What is the need for preferential tripping in marine electrical distribution system

 In marine electrical distribution system, a major consideration is that the supply of power to the switch board must be maintained, if possible. Otherwise a black out situation due to tripping of alternator breaker can endanger the safety of the vessel itself especially in restricted waters and maneouvring. The breaker therefore is to be arranged to trip instantly only in the event of fault such as short circuit. When over current is not so high, time delays with inverse definite Minimum Time relays are provided by way of preference trips which are designed to shed non-essential switchboard load. Preference trips are operated by relay set at about 110 percent of the normal full load, but with different time delays to open the breakers feeding non essential feeders such as galley load, ventilation fan, AC, etc, thereby reducing the load on alternator and saving the main breaker to trip

conditions for synchronizing of two electrical alternators (generator on parallel) on board ship

Three conditions must be fulfilled before an incoming alternator can be connected to the live busbar system.
 i) The voltage of the incoming alternator must be same or within acceptable limits of the busbar voltage. This to be checked from the Voltmeter.
ii)  The frequency of the incoming alternator must be same or within acceptable limits olthe busbar frequency.However in practice, the incoming alternator frequency is to be kept slightly higher, i.e. synchroscope to move clockwise at a slow speed, because when it will come on load there will be drop in speed and then it will match with the busbar frequency.
iii) Phase angle of the incoming alternator must be matched with that of the busbar, indicated by the synchroscope pointer at 12'0 clock position. Which means phase angle difference between corresponding phases of incoming alternator and that of bulbar must be zero or within acceptable limits.

Explain Skin effect and proximity effect in current carrying conductor. How it effect the system

Skin Effect :
When an alternating current flows through a solid conductor, the distribution of current across the cross section of the conductor found to be non-uniform and the current tends to flow around the surface of the conductor. This effect is called skin effect. It is more pronounced at high frequencies and with conductors of large cross sections. Since the inner core of the conductor is linked with larger amount of magnetic flux than the outer surface, it is having larger impedance to alternating currents than the Outer surface and hence current and current density tend to he greater in the outer surface.This reduces the effective area of section of the conductor. rherefore a hollow conductor bus is best suited and economical to transmit high current.

Proximity effect
When two conductors are placed near to each other carrying high currents. then half portion of the conductor links with more magnetic flux being nearer and the other half gets less flux linkage being further away. This causes non-uniformity of current flow across the cross-section of the conductors due to the increase in impedance in part of the conductors.

What all supplies are given from emergency generator in passenger ship

The emergency source of power shall be capable of operating simultaneously for a period of 36 hours following services, mainly :
a) the ship's emergency bilge pump, if it is electrically operated
b) the ship's watertight doors, if they are electrically operated
c) the ship's emergency lights at every boat station on deck and over side, in all alleyways, stairways, machinery space, radio station, main navigating and central fire recorder equipments.
d)the ship's navigational lights
e) all communication equipments, fire detecting system, and signals
f) the sprinkler pump for fire fighting
e) the ship's daylight signalling lamps.

Emergency generator power supply in cargo ship, its location and sources of load it supply

The emergency source of electric power shall be

a) located above the uppermost continuous deck;
b) readily accessible from the open deck.
c) located aft of the collision bulkhead.
d) so arranged that a fire or other casualty in the space containing the main source of electrical power and associated equipment do not interfere with supply, control and distribution of emergency supplies.
Every ship shall be provided with a self contained emergency source of electrical power which shall be so designed and arranged that it will operate at full rated power when the ship is listed 22.5° and when the trim of the ship is 10° from an even keel or any combination of or up to these limits.

Emergency Supplies

The emergency sources of electric power shall be capable of simultaneously supply the following services, including any starting current and for the following periods:

A)for a period of 18 hours, emergency lighting in

1) all service and accommodation, alleyways, staircases, exits. lifts and lift shafts.
2) In machinery space and main generating station including control positions.
3) in control station. machinery control rooms and at each main and emergency switchboard.
4)  at the storage position for fireman's outfit
5) at the firepump, sprinkler pump and emergency bilge pump and at their starting positions.

B)for a period of 18 hours

1)The fire detection and fire alarm system.
2) The daylight signaling lamps, ships whistle and manually operated call points intermittently.
3) The general alarm.
4) statutory navigational instruments and aids.

C) for the period of 18 hours :-
i) the navigational lights
ii) Any fire pump and emergency fire pumps dependent upon the emergency generator . for their source of power
iii) capable of giving power to main or aux steering gear

Attention seafarers- Uploading sea service in Master Checker by RPS

        As per the instruction from DG shipping,  recruitment and placement of seafarers (RPS) agents are required to submit the details of the sea service of the seafarers recruited/placed by them on board Indian-foreign flag ships, by the 12th of every month. Earlier so many incidents took place in which mmds refused to accept examination application from candidates since their sea service was not reflected in dg shipping website (e-governace). Therefore, it becomes imperative that the RPS entities  upload complete and correct data of sea service in the e-governance system  of dg shipping.

Dg told that it is clearly unfair to penalise the seafarers for a mistake on part of the RPS and decided that the RPS agencies which do not upload the data of sea service of their respective seafarers would face immediate suspension of licenses. The RPS must upload the data by twelfth of every month. If due to any technical problems, the data cannot be uploaded in the same month, then the system would remain open for another two months, thus giving three months time to upload the same. which is quite sufficient. However the RPS agents are instructed that they should not wait till last moment, but complete the uploading in the first month itself.

So it is advised to all seafarers that they should check whether their sign on and sign off date correctly entered in the data base. If it is not added, then they must inform the respective RPS agent to add the same into the database. The dates cannot be entered after a time period of three months. Since seafarers go for a contract of more than three months, they must confirm that their sign on date is entered in the data base. They can check the details in MASTER CHECKER after loging into e-governace and are advised that they should regularly check the Master checker, particularly after their sign on and sign off and ascertain whether the RPS has correctly uploaded the details in the system. If not, they may immediately bring the same to the notice of RPS concerned. In spite of the above measures, if the ,RPS fails to upload the sea service, the seafarer would be provided a facility to claim such missing sea service by uploading the relevant pages of CDC, Passport, letter in support of sea service from RPS and a self declaration on genuineness of the sea service in his profile on e- Governance.

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Spark Erosion in bearing and shaft earthing devices

Spark erosion is micro pitting related to main bearings and thrust bearings where the bearing material (HMO7 or AlSn40) is corroded by electrical sparks from the bearing shell to the main bearing journal due to electro-potential difference between the crankshaft and the bearing. Spark erosion in AlSn40 bearings may develop faster than in HMO7 bearings. micro pitting marks on the sliding surfaces of main bearings and journals might result in a slight transfer of white metal stripes to the journal and faster wear down of bearing material.

Pitting Corrosion

Without a device, voltage differences of up to 350-450 mV can be measured between shaft and bearing. It has proved that sparks are normally not generated at a voltage below 80 mV. To avoid spark erosion, the volt¬age have to be reduced to a value below 50 mV by a shaft earthing device placed on the propeller shaft close to the engine. This can most easily be done by using a solid silver track as a slip ring, and silver/graphite material as brushes. The silver track material is superior because of its low electrical resistance, and the silver/graphite brush ensures that the system is self-lubricating and self-cleaning. The strip is connected to the hull via brush system to reduce the potential between shaft and bearing.

shaft earthing device

 A voltmeter is installed to ensure a continuous display of the potential difference. Readings in excess of 50 mV are a sign of worn brushes or poorly maintained brush gear and/or slip rings. The functioning of the earthing and measuring device is checked, first by releasing the earthing bushes from their contact with the shaft slip ring, which should then give a "high" voltage pulse of 100 - 400 mV, read at the meter. This indicates that the monitoring circuit is in order. Secondly, when the brushes once more are brought into contact with the slip ring, the reading should again fall below the 50 mV limit. The silver track and the silver/carbon brushes should not need any regular cleaning, but if oil is found on the track, it should be cleaned off carefully, using an electro-cleaner or similar solvent'

Notice to All Indian Seafarers from DG

The below message is for all the Indian seafarers from DG shipping. DG shipping is taking strong initiative against all fraud manning agents in India. Read the below message carefully and share with your friends,

Directorate General of Shipping, Govt. of India has noted with concern, the rise in number of complaints received from seafarers who are cheated/ fleeced by unauthorized recruitment and placement agents under pretext of offering them placement on board foreign flag ships. Common problems reported by the seafarers are;

1. In an event of seafarers meeting with an accident, injury, stranding or any other mishap, the unauthorized agencies do not provide any kind of assistance for repatriation / medical aid / compensation etc. 

2. Such unauthorized agencies do not provide assistance or make intervention in disputes regarding wages/compensation, arising between the seafarers and the foreign flag ship owners.  

3. In case of demise of the seafarer, unauthorized agencies avoid taking up responsibility of bringing mortal remains or arranging compensation.  

2. In all the above mentioned complaints, it becomes difficult for DGS, GoI to directly take up the matter with the foreign flag ships, as they do not come under its jurisdictional control.  It is therefore, requested that placement through unauthorized agencies may strictly be avoided, as it may lead to complications/problems for the seafarers. It may be noted that sea service through an unregistered agency cannot be uploaded in the e-governance system of DGS and therefore will not be considered by the DGS for examination/ certification.  It is therefore advised that the seafarers may get their placement on foreign flag ships only through registered placement and service agencies. List of such approved agencies is available in the website www.dgshipping.gov.in. Before considering any offer of appointment from placement agencies, you must check the website of DGS and ascertain whether such agency is registered or not. 

3. There could also be a case of some fraud agency impersonating as a Registered RPSL. As a measure of precaution, before considering an offer of appointment from any Registered RPSL, you may also contact the said RPSL on telephone or by an e-mail and ascertain its genuineness. The contact details of all such registered RPSL are also displayed in the website for information.

  

4. The registered RPSL are required to upload the details of your sea service in the e-governance system of the DGS. Such uploaded details are visible to the seafarer in the master checker of the seafarers profile in the e- Governance system. You are advised to regularly visit the master checker and check whether the RPS has correctly uploaded the dates of your sign on/ sign off in the system in the same month to ensure that your sea-service records are up-to-date.

 (Message copied from dg website)
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