Shaftline Earthing Device; Scope And Field Of Application; Design Description - Man B&W L35MC6-TII 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. A shaft-to-hull monitoring equipment with
a mV-meter and with an output signal to the alarm
system must be installed so that the potential 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.

Design description

The shaftline earthing device consists of two silver
slip rings, two arrangements for holding brushes
including connecting cables and monitoring
equipment with a 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 sufficient
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 0.001 Ohm.
MAN B&W MC/MCC, ME/MEC/ME-GI/ME-B engines
Cabling of the shaftline earthing device to the hull
must be with a cable with a cross section not less
than 45 mm². The length of the cable to the hull
should be as short as possible.
Monitoring equipment should have a 4-20 mA
signal for alarm and a mV-meter with a switch for
changing range. Primary range from 0 to 50 mV
DC and secondary range from 0 to 300 mV DC.
When the shaftline earthing device is working
correctly, the electrical potential will normally be
within the range of 10-50 mV DC depending of
propeller size and revolutions.
The alarm set-point should be 80 mV for a high
alarm. The alarm signals with an alarm delay of 30
seconds and an alarm cut-off, when the engine is
stopped, must be connected to the alarm system.
Connection of cables is shown in the sketch, see
Fig. 5.17.01.
MAN Diesel MAN Diesel
5.17
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