Insulation Resistance Test for Three Phase Bus Bars

[nightfox1925]

Is the connection for conducting an insulation resistance test for 3 phase bus bars (in an MCC) similar to 3core cable test applications that is:

Example for Phase A to ground:

1. Connect the Line (+) lead of the megger to Phase A Bus.
2. Short both phases B and C and connect to the Earth lead
of the megger and ground bus.

If a Guard terminal is available, then 2.) becomes: short phases B and C and connect them to the Guard temrinal of the megger. The Earth terminal is connected to the ground Bus.

Example for Phase A and Phase B

1. Connect the Line (+) lead of the megger to Phase A

2. Connect the Earth terminal to Phase B

If guard terminal is available, then ????

Are the procedures stated correct...I would greatly appreciate if anyone provide me a link or a simplified diagram that would show me some connections.

Thanks

 

[oldfieldguy]

Your first example is the method commonly used,except that the negative lead should go to the specimen under test and the positive lead to ground.

This simultaneously tests the phase to ground insulation AND the phase to phase insulation. If you find questionable readings, then you can undertake tests involving the use of the guard lead to isolate paths.

old field guy

 

[nightfox1925]

Thanks for the prompt response oldfieldguy. The test record form of our client shows Phase A to Ground, Phase B to Ground, Phase C to Ground AND we also have separate table for Phase A to Phase B, Phase B to Phase C and Phase A to Phase C. It seems to me the procedure and connection is different for the phase to ground and phase-phase.

If I will perform the IR test for the phase to phase, what would be the correct connection if I do not have any guard terminal on my megger? What if I do have guard, what's the connection going to be?