• Electrical currents induced in the shafting of rotating machinery flow through the bearings, journals and the machine frame. As the current passes through the bearing oil film it can cause sparking (electric discharge machining), which pits and scores the bearing surfaces and the journal.
• This leads to rapid bearing wear, overheating, and eventual bearing failure. The pitting (frosting) of the bearing and shaft surfaces is characteristic of shaft currents.
• In d.c. machines, shaft currents can also cause sparking at the commutator and damage to the brushes.
• The currents are caused by magnetic asymmetry in the machine (e.g. unbalanced magnetic pull, eccentric rotor, segmented stator laminations, or a circulating flux linking the shaft) which induces an e.m.f. along the shaft.

(i) d.c. machines:

• The shaft is insulated from the frame at one end by fitting an insulated bearing (a bearing with an insulating layer between the bearing housing and the frame, or an insulated bearing liner). This breaks the circulating current path through the shaft and frame.
• The other bearing is left earthed (metallic) so that any residual current has a defined path and does not pass through the insulated bearing.
• A brush (earthing brush) may be fitted to the shaft to collect and earth any residual shaft current, preventing it from passing through the bearings.
• Ensuring the magnetic circuit is symmetrical and the air gap is uniform reduces the unbalanced magnetic pull that induces shaft currents.

(ii) Main shafting fitted with a bronze propeller:

• The bronze propeller and the steel shaft form a galvanic couple in seawater, and the shaft can carry current due to the propeller earthing effect and any stray currents.
• The shaft is insulated from the propeller (insulating coupling or insulating sleeve between the propeller and the shaft) to break the electrical path.
• An earthing brush (shaft earthing brush) is fitted to the shaft to provide a low-resistance path to earth, so that any current is conducted to earth through the brush rather than through the bearings and stern gland.
• The shaft earthing brush also prevents electrolytic corrosion of the propeller and shaft and reduces the risk of bearing damage from shaft currents.