Insulation Resistance Test For Wye Connected Generator

Ahem. I presume that you get zero ohms with a megger because you were just testing for insulation resistance. If you look at a Wye wiring diagram, you should easily see why you have a low resistance between phases.

I am agree with redfred,

If you are testing insulation resistance of a star connected generator with the isolated neutral, the insulation tester will never show a reading of 0 Ohm until and unless there is any short-circuit between the phases. I think you have forgot to isolate the neutral.

But if you have isolated the neutral, then remove all connections of all 6 leads and measure insulation resistance between the phases and ground.

Vinu_Answers Sure_Answers

"But if you have isolated the neutral, then remove all connections of all 6 leads and measure insulation resistance between the phases and ground"

Our OP has already tested 3 phases to ground and from what I can see got good readings. He does now need to split the star point to test between phases.

I assume you have a 3-terminal insulation tester . . . Hi - Lo - Guard.

Also assume that all your 3 phases - with 6 leads are disconnected from ground.

SO, to test between phase A to B, connect A to Hv, . . B to Lo, everything else to Guard and ground it.

hi,

greetings from India,

6 leads mean R (r1r2),Y (y1,y2) B (b1,b2)

1. you have tested Ph- Earth and got values( hope those are acceptable as you have not mentioned them (above 20 M ohm))

2. do you have removed N link between r2-y2-b2 ?

if so then you must get good IR between r1 & y1, y1 & b1 and b1 & r1.

if this shows '0' M ohm then there must be inter turn fault (or due to heavy moisture content)

Otherwise check the set for residual voltage by removing excitation.

this must be balanced and equal P-P and P-n wise.

if this test is ok then you may start the set.