Hamilton Jet HJ241, Manual

Page: 55 / 202

4.1
Section 4
P
RECAUTIONS
A
GAINST
C
ORROSION
4.1 General
4.1.1 Electrical Wiring System
The guidance of the local inspecting authority
rules should be sought, but in general note the
following for:_
D.C. Systems:— Every part of the DC system should
use Two insulated "normally conducting" wires, a
positive and a negative. The negative must not run
through the frame of any unit, through the Hull of
the boat, or through the bonding system.
Do Not
Use An Earth Return System .
It is recommended that engine starter motors or
other DC motors should be the two wire type with
an insulated negative terminal rather than having
the casing of the starter connected to the battery
negative. This is to ensure starting currents do not
pass through the earth bonding system.
In smaller vessels, it is common to use negative
ground engine systems in which the starter motor,
starter solenoid, and alternator are single pole
devices using the engine block as the local return
conductor. In This Situation, It is Important to
Connect the Engine Block to the Battery Negative
with a Heavy Battery Cable
.
In installations with two engines and two battery
banks with cross-connect starting capability, there
must be two heavy conductors between the
engines. Local standards if appropriate should be
complied with.
A.C. Systems:— For a vessel with both AC and DC
circuits, it is essential for safety reasons that the AC
system has a separate earth wire.
Dock potentials can be as positive as +350mV
relative to a silver / silver chloride reference
electrode. If a vessel with bonded aluminium Jets is
connected to such a dock potential through the
separate earth wire, without the protection of an
Isolation Transformer or a Galvanic Isolator , the
corrosion rate of the aluminium would increase to
a value far greater than the normal sea water
corrosion rate. This would occur regardless of
whether the aluminium was protected by anodes or
not.
A. Using an Isolation Transformer
If using AC shore supply, the recommended method
of preventing potentially serious galvanic or stray
current corrosion is to install an Isolation
Transformer on board at the incoming line.
When an Isolating Transformer is used, there must
be no connection between the shore supply earth
and the vessels earth bonding system. The primary
C
AUTION
An Isolation Transformer or a
Galvanic Isolator must be
correctly fitted to the vessel’s
electrical system if the vessel is to
be connected to an external AC
shore supply.