Megohm testing

[Joe Villani]

journeymenelctric,

I am by no means a meggering expert.

It sounds like the testing was a after thought or something just missed. What has happened to me in the past all the wiring was pulled in and terminated, and then your told to go out and test the conductors. Nobody wants to remove and terminate the conductors again so they leave them in place and try to test them. This is not the proper procedure and will lead to mixed results.

In your post you mentioned that you energized the conductors. When you tested the conductors, did you remove them from the terminations and isolate them? If the conductors are hooked up to equipment you will get all sorts of readings.

Just a thought.

Joe Villani

 

[brian john]

In your post you mentioned that you energized the conductors. When you tested the conductors, did you remove them from the terminations and isolate them? If the conductors are hooked up to equipment you will get all sorts of readings.

You'll get two types of readings acceptable or unacceptable. Now it may not meet spec for that particular job to perform readings this way. But the results are as stated good or bad.

 

[zog]

in a 60 second test infinity is great and you want to see that when testing wires but anything over 50megaohms in a 60 second test is good. but new wire should be infinity

AAArrrggg! Would everybody please stop saying "Infinity", first there is no such thing as infinite resistance, second you never get your needle all the way to that mark if you look closly it is above (Right) full meter deflection.

But most important, that nasty little mark is above the full defelection on many megometers and can mean different values, if some cases your "meggers" "infinity" mark may be less than the acceptable minimimum value for some equipment.

Please, please stop saying "infinity" before my head explodes.

 

[mdshunk]

Please, please stop saying "infinity" before my head explodes.

Infinity, infinity, infinity. :grin:

Zog, is there some doubt in your mind, when people say that, as to what it is they're talking about?

 

[electricalperson]

AAArrrggg! Would everybody please stop saying "Infinity", first there is no such thing as infinite resistance, second you never get your needle all the way to that mark if you look closly it is above (Right) full meter deflection.

But most important, that nasty little mark is above the full defelection on many megometers and can mean different values, if some cases your "meggers" "infinity" mark may be less than the acceptable minimimum value for some equipment.

Please, please stop saying "infinity" before my head explodes.

infinity = above the range of the megger. in the MJ159 infinity equals over 2000Mohms.

 

[zog]

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.

This is a difficult question to answer, and a difficult cable to test.

First, as Brian mentioned, you have to isolate the cable, no terminations or anything.

Second, as someone mentioned, what time was your test for? I think you said like 15 seconds somewhere, if that is true, your 40 Meg is really much lower at 60 seconds.

Third, you didnt mention if your 40M was temperature correted, makes a big difference.

The NETA ATS for this cable is 100M, at 20 Degrees C. Now this is a pretty long run, and is underground, so 100M might be a little hard to obtain. But you should be close to that value.

 

[zog]

Infinity, infinity, infinity. :grin:

Zog, is there some doubt in your mind, when people say that, as to what it is they're talking about?

Yes marc there is a doubt, that is the whole point. Instead say >999M, or >2000M, or >100G, or >5T. Big, difference.

 

[zog]

infinity = above the range of the megger. in the MJ159 infinity equals over 2000Mohms.

Right, and that would be unacceptable readings for some MV equipment. So it is not infinity, it is ONLY >2G.

 

[mdshunk]

Yes marc there is a doubt, that is the whole point. Instead say >999M, or >2000M, or >100G, or >5T. Big, difference.

You really think a guy that doesn't know the right word needs to test in the terraohms, or even owns the equipment to do it? I think it's better, at this point, to get people tuned into using meggers and worry about the proper use of the word infinity at some point in the future when everyone is pretty much on-board with the general concept of insulation resistance testing.

 

[electricalperson]

Right, and that would be unacceptable readings for some MV equipment. So it is not infinity, it is ONLY >2G.

but MV equipment will require the use of a more powerful megger. the MJ159 wont cut it. megger makes a nice one the MIT510, that puts out 15Tohms and 5000volts. but for testing THHN and romex the cheaper lower range models are just fine.

 

[mdshunk]

Right, and that would be unacceptable readings for some MV equipment. So it is not infinity, it is ONLY >2G.

Think about it in context here. We're talking about an insulation resistance test on underground THWN with a cheapo Sperry meter, as noted by the original poster.

 

[brian john]

Maybe it is just from habit, training or the mixed blend of meggers we own, but I seldom use the term Infinity. Full scale of your meter meg is my gig, ter and with digital it will go on and on like 2 to 3 to 4 to 5 blade razors.

 

[mdshunk]

...and with digital it will go on and on like 2 to 3 to 4 to 5 blade razors.

Funny you mention that. My wife got me one of those 5 bladed razors to try. They're pretty nice, actually. I've been using that Mach-3 thing for years. Anything's better than a BIC. I say, let's just skip to the 12 blade razor and be done with it.

 

[brian john]

Talk about off subject, went on vacation and decided to take Bics, shreaded my face, gigacuts.

 

[electricalperson]

id take a bic over any electric razor. the electric razor i have rips the hair out instead of cutting it normally.

 

[mivey]

id take a bic over any electric razor. the electric razor i have rips the hair out instead of cutting it normally.

This one?:grin:

 

[electricalperson]

This one?:grin:

that was actually pretty funny :grin:
the one im talking about i bought at wal mart. its one of those cheap remington razors with the 3 circles of cutters. it doesnt cut too well

 

[peter d]

I've been using that Mach-3 thing for years.

It doesn't look like you need to change your blades too often.

 

[76nemo]

zog, I already pointed out that "infinity" was a matter of origin from the tester at hand, so I'll say it again, there is no such thing as infinity, and I think that is now understood,......I hope. How well can anyone determine anything on a "spot" test

 

[76nemo]

I didn't want to be the first one to comment about Sperry equipment, but Marc did already. Since Marc brought it up, is anyone doing any commisions with a Sperry tool??????