measuring low voltage cable insulation 1

[emadmoad]

I have a problem on site , which is lasting for 4 days untill now and I did not figure out the cause or the problem .
the Situation is I had a short circuit and the main fuse broke down .
I checked Visual inspection for the contactor , overload , motor winding , cable and connections and i did not find the root cause , taking into consideration that I install a new fuse and start the motor and it is working now !!!!!
I still did not megger the cable yet ( check cable insulation) .
as i learned it is not so safe to megger the cable
Would you please help me with some steps to find the problem , and Also can u tell me how to measure the cable insulation in a safe way ????

 

[buzzp]

quadtech.com has a nice paper to download discussing cable testing.

 

[CarlPugh]

Sometimes if fuses are operated near their rating, they fail after a period of time. Time could be hours, days, weeks or even months.
Could the problem have been that the current through the fuse was too close to their rating?

 

[emadmoad]

I'd like to thank u sir for assisting me .
I have a motor 75 kw and the Fuse is 160A
Another thing I id like to ask about .
this motor is starting by a star delta contactors , I cancelled the star circuit and let the motor works direct on line and it is working now for 4 days without a problem or short circuit .
Do u think that the problem is in the star control circuit ???

 

[CarlPugh]

The problem could have been in the star control circuit.
There could be other problems with starting the motor directly on the line. However, I will leave it to the motor experts to advise you about starting the motor directly on the line.

 

[ShadyYehia]

Hi Emad :
I'm Electrical engineer in the Midor Refinery firs of all u mentioned that it's not safe to meggar the cable and i don't know why it's not safe to meggar the cable ?!!!!

Secondly, i believe that ur motor is not protected by a ring type current transformer to discover the residual current occuring in the motor starting current as i believe that hi rating motors like yours which are mainly working as a low voltage motors will face the same problem as yours

witing for the reason of not meggaring the cable ?

 

[emadmoad]

Hi Shady
Concerning meggaring the cable , I read this in some thread here as it may be danger to the other connected equipment to inject the cable with voltage , anyway it is just a precaution.
I believe that the current transformer is for step up/down current and not for protection .
please explain.
MY LAST DESICION IS TO USE A MECHANICAL INTERLOCK IN THE STAR DELTA CONTROL CIRCUIT .

 

[buzzp]

Cable testing is done all the time. Of course, all equipment should be disconnected so only the cable is tested and so that no damage will result with connected equipment (should not damage anyway but for only testing the cable, disconnect all equipment).
Based on what you have described, it is fairly obvious the problem was with your starter or circuit. If the cable was the cause of the problem it would NOT go away.
In your original post you said it worked after replacing the fuse. Was the star starting circuit bypassed at this time too?
What does a current transformer have to do with anything? Maybe the poster who mentioned this is merely pointing out that he suggests a CT around the neutral or one around all three phases.