Megger test results

[buddy91082]

We are getting megger test resulst back on new lightning arresters with the follwoing values:

Phase A - 6,000 megohms
Phase B - 56,000 megohms
Phase C - 7,000 megohms

Any reason to be concerned about on the large discrpeancy in Phase B or possible causes?

Thanks.

 

[rolmag175]

What type of arrestor do you have...(Gap type?)
If so possibly due to incorrect gap size, corrosion, poor connection somewhere in circuit?
Does manufacturer give installation/commissioning details.

rolmag175

 

[racookpe1978]

Phase to ground results, not phase to phase, correct?

What are specifications for each? (High resistance, or low?)

 

[buddy91082]

I would have to verify what type of arresters we have. Yes, the values are phase to ground.

 

[pwrengrds]

Are you meggering them the same way (positive or negative to ground)each time?
At what voltage are you testing?
What voltage are the arrestors rated for?

 

[ScottyUK]

Presumably this is an MOV type arrestor as this is an LV installation, judging from you using a megger rather than anything more sophisticated. If your megger test voltage is right up near the kneepoint of the arrestor then some leakage is quite likely. The V/I characteristic at the kneepoint is anything but linear, so manufacturing tolerances in the arrestor could easily explain the difference you are seeing.

Moral: don't do megger tests with MOVs on the system because the results are often confusing.

 

[buddy91082]

yup, MOV type of arrestor.

Called manufacturer. They stated that the phase B arrestor may be a different voltage rating (10kV vs 12kV) which may be causing the big swing. That doesn't make sense to me.

Still not sure if the big swing in megohm on phase b is a concern. I am having retesting perforomed to see if this was a typo or operator error.

Thanks.
b

 

[mmt019]

The readings appear to be acceptable in accordance with NETA ATS-2009. I'll reiterate what others are saying: Check with the manufacturer, verify that the 'B' phase arrestor is the same as the others, retest.