11.1 Accommodation power system.
11.1. Accomodation Power System
Electrical Distribution
Power Distribution The function of a ship’s electrical distribution system is to safely convey electrical power to every item of equipment connected to it. The most obvious element in the system is the main switchboard. The main board supplies bulk power to motor starter groups (often part of the main board), section boards and distribution boards. Transformers interconnect the HV and LV distribution sections of the system. Circuit breakers and fuses strategically placed throughout the system automatically disconnect a faulty circuit within the network.
The main switchboard is placed in the engine control room and from there engine room staff monitor and control the generation and distribution of electrical power. It is very important that every engineer has a profound knowledge of the electrical distribution of the ship’s power. The only way to acquire this knowledge is to study the ship’s power diagrams. Almost all oceangoing ships have an A.C. distribution system in preference to a direct current D.C. system.
Usually a ship’s electrical distribution scheme follows shore practice. This allows normal industrial equipment to be used after being adapted and certified where and if necessary, so it can withstand the conditions on board of a ship (e.g. vibration, freezing and tropical temperatures, humidity, the salty atmosphere, etc. encountered in various parts of the ship). Most ships have a 3-phase A.C., 3-wire, 440V insulated-neutral system. This means that the neutral point of star connected-generators is not earthed to the ship’s hull.
Ships with very large electrical loads have generators operating at high voltages (HV) of 3.3KV, 6.6KV, and even 11KV. By using these high voltages we can reduce the size of cables and equipment. High voltage systems are becoming more common as ship size and complexity increase.
The frequency of an A.C. power system can be 50 Hz or 60Hz. The most common power frequency adopted for use on board ships is 60Hz. This higher frequency means that generators and motors run at higher speeds with a consequent reduction in size for a given power rating. Lighting and low power single-phase supplies usually operate at 220 V. This voltage is derived from a step down transformer connected to the 440V systems.
GROUNDING SYSTEMS IN SHIPBOARD ELECTRICAL NETWORKS.
In electrical engineering, the ground means reference in electrical circuits from which other voltages are measured. The earth point means a solid connection to the earth, which due to its massive section earth becomes your ground. By absence of the earth on board of a ship, the ship’s hull can be used as a substitute for the earth. Depending on the construction of the electrical networks they may or may not be connected to earth potential. In general we can have solidly grounded, reactance grounded, resistance grounded and isolated networks. In isolated networks there is the challenge to detect earth faults. Ships distribution systems are typically isolated in low voltage systems (<1000V AC) and high resistance grounded in high voltage systems. High resistance grounding ensures the trip action in case of an earth fault and prevents short circuit faults in the network. High resistance grounding can therefore not guarantee continuity of service.
Electrical faults
There are three different kind of electrical faults
 copy.jpg)
(a) Earth fault
An earth fault is caused by loss of insulation allowing the current to flow to earth potential. Causes of earth faults are typically breakdown or wear of insulation. The majority of earth faults occur within electrical equipment due to an insulation failure or a loose wire, which allows a live conductor to come into contact with its earthed metal enclosure.
To protect against the dangers of electric shock and fire that may result from earth faults, the metal enclosures and other non-current carrying metal parts of electrical equipment must be earthed. The earthling connector connects the metal enclosure to earth (the ship’s hull) to prevent it from attaining a dangerous voltage with respect to earth. Such earth bonding of equipment ensures that its voltage in reference to earth always remains at zero.
(b)Open circuit fault
An open circuit fault occurs when a phase conductor is completely or even partially interrupted. Causes of open circuit faults are bad connections or a break in the wire. Open circuit faults when intermittent can cause flashes. Open circuit faults when not completely open (bad connection) can cause a lot of heat and are a fire hazard. Open circuits in three phase circuits can cause motors to run on only two phases and create a motor overload.
(c)Short circuit faults
occur where two different phase conductors are connected together. This can be caused by double break loss of insulation, human error or another abnormal situation. A large amount of current is released in a short circuit, often accompanied by an explosion.
Significance of Earth Faults
If a single earth fault occurs on the live line of an earthed distribution system it would be the equivalent to a short-circuit fault across the generator trough the ship’s hull. The resulting large current would immediately cause the line protective device (fuse or circuit breaker) to trip out the faulty circuit. The faulted electric equipment would be immediately isolated from the supply and so rendered safe. However, the loss of power supply, could create a hazardous situation.
The large fault current could also cause arcing damage at the fault location. In contrast a single earth fault occurring on one line of an insulated distribution system will not cause any protective trip to operate and the system would continue to function normally. This is the important point: equipment continues to operate with a single earth fault as it does not provide a closed circuit so no earth fault current will flow.
More important is that if a second earth fault occurs on another line of the insulated system, the two faults together would be equivalent to a short- circuit fault (via the ship’s hull) and the resulting large current would operate protection devices and cause disconnection of perhaps essential services creating a risk to the safety of the ship. An insulated distribution system therefore requires two earth faults on two different lines to cause an earth fault current to flow.
In contrast, an earthed distribution system requires only one earth fault to cause an earth fault current to flow. An insulated system is, therefore more effective than an earthed system in maintenance continuity of supply to essential services. Hence it’s adopted for most marine electrical systems. Note: Double-pole switches with fuses in both lines are necessary in an insulated single-phase circuit.
Earth fault indication by set of lamps
If there is an earth fault, when the test button is pushed lamp will not glow for the faulty phase as there is no potential difference across it. In each phase current will flow without hindrance. In phase having earth fault current will take an easier path than through the resistance. Both ends of the lamp will be in ground potential Instrument type earth fault indicator:
Instrument applies a small DC voltage into the distribution system. The resulting current being measured to indicate insulation resistance of the system. Instrument permits a maximum earth monitoring current of only 1mA and indicates IR directly in KΩ. It gives both visual and audible alarm.
Protection in an earthed system can also be provided using current transformer [CT]. If motor is healthy, phasor sum of current measured by CT is zero. If an earth fault occurs in motor phasor sum of current will not be zero. Current monitored by E/F relay trips the contactor in starter to isolate the faulty motor circuit.
SHIPS AUXILLARY SYSTEM
Ships are self contained units floating at sea and hence there needs to be a source of power generation, more than one in fact, as well as an emergency backup apart from the main power system. The alternators connected to the diesel generators generate electricity which is then distributed to the different systems of the ship via the main switchboard.
In this article related to Ships auxiliary system we will learn about the general arrangement of the diesel generators and main switchboard on ships. Please note that there are various types of ships, of different sizes, carrying different types of cargoes and so on. Hence there cannot be any hard and fast rule which covers each ship but on reading this article you would be able to grasp the main concept.
Main Switchboard
There are two different types of power requirements on ships namely the 440V and 220V. In fact most of us, even not related to marine engineering understand this pretty well, especially in Asia where normally the 440V is used for industrial connections and the 220V for domestic connections. This is simply because industrial equipments require more power for their operation than general domestic items. Similarly there are heavy loads on ships like say big pumps, winches, cranes etc which require heavy power, while the accommodation and other related uses are similar to the domestic requirements on land.
The main switchboard does the same function like a power substation on land that is to distribute electricity to different areas as per the requirements and also provide means to monitor and control the power distribution. There are also starter boards which contain the start, stop buttons for most major electrical motors on the ship.
Emergency Backup
Apart from the main power source, there is an emergency back up in the form of batteries as well as an emergency diesel generator. The batteries are sufficient to cover up for the time between the main power failure and the injection of power via the emergency system. Of course in smaller ships, there is not emergency generator and the battery backup is sufficient to keep things under control until the main power is restored.The emergency power is supplied mainly to equipments like the steering gear, bilge pump, fire pump, navigational equipment, alarm systems and so forth. In short these are the systems which are vital for the existence in the absence of anything esel.
440 power requirements in the accommodation for domestic use appliances ,to be detailed to the candidates giving specific details on safety & wiring practices.
440 Voltage required accommodation appliances.
Refrigeration plant
Air condition system
Washing machine
Drier
Galley blowers
Galley exhaust fan
Accommodation exhaust blower
sanitary exhaust blower
CO2 exhaust fan
Electrical shock
When we talk about accident on a ship, an electrical shock is the worst of all kinds. Electrical wires and connections are present everywhere on a ship and it is important to prevent yourself and others from getting a major electrical shock.
Steps to minimize the Risk of an Electrical shock.
Start with the first round of the day: check all electrical motors, wiring and switches, for abnormal sounds, variation in temperatures and loose connections.
Ensure that all electrical connections are inside the panel box so that no one can touch them accidently.
In accommodation area multiple socket plugs shouldn’t be used.
Turn off the breaker before starting any work on an electrical system.
Use ply cards and notice board as much as possible to inform others about the ongoing work to avoid accidental starts.
Double check electrical tools such as portable drills for any loose wires before attempting any job.
Always wear protective clothing, rubber gloves, rubber kneepads and safety shoes to avoid risk of shock.
Use electrical insulated handle tools for working or checking electrical systems.
Before working, remove jewelry, wrist bands and other conductive items.
When working or removing multiple wires, tape off all, other than one wire you are working on.
Try as much as possible not to work on live system and even if you do so. Be a professional and work carefully, taking all necessary safety precautions and with utmost concentration.
During working in group or pair, organize a tool box meeting and discuss the procedure, risk and hazards of the job in hands.
If you don’t know about the system, ask for assistance. Don’t work without knowing the system.
Always think first about your personal safety and safety of fellow seafarers while carrying out any electrical work onboard.
Electrical Fire safety
The root cause of any electrical fire is the insulation of the circuit or wire. If the insulation is weak or damaged, it may lead to spark, electrical shock of fire in the system causing major accidents and casualty. The best way to avoid electrical fire is to maintain the insulation of electrical wires and equipment.
The insulation of the electric cable is generally made up of rubber of plastic. He amount of smoke generated by the plastic in case of fire is dependent on factors suck as nature of plastic, type of additive used, flame of fire and ventilation arrangements In general, most plastic produce a very dense smoke when heated.
Some plastics burn very clearly, when subjected to heat and flame, producing very less smoke. If insulation used is of urethane foam a very dense smoke is produced and visibility in the room is lost. Some plastics contain Poly vinyl chloride (PVC), which is produces pungent, and irritating odor.
Rubber when used for insulation produces a dense, black, oily smoke and has some toxic qualities. The most common gases produced during combustion of rubber are hydrogen sulphide and sulphur di- oxide. Both these gases are dangerous for health and can be fatal in certain cases.
Ways to Reduce these Hazards.
The following steps should to take as preventive measures.
Cables having E.P.R (Ethylene Propylene Rubber) insulation with necessary sheathing of Poly Chloro Prene or Chloro Sulphonated Polyethylene (PCP or CSP) may be used to protect the insulation against fire.
G.I armor may be used to protect insulation from fire but needs to be earthed.
By using cables having high oxygen index number, the number allotted to material depending on minimum percentage of oxygen required to sustain combustion.
If the material used is having oxygen index number 27, it means that minimum percentage of oxygen required to burn the material is 27% which is well above the normal atmospheric oxygen percentage of 21%. Thus, the insulation material will not catch fire.
Important Precautions for Installation of Electric cables.
The cables and wiring external to the equipment must have flame retardant properties and should be installed in such a manner that is should not interfere with the original flame retarding properties. Cable and wirings for emergency equipment, lighting, communication and signal should be kept away from spaces such as galley, laundries, machinery spaces of category A &other high risk areas. Special precautions are to be taken for cable installation in hazardous area as it might lead to explosion in case of electrical faults.
Terminations and joints are to be made in such a manner that they should retain their original fire resisting properties.
Avoid cable for damage and chaffing during installation.
Fireproof gland to be used in case of cables passing through the bulkhead to prevent fire from one compartment to other.
