Marine Installation Manual
2021-08
3-16
3 Engine Installation
3.10 Propulsion shaft earthing
X72DF
3.10
Propulsion shaft earthing
Electric current flows when a potential difference exists between two materials.
The creation of a potential difference is associated with
thermoelectric
by the ap
-
plication of heat,
tribo-electric
between interactive surfaces,
electrochemical
when
an electrolytic solution exists, and
electromagnetic induction
when a conducting
material passes through a magnetic field.
Tracking or leakage currents are created in machinery by any of the above means
and
—
if they are not adequately directed to earth
—
can lead to component fail
-
ures, or in some cases result in fires and interference with control and monitoring
instrumentation.
3.10.1
Preventive action
Using earthing brushes in contact with slip rings and bonding the chassis by
braided copper wire are common ways of protecting electric machines. Where
operating loads and voltages are comparatively low, the supply is isolated from
the machine by an ‘isolating transformer’, often with handheld power tools. The
build specification dictates the earthing procedure to be followed and the classi
-
fication society is to approve the final installation.
On vessels with star-wound alternators the neutral is considered to be earth, and
electrical devices are protected by automatic fuses.
Isolation of
instrument wiring
Ensure that instrument wiring meets the building and classification society spec
-
ifications and that it is shielded and isolated to prevent induced signal errors and
short circuits.
In certain cases large items of machinery are isolated from their foundations, and
couplings are isolated to prevent current flow, for instance when electric motors
are connected to a common gear box.
Retrospective fitting of earthing devices is not uncommon, but due consideration
is to be given at design stage to adequate shielding of control equipment and
earthing protection where tracking and leakage currents are expected. Magnetic
induction and polarisation are to be avoided and degaussing equipment incorpo
-
rated if there is likely to be a problem.
3.10.2
Earthing device
shows a typical shaft earthing device.
The slip ring (1) is supplied as matched halves to suit the shaft, and secured by
two tension bands (2) using clamps (12). The slip ring mating faces are finished
flush and butt jointed with solder. The brushes (4) are housed in the twin holder
(3) clamped to a stainless steel spindle (6), and there is a monitoring brush (11) in
a single holder (10) clamped to an insulated spindle (9). Both spindles are at
-
tached to the mounting bracket (8).
Conducting material
for slip rings
Different combinations of conducting material are available for the construction
of slip rings. However, alloys with a high silver content are found to be efficient
and hard wearing.
