So I bought a megger... - Electrician Talk - Professional Electrical Contractors Forum

nrp3 · 10-13-2009, 07:37 PM

I bought a used megger on Ebay. Its a megger MIT 420. I also realize I don't know how to interpret the results too well. I have three of these annoying intermittent tripping AFCI/GFCI, UF, problems. I tried it out today on the UF fed pole lights. Of course, getting there, there breaker resets with no problem and stays on. I seperated out the two runs of poles. I got a reading of .1 meg and 10 volts applied and the other was .2 meg with 120v applied. This was on both 500v and 1000v settings. Trying it out on some romex in the truck and it was off the scale at 551 applied volts on the 500v setting. I realize I don't have any benchmark to go by.

The history of this particular problem is that has been an ongoing thing for a year. The circuit seems to trip when it rains. It takes time (hours?) for it to trip and will easily reset and stay on when you get there to check it.

The other two problems involve a basement light circuit and an AFCI. Thats a small area and can easily be rewired, probably faster than I can figure out how to use the megger. The other is a finished garage with an intermittently tripping GFI breaker. This one has all kinds of three and 4 ways. One of the runs was damaged and I replaced it with short run of surface conduit. All plugs and switches were replaced. I removed some romex from the attic that went nowhere but was live.

What kinds of readings would I expect to get or is there a range of readings I should get? Maybe I should send this in for calibration too.

BryanMD · 10-13-2009, 08:03 PM

before anyone else gets to jump in.. RTFM!

that's a current model.
did it not come with a manual?

nrp3 · 10-13-2009, 08:21 PM

It didn't. The users guide I think is on cd. I know, thats what I get for buying used. It is available online and I have looked at it, however it is more about features and less about interpreting the readings. I am new to using a megger. I bought it to figure out these sorts of problems. I have the megger supplement about using meggers. Most of the pamphlet is about testing motor and transformer windings. I bought this unit because it seemed to be able to do everything I wanted. I may not be using it correctly either. Hoping those with experience will shed some light.

480sparky · 10-13-2009, 08:57 PM

Uh, I don't think you're supposed to have the AC power on when you megger a conductor.

Users' Guide.

Safety warning # 1 from that very guide:

Quote:

The circuit under test must be switched off, de-energised, securely isolated and

proved dead before test connections are made when carrying out insulation and

continuity tests.

BryanMD · 10-13-2009, 09:12 PM

Quote:

Originally Posted by 480sparky

Uh, I don't think you're supposed to have the AC power on when you megger a conductor.

there goes Ken... actually reading through the post again.

nrp3 · 10-13-2009, 09:30 PM

Ok, I'm a little slow, what did I miss?

RIVETER · 10-13-2009, 09:34 PM

I agree with 480sparky...Put that in your megger and crank it. Seriously, though, I may have not been fully utilizing the meggers that I have used, but all I have ever looked for is very, very close to infinity.

RIVETER

JohnJ0906 · 10-13-2009, 10:05 PM

Quote:

Originally Posted by nrp3

Ok, I'm a little slow, what did I miss?

Turn off AC power.
Remove all lamps from light fixtures, unplug everything.
Remove GFCIs, Dimmers, or any other such device, and wirenut them through.
Your megger probably has a battery voltage setting - Mine has a 200Kohm setting at battery voltage, before the 250vDC, 500vDCand 1000vDC settings. Use it first. That way you can find out if you have removed all the above items without frying them.

BryanMD · 10-13-2009, 10:05 PM

Quote:

Originally Posted by nrp3

Ok, I'm a little slow, what did I miss?

Quote:

Originally Posted by nrp3

I seperated out the two runs of poles. I got a reading of .1 meg and 10 volts applied and the other was .2 meg with 120v applied... I removed some romex from the attic that went nowhere but was live.

your assignment nrp, is to compare and contrast two common terms: voltage & continuity and apply same to the function of a megger.

Lone Crapshooter · 10-13-2009, 10:13 PM

To use a megger the power must be off and every thing that is normaly connected to ground must be isolated from ground all line to neutral and line to line loads must be removed from the circuit. The only thing you want to test is the hot conductor. Then with all of the loads disconnected check the neutral conductor. If you just removed the hot wire and did not isolate the rest if the circuit you are reading to ground through loads . One of the main things about megging is isolating from ground then if it still reads to ground that is your problem.
One thing you could do to check for isolation is check the circuit with a ohmmeter first then when reads several megs to ground then put the megger on the circuit. Working as a industrial electrician there is hardly a day goes by that I dont have to meg something. Most of the time it is a 480 0r 2400 volt motor and it and by their very design they are isolated from ground.Sometimes we will have to meg a switchrack and that is when the fun comes in trying to isolate everything from ground we may work for a hour or more trying to get everything off the bus so we can check for grounds.
Listen Think Solve
LC

nrp3 · 10-13-2009, 10:39 PM

Those voltage applied readings I mentioned are what the megger was applying to the circuit at the time. That romex I removed from the attic was part of the same circuit in the garage. It went to something that was associated with some previous garage door openers. I tore everything apart and seperated the neutrals, grounds, black and red conductors on the sections of 12-3. The circuit was off and got readings if remember correctly 25 meg or off the scale at 276 applied volts (250 volt setting). All the sections I checked within the garage had similar readings. A portion of the home run was the only different one, lower, though I can't remember the exact figure.

The UF readings I took today were radically different, though it would make sense that different lengths of different insulation on a rainy cold day might be different.

I know there are degrees of continuity. This meter has a couple of settings. In the garage, there was a section of 12-3 that had a red conductor that was open that I replaced. The whole house was re sided with new wood clap boards and the nails they were using seemed a bit long...

nrp3 · 10-14-2009, 07:47 PM

I gave up on the basement AFCI problem. It took less time to remove the wiring and replace it than to test it. I altered the garage wiring some so I will see what happens with that.

ampman · 10-26-2009, 04:44 PM

go to meggers website and log in then download "a stitch in time" its a good read on using a megger

nrp3 · 10-26-2009, 06:34 PM

I have the booklet version. I have read it through and still a little uncertain. I'd like to know what readings people get. That basement section I rewired still was tripping the breaker. All that was left was the four cfl bulbs and the breaker. I changed the bulbs and the breaker. Nothing left. Still would like to know what the problem was.

I took another stab at the garage today. The breaker trips after a period of time, like after I went to lunch. It tripped a couple of times while we were testing the new generator and transfer switch. The megger readings for today were 6-10 gig ohms on the 50 v setting and 20 g in the 500v range. There is a section that seems to be in the 650-700 meg range that I will check further tomorrow. The pamphlet that came with the CH GFI breaker mentions that a cause of tripping could be more than a 250 run one way in the wiring. This would qualify for that.

The pole lights seem to be holding with the two sections isolated.

nolabama · 10-26-2009, 06:51 PM

Quote:

Originally Posted by nrp3

The breaker trips after a period of time, like after I went to lunch.

thermal
just read your OP
prolly not thermal but could be

nrp3 · 10-26-2009, 07:14 PM

I checked the current with the clamp on. I had three 4 bulb t-8 strips, six cfl floods, a 500w work light, and three garage door openers plugged in. It came to 6.63 amps. I couldn't force it to trip. I moved the work light around to different receptacles, tried moving some of the three and 4 ways, and couldn't get anything to happen. It must have taken at least an hour for it to trip and it happened while I was out. The only time I saw it trip was when the transfer switch went from utility to gen power and back.

ElectricBill · 10-26-2009, 09:50 PM

I assume you have tried a new GFCI breaker for the garage? Well worth a shot if you haven't.

nrp3 · 10-26-2009, 10:13 PM

I have another new breaker to try. It made it a week or so without tripping. I'll meg some more tomorrow. I am trying to not change to many things at once, but you are right, its worth a shot. The one section that seems to have a lower reading is in the same area that I already replaced a section of 12-3 romex. That one was obvious though, no continuity, the switch wouldn't work.