Electrically insulated bearings typically have an aluminum oxide-coated outer ring OD face and end faces. Insulated bearings with a coated inner ring bore and end face, these bearings offer increased protection against high frequency currents due to the smaller coated surface area of the inner ring than the outer ring.

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Insulated bearings prevent premature bearing failures eventually caused by stray electrical currents. Insulation properties must remain stable regardless of environmental conditions, in particular when bearings are stored, handled and operated in humid climates.

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Bearing damage can occur when electrical currents use the rolling contact as a conducting path.

Today, a number of publications are available that deal with this matter, including discussions about root cause and counteractions, for example [1, 2, 3, 4].

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The tribological regime of a bearing determines the electrical behaviour of the rolling contact and the possible outcome.

In the conductive state the bearing is at a standstill and shows a low ohmic resistance. Due to the relatively good metal-to-metal electrical contact, only very high amperage currents such as welding currents are able to destroy the raceway surface.

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The resistive state is present if the bearing is in a mixed lubrication regime with increased ohmic resistance. In this regime, low electrical currents in the range of a few amperes have the potential to be dangerous.

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In the capacitive state the bearing is in a full film lubrication regime, and it acts like an electric capacitor with a specific breakdown voltage. If the applied electric field strength present in the lubrication film of the contact zone is high enough (in excess of the threshold value) electric discharges, called EDM (electric discharge machining) currents, will occur.