ELECTRICAL SAFETY

&

PROTECTION

By Oladokun Suleiman Olanrewaju

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

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

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

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

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

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)

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.

Types of Neutral Connections

• Insulated Neutral System

• Earthed Neutral System

• Earthing Resistance Neutral System

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

Insulated Earth Neutral

Gen

Earth

Load

Insulated Earth 3-Phase

Earth

LoadGen

Single Earth Fault

no effect

Insulated Earth 3-Phase

Earth

LoadGen

Double Earth

short Circuit

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

Neutral Earth 1-Phase

Gen

Earth

Load

Fault

Neutral Earth 3-Phase

Earth

LoadGen

Fault in

Line

Operational Consideration

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

3 basic circuit faults

An open-circuit fault is due

to a break in the conductor,

as at A, so that current

cannot flow

An earth fault is due to a break

in the insulation, as at B,

allowing the conductor to touch

the hull or an earthed metal

enclosure

A short-circuit fault is due

to a double break in the

insulation, as at C,

allowing both conductors

to be connected so that a

very large current by-

passes or "short-circuits"

the load.

Insulated neutral system

Insulated system - totally electrically

insulated from earth (ship’s hull)

Earthed neutral system

Earthed system has one pole or

neutral point connected to earth

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

• 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

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

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.

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.

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