Help with megger results of NM-B

[ptonsparky]

As I read this thread, I can't help but wonder 2 things (honestly):

1) How do you become an electrician without knowing how to test insulation?

2) Why wouldn't you megger all the wire you run? It's cheap insurance against a lot of headache later, should there be a nicked wire, hammered wire, or sloppy connection.

Keep in mind, I didn't go through a fancy union apprenticeship, so I'm honestly perplexed. I was taught never to make a connection you hadn't tested. If I have wire nuts in one hand, I have my megger in another. I'm a lot happier re-running a 50' section of NM in open framework than trying to troubleshoot a tripped breaker after all the drywall has gone up.

1. Using a megger was never on any of my tests.

2. We all should test our wiring for faults prior to energizing, but in a residence a simple continuity check would suffice for most of it.

Using a megger is a waste of time for checking connections.

In my post with the pictures I have two screws in a work bench checking the resistance between them and to earth. Passed the test for Romex at a 1000 volts. A Meg test will not find all the problems. It will not find open air splices. It will not neccisarily find bare conductors stapled to a 2x4. It will not find poor splices.

 

[mbrooke]

As I read this thread, I can't help but wonder 2 things (honestly):

1) How do you become an electrician without knowing how to test insulation?

Its not code required or ever taught.

2) Why wouldn't you megger all the wire you run? It's cheap insurance against a lot of headache later, should there be a nicked wire, hammered wire, or sloppy connection.

Keep in mind, I didn't go through a fancy union apprenticeship, so I'm honestly perplexed. I was taught never to make a connection you hadn't tested. If I have wire nuts in one hand, I have my megger in another. I'm a lot happier re-running a 50' section of NM in open framework than trying to troubleshoot a tripped breaker after all the drywall has gone up.

Im perplexed as well. Last few times I brought this up in other forums I was met with as much enthusiasm as pork during bar mitzvah. Almost everyone had very strong opposition, several going as far as questioning my credentials. I cant figure it out either.

Just to add, imo the best time to test is after all the rock is up and cabinets are mounted.

 

[big john]

But doesn't the extra length lower the total readings? 2000 feet on NM would meg lower then say 25 feet?

Yes, distance makes a difference. Your charging current goes up with distance, and leakage current is directly related to surface area, which obviously increases the bigger/longer the cable.

This is one of the reasons DAR and PI tests are valuable because they are supposed to be dimensionless quantities. So for a given run of insulation while your leakage will increase with distance the ratios of your timed tests should be about the same regardless of whether it's 50 feet or 500 feet.

 

[luckylerado]

Yes, distance makes a difference. Your charging current goes up with distance, and leakage current is directly related to surface area, which obviously increases the bigger/longer the cable.

This is one of the reasons DAR and PI tests are valuable because they are supposed to be dimensionless quantities. So for a given run of insulation while your leakage will increase with distance the ratios of your timed tests should be about the same regardless of whether it's 50 feet or 500 feet.

Not applicable to this scenario. No one is doing dielectric absorption and calculating polarization index for 240V residential branch circuits. There is no reason to confuse this further and there are no adjustments applied for length of conductor.

 

[ptonsparky]

Not applicable to this scenario. No one is doing dielectric absorption and calculating polarization index for 240V residential branch circuits. There is no reason to confuse this further and there are no adjustments applied for length of conductor.

The question was asked.

 

[big john]

Not applicable to this scenario...

It's always applicable that people using a piece of test equipnent have an understanding of the principles of operation. Otherwise you just have guys pushing buttons with no idea how to interpret the results they get.

 

[luckylerado]

It's always applicable that people using a piece of test equipnent have an understanding of the principles of operation. Otherwise you just have guys pushing buttons with no idea how to interpret the results they get.

:happysad: If you say so professor. I think you are just muddying the waters even further for the typical electrician. Throwing acronyms around out of context does not provide a better understanding.

How will the OP get a meaningful DAR or PI with a meter that stops at 2000KΩ? If the IR value is not allowed to increase over time then at best the result will be 1:1 which would indicate unsafe conditions for either test while still getting a passing result from the spot test. Does that mean that the 600V THHN on a 240V system are unsafe?

The question was asked.

Yes in the context of NM cable in residential setting. Not distribution or windings.

 

[big john]

...I think you are just muddying the waters even further for the typical electrician....

If a "typical electrician" isn't interested in knowing this, they are free to ignore it.

...How will the OP get a meaningful DAR or PI with a meter that stops at 2000KΩ?

He may not in this case. But anybody that plans to use a megger as a diagnostic tool will eventually get readings significantly below the maximum scale, and guessing isn't a good troubleshooting method.

...If the IR value is not allowed to increase over time then at best the result will be 1:1 which would indicate unsafe conditions...

Perfect example of why it's important to understand the test: A flat PI does not automatically mean that insulation is unsafe. It means that leakage current is the dominant component of the test. Most insulation is hygroscopic, old insulation generally more than new, so if it's been exposed to a lot of moisture (like temperatures below the dew point), the readings may be flat without indicating the insulation is actually breaking down.

That's where you consider the environmental conditions, and do a quick step test and see if your current linearly follows your applied voltage. Even if your DAR or PI is flat, if your leakage doesn't increase, you've likely still got perfectly serviceable wiring.

Any circumstance where your leakage disproportionately increases with time or voltage indicates wiring that needs to be investigated.

 

[mbrooke]

Not applicable to this scenario. No one is doing dielectric absorption and calculating polarization index for 240V residential branch circuits. There is no reason to confuse this further and there are no adjustments applied for length of conductor.

Its not confusing me, I think Big John's advice is worth gold. :thumbsup:

 

[mbrooke]

If a "typical electrician" isn't interested in knowing this, they are free to ignore it. He may not in this case. But anybody that plans to use a megger as a diagnostic tool will eventually get readings significantly below the maximum scale, and guessing isn't a good troubleshooting method. Perfect example of why it's important to understand the test: A flat PI does not automatically mean that insulation is unsafe. It means that leakage current is the dominant component of the test. Most insulation is hygroscopic, old insulation generally more than new, so if it's been exposed to a lot of moisture (like temperatures below the dew point), the readings may be flat without indicating the insulation is actually breaking down.

That's where you consider the environmental conditions, and do a quick step test and see if your current linearly follows your applied voltage. Even if your DAR or PI is flat, if your leakage doesn't increase, you've likely still got perfectly serviceable wiring.

Any circumstance where your leakage disproportionately increases with time or voltage indicates wiring that needs to be investigated.

Awesome info!

So in a way wire can be represented as a fixed parallel capacitor and a resistor which is dependent on the environment?

 

[kwired]

I have only used a megger as a go/no go type of tester. For most troubleshooting I do that is all that is needed from it.

I have a relatively inexpensive analog type megohmeter.

When you hit test button the pointer will move as it charges the line, but if the line tested has good insulation then it bounces back rather quickly to near infinite reading. If insulation is bad the pointer doesn't bounce back or at least doesn't move back very much. I hardly ever read what value is being indicated, just watching the pointer move tells me go/no go for an insulation test. No go usually means double check that you don't have some conductive path in the circuit being tested that you might have overlooked before you declare something is bad.

 

[peter d]

Its not code required or ever taught.

Im perplexed as well. Last few times I brought this up in other forums I was met with as much enthusiasm as pork during bar mitzvah. Almost everyone had very strong opposition, several going as far as questioning my credentials. I cant figure it out either.

1) Most electricians are opposed to meggering new wiring because it's unnecessary.

2) Your credentials were being questioned because you gave strong reasons for them to be questioned.

 

[mbrooke]

1) Most electricians are opposed to meggering new wiring because it's unnecessary.

2) Your credentials were being questioned because you gave strong reasons for them to be questioned.

I know most electricians are apposed.

Can you show me those reasons?

 

[kwired]

1) Most electricians are opposed to meggering new wiring because it's unnecessary.

Until you have a bad batch of cable. But usually you feel you are too busy to think such testing is worth the time and seldom have a situation that would fail such a test.

I did one time run 10-3 all across a basement for a dryer receptacle, did some other wiring as well, but when I connected everything up and turned on breakers the dryer circuit had a fault in it. I checked the outlet first to make sure nothing wrong there - it was good. Luckily all of what I ran was fairly exposed and easy to inspect it - still found nothing obvious. Did not have a megger at that time, but connected my digital meter and had continuity between conductors (been a long time ago could not tell you what the reading actually was). Tested the unused portion of the cable sill on reel - had similar results. Was just glad this hadn't been installed in a new home and was mostly concealed before finding that problem.

 

[peter d]

I know most electricians are apposed.

Can you show me those reasons?

This subject has already been beaten to death and my views have been stated already. I'm not answering the same questions over and over again. But you're more than welcome to show me statistics that demonstrate that new wiring is a major contributor to building fires. When you do that, I'll start believing that testing is necessary.

 

[mbrooke]

This subject has already been beaten to death and my views have been stated already. I'm not answering the same questions over and over again.

Then don't keep repeating: "Your credentials were being questioned because you gave strong reasons for them to be questioned" Ive read your reasons behind that and so far none appear valid or have any proof behind them.

As is both you and the one who initiated this character assassination campaign are breaking forum rules:

To all forum members, I am formally requesting that you no longer publicly question new members qualifications for posting here.

http://forums.mikeholt.com/showthread.php?t=81328

But you're more than welcome to show me statistics that demonstrate that new wiring is a major contributor to building fires.

You are correct here, I have no statistics, and in truth I would venture to say only a very small percentage of fires stem from the actual conductors themselves. The only real risk of fire imo is where a staple is energized by the hot without involving any other conductors.

When you do that, I'll start believing that testing is necessary.

I have no statistical evidence, and in truth most wiring errors will never result in a fire, however I always like to go back and double any work. Also a good idea in residential after the cabinets have gone up (other trades can be a problem).

 

[romex jockey]

I've little patience or faith in a forum who's maverick mods run off posters , summarily having said threads closed down by other mods

Insisting American electricians do not pretest , know how to use testers , or that it is NOT code required is just plain wrong all around

Grow up

~RJ~

 

[iwire]

I've little patience or faith in a forum who's maverick mods run off posters , summarily having said threads closed down by other mods

Insisting American electricians do not pretest , know how to use testers , or that it is NOT code required is just plain wrong all around

Grow up

~RJ~

If you don't like a website there is a way to avoid it.

 

[peter d]

Then don't keep repeating: "Your credentials were being questioned because you gave strong reasons for them to be questioned" Ive read your reasons behind that and so far none appear valid or have any proof behind them.

As is both you and the one who initiated this character assassination campaign are breaking forum rules:





http://forums.mikeholt.com/showthread.php?t=81328

The exact wording is "new member". You're far from a "new member" at this point, therefore no rules have been broken.

 

[peter d]

I've little patience or faith in a forum who's maverick mods run off posters , summarily having said threads closed down by other mods

Insisting American electricians do not pretest , know how to use testers , or that it is NOT code required is just plain wrong all around

Grow up

~RJ~

I have little patience for a person who deliberately lies about being an electrician, then has his buddy help defend his lie.