challenges of field testing for shaft voltages

The following diagrams show how complex it is to test for shaft voltages:

Machinery bearing train (top diagram) and electrical equivalent (bottom diagram) showing multiple parallel electrical pathways with variable resistance from shaft to frame through bearings. Also shown is low impedance pathway through shaft grounding system provided to control shaft-to-frame potential that can cause capacitive electrical discharge damage to bearings and other connected equipment.

The lower diagram shows how challenging it can be to test for shaft voltages in the field due to multiple parallel pathways for current to flow. Also impacting accurate measuring is the environment may contain other magnetic fields affected by rotating equipment which will be detected by testing equipment. Bearings that are already damaged will act as a low impedance pathway further challenging accurate field testing. The best way to determine if electrical bearing damage is occurring is to pull the bearing and examine for fluting and/or frosted wear patterns.