Electrolytic Corrosion - Testing an Inshore
Fishing Vessel's Electrical System for
Security
1988/7/FG
1. Hazards of an insecure electrical system
Most inshore fishing vessels have a two-wire insulated electrical system to distribute power to the
equipment and services. If the security of this system is broken, i.e. if leakage occurs between
either wire and earth, then an electrical current can flow through the metalwork of the vessel
causing rapid corrosion with subsequent failure of pipework and loss of metal from valves, glands,
propellers, etc. As this can have disastrous consequences, it is advisable annually to test the
electrical system for security as outlined here.†
2. Causes of electrical leakage
Electrical leakage can be caused by:(1)
(2)
(3)
(4)
Broken or perished cable insulation.
Exposed terminations caused by broken or missing covers on terminal blocks.
Moisture ingress into distribution boxes, lights, switches and deck fittings.
One pole of the supply being connected to the case of an item of equipment (this is rare
nowadays but can still be found on older equipment).
3. Electrical system security test
For this test, the electrical system is in effect divided into two sub-systems; the primary power
circuit, comprising the cables, distribution boxes and fuse boxes; and the electrical and electronic
equipment, i.e. the sounders, radars W/T equipment, lights and electrical services.
3.1 Tools and equipment
Avo Model 8 test meter or other high quality moving coil meter with 20,000 ohms/volt movement.
Two six foot test leads.
An independent shore light or hand lamps.
An electrician's tool. kit.
3.2 Preparation
(1) Ensure that all engines are stopped.
(2) Ensure that all equipment and services are switched OFF, i.e.
(a) Radar, sounders, etc., switched off at the equipment cabinets.
(b) Lights switched off at their appropriate switches, usually located in the wheelhouse. Do
not forget the accommodation deckhead and berth lights and any others controlled
from the deck, e.g. whaleback lights or power block floodlight.
3) Ensure that all circuit breakers or switches on the switchboard are selected ON except
where the breaker or switch controls a service or equipment direct. In this case, isolate the
service or equipment by removing the supply fuses from the appropriate fuse box, then
select the circuit breaker or switch to ON.
†
Tests on higher voltage systems should only be carried out by qualified personnel
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3.3 Primary power test
In a secure electrical system there should be no voltage potential between the engine room
metalwork and the basic power source (it is assumed that this is a bank of 24V batteries). To
test the security of the primary power circuit:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)
(22)
Set the test meter to the 25V d.c. range.
Using one of the test leads, connect the positive terminal of the meter to the positive
terminal of the battery, ensuring that good electrical contact is made by removing any
grease and sulphate from the battery terminal.
Choose a convenient bolt head or stud on the main engine, removing all paint, grease, etc.,
from the stud, then connect the negative terminal of the meter to the stud using the other
test lead.
Note whether any voltage reading is obtained on the meter then disconnect the test leads.
Connect the negative terminal of the meter to the negative terminal of the battery using one
of the test leads. Again, ensure that good electrical contact is made by removing any
foreign matter from the battery terminal.
Using the other test lead, connect the positive terminal of the meter to the stud on the
engine.
Note whether any voltage reading is obtained on the meter, then disconnect the test leads.
If a voltage reading is obtained on the meter for either terminal, continue with the following
steps:
Disconnect from the battery the cables leading to the switchboard.
Set the test meter to the ohms range, then connect it between the positive feed cable and
the main engine and check whether a circuit to earth is present.
Repeat step (10) for the negative feed cable.
If during steps (10) and (11) a circuit to earth is evident there is a fault in the vessel's wiring
at some point. To locate the fault continue with the following steps:
Connecting the test meter as detailed in steps (10) and (11) once again, isolate each
service in turn at the switchboard until the resistance indicated on the test meter becomes
acceptably high, i.e. above 500 kilohms.
Having found the service causing the resistance to earth, the fault can then be pinpointed
as follows:
Find the distribution box connected to that particular isolator on the switchboard.
Connecting the test meter as detailed in steps (10) and (11) again, remove the distribution
box fuses one-by-one noting the resistance on the meter after each removal.
If the resistance disappears on removal of one particular fuse, identify the service it
supplies, then, connecting the test meter between the output terminal at that fuse socket
and the main engine, verify that the circuit to earth is in fact in that service.
Examine the condition of the output feed cable to the service, and all junction boxes up to
the service switch, the latter being switched off as detailed in paragraph 3.2, step (2).
Typical faults to look for are:
Cracked, perished or burnt cables
Heavy condensation or water in junction boxes, fuse boxes and switch units.
Exposed terminals.
Note:
If after removing all the fuses a resistance is still present, then the fault will be
in the cable between the switchboard and the distribution box.
Rectify any fault found, then re-test.
3.4 Electrical and electronic equipment test
Having tested the primary power circuit as described in paragraph 3.3, thereby eliminating it
from the investigation, the electrical and electronic equipment should be tested as follows:
(1)
(2)
(3)
(4)
(5)
(6)
Reconnect the battery to the switchboard.
Select all circuit breakers and switches on the switchboard to ON.
Set the test meter to the 25V d.c. range.
Using one of the test leads, connect the positive terminal of the meter to the positive
terminal of the battery.
Connect the negative terminal of the meter to the previously cleaned stud on the engine
using the other test lead.
Switch on each item of equipment in turn, note whether any voltage reading is obtained on
the meter, then switch off again. Disconnect the test leads.
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Connect the negative terminal of the meter to the negative terminal of the battery using one
of the test leads.
(8) With the other test lead, connect the positive terminal of the meter to the stud on the
engine.
(9) Repeat step (6).
(10) If a voltage reading is obtained for any equipment, examine the equipment, particularly for
moisture ingress, incorrect terminal wiring, corrosion of switch contacts and sulphation
around terminal connections.
(11) Rectify any fault found, then re-test.
(7)
This Data Sheet was produced under the MAFF Funded R&D Commission
by Sea Fish Industry Authority, Technology Division,
St Andrews Dock, HULL, HU3 4QE
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