2. Power Systems
• Majority of merchant ships have a 3-phase 3
wire, 440 V insulated neutral to earth power
systems
• This power system falls in the category of LV
and meets the power demands of medium
capacity motors up to 200 kW
3. contd
• When large loads are connected to the LV
system the magnitude of current flow becomes
too large resulting in overheating due to high
iron and copper losses
4. Switch Boards
• Switchboard is the main electrical power control
center of the ship
• All generators deliver their electrical power to the
main switch board.
• Main switch board supplies power to section boards,
motor starters, lighting boards and other distribution
lines
• Main switch board is connected to emergency switch
board/ generator/battery back up
5. Types of Switch Boards
• Open front Type
– All essential switch gears are exposed in front of
the panel. Used for dc power system
• Dead Front Type
– All switch gears and live parts are concealed
behind the front sheet panel
– Only the operating handle and instruments appear
on the front. Mandatory for ac power system
6. Contd
• Dead front SB is made accessible by providing
hinged doors in the front panel
• Some times too many equipment crowd the SB
which make maintenance difficult
• Indication Lamp is provided in the
switchboard to show that the shore supply is
available for connection to bus bar
7. Safety Requirement for Electrical System
• The most common accident is the one caused by
touching a live part – whereby the human body
becomes a part of the circuit.
• A current of 25 mA flowing from hand to hand is
already sufficient to cause irregularities in the
heartbeat.
• At about 50 mA a person can lose consciousness
and his heart can stop beating resulting fatal.
• Resistance of the human body (of the parts
through which current is flowing) from hand to
hand is about 1300 , contact resistance of the
skin from 500 (dry) to 5 k (wet)
8. Safety Requirement for Electrical System
• Dangerous Contact Voltage:
- It is the result of the product of the resistance of the
body and the current intensity: (ex: 1300 x 0.025 A
= 32.5 V)
- An AC voltage of 50 V is considered dangerous .
- Children, owing to their lower body resistance are not
allowed to work with voltage exceeding 24 V.
9. Types of Neutral Connections
• Insulated Neutral System
• Earthed Neutral System
• Earthing Resistance Neutral System
10. Insulated Neutral to Earth
• This system is totally insulated from the ship’s hull
• This system maintains continuity of power supply to
the equipment even in the event of single phasing
fault.
• This ensure power supply to critical equipment
• The power supply to the equipment can disrupt only
if two single phase faults occur simultaneously in two
lines which is then equivalent to short circuiting
faults
• But such fault occur very rare
14. Neutral Earthed System
• Earthed Neutral is done in 3-phase system for supply
voltage of 3.3-6.6 kV and above
• If a solidly earthed system is used for such high
voltage then magnitude of earthed current will be
extremely high which can damage the equipment
• The earth current is limited by connecting a resistor in
series between earth and neutral point
• The earth current due to single phasing is limited not
to exceed rated current at maximum load
18. General
• Shipboard systems - insulated from earth (ship's hull)
• Shore system - earthed to the ground
• HV systems (>1000V) - earthed to ship's hull via
neutral earthing resistor (NER) or high impedance
transformer to limit earth fault current
• Priority for shipboard - maintain electrical supply to
essential equipment in event of single earth fault
• Priority ashore - immediate isolation earth-faulted
equipment
19. 3 basic circuit faults
An open-circuit fault is due A short-circuit fault is due
to a break in the conductor, to a double break in the
as at A, so that current insulation, as at C,
cannot flow allowing both conductors
to be connected so that a
An earth fault is due to a break very large current by-
in the insulation, as at B, passes or "short-circuits"
allowing the conductor to touch the load.
the hull or an earthed metal
enclosure
22. The preferred system??
• If earth fault occurs on insulated pole of ‘EARTHED
DISTRIBUTION SYSTEM’ - equivalent to ‘short
circuit’ fault
• Large earth fault current would immediately ‘blow’
the fuse in line conductor
• Faulted electrical equipment immediately isolated
from supply & rendered SAFE, but loss of equipment
• Could create hazardous situation if equipment was
classed ESSENTIAL
23.
24. • If earth fault ‘A’ occurs on one line of ‘INSULATED
DISTRIBUTOIN SYSTEM’ - not trip any protective gear &
system resume function normally
• Thus, equipment still operates
• If earth fault ‘B’ developed on another line, 2 earth faults
would equivalent to a short-circuit fault & initated protective
gear
• An insulated distribution system requires TWO earth faults on
TWO different lines to cause an earth fault current.
• An earthed distribution system requires only ONE earth fault
on the LINE conductor to create an earth fault current.
• Therefore an insulated system is more effective than an
earthed system - maintain supply continuity to equipment, thus
being adopted for most marine electrical systems
25. High voltage system
• Shipboard HV systems - ‘earthed’ via resistor
connecting generator neutrals to earth
• Earthing resistor with ohmic value - chosen to limit
maximum earth fault current < generator full load
current
• Neutral Earthing Resistor (NER) - assembled with
metallic plates in air – due to single earth fault will
cause circuit disconnected by its protection device
26.
27. DO`S of Electrical Safety
• DO operate and maintain equipments according to
manufacturers recommendation or ship owners
procedures
• DO ensure that al guards cover are securely fitted and
that all bolt and fixings are in place and tight.
• DO switch off and lock off supplies, remove fuses
and display warning notices before removing cover of
equipment for maintenance
• DO confirm that circuits are DEAD before touching
conductors and terminals.
28. DO`S of Electrical Safety
DO Get to know the ships electrical system and
equipments-
• Study the ships to mark the location of switches and
protection devices supplying distribution boards and
essential items of equipments
• Write down these information in note book ,
• Become familiar with the normal indications on
switchboard instruments so that abnormal operations
can easily be detected.
29. DON`T of Electrical Safety
• DO NOT touch live conductors under pretext
• DO NOT touch rotating parts
• DO NOT leave live conductors or rotating
parts exposed
• DO NOT overload equipments
• DO NOT neglect or abuse equipments
