Megohm testing

[journeymenelctric@msn.com]

I am currently working on a job that requires a insulation test and report.Testing all of the shorter runs of 200 feet or less, I get a reading of infinity. On one circuit that is fed underground 600 feet with 350 mcm copper wire, I get a reading of approximately 40 Megohms. Is this normal. The wire pull went well and it has been energized. What is a normal reading.I have never had to megger anything before, where can I get a test report form.

 

[electricalperson]

a few other experts on meggers will come soon but heres my input. anything less than 50 meg on a conductor means the insulation might be damaged. i would continue to monitor that for any changes.

 

[electricalperson]

biddlemegger.com will have your test record forms for download too. get the book a stitch in time and you will understand insulation testing a lot more

 

[76nemo]

I am currently working on a job that requires a insulation test and report.Testing all of the shorter runs of 200 feet or less, I get a reading of infinity. On one circuit that is fed underground 600 feet with 350 mcm copper wire, I get a reading of approximately 40 Megohms. Is this normal. The wire pull went well and it has been energized. What is a normal reading.I have never had to megger anything before, where can I get a test report form.

40M is pretty low. What V did you use? You can make your own results spreadsheet with Microsoft Word.

 

[journeymenelctric@msn.com]

I am using a A.W. Sperry Meg-o-volt series 530 which is 1000 volts, it uses 6 AA batteries

 

[76nemo]

40M to ground or to another conductor? Chris, I don't ever remember forms on Megger's site, I have to go look for them, that's pretty neat. THANKS Chris!!!!

 

[journeymenelctric@msn.com]

That was between conductors. I was thinking there wasnt enough power in the meter for the 1200 feet of conductor.I dont have enough experience with a megger.

 

[mdshunk]

That was between conductors. I was thinking there wasnt enough power in the meter for the 1200 feet of conductor.I dont have enough experience with a megger.

You apparently had enough "power" to get a reading that low. More "power" might get you an even lower reading.

How long did you hold the test for? 1200 feet is a long wire to charge up. Darned near a quarter-mile. Could be as simple as that. Hold the test for a couple minutes and see if things start going your way.

BTW... I'm somewhat surprised you're allowed to do your own testing for commissioning. Generally, when reports are spec'd to be done, they want them done by a 3rd party. Doesn't really matter who, but just not the company who installed it.

 

[76nemo]

So you got 40M between all three test points:-?

 

[mdshunk]

I get a reading of approximately 40 Megohms. Is this normal. The wire pull went well and it has been energized.

Wow, I just noticed this. You didn't know if this was a bad reading or not (it definitely is) and you energized it anyhow? It is possible that you conducted the test wrong, but it is equally possible your feeder has an issue. What caused you (or someone) to energize it anyhow?

 

[electricalperson]

40M is no good when you test from phase to ground and phase to phase. on THHN conductors you would want to see infinity. all THHN conductors ive tested and known to be good start at usually 1000Mohms and shoot up into the gigaohm range. anything under 50M has a problem. like mdshunk said you need to keep the test button pressed for at least one minute to get a true reading. you need to let the conductors fully charge before you take a reading

 

[journeymenelctric@msn.com]

They varied a little between conductors ranging from 40 to 60.

 

[electricalperson]

They varied a little between conductors ranging from 40 to 60.

you need to figure out why the reading were so low. what kind of insulation was it? was it THHN?

 

[journeymenelctric@msn.com]

I will Have to try letting the wires charge longer, probably only left it on for 15 seconds.

 

[electricalperson]

I will Have to try letting the wires charge longer, probably only left it on for 15 seconds.

that might be helpful. if the reading stays teh same for the entire test then there might be a problem but if it starts at 40M ohms and rises to above 50 then its fine.

leave the test button pressed for at least 60 seconds

 

[brian john]

Send me an IM with your email I forward you the forms.

 

[journeymenelctric@msn.com]

We had no problems pulling it,it pulled easily, and I checked everything with a regular ohm meter, saw no problems.

 

[mdshunk]

... and I checked everything with a regular ohm meter, saw no problems.

You typically would not. By the time you can measure a nonconformity with an ordinary ohm meter, you've got a really bad problem. As it stands now, it seems you have a feeder that will have a short life.

 

[76nemo]

Chris, do you have a link to the spreadsheets by Megger? I see their cards, but they suck. I have no business saying that, excuse me.

I like Time/Date, V, S/PI/DAR, Temerature/Humidity, what instrument, what machine, and notes, to start with. I don't want a plotting graph that I can see in my head. If you look in one ear, you can see out through thee other

I just looked to no prevail. I am not one to "surf" with a goal in mind, it frustrates me, and rattles my cobwebs Are you talking of the Insulation tests and Hi Pot cards???

 

[76nemo]

I will Have to try letting the wires charge longer, probably only left it on for 15 seconds.

Are you sure it's not just a "spot" checker?