EEVblog #38 2of2 – Seismic Survey Boats & Insulation Resistance MeasurementPosted on October 16th, 2009 11 comments
Dave explains errors and problems associated in insulation resistance measurement through a relay test matrix.
Betcha no one else has covered that one before!
interesting blog, and yeah i bet no one else covered that.
Hey great, really feel like im learning something now! I have never gotten the chance to utilize a relay, do you remember what your first project was involving relays Dave?
Interesting! But… When you said “bang, bang”, I thought: “poor fishes”. This brings me to a question: How do you feel about our passion, and the way it impacts our enviroment? I’m always on a fight against myself, because I love electronics but I also love.. well, being alive!!! And all the stuff that we build, and trash, and “consume”, it’s all going to kill us. I know, it’s a big question, but I’d like to know your opinion!
I love relays.
There are a lot of things I don’t understand, but I understand relays. LOL
I keep wondering why there is a need for a 100Mohm meter.
Now I see why insulation meter is so useful for cables.
You can roughly measure multi 100 M? resistances with a bog standard $10 no-name, 3 1/2 digit multimeter with the same trick.
The highest resistance range of a 3 1/2 digit multimeter is usually 2 M?. You need to get a resistor as close to that value as possible. Lets say Rr = 1.8 M? (best would be one giving you a 1.999 M? reading, but that would be like winning the lottery). You connect that reference resistor Rr to the multimeter and measure it. Best would be if you fix the resistor with some screw terminals to the meter.
The you connect the unknown high resistance resistor Rx in parallel to Rr and measure the resulting resistance Rp. Rp is less than Rr.
The value of the unknown resistor Rx is
Rx = (Rr * Rp) / (Rr – Rp)
Lets say you measure
Rr = 1.800 M?
Rp = 1.790 M?
-> Rx = 320 M?
You have just measured a 320 M? resistance with a $10 multimeter and a 5ct resistor, 160 times larger than the specified maximum value the meter is supposed to be good for. The error gets larger, the larger the unknown resistance Rx is.
If only the last digit differs between Rr and Rp you are in the Giga ? range, but you really can’t say if it is one, two or three Giga ?.
Great explanations, although one thing did make me smile. High resistance is all a matter of perspective. My first task in my current job was to help design an insulation tester. It measures 35T?. Mind you, one of my colleagues remarked that it was
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