What Is a Megohmmeter?

TECHNICAL FOCUS: TOOLS

By: Richard Cadena

I have a new meter, and it goes to 11. It looks like an ohmmeter, and in a lot of ways it acts like an ohmmeter, and in fact, it is a type of ohmmeter, except it’s designed to measure resistance into the millions of ohms. Can your meter do that? Ordinary ohmmeters and multime - ters have much more limited range. For example, the specifications for my Fluke 323 multimeter say that the ohmmeter function has a range of up to 400.0W or 4,000W, depending on the setting. In the old days of analog meters, you had to select the range from a number of options, because these meters are more accurate in the higher end of the scale than in the lower end. That’s still the case for some digital meters but, today, most meters are autoranging, so you don’t have to worry about guessing which setting you should use. They still, however, have limited accuracy. I think the 323 will display resistance values higher than the given range but with very limited accuracy. And because it’s a digital meter, the reso - lution is given as 0.1 or 1, again depending on the setting. It also says it has an accuracy of 1% +/-5 digits. I could have paid more money to get the Fluke 325, and a lot of my friends have because I see them a lot, and that would allow me to read 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. resistances up to the range of 40,000W. For the vast majority of my work, I don’t need the extra range, ity; it’s one of the main things that when it comes to live event production, resolution, or accuracy. But for cer - keeps us safe when we set up power we are the kings and queens of pun - tain tasks, like evaluating the effec - distribution and power a show. ishing portable power cable in very tiveness of the insulation of cables or Insulation is typically made of ther - creative ways. We heat them by using equipment, not even the extended moset, thermoplastic, or thermoplastic them for outdoor productions and run - range of the 325 is enough, so you elastomer, all of which are all polymers, ning lots of current through them; we need specialized equipment, like a and polymers start deteriorating as let them stand in the rain and run them megohmmeter. soon as they are manufactured. Heat, through wet streets and gutters; and Insulation is one of the first lines of moisture, and mechanical shock all we toss them around like we are defense against the hazards of electric - contribute to the aging process, and Olympic hammer throwers. It’s a won -

140 • October 2018 • Lighting &Sound America TECHNICAL FOCUS: TOOLS

der they last more than a few shows. The questions are, How long do they last and how long should they last? The answers are probably best achieved by testing the resistance of the insulation. If a power cable has good insulation, it will minimize leak - age current between conductors and earth. But as a cable ages, it devel - ops small cracks, pinholes, cuts, and scrapes that contribute to leakage Reviews of the current. If you examined your cable print edition: under a microscope, you might be "Huge and superbly surprised at what you find. Another illustrated... It's an way of examining it is to read the absolute joy to read" dielectric strength or the resistance of —Michael Coveney, the insulation. WhatsOnStage.com A megohmmeter looks a lot like a "Brilliant... &e ultimate regular ohmmeter—it has a mode book on the creative team selection switch and two leads—but side of theatre." there are some important differences. —Musical $eatre magazine, UK Besides having the ability to measure resistance into the millions of ohms, it also works much differently than an ordinary ohmmeter. It applies very high voltage between the leads, typi - cally 250VDC, 500VDC, or 1,000VDC, A THEATRE and the current that is generated is then used to calculate the value of resistance. That makes it a bit trick to PROJECT use than an ordinary ohmmeter because the meter could give you a 2ND EDITION shock, so you have to be careful not to get your fingers across the leads A BACKSTAGE ADVENTURE • EXPLORE THE WORLD OF THEATRE when you’re using it. I always wear leather or rubber gloves when I’m By Richard Pilbrow metering power, and I recommend with David Collison the same when you’re using a megohmmeter. Before you test the conductors, A THEATRE PROJECT is a spellbinding backstage adventure you have to disconnect them from that will challenge, provoke, entertain, and inspire all who love the circuit; otherwise, you can dam - the magical world of theatre. age the circuit. Besides, you want to measure the resistance between two Personalized autographed copies of the new Second Edition… a perfect gift! conductors separated by the insula - tion, not through the circuit. Then you connect the two leads to the two Soft Cover: $29.99 $15.00 (plus shipping) conductors and press the test button Hard Cover: $39.99 $25.00 (plus shipping) for a minute or more. The meter will then apply the voltage to the conduc - To order, contact [email protected], tors, and if there is any capacitance 212-244-1505, ext. 712 between them, you can watch the voltage rise before the meter yields the final results of the test. You should repeat this procedure with the leads connected between one of the

142 • October 2018 • Lighting &Sound America TECHNICAL FOCUS: TOOLS

Clear-Com Wireless. ;m;;uĺ conductors and the grounding or earthing conductor, and then again u;;"r;-hѴ-oul&r]u-7;ĺ with the leads connected between the other conductor and o;uou_oѴ;;m;b|_ ground/earth. om; 0-v;v|-ঞomĺ ;Ѵ;0u-ঞm]ƔƏ+;-uv If you are testing a circuit with an o=m|;u1olmmo-ঞom Ŋ ;rѴo|u-mv1;b;uv_;u;o operating voltage in the range of m;;71o;u-]; 50VAC to 500VAC, then the insulation Ŋb;b;7b-]movঞ1v-m7|u-1hbm] resistance should read over 1MW; www.clearcom50.com otherwise, it doesn’t meet the require - Ŋ"-l;=-lbѴb-u;Ѵ|r-1hŋlbmbl-Ѵ |u-bmbm]=ouv;uv ments for BS 7671: Requirements for Electrical Installations (a.k.a., the Visit us at: +ou;m;Ľvor;u-ঞoml-7;v;-lѴ;vvĺ Regs), which is a British standard. As LDI 2018 far as I know, there is not an equiva - Booth #2010 Join Us! lent standard in the US but it’s still valuable to know how well the insula - tion is doing its job. We typically want to measure the resistance of portable cables when they are new and log the values, then repeat the test annually to monitor the effectiveness of the insulation. At some point, the value will drop low enough that they should be replaced. www.clearcom.com 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 turning a crank, which generated high volt - age that was applied between con - ductors. They still make those type of meters but they also make digital for ones that do the work for you. The one I bought recently is an Extech 380260 autoranging digital megohm - January 24, 6pm – 8pm at the Anaheim Marriott meter that measures insulation resist - ance up to 2,000M , and it was just Advanced tickets are $65 and go on sale October 15 Ω over $200. There are a variety of www.esta.org/btshh other megohmmeters that cost as much as $5,000 and up. The main A SPECIAL THANK YOU TO OUR SPONSORS difference between these meters is EXEC UTIVE DIAMOND PLATINUM their range, and if you need to meas - ure into the gigaohms or teraohms, and you’re willing to shell out the clams for them, they will gladly take your money. I’m happy knowing GOLD whether or not a cable meets the 1MW mark, and I’ll save the differ - PRODUCTION SERVICES ence to buy more tools. Production companies should be testing their feeder cable periodically and repair or replace it when the SILVER BRONZE resistance of the insulation is too low. Taylor & Taylor Associates Columbus McKinnon Corp., Earl Girls Inc., InCord, iWeiss Solutions, OSRAM, It’s a quick-and-easy thing to do and 2IÀFLDO0HGLD6SRQVRUVProtocol and Lighting&Sound America it could help to avoid serious prob - lems.

144 • October 2018 • Lighting &Sound America