need to measure winding temp are correct your IR readings to 40C otherwise the readings are worthless.


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## oliquir

JRaef said:


> I have a little 208V 1/2HP 3 phase motor that I picked up (literally) from the lot of a flea market after everyone had left. It megs out at under 50k, but it runs just fine on my drill press from a VFD. Now mind you, I get a "tingle" when I touch the case while it's running, so it's not really "fine" from a safety standpoint, but I just made a mental note not to touch it...


You dont have a ground wire going to the motor? :blink:


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