Does anyone still use 1/10,000th as the basis for calculating minimum insulation resistance?
1/10,000th
- Thread starter Tony S
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Doubt it Tony ....... we used to declare tests to be infinity until it was decided an actual reading should be taken.
Kurt
Well-known member
as in measuring leakage as a fraction of load current ?
That’s it, 1/10,000th of the full load current. Large motors and cables that are a few years old can hardly be expected to give ∞ readings. Weather conditions also play a part in expected readings, if a motor had been stood for a while damp will get in to the windings lowering the IR value. Let the motor run for a while and test again, the readings will be far higher.
One motor in particular had to have an IR test every week, I added weather conditions to the report sheet and plotted a graph of the results. Over the year I could look out of the office window and more or less predict the readings.
I diverted an old redundant 550V 0.3” PILC to feed a new board, the feed was via 400A fuses. Before making live I did a quick test and got about 100kΩ, a bit low even by the old standards. Options, make it live and see what happens or replace the cable, anyone fancy telling the gang there’s 200m of 195mm² SWA to pull in? I made it live, after 6 months with about 200A on it the readings were in the MΩ’s.
A “one size fits all” value for insulation resistance may be OK for small installations, large and particularly old stuff needs a different approach.
One motor in particular had to have an IR test every week, I added weather conditions to the report sheet and plotted a graph of the results. Over the year I could look out of the office window and more or less predict the readings.
I diverted an old redundant 550V 0.3” PILC to feed a new board, the feed was via 400A fuses. Before making live I did a quick test and got about 100kΩ, a bit low even by the old standards. Options, make it live and see what happens or replace the cable, anyone fancy telling the gang there’s 200m of 195mm² SWA to pull in? I made it live, after 6 months with about 200A on it the readings were in the MΩ’s.
A “one size fits all” value for insulation resistance may be OK for small installations, large and particularly old stuff needs a different approach.
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Kurt
Well-known member
Thought so.... BTW its P&S have changed my name. If its industrial and (not ideal conditions) , I measure the IR simply to make sure it does not go bang. I like the idea of 1/10000 though as its at least a guide. Did I tell you the story where I IRd a new drum of cable just before install and got a belt of it, the day after, cos I did not discharge it... lol........ happy days.
cheers Tony
P&S of Derbsyhire
cheers Tony
P&S of Derbsyhire
I also remember the days when if a motor had been standing for long periods we'd disconnect a phase and run it on 2 phases for a couple of minutes just to warm it up before IR testing.
It's a bit different nowadays, insulation materials and winding techniques have improved even in standard motors over the last 10 years mainly due to the high peak voltages in supply waveforms from drives/inverters/VFD's.
It's a bit different nowadays, insulation materials and winding techniques have improved even in standard motors over the last 10 years mainly due to the high peak voltages in supply waveforms from drives/inverters/VFD's.
Kurt, did you ever find the cat after testing the motor capacitor? :slap
Kurt
Well-known member
Cat has never been the same since I shorted the cap in the lounge when he was fast asleep. Pure Kurt.
