Hamilton Jet HJ241, Manual

Page: 56 / 202

4.2
4 PRECAUTIONS AGAINST CORROSION
4.1 G
ENERAL
HJ-241 / HSRX
winding shield is earthed to shore while the
secondary winding should be grounded on board
the vessel. Only one side of the secondary winding
is grounded on the secondary side of the
transformer and the vessel’s grounding circuit is tied
in at this point. There must be no DC electrical
connection between the shore supply and the on
board AC circuit. Further information (including
circuit diagrams) can be found in American Boat and
Yacht Council (ABYC) publication E-11
AC and DC
Electrical Systems on Boats.
B. Using a Galvanic Isolator
Alternatively with AC shore supply, a galvanic
isolator can be installed on the AC earth wire just
after the shore power inlet. This isolator isolates the
vessel from low voltage D.C. galvanic currents,
while allowing any short circuit to be safely
conducted back to shore.
Isolators with capacitors are preferred over isolators
with diodes only.The Galvanic Isolator must have an
indicator or alarm that shows whether the earth
current is being blocked or not. This indicator is
required since the Galvanic Isolator only blocks low
voltages (typically below 1.2V) and higher voltages
will pass through the isolator and cause vessel
corrosion damage. If the indicator was not present
then the vessel operator would be unaware of the
corrosion problem. Monitoring of the Galvanic
Isolator is important to ensure proper operation of
the isolator. The isolator must also have sufficient
fault capacity to allow circuit breakers to trip under
fault conditions. Galvanic isolators that comply with
American Boat and Yacht Council recommendations
(ABYC A-28,
Galvanic Isolators ) will meet the above
requirements and are recommended by Hamilton
Jet.
A correctly wired polarization transformer in
conjunction with a galvanic isolator is acceptable
for connecting to an AC shore power system.
4.1.2 Earth Bonding System
In aluminium and most GRP hulls, the Jet Unit, Hull
(if aluminium), all metal objects, electrical
equipment casings and Hull anodes should be
connected with a low resistance bonding system
(separate from normally current conducting 2 wire
electric system).
The bonding strip and connecting wires should be
aluminium or
insulated copper of at least 14.5
sq.mm. cross section area (e.g. 5mm diameter.) to
give very low (e.g. 0.01ohm) electrical resistance.
If a copper bonding strip is used, it should not be
connected directly to the Jet Unit as galvanic
corrosion will occur. The copper bonding strip
should be connected to the Jet Unit via a stainless
steel terminal connector.
The bonding wire or strip which runs the length of
the Hull, should be kept clear of bilge water.
The main function of the bonding system is to
provide a path to battery negative, for stray
currents.
An exception exists for Steel and GRP Hulls whose
reinforcement is carbon fibre; the Jet Unit must be
totally insulated from the Hull and machinery thus
relying totally on its own Anodes for protection.
When a bonding system is used, it is essential that
cathodic protection is provided. This cathodic
protection can be in the form of
Sacrificial Anodes
or an Impressed Current System .
To minimise corrosion from stray current emanating
from within the vessel, all power sources (battery
and battery charger negatives, AC generator and the
ship side of the shore supply earth) should be
connected to the earth bonding system at a single
common earth point. This will hold these circuits at
a common voltage. Any stray currents will then
C
AUTION
Prevention of corrosion
Vessels using Hamilton Jet Units,
must be bonded and wired as
described in Section 4
"Precautions Against Corrosion"
section of the jet unit manual.