megger

rmonroe · May 14, 2004

I haven’t had the opportunity to use a megger meter in the passed. Apparently we have a short to ground on some control wires ran in ½” E.M.T. I cannot pick this up with a standard V.O.M., but I’m pretty sure that we have a mirror short in the wire.

My question is…what Meg value should I see if I have a short. And what value should I see if the wires are good.

Thanks

rmonroe

jshiepe · May 14, 2004

Some of the ones I have worked with in the past have different scales, but the easiest way to figure it out is to just short the megger leads together and give it a spin. This will be an indicator for your dead short. Separate the leads and give it another spin and this will be your indication of a no-short(open) condition, which is what you would see if the wires are good, that is if you are meggering to ground.

WARNING!!! Be sure not to touch both leads simultaneously while the megger is still spinning! It is truly a hair-raising experience!

HTH,
Jeff

The Dark Knight · May 14, 2004

If the wires are shorted, you should read close to zero ohm...

If the wires are ok, you should read 100's of megaohms(depending on the megger scaling).

Just make sure you use the appropriate voltage for the wires you are testing. Running 5kv thru 120volt wiring will likely cause insulation breakdown of your wires.

CaseyK · May 14, 2004

I have never been a fan of meggers. Had to go in and replace to much wire and equipment over the years when the test person used to high a range or left equipment connected. Hopefully it is a given that you will have all rquipment off the wires. As stated earlier, not to use the 5kv setting. Your control wiring should be rated for 300 volts if not 600 volts as far as the insulation. I normally use a digital vom for checking resistance and tracking down shorts, but then, I seldom have any wires over 100 foot long. Another test, if you have a 24 volt supply for control voltage, put one lead of a pilot light in the hot side, and connect the other to each of the leads to see if it lights up.

regards...casey

Rocco · May 15, 2004

I agree with The Dark Knight if the wires are shorted you will see a reading of 0 ohms. Although any reading above 1 Megohm should be ok. But I usually work on the meter reading infinity as a safe bet.

ChuckM · May 15, 2004

kc9ih said:
I have never been a fan of meggers.

They are great for testing motors and such. But as you pointed out, they can (and will) knock the bejebus out of a lot of equipment if not used properly.

juad · May 15, 2004

HI
i have some notes may it usufuel
1-for low tension cabel (wire) you need 500V or 1000V as maximu megger voltagefor test it is not good to use higher than this voltage because may it damage the insulatiuon.
2-for short circut you dont need to use megger it must be clear by using multimeter, the megger is used to test insulation .
3- it is very important to know that megger may damage the electronics equipment if thy still connected with tested wire even if the dc voltage(test voltage) come from output of that equipment.
4- the value of insulation depend on the test voltage, for you it is very easy to compare the value of insulation between many wires (take in mind that the leakage may be between wire and earth or betwwen the wires).
5- practically by using 1000V megger for testing 380V cabel i find
the insulation more than 10M.

GOOD LUCK

Mike Williams · May 16, 2004

0

bonkhead

Mikey Wiliams · Jul 9, 2004

logboss1 · Jul 9, 2004

If the control voltage is 110 or more with dry conduit, you should be able to find the problem with the ohm meter. Be sure to isolate the wires at each end of the run, and the check them at each end. If there was a dead short it could burn the wire in half leaving one isolated and the other shorted. Vibration can also be a factor in bad wiring.

On the other hand if you are sure the short is in the conduit, there can't be that many wires in there, it might be quicker to just pull another run.

My thoughts on meggers... motor applications only.
The trusty ohm meter works for everything else.

waggs · Jul 10, 2004

shorted control wiring

1) Control voltage?
2) Symptom?
3) Blowing control fuses or controls inoperative?
4) Are these wires to input devices or outputs to contactors, valves, etc.?
5) Output devices not functioning?
I agree, don't use the megger, stick with a digital meter. As logboss stated, isolate wires from both ends of circuits and check to ground. Don't overlook dirt and oil or tracking at devices. Loose terminals will drive you nuts as will an occcasional burnt off ring or spade terminal, or even the wire itself. Good Luck...been there...done that. banghead

Bob

allscott · Jul 10, 2004

A megger can be a valuable tool if you know how to use it. As previously posted don't use a voltage higher than the insulation rating of your conductors (you'll break down the cables and the readings will be useless) Do disconnect the wiring at both ends because you can smoke stuff if you don't.

I use a megger when my multimeters readings are inconclusive. Quite often if you have nicked or burnt up wiring a multimeter will give you confusing readings. Remember your meter is only putting 9V on the wires and sometimes it takes more to find a fault. A megger will tell you for sure if you have a short between wires or to ground.

Meggers are also good for showing mechanics who holds the power "Hey Bob hold these leads while I crank the knob" just kidding, don't do that these machines generate enough voltage to stop a heart so be careful.

waggs · Jul 10, 2004

"Hey Bob hold these leads while I crank the knob"

allscott, did you have me in mind when you said to 'hold these leads'? HAHAHA....Nope...never got caught on that one. I do like the fact you pointed out that the digital only supplies 9vdc. I sure love my old Triplett anaglog with that 30vdc battery in it.

Bob

allscott · Jul 10, 2004

I held the leads once (younger and dumber) damb smart a.. electrician.

These fancy autoranging multimeters that everyone is using these days are nice but more often than most people would think an old fashioned VOM with a needle will tell you alot more.

corkers · Jul 10, 2004

Downunder

Just thought id add that in Australia we are required by law to megger any new installation (full or partial) and the minimum result is 1 Meg Ohm. (A couple of exceptions for heating elements and the like) the other important thing to note is you should look for differences eg if you test a three phase supply and 2 of the phases are 5 Meg Ohm and the remaining phase is 1 Meg ohm to earth, while technically compliant, this indicates some sort of problem.

$0.02...

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