To Meg or not to Meg...how does one meg??

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ledolectric

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... Hi all,... I'm a licensed electrical contractor in Jersey, and I've recently been contracted to restore power and re-wire a residential home that had been involved in a fire..the extent of the fire damage was somewhat limited to the very center of the house,..or roughly less than 1/3 of the ediface.. I had met with the PoCo official as well as the AHJ the other day, and PoCo was ok with restoring power as long as we didn't re-energize ANY existing wiring..I was fine with that,..then the AHJ said he'd like the CB panel changed...ok,..no problem...I then told the AHJ that I needed to install about 6 to 8 temporary 20 amp. 120 volt circuits for utilization of de-humidifiers and lighting..AHJ was cool with that as well, as long as I provided GFCI protection..then he tells me that he wants the ENTIRE house re-wired and to call him back for the rough-in inspection..

... I then tried to show him the extent of the damage, and where it was located, when he thought a minute, then told me that I would have to "meg-out" all of the wires and certify in writing as to their status.. that is,..if I didn't want to re-wire the premises..

... OK,...long story longer,..I dont own a megger,..and secondly, I wouldn't know how to conduct such a test.. It would be easy for me to toss the job to someone else,..but I really want to learn how to conduct this test, if I could just obtain the parameters and protocol.. HELP!!!
thanx,.... Russ
 

dmplec

Member
I would disconnect all equipment and light fixtures and any bonding jumpers and test for shorts and grounds on all circuits.
 

ledolectric

Member
dmplec said:
I would disconnect all equipment and light fixtures and any bonding jumpers and test for shorts and grounds on all circuits.
... Ok,... do I then test from the panel (homeruns) to the branch circuits upstream?? I need to understand the procedure and what exactly I'm testing for..

... I understand that the "megger" sends high potential (voltage) through the conductor and the meg/ohmmeter will measure the "leakage" voltage from the di-electric (insulation), but once I have this piece of machinery in my hands, where do I place the leads??..if the cable is "BX",..do I attach a lead to the shielding and one to the conductor?? do I test the conductors one at a time?? for instance,..14/3 bx...does each conductor have to be tested against/across a known ground path?? Similarly,..if the cable to be tested is Romex,..where do I clip the leads to? and what voltage should I set to test with?? and lastly,..what are acceptable readings, and what reading is prohibitive (failure)... Thanx for your initial reply,..Russ
 
ledolectric said:
... OK,...long story longer,..I dont own a megger,..and secondly, I wouldn't know how to conduct such a test.. It would be easy for me to toss the job to someone else,..but I really want to learn how to conduct this test, if I could just obtain the parameters and protocol.. HELP!!!
thanx,.... Russ

Using the search function in the menu bar I asked "megger" and got about 240 hits.

This is probably good (until MDShunk comes galloping up):
http://forums.mikeholt.com/showthread.php?t=93850&highlight=megger
 

dmplec

Member
I would just use an ohm meter and test each wire to ground to check for grounds and then each wire to each other to test for shorts. But be sure all light fixtures are removed and all equipment disconnected from the circuit that is being tested. I don't think the inspector really wanted a megger used. I would check with him and ask what exactly he wanted.
 

ledolectric

Member
dmplec said:
I would just use an ohm meter and test each wire to ground to check for grounds and then each wire to each other to test for shorts. But be sure all light fixtures are removed and all equipment disconnected from the circuit that is being tested. I don't think the inspector really wanted a megger used. I would check with him and ask what exactly he wanted.

... He specified and I quote.." I want you to meg-out ALL of the existing wiring, and submit to the borough, a written certification affirming this"...
 

mdshunk

Senior Member
Location
Right here.
Megging a house that has suffered some sort of act of God is a somewhat common request from inspectors. The process is rather simple, but time consuming. Pretty much the only tools needed are a notepad, pencil, megger, and a 10-in-1. My process, in a dwelling, is more or less as follows:
  • Unplug everything that is cord and plug connected
  • Disconnect by some means all hardwired equipment like furnaces and well pumps
  • Remove all GFCI's and wire nut them line to load
  • Remove all dimmers and wire nut them line to load
  • disconnect all hardwired line voltage smokes
  • turn on all single pole switches
  • wire nut together all three wires of any 3-way switches and wire nut the travellers on 4-way switches
  • remove all light bulbs from any incandescent fixtures
  • disconnect any fluorescent ballasts at the ballast
  • disconnect any electric baseboard heat at the baseboard heater
  • Remove all the branch circuit hots off every breaker
  • Remove all the branch circuit neutrals off the bars
  • (leave the grounds on the bars)
  • Meg the hots first. Put one megger lead on the first hot wire, and check that wire to the ground bar first, and record the reading. Megger that hot wire to every single other hot wire, and to every single other neutral. Note any failed readings. Do the same for each hot wire in the panel.
  • Repeat the above step for each neutral. Check each neutral to the ground bar first, then to each other hot wire, and note any failed readings.
  • Investigate any circuits with failed readings. Perhaps you forgot to unhook some piece of equipment? Perhaps there is a failed piece of cable someplace.
  • Check any suspect equipment that you disconnected at the beginning of the procedure from each of of the equipments hot leads to the ground of the equipment. Do not megger equipment line to line, or line to neutral, or you can blow stuff up.


Keep you notepad of recorded readings, and also make notes of what you did to correct any deficiencies found. That's good enough for any inspector that I've run across. Expect to spend a good day, or sometime two or three if you find problems, doing this. When you put things back together, it's a good idea to put on new GFCI's and new smokes if you can figure out a way to get someone to pay for it. Simply declaring them condemned is generally good enough to get insurance to pay for them if it's an insurance claim. They do have a life span.

I typed this as more or less a stream of consciousness, so if I forgot something, you'll have to forgive me, but I guess you'll get the idea.
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ledolectric

Member
... Thank you Marc for your very poignant reply... the only question I have left is..where you said that about a cable "failing"...what reading should I be vigilant for?? and what is a good reading,..and what voltage should I set on the meter?? otherwise you've been a great help...Thank you,..all!!
 

mdshunk

Senior Member
Location
Right here.
ledolectric said:
... the only question I have left is..where you said that about a cable "failing"...what reading should I be vigilant for?? and what is a good reading,..and what voltage should I set on the meter??
The exact reading is less of a concern than deviations from a trend. With residential wiring, that is wiring done in cables, you'll typically either get full-scale readings or really low readings. The low readings are what will concern you. I have a personal cutoff of 20 meg, below which I will condemn the cable or equipment.

With regard to voltage, if your megger has a 50 volt setting, use that first to determine if you've missed disconnecting something, lest you accidentally blow something up. A roof ventilator, for instance, might have never crossed your mind, or perhaps the outdoor AC unit. Trust me, you don't want to buy a defrost control board for a heat pump, if you can help it. If any of your 50 volt readings fail, go check out and see what's going on... did you miss disconnecting some load, or is there a fault so bad that it shows up on 50 volts? Assuming you've got all your 50 volt readings squared away, now you can take your "real" readings. My rule of thumb is twice operating voltage. For resi, I check everything at 500 volts, but certainly not any more. More than that, and you can make old braided romex and old BX fail by blasting a spark from conductor to conductor.

That's really all I know about that, which ain't much compared to others. Download that "Stitch In Time" .PDF from the megger people, and do a Google search for Brian John's posts both here and on other sites. He's written some very informative stuff on the net on the topic. People tend to keep this sort of info close to the vest, but Brain is quite generous. I would add, however, that the "Stitch In Time" book has a 1 meg cutoff for a lot of things, which I think is much too liberal. By the time a cable or conductor is leaking down that much, it has gone through a pretty dramatic change from the day it was installed. For that reason, I'd use that book as more of a 'how-to' and would advise you to ignore the numbers they use for the most part.
 

mdshunk

Senior Member
Location
Right here.
I just thought I should add that I am aware that some electricians simply refuse to "certify" any old wiring, no matter what sort of tests they do. This is a personal decision that you and maybe the people who give you legal advice might want to discuss. Some electricians are uncomfortable giving the green light to an installation that they can't substantially put their two eyes on. Different strokes for different folks, as they say. This "certification" is a normal part of electrical work for old installations in the UK, and it seems to work out well for those guys.
 
The hardest process in the meggering of a house is the prep/cleanup.
In the prep, someone has to go around the entire house and outside property disconnecting loads, every load. When you miss a load, you will know it for sure.
Do not forget to look in every nook and cranny. The bell transformer and transformers for the heat/A/C need to be disconnected from the circuit.
You will be surprised what you will find.
Here is a nice piece by Fluke and it is an easy read.
Newer cables will most likely read very high. The old cloth braid conductors will most likely give you the readings all over the board. This is also where I agree with Marc that the numbers you will see for go-nogo us usually 1 meg. I think the conductors that test that low (especially the cloth braid or older plastic conductors) are in need of replacement.
...and when you are done meggering, most of the circuits will probably be good, tag the bad. And then go and put it all back together again.

http://support.fluke.com/find-sales/Download/Asset/1579160_6115_ENG_C_W.PDF


I have designed a simple recording sheet for the testing of circuits. It is in Word format. If you want, I will email you a copy, you will just have to replace the title on the top of the page...I am pretty sure the name of my company is not the same as yours. :grin:
 

mdshunk

Senior Member
Location
Right here.
Oakey said:
What a great thread as I have to do this test soon. Thank you guys
Good. I always wonder when I type a lot and ramble on like that if anyone actually reads any of it. I recall one contractor saying that he used to Xerox interesting educational tidbits and pass them out to his men, and became disheartened when he found them in the wastebasket. At least on the 'net, there's no visable wastebasket.

I'm curious to know why the test is being done in your case? I find that I'm normally called to do it after water damage from a broken pipe, a leaking roof, or fire fighting efforts. After a lightning strike is another time. Also, the local PoCo's in my area will require that an owner have an electrician certify that the interior wiring is "safe" in a dwelling that has had the power shut off for one year or more. Those can be a little hairy, since I want to install GFCI's, smokes, and AFCI's, but can't always convince the owner to do that. In those cases, no signature from me. Get another guy.
 
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Marc Shunk.,,

You did wrote very good part and yes i am well aware with Brain too he have good article to tell us and in other forum i did make a copy for other type of system and it did save myself a major headache more than once.

Anyway back to the topic i do few meggers as well both resdentail and commercal side [ on commercal side more common to meggered the motors kinda common anyway.] and i used the same rules as what Marc Shunk mention 2X over the max operating voltage on resdentail side 500v is the top and commercal it will be either 500 or 1000 volts [ if you have 750 v setting that fine use that as well ]

first thing is make a quick go/ no go test with 50 or 100 v setting sometime i can find it pretty quick before i can actally ramp up.

also what others mention is the biggest thing is prep/ clean up time that will really eat up the time so might want to plan this ahead of the time and expain to the comuster they will be out of power min of full day sometime more depending on the test it get involded.

Merci, Marc
 
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