Shaft voltage is a necessary evil in generators and is caused by various stray magnetic fields generating an EMF from one end of the shaft to the other end. Now the problem is that this EMF would cause a circulating current through the bearing metals, resulting in damage to the surfaces in time, and ruination /pitting/ of these.
To avoid this current flowing the loop is broken by insulating one pedestal /usually the non-driving end/or the top bracket in a vertical machine. The voltage will still exist, but no current will flow. Theoretically that is.
To check the integrity of the bearing insulation some clever methods have been used-
1.- A small slip ring or surface is provided on the shaft end to measure the presence of this voltage, if it exists then the bearing insulation must be ok. A few volts to millivolts.
2.- a special split core CT is put around the shaft at the driven end to detect the presence of current. No current: ok. A special relay type was developed by ASEA for this a couple of decades ago.
