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#54210 07/22/05 08:38 AM
Joined: May 2005
Posts: 706
Tiger Offline OP
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?


#54211 07/22/05 11:46 AM
Joined: Apr 2005
Posts: 65
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.

#54212 07/22/05 11:55 AM
Joined: Apr 2005
Posts: 65

#54213 07/22/05 08:46 PM
Joined: Oct 2004
Posts: 55
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.

#54214 07/22/05 09:12 PM
Joined: Jan 2005
Posts: 5,362
Likes: 1
Cat Servant
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.

#54215 07/23/05 02:27 AM
Joined: Jul 2002
Posts: 8,432
Likes: 3
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]

#54216 07/23/05 06:40 AM
Joined: Dec 2004
Posts: 1,064

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.....


#54217 07/23/05 08:08 AM
Joined: Apr 2005
Posts: 65
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.

#54218 07/23/05 01:28 PM
Joined: Oct 2004
Posts: 55
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.

#54219 08/21/05 03:18 AM
Joined: Jul 2002
Posts: 8,432
Likes: 3
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.

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