Two Cores of Power Cable are Shorted with Ground and Still Motor is Running

Thanks for your interest.

There are several ranges in Megaohm meter. For example if 200 Mohm is selected and IR is greater than 200 Mohm then it shows OL, similarly for 1,000 Mohm and finally for 2,000 Mohm. it also have option of auto mode.

Don't worry. Tool is properly used and results are verified 2 to 3 times with another instrument also.

I don't think you fully comprehended and considered tcmtech's comment.

I comprehend your statement and fully agree.

You are aware that the big 'M' in front of the Ohms is for millions (1,000,000) of them right?

And that what I am talking about is using the low resistance range, 200 or 2000 ohms not 200 or 2000 million (200,000,000 or 2,000,000,000), to do the testing between the lines and the ground.

Besides moisture in the cable, there could be an arcing ground fault. These can be troublesome to track down and/or intermittent.

<...Two cables are short with ground and still motor runs...expert opinion...>

There are a number of serious safety faults and the current installation is best de-energised immediately before someone gets hurt/killed.

Re-energisation is only to take place once all faults are located and eliminated.

Saw a motor in 1979 that had similar symptoms. Driving belt conveyor in mine. Would run for a few hours then trip breakers (not over load relays). The motor was pulled and inspected multiple times.

Eventually discovered a broken rotor bar. Centrifugal force during running caused bar to swing out and trip the earth leakage.

Fault only hapened on low loaded time in shift. Low load on conveyor allowed an incremental increase in speed. Just enough for the bar to short.

One stopped, the bar returned into position and was unable to be detected visually. Finally showed on dye penetrant.

As far as the cable, does need to be confirmed actual reading on appropriate meter scale, or alternate measurement process.

That would be very rare today with modern rotor construction.

When I was a young engineer -specialty electrical machines- I was sent to receive a new supply cable of about 200 m length [I think was a 3.5* aluminum 120 sqr.mm p.v.c. insulated [0.6/1kV rated].

The installation delivering man was a faculty colleague of mine-also very young and machine specialist.

I tested the cable with 500 V megger and all 3 phases against neutral-and between phases- was 0.

After a long hesitation, after checking again and again entire installation, I decided to try to introduce [with all required protection equipment] a single fuse and to wait one minute. And at the end all 3 fuses were in place and the cable stay energized at no-load for 72 hours.

At the end, after 72 hours, I checked the cable insulation resistance: all the above measured was now above 1 megohm.

The moisture penetrated the ends and the no-load current was enough to dry it. In my opinion, you have to dry the cable keeping it energized for at least 3 days continuously.

After then you have to seal the ends. See for sealing appurtenances at Raychem[Tyco], 3M and other.

Entry of moisture in cable insulation(either at M.C.C. panel end or motor T.B. end due to improper terminations or sealing, and also if cable runs underground due to physical injury of cable insulation) may cause the above situation. Also there is a high probability of development of very high resistance fault( Arcing fault) due to physical injury of cable insulation and in such case, during insulation test by application of high voltage D.C.(500 volt and above) result may show zero reading. You may test the fault resistance by analogue Ohm-meter having high resistance scale, Meg-ohm range. For detection of such type of fault, special cable fault locator coupled with TDR, HV Thumper, Magnetic amplifier with head phone etc. are required with special expertise.

Manindra