Megger - ECN Electrical Forums

What's involved in testing insulation with a megger. I'm not looking for an instruction manual, but approx. method and time to test 20 residential circuits. Also any preferred equiptment?

Thanks
Dave

I tried to get the same information a couple of weeks ago. I explained that I new how to use a megger but I could find specs. for Romex. If you go to the southwire web site they will tell you that 2-50 Meg. is inconclusive. Leading you to believe that over 50 is good. I didn't get the job anyway. Some good hearted person left a site address about a different insulation test that looks to be easier because you can leave everything hooked up. I have not bought or rented this equipment but the site looks interesting. Check out the hipot tester. http://www:hipot.com/index.asp

http://www.hipot.com/index.asp

I'm not recommending that you buy anything for sale on Ebat but you can learn a lot about the different test equipment product lines, and if your lucky find a deal.
Biddle and AVO are good companies that make digital units. As for Romex and most other cables and cable assemblies, if you call the manufacturer you will find that it is rated for 600V and it can be meggered at 500V. More on megger results later.

For 'quick and simple" megger testing, Grainger sells a cheap ($120 or so) megger that has a simple bank of LED's for a display. Press the button, and some of them light up, starting with the green and progressing down the scale to the red.
Such a megger won't let you do the 'fancy' tests you would do for preventative maintenance, but will provide a quick check of wiring.

Good God!,
What is it with you US sparkies?.
Look, I would have thought that it was a pre-requisite to getting a Licence as an Electrician.
A Meg-ohmmeter here is a standard piece of gear here.
You have to be able to use one and understand the readings to get registration here.
Hmmm. [Linked Image]

Trumpy,

Pls, there are so many different pieces of equipment and test equipment out there, most guys don't need or know how to use. Does that make them inapt?

Why does a residential guy need to know how to use a highpot tester?
Why does a lineman need a low volt wire tracer?

Some guys here do different things and need different equipment than others.

Does that make all of us "US Sparkies" knuckleheads?

That wasn't nice.....

Dnk.......

Trumpy, using the megger is not the problem.
Trying to find the acceptable test levels is. Also finding acceptable test proceedures. Sure using the little megger from Grainer is probably just great. I'm sure that in reality it works just fine. But you meg a house after some water damage and later something goes wrong. You find yourself in court. They will want to know the calibration date on the megger. They will want to know the proceedure and how it was documented, are you trained & certified ( sure I was certified 30 years ago in the military, but that was to meg cables in the nuclear field not Romex). I have asked 6 different engineers and got 6 different answers, I checked with the cable manufactures and they are not much help. It seems you need a team of highly trained engineers to keep from voiding the warrenty on the cable. Now if you want to meg industrial cables that is a whole different ball game because there are agreed upon tolerances ( set limits ). That's why most people just replace the cable after water damage or flood. That way you are sure that it want come back on you. We are a litigation crazed society and no one wishes to make a definative statement for fear of being sued. I can meg cables all day long and tell you if they are good for all practical purposes but I not so sure I can out wit a high priced lawyer. The reason I think the hipot test might be a good one is that it is accepted for the mobile home industry, and thus a precedent. If you have a documented proceedure that you are sure will stand up in an Amercian court ( remember that if the coffee is a couple of degrees to hot someone gets a million bucks if they are dumb enough to spill it on themselves) then by all means, post it.

Hey I had to break out an old training manual to make sure I had the info correct but here's what I was taught.
First the purpose of insulation resistance testing is to insure the integrity of the insulation. Applying a DC voltage across the insulation medium a current of three components is set up; Capacitive charging current, dielectric absorption current and leakage current.
Leakage current is the single most important component used to determine the reliability of the wire and or equipment under test.
By applying 500 DC volts or more to the system under test and reading the leakage current in milliamps or meg-ohms insulation quality can be determined.
An operational standard (and I can't seem to find a source other than "that's what has been widely used in the industry for years") is that one meg-ohm per 1000 DC volts applied is an allowable lower limit for ordinary situations (68F and 50% humidity).
Applied test voltages are as follows, for a Maximum Rated Voltage (MRV) or 250 VAC apply no more than 500 VDC to the system under test. For and MRV of 600VAC apply 1000 VDC and for an MRV of 5000V AC to 15,000VAC apply no more than 2,500VDC.
In the case of residential wiring, I have not found an industry wide standard that is different than the one above. If you are worried about the test damaging the installed wiring you can test short length (at least 25 ft) of the wire to insure that it will not break down. And that will also establish a benchmark for reading the installed wiring.
In a residential system member that there is a bell xfmr that is directly connected to the circuit and I would disconnect that and other devices that any other device installed prior to the test.
At the end of the test be sure that the applied capacitive charge has been removed from the circuit.

Sorry to bring this up again but I think that something has to be mentioned here.
A Megohm meter does not work by magic and it is a simple bit of test gear to use.
Like a few test instruments, it works with Ohms Law in mind, having to find relevant results is a no-brainer.
You should be able to work out in your head what the relative condition of the installation is by the test results.
There are fewer simpler test instruments, apart from a Bell Set.

Thanks guys. I appreciate your responses. I've only been asked to do this once by an insurance company after lightning damage. Liability is a major concern (insurance companies have almost as many lawyers as agents).

Dave

Mike I think you are insulting most of us.

In 23 or 24 years in the trade I can only think of a few times where a Mega was needed in the work I do.

There is a lot of mystique surrounding megger use. I think this is because most folks have only seen them used by a "special" person, in an industrial setting, and were not allowed anywhere near the tool. Of course, this was because of two factors:
-Meggers were once only one price: expansive; and,
-The sort of testing done in an industrial setting is very dependent on technique and documentation.

We're not talking about that sort of testing here....we want a quick & easy pass/fail test for our circuits before we power up. Sort of a souped-up continuity test. The value? Well, sometimes the wire gets damaged, sometimes we forget to tighten all the screws we don't use, and sometimes that $##@ ground wire get pushed too close to a 'hot' screw when we push the device into the box.

That's why I suggested the cheap meter with the LED display. There's no need to translate a numerical value into "good" or "bad;" the meter does that for you.

Another approach might be to power up for the first time through a GFI. 5mA of leakage current is a pretty good indication that there is a problem somewhere- and without making scorch marks on the walls!

Now, this is for "new work" only. If you are checking for insulation that has degraded from age, been damaged by lightning or fire, etc., then the fancier tests have merit. I just would not ordinarily do a "polarity index" or "saturation" test for the usual circuit.

John,

Well said, no need for Hi Pot. or breakdown stats, just a simple megger test, when i started the trade, we were required to megger all new wiring, now today with hurry up just get it in, breakdown testing is a lost art.
Yes, the testing was done by the manufacture of the cable, that was before it was thrown off the truck, stepped on, scraped, pulled, and hammered on.
As i look at the new construction wiring installed today, i wonder how much worse it can get.

Megger readings are still common here, and in fact are required by our "code" for a new installation.

I also find a megger very useful for leakage tests on motors, heating elements, and such like.

Thanks to George Corron for supplying the following scans from a 1950s code book:

[Linked Image]

Bob(Iwire),

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Mike I think you are insulting most of us.

If I insulted anyone I apologise,
But I thought that using Basic Test gear was a Requirement for passing your exams?.
I could be wrong too.
However, it seems to be clear that training for Electricians seems to be lacking in the US when Licenced Electricians have to ask how to use a Megger.
A Megger is a simple tool to use and gives any Electrician that uses one peace-of-mind before leaving a roughed-out house or building.
Come back and re-test at the panel and check if your cables are the same as you left them.
Any drop in Insulation Resistance will indicate problems.

[This message has been edited by Trumpy (edited 08-24-2005).]

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What's involved in testing insulation with a megger. I'm not looking for an instruction manual, but approx. method and time to test 20 residential circuits. Also any preferred equipment?

Thanks
Dave

Dave: Here's what is required based upon the NEC including the commentary from the 2005 NECH.

110.7 Insulation Integrity

Completed wiring installations shall be free from short circuits and from grounds other than as required or permitted in Article 250.

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Commentary from NECH

Insulation is the material that prevents the flow of electricity between points of different potential in an electrical system.

Failure of the insulation system is one of the most common causes of problems in electrical installations, in both high-voltage and low-voltage systems.

Insulation tests are performed on new or existing installations to determine the quality or condition of the insulation of conductors and equipment. The principal causes of insulation failures are heat, moisture, dirt, and physical damage (abrasion or nicks) occurring during and after installation. Insulation can also fail due to chemical attack, sunlight, and excessive voltage stresses.

Insulation integrity must be maintained during overcurrent conditions. Overcurrent protective devices must be selected and coordinated using tables of insulation thermal-withstand ability to ensure that the damage point of an insulated conductor is never reached. These tables, entitled "Allowable Short-Circuit Currents for Insulated Copper (or Aluminum) Conductors," are contained in the Insulated Cable Engineers Association's publication ICEA P-32-382. See 110.10 for other circuit components.

In an insulation resistance test, a voltage ranging from 100 to 5000 (usually 500 to 1000 volts for systems of 600 volts or less), supplied from a source of constant potential, is applied across the insulation.

A megohmmeter is usually the potential source, and it indicates the insulation resistance directly on a scale calibrated in megohms (M). The quality of the insulation is evaluated based on the level of the insulation resistance.

The insulation resistance of many types of insulation varies with temperature, so the field data obtained should be corrected to the standard temperature for the class of equipment being tested. The megohm value of insulation resistance obtained is inversely proportional to the volume of insulation tested. For example, a cable 1000 ft long would be expected to have one-tenth the insulation resistance of a cable 100 ft long, if all other conditions are identical.
The insulation resistance test is relatively easy to perform and is useful on all types and classes of electrical equipment. Its main value lies in the charting of data from periodic tests, corrected for temperature, over a long period so that deteriorative trends can be detected.

Manuals on this subject are available from instrument manufacturers.

Thorough knowledge in the use of insulation testers is essential if the test results are to be meaningful.

The way I understand things is that A HiPot test is a Destructive Test. Meaning That if it Fails and the Milliammeter Pegs then you have "Punched through the insulation" We Have only used A HiPot tester on High Voltage Cables (5 to 15Kv) I've Personally only used it twice. The last time I HiPotted the wind was blowing and the other end of the cable was exposed and I could tell the wind gusts by the milliamp readings. (the manual says to use "SaranWrap" and cover the ends to the cable under test to reduce Corona Loss But it Blew off and it was a slow 7 mile trip to get to the other end of the HV Cable)
I Use a 500 Volt Megger allot, Mostly for checking motor windings. One Customer asks us to test their cooling tower motors yearly. If you have moisture in the windings it will probably fail shortly
Most of us use the old style Biddle AVO hand crank meggers. I Prefer them to fancy digital Meggers, It's like a wiggy compared to a Multimeter.

J Spade

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However, it seems to be clear that training for Electricians seems to be lacking in the US when Licenced Electricians have to ask how to use a Megger.

Sorry, that's just plain wrong. I do residential work primarily so I've never used a set of hydraulic crimpers. Does that mean my training is lacking? No. It means that I've never been exposed to that type of work to get the opportunity to use one.

The same goes for meggers. Many electricians will go through their whole career and never use a Megger here. I am certain I could figure out how to use one if the need arises but so far that hasn't happen. The bottom line is that there are extremely few cases in residential and commercial wiring where they are needed.

In fact, the only place where I ever saw the need for one was at a new power generation facility where the job specs required all cables to be Megged upon installation. That was highly specialized industrial work, and very few electricians get that type of experience.

Peter

There are many cases in residential, and commercial wiring where a megger should be used but is not.
_____________________________________________
"110.7 Insulation Integrity

Completed wiring installations shall be free from short circuits and from grounds other than as required or permitted in Article 250."
_____________________________________________

There is a difference between an installer, and a craftsman, the installers today, seem to have a just get it done additude, where the crafstman, will take the time to test the installation, As i see it, their has been a continous dumbing down of the trade, since the mid 60's, It's up to everyone in the trade, to raise the bar.

[This message has been edited by LK (edited 08-26-2005).]

I do not see that 110.7 requires the use of a Megameter.

I see that it requires that circuits are free from short circuits and from grounds.

That can easily be determined without a mega.

Maybe this is just a local thing for Pete and I but I have been doing this a while in many different types of electrical construction and very few electricians or ECs own a Mega.

The company I work for now has quite a few, some that interface with a lap top to give very detailed reports.

This is used when the job specifications require it, or if we have to verify damaged wiring.

LK, I can figure out if I have a short or a ground fault with a continuity tester. Why do I need a Megger to comply with that code section?

Furthermore, let's say I skin some insulation off a wire as it's pulled into PVC. Is a Megger going to be able to tell me that I compromised the insulation? My guess is no.

Peter

Peter,

What if it is RMC? not PVC, the megger will find faults, where an ohmmeter will not, it's breakdown your looking for not continuity when testing for insulation.

One handy piece of equipment to have, is a megger, with AFCI's and GFCI.s in branch circuits, the megger sure helps find circuit problems that cause these devices to trip.

[This message has been edited by LK (edited 08-26-2005).]

Fair enough. But even RMC or EMT is not a guarantee that it will work. If the nick is on the inside of a bundle or wires, and there's no wire lube to "short" the nick to the pipe, I can't see how it would work either.

All I'm saying is that a Megger has limitations, and very few electricians used them in my area.

Yes you are right, not all insulation failures will show, but use of a megger will avoid serious faults, from becomming a hazzard.

I feel, that using a good insulation breakdown tester, can prevent serious injury and property damage, we tend to speed jobs up, and loose site of some of the basic things that should be done.

Peter,

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Furthermore, let's say I skin some insulation off a wire as it's pulled into PVC. Is a Megger going to be able to tell me that I compromised the insulation? My guess is no.

I would disagree with that.
A Megger test shows you the Resistance of the Insulation between conductors and between conductors and ground.
The use of a higher voltage is to adequately stress the insulation.
Now, usually there is a capacitive effect between wires either in a cable or in a conduit to some degree anyway, causing (albeit small) leakage currents and that is with all of the insulation intact in it's original state.
If however, you skin one of the wires, the capacitive effect is going to be a lot higher (ie: less insulation, more current flow) in that particular spot.
A megohm meter would pick that up regardless of the type of conduit.

Mike,

That is the reason we use a megger, not an ohm meter, we are looking for potential insulation failures.
We found a wire fault, in underground pool feed circuit, that was in PVC, the megger indicated leakage, sure eniough, when we pulled out the old wires one was nicked.

OK, I'm listening and learning. Keep it coming. Maybe one day I'll use one these things. [Linked Image]

Peter

Peter,

I was going to send you some more megger information. but then i thought, you might be out shopping for one tonight.

Yeah, that's true. Thanks for your consideration. The problem now is trying to decide which one to buy. [Linked Image]

I was lucky to find a good deal, I borrowed a megger for testing at a fire job, it worked out great, found all the faults, it was then i decided to buy one, after pricing the new ones, i cooled off for a while, then a local EC was selling everything in the shop, I knew he had a megger, so i showed up 5:00 AM thinking i would be there first, trucks all around the block, so at 7:00 they open the doors, well i watched as the first group came out arms loaded, by the time i got in there, not much was left, a few pipe benders, a wire rack, and a megger, i picked it up and looked at the tag $25, when i paid for it, he said, what are you going to do with that?, So you have a point when you say not many use them, But the best place to buy, is an EC's sell out.

Mike & LK, I agree a megger is a very useful tool to own. On Sunday myself and another electrician will be testing the secondary conductors on a 3000 amp 480/277 underground service. We will be using a megger to do insulation resistance tests on each conductor. The facility we are testing at checks their underground cables yearly. Test results can be compared from previous years to see if the insulation is breaking down.
A good 73 page booklet on the use of Meggers is "A stitch in time" the complete guide to insulation testing. It is available at www.megger.com .

Just a slight re-traction here guys.
Look, I think I may have come over too strong in my posts above.
I'm all for Megger testing and as PaulUK said it is still in our regulations.
However, to call another countries training lacking is beside the point.
I view the Megger as just another tool in our arsenal, to give the customer (and ourselves) peace of mind, not only during the building process, but afterwards as well.
You don't have to spend Mega-bucks to get a good Megohm-meter, although the marketing branch of the larger Test Equipment companies will tell you different.
All a Megohm-meter has to do is send a DC voltage of twice the working voltage of the wires (ie: 120V= 250V, 240V=500V)down the circuit under test.
What's more, the current is so low, there is no way that anything could be damaged.
It isn't a Hi-Pot test, therefore it isn't destructive, I've worked with Hi-Pot tests as a Line Mechanic and they are only used on MV and HV cables, before they are connected to anything.
Comparing a Megger test with a Hi-Pot test is like comparing Oranges with Bananas.
Also,
I just go with what I've been taught, if I can pass that on, so be it.