Scope And Field Of Application; Design Description Of The Shaftline Earthing Device - Man B&W S80ME-C7 Project Manual

MAN B&W
Shaftline Earthing Device

Scope and field of application

A difference in the electrical potential between the
hull and the propeller shaft will be generated due
to the difference in materials and to the propeller
being immersed in sea water.
In some cases, the difference in the electrical
potential has caused spark erosion on the thrust,
main bearings and journals of the crankshaft of
the engine.
In order to reduce the electrical potential between
the crankshaft and the hull and thus prevent spark
erosion, a highly efficient shaftline earthing device
must be installed.
The shaftline earthing device should be able to
keep the electrical potential difference below 50
mV DC, and a shaft-to-hull monitoring equipment
with an mV-meter and with an output signal to the
alarm system must be installed so that the poten-
tial and thus the correct function of the shaftline
earthing device can be monitored.
Note that only one shaftline earthing device is
needed in the propeller shaft system.
Fig. 5.17.01
MAN B&W MC/MCC, ME-B/ME/MEC engines
Design description of the shaftline
earthing device
The shaftline earthing device consists of two silver
slip rings, two arrangements for holding brushes
including connecting cables and monitoring
equipment with an mV-meter and an output signal
for alarm.
The slip rings should be made of solid silver or
back-up rings of cobber with a silver layer all over.
The expected life span of the silver layer on the
slip rings should be minimum 5 years.
The brushes should be made of minimum 80%
silver and 20% graphite to ensure a sufficiently
electrical conducting capability.
Resistivity of the silver should be less than 0.1μ
Ohm x m. The total resistance from the shaft to
the hull must not exceed 0.005 Ohm. For a well-
functioning shaftline earthing device, the resis-
tance is expected to be approximately 0.001 Ohm.
MAN Diesel MAN Diesel
5.17
Page 1 of 3
079 21 82-1.2.1.0
198 49 292.3