megger readings

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stjohnbarleycorn

Senior Member
I am getting 25 meg between one leg and ground the others go over 1 gig. Is this and indication of minor breakdown or some problem that needs to be dealt with? It is an underground feeder 240, volts, I used the 250- 500- and 1000 ohm settings, on the 250 I got 16 mega ohms. thanks for any help.
 

zog

Senior Member
Location
Charlotte, NC
I am getting 25 meg between one leg and ground the others go over 1 gig. Is this and indication of minor breakdown or some problem that needs to be dealt with? It is an underground feeder 240, volts, I used the 250- 500- and 1000 ohm settings, on the 250 I got 16 mega ohms. thanks for any help.

What was the duration of the test and the tempature of the cable at the time of the test?

Either way, should be >100M @20C.
 

stjohnbarleycorn

Senior Member
its underground, so I would say that it is around 55 degrees, the durations I would say only 30 seconds to a minute at each voltage, the length was about 200'.

The other leg to ground as I said was infinity so I am suspicious.

thanks for the info. kevin
 

zog

Senior Member
Location
Charlotte, NC
its underground, so I would say that it is around 55 degrees, the durations I would say only 30 seconds to a minute at each voltage, the length was about 200'.

The other leg to ground as I said was infinity so I am suspicious.

thanks for the info. kevin

At 50F the correction factor is 0.63, so your 16M is really 10M.
 

brian john

Senior Member
Location
Leesburg, VA
Based on the fact that your other conductors are reading infinity (full scale for your megger). The low reading is highly susceptible. Is this an existing job or a new install?
Either case at 208 or 240 you MAY have time to plan replacement, but no guarantees.

Verify your readings at minimum one minute and correct for temperature.
Isolate the conductors on both ends prior to testing and clean all ends to insure you are not reading the cable exterior at the ends.

Is this a 3 phase 3 wire feeder
A 3 phase 4 wire feeder, if so did you megger the neutral with this conductor isolated?
Or is this a 3 wire 240 single phase feeder?
 

stjohnbarleycorn

Senior Member
This is a 3 wire 240v, its 2/0 aluminum URD, Its a circuit for pole lights.

The install is somewhere around 10-15 years old.

One set of conductors to another circuit has already been replaced.

I did not isolate the EG, I left it on the the lug in the control panel on the pole.

Thanks for you help here, I will re-check and advise a new set of feeders to the pole. kevin
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
It is an underground feeder 240, volts, I used the 250- 500- and 1000 ohm settings, on the 250 I got 16 mega ohms. thanks for any help.

Do you mean voltage setting?

I usually isolate both ends and test around 500-1000 volts myself if it's the 600v rated URD I'm thinking of. I'm no where near an expert like Zog and Brian John, but if one wire tested extremely low and another read full scale on your megger, it definitely sounds like there is a problem.

I'd probably put our Progressive pinpointer on the circuit and see if I could locate and repair the fault first. But sometimes it ends up being a bandaid because there will be more faults in the future if the wires were nicked in more than one spot. We always offer it as an option if they don't want to pay to replace the whole feeder though.
 
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