Megger is a generic term for mega-ohm meter, like Romex is commonly used for non-metalic sheathed cable. Megger is a mfg.  It is used to test the dielectric quality of an insulated conductor. It can also be used to drive worms from damp earth and cause fish to jump out of water (like a crank telephone).

 

It is pretty simple to use. The wire is disconnected from all devices, isolated from any grounding system or device connected to the megger, and then a voltage is hankcranked into it. The better the insulation, the longer the wire (now a capacitor) will hold the potential voltage.

 

The voltage selections for a good megger are high enough to kill and can destroy any equipment accidently still connected to a circuit, so it is not a test for a novice.

 

Usually, only high current conductors, large service or supply conductors are megged, but some picky commercial or tele-communication companies require a meg test for even small and low voltage (120 volt) systems before they will entrust connection to their equipment.

 

For residential use, systems of 400 amps at 240 volts or less, meggers can be used to test questionable insulations but generally a simple continuity test will seperate the shorted wires from the usuable ones.

 

Sometimes a megger is used to Hi-pot large high voltage wires that have been in a state of disconnect or non-use for a period of time. Water exists in all raceway systems to some extent. Conductors in use will keep transient moisture dried out, but over time it can creep into systems disconnected for construction or maintenance. A skilled electrician or engineer might be employed to crank increasingly higher voltages into one of these at rest conducting systems to dry it out over a day or two before use to avoid damage to a damp leaky but otherwise useable system with a sudden high voltage connection. Note:this procedure is very rare and usually involves an engineering company even for very large sophisticated electrical contractorsl.

 

I suppose it could be used to dry out a wet residential system, but I've never heard of it. It would be cheaper and more reliable to just replace a residential wiring system exposed to that kind of moisture.

 

How long does it take. It takes about sixty seconds to crank a test plus ever how much time it takes an electrician to disconnect all devices to each wire tested. Multiple device circuits would require disconnecting each device and testing each segment of that circuit.

That should be about all you should ever need to know about meggers, except don't touch a recently megged wire end until it has been drained of potential.  They can hold residual voltages for a while.

Hope that helps,

Randy Cox

 

 

 

 

Wow, great info. Thanks. Maybe you can get  Xactimate to correct their mispelling.

If there is a fire that melts/shorts out the service panel and destroys part of the building wiring, does it need a megger test after rewiring?

Every time? Some of the time?

The reason I ask is I want to put this in my estimate if it is really needed/actually done but I don't want to just throw things in. My coworkers usually put this in the estimates so I started doing it also.

Do city inspectors require this testing? It seems like it might be a good idea for liability reasons, both for the electrician and carrier. We've probably all had claims where the carrier told us to put something in that might not absolutely neccesary but doing so would reduce liability exposure.

I have a claim right now where 220 volts is coming out of an outlet that is supposed to be 110. I don't know much but I'm guessing that means some wires are shorting out inside the wall. The service panel started on fire after the transformer on the pole off the property started on fire.

I think I will end up paying for some rewiring, including drywall etc. If you were doing the estimate would you put in the Megger test?

Under what circumstances would you NOT allow for the Megger test?

Thanks

I failed to answer a couple of questions the first post. I'll try again. I'm trying to keep my posts shorter; I exhaust people sometimes with detail.

First! "Sparky!" friendly tradesmen often call us that before they know our name. When an electrician makes an unsuccessful "smoke test" co-workers call him that for a few days to remind him to be more careful :-)

We we sometimes write short Meg test reports as required by customer, specifications, or engineer as we meg components of a wiring system. Sometimes it is just something signed off on a punch list as "done". When required on every circuit, rarely, we sign and date a tag at termination point. More often we do it because some event happened as the wires were being pulled in that cause us to question the integrity of the insulation. Those we do on our own as a quality control measures to prevent sudden failures latter as the building vibrates or expands and contracts.

Is it likely to be a PA add on? Yes! Now that this has even been discussed will result in some PA's out there having the light bulb go on--especially in a residential situation. Let them pay for it like they would any other testing that the insurance company didn't feel necessary. If their test shows a problem; pay to fix the problem and maybe the test, but if the megger shows no problem he should pay for it out of his fee.

Megging is always done on high voltages like 4160 volts or something like that, but 240 volts or less just do not really need it in my opinion. A simple continuity test will reveal direct shorts. After a fire, it wouldn't hurt to meg any wiring that was proximate to the fire, but IMHO not required. (Actually, if the test is done at high voltage settings for low voltage wiring, it can harm the wiring)

If one of the wires was burned severly at one end, I'd recommend replacing the whole length of wire. Heat conducts so one end in a fire would send that heat to the other end unless it was over in a flash. As a wire bends it stresses the insulation on the outside of that bend. If heat is applied to one end, that heat will travel to the other end. At stress points like the curve of a bend, it may fail or stretch even thinner and fail later. Again, the megger wouldn't show that, but a physical inspection would.

In commercial installations like the equipment at EDS that linked to most ATM machines in the United States...they might require a meg test because of the extreme high dollar loss of an accidental shutdown. Same thing with tele-communications where 1/4 of the country might be shut down for a failure. Remember, though megging requires disconnection and subsequent re-termination. It will not reveal problems in the terminations. It only checks the integrity of the insulation.

Even on residential, if specification documents from the original design show a megging requirment, I would say whatever test was required when the system was new, should be paid for on replacement or repair to indemnify... otherwise there should be some particular requirment that passes the common sense test. Remember I'm an old master electrician/new adjuster. You'd know this better than me.

I did after-fire inspections and lightening inspections in the 1970's and early eighties and I didn't use a megger. If one end was fried...I'd replace the whole length. Occassionly in residential, part of the wire that went into undamaged walls could be saved with a junction box, but if that wire where cut revealed any brittleness at all.....pull it out...replace it!

Is it required by NEC? It was not required when I was an up-to-the-minute expert some years ago. I just checked my 1999 code book and found no mention of megger. I went through a six hour CE course three weeks ago and no mention of a new provision there.
I don't think it is even mentioned.

For a time, the code had some insulation specifications, but they dropped those because ambient temperatures and enviornments caused field readings to vary so much. Testing is done by various facilities like UL and requirments changed as problems in the field present themselves. Some local enforcement authorities might well require them. I had a new inspector require white romex rather than black on his first day on the job...but following week black was okay :-) Electricians often report requirments that are more in their head than real. Check and ask for a written memorandum from the authority. Off the cuff requirments are often waived when requested in writting, both from the electrician and the inspector; not just construction requirments but all kinds of demands are dropped when a written memo is requested. Lots of people would rather drop it than write it!

If a panel was melted down after a transformer fire, megging all the home runs into that panel would not be an unreasonable request. I'd personally prefer a physical and visual test and skip the megger. Here's why! A dry brittle insulation might pass the meg test, but crumble under normal stress later. A visual inspection would reveal what the megger would not.

Still, in court an expert with high technical vocabulary might trump my field experience at least in persuasion quality. If only the insulation at the termination was burned or cracked while the insulation at the wire leaving the panel was supple and normal...then I'd expect it to be that way at the next junction point...but would check there as well before using that length of wire. Because low bidders might skimp, a megger test requirment at the end of the job might scare them into doing a better job...even if the actual megger requirment was unneccessary, but that should be a simple field test with the qualified wireman signing off and not some high dollar engineering outfit, unless it was appropriate to the size of the loss. Last month we paid a test firm $200 a piece to check (at their location) breakers for tripping under load. One breaker returned to us tagged and signed off on as good. It was mechanically impossible to turn it on. How did they test that under current if it would not turn off or on?

Megging is more appropriate for possible abrasion problems or physical damage, maybe moisture for high voltage situations--480 and above. IMHO.

Should you put it in your estimate? Tom Toll or you would have the better answer than me.

A lot of things get in estimates that I think are crazy. My Dad was an aluminum siding applicator for decades. He always carried ladders and jacks. To charge extra for that would have been like a carpenter charging a hammer fee! I'd put a megger fee in if folks at the higher level wanted it. I'll be watching responses to this to learn that answer myself.

Now for the fun part that really is my expertise...sometimes even at a distance. You say you have a claim with 220 volts coming out where it is supposed to be 110. If you've got 220 volts from one wire to ground....there is a major transformer problem and it would show throughout the whole system. If you've got multiple wires at a box with 220 between any two of those, these are the possibilities: It was a multi-wire feeder designed to feed two circuits or more. It may have been a 220 circuit converted to 110 volt as the point of use device requirments changed. It could be that one wire remote from the box melted or burned into another wire designed for a seperate circuit elsewhere resulting in a 220 volt potential, but more likely it is just an identification problem as the wires were re-terminated during repair. If there is a continuity of connection due to something other than termination error....it indicates that those down wire inspections were not done well and the cause needs to be determined before the system is released for use. Sometimes during massive re-wiring, especially where portions of the wiring are reused and refed with new homeruns, circuits will unintentionally get feed from two feeders. If both feeders are of the same phase, all works fine except it takes two breakers to turn it off. Checking the circuits with all breakers on and turning them off one at a time will result in all breakers being turned off, but the circuit continually energized. Turning all off and one back on at a time will reveal two feeders...disconnect one. Same situation, but each feeder from a different phase will trip two breakers...and probably destroy the second one turned on into that situation. If the whole system is piecemeal repaired and one circuit double fed...some outlet box might then show 220 when it should only show 110. This case would be one of those double feeder situations. Sometimes double feeders of the same phase exist for years undetected.

Okay, i'm taking off my electrician hat and putting my adjuster hat back on...........checking the weather channel for the next storm.

Randy Cox