Author Topic: Ohmmeter and Insulation Testing  (Read 1739 times)

Offline K9DJT

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Ohmmeter and Insulation Testing
« on: June 06, 2014, 11:23:47 AM »
Last month I presented the idea of diagnosing electrical problems by measuring, or should I say looking for voltage drops in unlikely places, e.g., simple components such switches, connectors, lugs, terminals and wires.  One might ask why I wouldn’t recommend using the ohmmeter in the DMM (Digital Multimeter).  The main reason is that a higher energy component, whatever it might be, is not being stressed by the ohmmeter in contrast to being under a full load or operational stress.  So when would I use an ohmmeter?  I only use it for very simple, extremely low voltage and/or signal carrying connections.  The best example I can think of would be a mic cable, connector combination.  Basically from one pin or wire to the other end of the wire/connector.  When doing so, I would normally use alligator clips on the test leads so I am able to use both hands and flex the cable at both ends while looking for an intermittent open or short.  It’s a great measurement for such things.  Could I use it to measure the contact resistance of a relay?  Yup!  It would be perfect for antenna relay…you’re not going to look for a voltage there as you might on a DC circuit.  What you need to remember though, is to take the test lead resistance in consideration with the measurement.  A good set of test leads when shorted together should read anywhere from .1 to .5 ohms max.  You can mentally subtract that from your test measurement, or use the “Delta” feature I explained in a previous article, which will cause the meter to display the difference of the first measurement to the second.
OK, let’s say you want to check a transformer or choke in that old radio you just bought at Dayton before turning it on.  It isn’t that you don’t appreciate the adrenaline rush with a smoking transformer, but prefer not stinking up the whole house with the smell.  What you really want to do is measure the resistance of the insulation of the wire used in the transformer.  So, is this a place to use the ohmmeter?  Yes and No…meaning you would be able to detect a direct short to the transformer housing…maybe.  Think of what type of stress a standard ohmmeter places on that winding.  If it isn’t an old analog Simpson, it’s less than a volt and a half.  Do you think that is even close enough to detecting a failure in a questionable winding?   You’re right, it isn’t!  We need to stress this thing with some real voltage to see if the insulation has any leakage or will actually break down.  Hence, the “Insulation Tester” which is aka “Megger”.  Basically, an Insulation Tester is an ohmmeter which uses a very large voltage, i.e., up to 5,000 VDC, but a low current which is limited to about 2 mA.  It has selectable  voltage ranges and will detect leakage between windings and/or the housing.  The display reading is typically in the meg ohm range.  You normally will test a winding at twice its operating voltage, e.g., 480 VAC would be tested at 1000 VDC and a good winding would be 1000 ohms per volt.  In this case, 1000V X 1000 ohms = 1 meg ohm or greater.  Any value less than 1 meg would be considered a questionable device.  I just presented the very basic test.  We can make it more complicated in a future article!

See you on the radio!

73, Gary
K9DJT