EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: exe on August 19, 2018, 10:32:41 am
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Hello!
I wonder if megohmmeter are safe to use. Many of them have output voltage up to 1kV.
If they are safe, how can I make one that would also be safe to construct and use? I think charge pumps are inherently dangerous as they capacitance ready to quickly discharge through the body. One idea came to my mind is to put a resistor in series with each cap to limit the current...
PS my final goal is to make a cheap and safe high-resistance meter from a high-voltage generator and current meter.
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I think commercial insulation testers already include current limiting measures and drop to zero when turned off. Of course there's still the possibility of charging up external capacitances.
My 10kV insulation tester discharges itself, but if I've been using the non-clip probe to check things like SMPS transformers it can still leave 'slightly painful' charges on the capacitance between windings (a good sign of low leakage insulation).
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They can be extremely dangerous (due to stored energy in the D.U.T) if you use them to test the insulation of a large capacitor or long cable and make contact with the D.U.T. during the test sequence. All commercially available modern ones will ensure the D.U.T. is safely discharged at the end of the test sequence as long as you do not disconnect the test leads while the test is in progress. Old ones may not have safety features like auto-discharge.
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Of course they are safe to use as intended, but do not hold the probe and hit "test", you will get hurt.
The purpose of these high voltages is to catch any breakdown in insulation.
If you only want to measure high resistances use a couple of (9V) batteries in series and your dmm on the mV range
as a current meter.
Batteries are preferred because of the low noise.
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Part of my normal work duties include ac dielectric testing up to 80kv. The procedures used determine how safe you are. I have an old Biddle with the hand crank & a modern fluke megaohm meter for home use. The instruction manuals for these include procedures for their safe use. Most important is not to come into contact with the specimen under test & to lower the voltage back to zero. Danger exists if test is interrupted, by failure or the operator, as cables, bus bars, arrestors, capacitors, can store energy.
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Attached is a little calculator that I've thrown together in excel when using a dmm as current meter on the volts or milivolts range
together with a external voltage source ("spanning test"), "weerstand X" is the resistance measured.
Have fun