Three phase motor ( Power cable faulty phase )

[SoonIWill]

Hi gentlemen,

I have three phase motor has an issue. This motor is star connection. When I just disconnect the power cable coming from MCC through breaker, and did megger test I found the reading as follows:

Phase1 with ground = 0.17 MOhm
Phase 2 with ground = 250 Mohm
Phase 3 with ground = 4.5 Mohm

Between phases each one with other = 550 Mohm

And I checked the motor winding they are in good condition

The question here due to power cable reading it shows me that conductor 1 ( phase 1 ) is faulty and the whole cable need to be replaced right !!!!?

Now, the big question is

While I have this bad reading for the cable why when I energized the motor the breaker not tripping ( the motor still working healthy without any trip ) !!!!!!!

Please note that in my breaker in MCC room I don’t have ground fault relay and no CT only breaker fuse for protection and overload contacts

Is this the cause why the breaker not tripping due to there is no ground fault relay !!!?

Thanks all

 

[pc1966]

Phase 1 with ground = 0.17 MOhm
Phase 2 with ground = 250 Mohm
Phase 3 with ground = 4.5 Mohm

Between phases each one with other = 550 Mohm

Are you sure that is only the cable you are testing, and no other control gear, etc, present?

I am just wondering how P1-G and P3-G is not reading around 4.7M if you measure P1-P3 unless there is some voltage-dependant device present somewhere (e.g. some surge protection in some electronic control system, etc). Though it could be failing insulation at (presumably) 500V test voltage which is not apparent and lower voltages.

Are the end points of the cable clean and free of any conductive dust or moisture? (If DC motors in use then graphite dust from the brushes wearing can drop IR reading but can be cleaned up with a bit of care and solvent to being things back to good).

And I checked the motor winding they are in good condition

The question here due to power cable reading it shows me that conductor 1 ( phase 1 ) is faulty and the whole cable need to be replaced right !!!!?

If it is an internal cable fault then you probably should replace the whole cable as quite possibly the others are going as well. E.g. P3 is heading down towards P1 reading it seems..

While I have this bad reading for the cable why when I energized the motor the breaker not tripping ( the motor still working healthy without any trip ) !!!!!!!

Please note that in my breaker in MCC room I don’t have ground fault relay and no CT only breaker fuse for protection and overload contacts

Is this the cause why the breaker not tripping due to there is no ground fault relay !!!?

You don't give other details of motor size, operating voltage, or even which country you are in to know what conventions are in use.

Here in UK (and similar in EU) most LV supplies are 400V/230V 3-phase so 0.17M to ground is only going to have 1.4mA or so flowing and that is not going to trip any common RCD/GFCI, certainly not the higher threshold delay-type seen for bigger power system protection.

But it is worryingly low and could indicate a fault will be coming your way at some point soon, and depending on the size of your system and the consequences of short circuit or an unplanned shutdown that could be very expensive or possibly dangerous.

 

[Lucien Nunes]

As per @pc1966 the faults from L1 and L3 to ground should appear in series as a fault L1-L3. The measured resistance L1-L3 might not be exactly the sum of the two, but one would expect it to be lower than the measurements for L1-L2 and L2-L3. It might be worth double-checking those readings before choosing what to do next.