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Joined: May 2005
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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
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Joined: Jan 2003
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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.
Bob Badger
Construction & Maintenance Electrician
Massachusetts
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Joined: Jan 2005
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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.
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Joined: Jan 2003
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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.
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Joined: Aug 2001
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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.
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Joined: Aug 2001
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Joined: Jul 2002
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Bob(Iwire), 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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Joined: Oct 2000
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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. 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.
Joe Tedesco, NEC Consultant
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Joined: Aug 2005
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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
It's Not The Fall That Kills You...
It's That Sudden Stop At The End
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Joined: Feb 2002
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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
Peter
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