Insulation Resistance of Electrical Cable
- Thread starter pasha
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- Illinois
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- retired electrician
Re: Insulation Resistance of Electrical Cable
The rule of thumb that I have been taught is 1 meg per 1000 volts, or in your case 1 meg. I have rarely seen readings that low unless the run was very long or wet.
Don
The rule of thumb that I have been taught is 1 meg per 1000 volts, or in your case 1 meg. I have rarely seen readings that low unless the run was very long or wet.
Don
Re: Insulation Resistance of Electrical Cable
If you can try obtaining the Dielectric Absorption Ratio (DAR). An example of a DAR is Meg reading at 60 sec/ meg reading at 30 sec. The following table was obtained from Recommended Practice for Electrical Equipment Maintenance, NFPA 70B, 2002 edition. The table serves as a guide to interpreting DAR ratios.
Table 1: Dielectric Absorption Ratio Testing:
Condition DAR
Poor < 1.1
Questionable 1.1 ? 1.25
Fair 1.25 ? 1.4
Good 1.4 ? 1.6
Excellent > 1.6
DAR tests are preferable because they are a ratio of changing resistance over time and are not subject to cable lengths. The Megohm value will be inversely proportional to the volume of insulation. Therefore a cable of 1000 ft length should have half the insulation resistance of a cable of 500 ft.
I was looking through NFPA 70B and I don't see a rule of thumb for "spot" megging. Hope this helps.
I see rules of thumb for motors but not cable.
If you can try obtaining the Dielectric Absorption Ratio (DAR). An example of a DAR is Meg reading at 60 sec/ meg reading at 30 sec. The following table was obtained from Recommended Practice for Electrical Equipment Maintenance, NFPA 70B, 2002 edition. The table serves as a guide to interpreting DAR ratios.
Table 1: Dielectric Absorption Ratio Testing:
Condition DAR
Poor < 1.1
Questionable 1.1 ? 1.25
Fair 1.25 ? 1.4
Good 1.4 ? 1.6
Excellent > 1.6
DAR tests are preferable because they are a ratio of changing resistance over time and are not subject to cable lengths. The Megohm value will be inversely proportional to the volume of insulation. Therefore a cable of 1000 ft length should have half the insulation resistance of a cable of 500 ft.
I was looking through NFPA 70B and I don't see a rule of thumb for "spot" megging. Hope this helps.
I see rules of thumb for motors but not cable.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Insulation Resistance of Electrical Cable
There are NETA (National Electrical Testing Spec.s) regarding testing and acceptable values.
From expierence I can tell you any cable megging 1 megaohm will fail in the immediate future and 5 megaohms is on it's way to failure. NETA Specifications is 1000 VDC for 600 volt cables for one minute minimum acceptable value 50 megohms.
www.netaworld.org/
I like 50 megaohms or better there is also the following considerations; the age of the cable, humidity, direct burial, cable tray, in conduit, how adjacent conductors of the same age (particular the other phase and neutral conductor) megger.Is this a go no go test or a specific time duration?
There are NETA (National Electrical Testing Spec.s) regarding testing and acceptable values.
From expierence I can tell you any cable megging 1 megaohm will fail in the immediate future and 5 megaohms is on it's way to failure. NETA Specifications is 1000 VDC for 600 volt cables for one minute minimum acceptable value 50 megohms.
www.netaworld.org/
I like 50 megaohms or better there is also the following considerations; the age of the cable, humidity, direct burial, cable tray, in conduit, how adjacent conductors of the same age (particular the other phase and neutral conductor) megger.Is this a go no go test or a specific time duration?
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