What is an acceptable megger reading?

[Controls Guy]

I have been using meggers for well over 25 years. Having an aviation background before switching to industrial in 1997, 100 MegOhms was accepted as "good" when testing aircraft wiring to ground.

The 100 MegOhm or above rule has served me well to date.

I have recently come across a situation with extremely long 480 3 phase runs - > 2000 feet measuring low phase to phase and phase to ground. The feeders are underground, with vaults and conduit extremely wet and even submerged because of the water table.

Readings vary from >4000 Megs down to tens of thousand of ohms. Megger results are consistent on 500 and 1000 volt tests. With my background in aviation and the 100 MegOhm go/no-go rule, this has worked for 25 years.

However, given I have not yet had to address such a bizzarre underground installation before, this old dog needs to learn some new tricks from someone that has an extensive background with underground feeders.

Any feedback would be greatly appreciated!

 

[zog]

100M for LV cables is the minumum spec per ANSI and NETA. (Corrected to 20 degrees C)

Other equipment and voltage classes have different specs but it seems you are only asking about LV cables.

 

[ohmhead]

Well we run lots of underground and pull lots of feeders and we test our own conductors every size from 1/0 to 1000 mcm mostly 250 mcm to 600 mcm lots and lots you already know how to test but i know what you are going thur .

Example a run of 500 mcm 3 phase testing you get odd readings like 400 meg on b to c or 25 meg on a to b lets say different readings not the same .

Most electricians pull it out and say its too low dry up the ends recheck it will come up that 25 now becomes 125 meg or it maybe get higher let it dry out one day or cut it back a few inches it has water in jacket during pulling .



It may have soap now under jacket you dont have to get the same perfect readings on each as we normally read .

Ive turned on 600 mcm feeders with as low as 50 megg ohms and its been 10 years since that project .

Ive had readings mostly in the same conduit 3 phase 4 wire feeders at 100 megg and 400 megg in same raceway its ok.

After you turn on power shut down after a load is applied for a day or two and just for fun recheck it guess what your now at 500 megg ohms and climbing the moisture has burned its way off the conductor by heat .

We test at 1000 v only for each test 1 minute if its slow coming up if its coming up fast and pegging we just go on to the next .

 

[Controls Guy]

Thanks Zog & Ohmhead

Thanks Zog & Ohmhead

Thanks for your input. I mostly stick to instrumentation and automation, but when on the job everything seems to be dragged out of the closet.

Very much appreciate your technical views and looks like Air Force nuke guy was bailed out by two very sharp Navy nuke guys!

SAC motto: Peace is our profession - war is only our hobby!