When to use electrically insulated beatings on electric motors?

[ssbb2]

In my companies standard for motors 250HP or greater it is stated “Whenever necessary, the bearings shall be insulated to prevent the passage of shaft currents through the bearings. If insulated bearings are necessary, both bearings on the motor shall be insulated to facilitate testing.” My question is what are the deciding factors for needing an insulated motor bearing? Is it the line voltage or the RPMs or what it is coupled to? I am involved in the purchase of two Low Head pump motors. Two 300HP 514RPM and two 400HP 1200RPM, antifriction ball bearings, 480V, 3PH, coupled to bearing cooling water pump. Any information on this would be appreciated.

 

[KllrWolf]

Insulated bearings are needed anytime you can get shaft current that would discharge through the bearing. The only time I see them required is when the motor is operated on a VFD, but there are probably other circumstances that require it as well.