Hi Scott:


Analog resistance measuring equipment puts a known voltage across an unknown 
resistance and measures current (the meter is calibrated in ohms).  It is an 
implementation of Ohm’s Law.


Measuring high resistances, e.g. more than 10 megohms, requires a higher 
voltage to get enough current for operation of the analog current meter, which 
was on the order of 0.1 milliampere or more full scale.   Hence, 500 volts dc.  
The insulation resistance meters I’ve used had several voltage settings, mostly 
greater than 500 volts.   


Don’t confuse insulation resistance measurements with electric strength 
(flash?)  measurements.  They are two different and independent measurements.  


The insulation resistance measurement measures the resistance of the parallel 
solid insulations in the system or the product.  It’s a dc measurement.  The 
insulation resistance can be hundreds or thousands of megohms.  Insulation 
resistance was a popular measurement when transformers were insulated with 
papers and wire was insulated with cotton and asbestos.  Today, insulations are 
very good with very high, stable, and non-hygroscopic insulation resistance.  
So, we are seeing fewer requirements for insulation resistance measurement.  


Electric strength measurement (usually a pass-fail test and not a measurement) 
is to prove that air and solid insulations (in parallel and in series) will 
withstand the normally-occurring transient voltages that appear on the mains.  


A circuit can pass the insulation resistance measurement yet fail the electric 
strength measurement.  Or, it can fail the insulation resistance measurement 
yet pass the dielectric strength measurement.


With today’s modern insulations, I see no safety benefit to measuring product 
insulation resistance.


Best regards,





From: Scott Xe [mailto:scott...@gmail.com] 
Sent: Sunday, February 18, 2018 9:46 PM
Subject: [PSES] Insulation resistance test


Insulation resistance test is one of most common safety tests nowadays: Flash 
test, earth continuity test, leakage current test and insulation resistance 
test.  Can someone share the history of this teat to use DC and 500 V.  The 
products are working on AC and test voltage is higher than normal operating 
voltage but much lower than the flash test.  It is a trend to skip this test on 
production line.  What are the distinct benefits for this test?


Thanks and regards,



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