Mininmum insulation resistance to earth

For testing quality of the insulation or how low can you go before breakdown ?

sQuick..

As always in this newsgroup "It depends". I'd feel very nervous firing up a 13.8 kV motor with less than 50 megohms to ground on each phase, cold, after 10 minutes - but at other voltages, other criteria apply.

Bill

Thanks ,It was a general question really which i had been asked at an interview and felt that it was a "it depends "kinds of answer but wondered if there was a text book value which i should of given.I answered the question with 1meg ohm and the guy seemed happy but i have asked the question to others with varying values.

Well, as others have said, "It Depends".

We used 1Mohm as the lower end for 'corrective action required' for 600V and lower. 10Mohm for 4160. I don't do anything higher than 4160 in the way of motors.

But a new one in good condition, I'd expect 50Mohms or higher. In the Navy we used to plot the readings in 'material history' for trending purposes. Any sudden drop meant we should go look things over and see what happened.

But some DC machinery, we would go as low as 50kohms. DC machinery is notorious for having low insulation resistance if they have the brush rigging ventilated with the same fan as the motor. Carbon dust gets into

daestrom