Insulation Resistance per IEC 60439-1 8.3.4

The IEC standard should tell you somewhere what the minimum safe insulation resistance should be for all paths. Likely this value must be derived from some relationship for the standard to apply to so many potential scenarios. So to obtain a usable number, one must understand electricity and its units of measure. The busway manufacturer gives the expected minimum resistance for their product which will be only one of the circuit paths. The manufacturer specifies their product's resistance by some fixed mega-ohm value per unit length. This makes sense since a longer busway will have more material for current to travel across and therefore more resistance.

I'm very worried that you have an industrial standard in front of you and cannot figure out how to accurately transfer all of the pertinent information to us, let alone understand what that information means. Going back to basics, Ohm's law states V=IR with V being voltage in volts, I being current in amperes and R being resistance in ohms. So applying a little simple Algebra to this equation to get the units your IEC statement identifies, one gets 1/I=R/V. The units you have posted do not produce a resistance value, it is the inverse of the current. When one considers insulation characteristics that should produce very little current this almost makes sense for instead of a number much smaller than one but above zero for the current, you will get the inverse of the current which will be much larger than one. Regardless, this does not give you a resistance. To get a resistance you must multiply this puzzling unit with presumably your test voltage. R=(R/V)*V

Dear friend,

This is what the manufacture explained to me. (But i don't understand 66 MΩ per 100m):

"Dear sir,

According to IEC formular, our busway is designed for rating voltage 660V.

The calculation should be 660*1000ohm=660000 ohm. 1megaohm(MΩ) = 1million ohm.

So, according to above should know our busway insulation resistance between circuits and exposed conductive parts must be have least 0.66MΩ per meter. 66 MΩ per 100meters."

So, if i measure a busway run 100m length, the minimum insulation resistance should be read at least 66MΩ?

Actualy, it's very hard to get that value after install busway system. When i mearsure each part of busway before installation (about 3m) the insulation was around 1000 to 2000MΩ, even higher. But after install the whole run, the insulation become to 1 or 10, or 20, 60MΩ....etc

Do you know why?

Thank you so much!

The details that your manufacturer gave you are very back-to-front. If the insulation resistance is a minimum of 0.66MΩ per meter then this does NOT calculate out at 66mΩ for 100 meters. I would suggest you find a contact for the manufacturer who can give you better information. Incidentally your megger testing results sound about right.