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 3-Phase Gen Load Earth Single Earth Fault no effect
Insulated Earth 3-Phase Gen Load Double Earth 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
General• Shipboard systems - insulated from earth (ships hull)• Shore system - earthed to the ground• HV systems (>1000V) - earthed to ships 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 faultsAn open-circuit fault is due A short-circuit fault is dueto a break in the conductor, to a double break in theas 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
Insulated neutral systemInsulated system - totally electricallyinsulated from earth (ship’s hull)
Earthed neutral systemEarthed system has one pole orneutral 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 SafetyDO 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