This document discusses electrical safety and protection on ships. It notes that most merchant ships have a 440V 3-phase 3-wire power system. Larger loads require higher voltage systems. Switchboards distribute power and include open and dead front types. Safety requirements to prevent electric shock are described, including types of neutral connections and operational considerations when connecting to shore power.

1. ELECTRICAL SAFETY
&
PROTECTION
By Oladokun Suleiman Olanrewaju

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

11. Insulated Earth Neutral
Gen
Load
Earth

12. Insulated Earth 3-Phase
Gen Load
Earth Single Earth Fault
no effect

13. Insulated Earth 3-Phase
Gen Load
Double Earth
Earth short Circuit

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

15. Neutral Earth 1-Phase
Gen
Load
Earth Fault

16. Neutral Earth 3-Phase
Gen Load
Fault in
Earth Line

17. Operational Consideration

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

20. Insulated neutral system
Insulated system - totally electrically
insulated from earth (ship’s hull)

21. Earthed neutral system
Earthed system has one pole or
neutral point connected to earth

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

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

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