It consists of a silver alloy metal band (SlipRing). Silver graphite brush and shaft – Hull mV meter. The purpose of this system is to prevent spark erosion by the continuous connection between the Propeller shaft and hull, which lessen resistance between 2 different kinds of metal.
As Propeller rotates, electric current arises within the hull – seawater – Propeller – shaft – shaft grounding – hull, which may cause spark around shaft grounding area, & this may cause erosion within shaft bearing metal and crankshaft journal.
READING OF SHAFT – HULL mV meter should be always under 50mV.
What if Reading Exceeds 50mV (As Per Makers Manual) – Clean surface of slip ring, Remove Brushes from brush holders if u use electric instrument cleaner or alcohol.
If under 50mV reading cannot be achieved after the cleaning, the condition of the brush stopper’s spring, brush holder, wiring, etc should be checked.
Once a week check the following: SlipRing, soiling, loosening, Brush, Earth cable, connection, and brush stopper spring.