insulation testing

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Take the leads seperated in open air turn on the tester readings should be full scale 50 mega-ohms. Touch the leads together the readings should be Zero "0".

Take a piece of paper and scribble on it with pencil lead the lead should give you a conductor, of some unknow resistance, maybe spark or catch on fire, then clear to a full scale readings.

Full scale readings are in mega-ohms, giga-ohms or tera-ohms, this particular meter has a full scale reading of 50 mega-ohms, or 50,000,000 ohms.
 
brian john said:
Take the leads seperated in open air turn on the tester readings should be full scale 50 mega-ohms. Touch the leads together the readings should be Zero "0".
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Ok, I did that, and it did exactly as you described. When Im trying to test the insulation on a 10' scrap of 1/0 wire I attach one lead with the aligator clip to the insulation and touch the other test probe to the wire, and nothing happens. Do you know what Im doing wrong? TY
 
Probably doing it right. What you are describing is as close to an open circuit as you can get.

carl
 
Thank you for the replies. I will do the tests you referenced this weekend when I have time, and report back. Oh, I forgot to mention that in the piece of wire I was testing a took a razor knife and sliced a 4" chunk out of the insulation, down to the copper. Shouldnt this meter have picked this up, or not?
 
GG,
took a razor knife and sliced a 4" chunk out of the insulation, down to the copper. Shouldnt this meter have picked this up, or not?
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No. There has to be a conductive path between the points where the two test leads are connected to the wire. Air is a good insulator, bare wire not touching anything will not have a current path. You could even put that wire with a section of its insluation removed in a metallic raceway and still get a good reading as long as copper is not touching the conduit, assuming clean and dry conditions.
Don
 
GG said:
Oh, I forgot to mention that in the piece of wire I was testing a took a razor knife and sliced a 4" chunk out of the insulation, down to the copper. Shouldnt this meter have picked this up, or not?
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This is exactly why an insulation test is not a true indicator of a damaged conductor.

I have always wondered why some people will put so much faith in a megger on NM in a house that has been hit by lightning.

Roger
 
As I have stated before nothing is foolproof, BUT In my experience, when you have conductor insulation damage (BARE SPOT/blow out) from a lightning strike you have carbon from the explosion/short/arcing/flash over. I assume the aforementioned BARE SPOT most likely occurs at a weak spot in the insulation and the short is from the neutral or hot conductor to equipment ground conductor. At this point there will be carbon, I have never seen a bare spot/blow out that was not accompanied by this carbon, meg that you get a low megger reading.

Not condoning this, BUT assume you pull Romex/NM nick the wire and not notice it. You meg all the branch circuits and they all meg full scale of the megger you are using at 1000 VDC, you/I feel fat dumb and happy we did a good job. Ahhh but there is a bare hot conductor buried in the wall, is this safe? Not a good idea, not something I want, but what is this bare spot going to do in the next year, 5 years, 10 years..... This goes for the bare spot in the conduit also IMO.....

In addition to all this experience, common sense and your companies liability play into making any calls on lightning jobs or heck for that matter any jobs.
 
brian john said:
As I have stated before nothing is foolproof, BUT In my experience, when you have conductor insulation damage (BARE SPOT/blow out) from a lightning strike you have carbon from the explosion/short/arcing/flash over. I assume the aforementioned BARE SPOT most likely occurs at a weak spot in the insulation and the short is from the neutral or hot conductor to equipment ground conductor. At this point there will be carbon, I have never seen a bare spot/blow out that was not accompanied by this carbon, meg that you get a low megger reading.
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And I would agree with that if the stroke came in on the power conductors and caused a "Blow Out", however, in my case (my own home) the phone line at my service took a hit and the phone conductors literally vaporized in my basement.

Every where the glowing phone cable crossed over a piece of NM it simply melted the sheathing off, and in two cables, it also melted through the insulation of an individual conductor inside the sheathing.

BTW, I have had 7 hits that have entered my house in the last 14 years, and even more that have hit and killed trees including some Oaks that have been turned in to furniture.

I live on a hot spot. :D

Roger
 
Brian,
This goes for the bare spot in the conduit also IMO.....
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If it is an interior conduit, it may never fail...if it is outside or in a wet location it will fail. This is one reason to use a conductive wire pulling lubricant and meg as soon as possible after completing the pull.
Don
 
Can this tester be used to tell you if a breaker is good or not? It does not mention anything about using this meter for checking a breaker in the directions, but I know I have heard about using a megger to check a breaker. What type of megger do most of you use? I guess I should have asked before I flopped out $500 for this Fluke 1587.
 
As part of circuit breaker testing, we do megger the CB, that is with the CB open you would meg across the contacts, and if the CB is mounted in a panel you should meg line to ground and load to ground, plus pole to pole with the CB closed.

After opening under fault the CB should be meggered and if possible the resistance of the contacts should be measured with a DLRO (digital low resistance ohm-meter) also called a ductor and micro-ohm meter.

The previous house I lived in was also a hot spot. One time tripped several GFCI's went to reset them just as lightning hit again, KNOCKED the living stuff out of me.
 
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I just lately popped open the 220V breaker box for my 1981 installed AC compressor. Using a thermal imager, I found that one leg on service side directly from a seperate meter was 10F warmer than the other leg and both warmer than the load side. The service side is aluminum too, not sure about the size!

The breaker contact on the one side was arced enough where the plating was gone. I have a Fluke 1507 insulation tester and the breaker tested fine. I also have a milliohm meter and the bad contact read only .006 ohms and the other side read .004 ohms. Not all that bad but under load, it's a different story.

I replaced the breaker and the service/load side are all equal in temperature now. I'm really not sure how equal they should be though. That old compressor motor has a power factor of .97 too. Since then I found a repair manual on-line where it listed the PF at .97
 
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