Lightening Arrestor IR value

[kssschsekhar]

Dear all

Could you anybody tell the reasons for low Insulation resistance of 220kv/400kv Lightening Arrestor?

In our substation we got the following values

Test Voltage: 5000v dc
Meggar make Insulation Resistance instrument

Top stack : 7.5 Mega ohms
Bottom stack : 175000 Mega ohms
Total stack: 200000 Mega Ohms

LA is in service in 220kv Main Bus-II. However during Preventive Maintenance checks we identified above values recently.

As LA is completely sealed. The what are the factors effect this Low Insulation resitance values.

Regards

 

[magoo2]

Have you made comparative measurements on a good arrester? I'm not sure what values would be appropriate for your rating.

The most common failure mode for arresters is moisture. The comparative test on a good arrester would help determine this. I've observed seals in which a gasket was improperly installed that allowed moisture to enter the arrester housing. Unfortunately, from a forensics standpoint, you usually have to chisel off the sealing area (assuming it's a porcelain housing) to find this.