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I have a new meter, and it goes to 11.It looks like an ohmmeter, and in a lotof ways it acts like an ohmmeter, andin fact, it is a type of ohmmeter,except it’s designed to measureresistance into the millions of ohms.Can your meter do that?Ordinary ohmmeters and multime-

ters have much more limited range.For example, the specifications formy Fluke 323 multimeter say that theohmmeter function has a range of upto 400.0W or 4,000W, depending onthe setting. In the old days of analogmeters, you had to select the rangefrom a number of options, becausethese meters are more accurate in thehigher end of the scale than in thelower end. That’s still the case forsome digital meters but, today, mostmeters are autoranging, so you don’thave to worry about guessing whichsetting you should use. They still,however, have limited accuracy. Ithink the 323 will display resistancevalues higher than the given rangebut with very limited accuracy. Andbecause it’s a digital meter, the reso-lution is given as 0.1 or 1, againdepending on the setting. It also saysit has an accuracy of 1% +/-5 digits. I could have paid more money to

get the Fluke 325, and a lot of myfriends have because I see them a lot,and that would allow me to readresistances up to the range of40,000W. For the vast majority of mywork, I don’t need the extra range,resolution, or accuracy. But for cer-tain tasks, like evaluating the effec-tiveness of the insulation of cables orequipment, not even the extendedrange of the 325 is enough, so youneed specialized equipment, like amegohmmeter. Insulation is one of the first lines of

defense against the hazards of electric-

ity; it’s one of the main things thatkeeps us safe when we set up powerdistribution and power a show.Insulation is typically made of ther-moset, thermoplastic, or thermoplasticelastomer, all of which are all polymers,and polymers start deteriorating assoon as they are manufactured. Heat,moisture, and mechanical shock allcontribute to the aging process, and

when it comes to live event production,we are the kings and queens of pun-ishing portable power cable in verycreative ways. We heat them by usingthem for outdoor productions and run-ning lots of current through them; welet them stand in the rain and run themthrough wet streets and gutters; andwe toss them around like we areOlympic hammer throwers. It’s a won-

What is a Megohmmeter?By: Richard Cadena

A megohmmeter is accurate in the millions of ohms, which allows you to test the resist-ance of insulation and determine whether or not it is doing its job.

Copyright Lighting&Sound America October 2018 http://www.lightingandsoundamerica.com/LSA.html

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der they last more than a few shows.The questions are, How long do

they last and how long should theylast? The answers are probably bestachieved by testing the resistance ofthe insulation. If a power cable hasgood insulation, it will minimize leak-age current between conductors andearth. But as a cable ages, it devel-ops small cracks, pinholes, cuts, andscrapes that contribute to leakagecurrent. If you examined your cableunder a microscope, you might besurprised at what you find. Anotherway of examining it is to read thedielectric strength or the resistance ofthe insulation.A megohmmeter looks a lot like a

regular ohmmeter—it has a modeselection switch and two leads—butthere are some important differences.Besides having the ability to measureresistance into the millions of ohms, italso works much differently than anordinary ohmmeter. It applies veryhigh voltage between the leads, typi-cally 250VDC, 500VDC, or 1,000VDC,and the current that is generated isthen used to calculate the value ofresistance. That makes it a bit trick touse than an ordinary ohmmeterbecause the meter could give you ashock, so you have to be careful notto get your fingers across the leadswhen you’re using it. I always wearleather or rubber gloves when I’mmetering power, and I recommendthe same when you’re using amegohmmeter.Before you test the conductors,

you have to disconnect them fromthe circuit; otherwise, you can dam-age the circuit. Besides, you want tomeasure the resistance between twoconductors separated by the insula-tion, not through the circuit. Then youconnect the two leads to the twoconductors and press the test buttonfor a minute or more. The meter willthen apply the voltage to the conduc-tors, and if there is any capacitancebetween them, you can watch thevoltage rise before the meter yieldsthe final results of the test. Youshould repeat this procedure with theleads connected between one of the

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conductors and the grounding orearthing conductor, and then againwith the leads connected betweenthe other conductor andground/earth. If you are testing a circuit with an

operating voltage in the range of50VAC to 500VAC, then the insulationresistance should read over 1MW;otherwise, it doesn’t meet the require-ments for BS 7671: Requirements forElectrical Installations (a.k.a., theRegs), which is a British standard. Asfar as I know, there is not an equiva-lent standard in the US but it’s stillvaluable to know how well the insula-tion is doing its job. We typically wantto measure the resistance of portablecables when they are new and log thevalues, then repeat the test annuallyto monitor the effectiveness of theinsulation. At some point, the valuewill drop low enough that they shouldbe replaced.The first time I used a megohm-

meter was during the Dark Ages,when I was in college. The megohm-meter was hand-operated by turninga crank, which generated high volt-age that was applied between con-ductors. They still make those type ofmeters but they also make digitalones that do the work for you. Theone I bought recently is an Extech380260 autoranging digital megohm-meter that measures insulation resist-ance up to 2,000MΩ, and it was justover $200. There are a variety ofother megohmmeters that cost asmuch as $5,000 and up. The maindifference between these meters istheir range, and if you need to meas-ure into the gigaohms or teraohms,and you’re willing to shell out theclams for them, they will gladly takeyour money. I’m happy knowingwhether or not a cable meets the1MW mark, and I’ll save the differ-ence to buy more tools.Production companies should be

testing their feeder cable periodicallyand repair or replace it when theresistance of the insulation is too low.It’s a quick-and-easy thing to do andit could help to avoid serious prob-lems.

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