POWER SUPPLYThe force responsible for electric current is called as Electromotive Force or E.M.F ( E ). Unit is VOLTAGE (V).The rate of flow of electrons is called as Electric Current ( I ) Unit is Ampere (A).The opposition to the flow of any current is Resistance (R)Unit is Ohms ( ).

The opposition to the flow of alternating current (AC) is Impedance ( Z ).Unit is Ohms.

OHMS LAW:

At a constant temperature, a current flowing in a conductor is directly proportional to the voltage and inversely proportional to the resistance of the conductor.

I = V/RR = V/IV = IR

Eg. If I = 5A & R=10, then V=5x10=50V

When resistors are connected in series, the total resistance increases.Rt = R1 + R2When resistors are connected in parallel, the total resistance decreases.Rt = 1 _____________ 1/R1 + 1/R2 Eg. 2 resistors of each 10 when connected, in series total resistance will be 20 in parallel total resistance will be 5

POWERPower (P) is the product of Voltage and the current: Unit Watts (W)P = V x A e.g. If V = 25V & I = 4A then P = 25 x 4 = 100WLoads of same rating, If connected in parallel, then total power consumption will increase.If connected in series then total power consumption will decrease.

eg. If two 100W bulbs are connected in parallel, then the total power consumption, is 200 W.

If two 100W bulbs are connected in series, then the total power consumption, is 50 W.If a bulb of 100 Watts lights for 10 hours, then the total power consumed is 1000 Watt hour or 1 unit.

FUSES AND METERSFuse is a conductor of low resistance made up of an alloy lead and tin. It protects the equipment from excessive current flow.Fuse rating is based on the load connected and not based on the supply Voltage.Fuse rating is measured in amperes, it means, above which current, it will melt and blow.

The meter reading:

Voltage has V printed on the panel and The Current will have A printed on the panel.

The meter measuring all the voltage, current & resistance is called as multimeter.

To check fuses, use the multimeter in resistance and check between the ends.

If zero ohms, then the fuse is OK & if infinity ohms then the fuse is blown off.

Always replace the fuse with same rating.

If not available, replace with any other closest rating, preferably on the lower side.

Do not replace with a higher rating fuse because it will not protect the load. The load may be damaged.

SHIPS POWER SUPPLY

In ships the power supply used for the GMDSS console are :

Ships mains from the main generator.

Reserve supply from the emergency generator.

Emergency supply from the GMDSS battery.

As per SOLAS requirement, an emergency generator should give power supply continuously for 18 hours and should be fitted on an open deck area.

If emergency generator, Complies with the SOLAS, then the GMDSS console battery should work for at least one hour.does not comply with SOLAS then, GMDSS console battery should work for 6 hours.

BATTERYTwo types of cells are available :Primary Cell and Secondary Cell.

Primary Cell :

Not chargeable Use and throw.

Carbon zinc ( Le-Clanche ) cell, of 1.5 VAlkaline Cell of 1.5 VLithium Cells of 2.6 to 3.6 V commonly available as button cells of 3 V. Lithium cells have shelf life of 3-5 years. Never try to charge it, will explode.

In all 3 Safety Equips 18v Lithium cells are used

Secondary Cell : Chargeable Use, charge and reuse.

Ni-Cad cell of 1.2 V shelf life 4 years onboard ships 15 cells are connected in series to get 18V. The electrolyte used is potassium hydroxide.Used for walkie talkies.

Lead acid cell each of 2V shelf life 3 yrs For GMDSS console, 2 blocks of 12V battery are connected in series to get 24 V. each block have 6 cells connected in series to get 12 V.

LEAD ACID BATTERY CONSTRUCTION

In a lead acid cell, the positive plate is Lead Peroxide. the negative plate is spongy lead.the electrolyte used is diluted sulphuric acid.

This cell will give a voltage of 2.2 V (potential difference between the +ve and ve plates.)

The specific gravity of a fully charged cell will show about 1280.

A fully discharged cell will show about 1230.

Specific Gravity : The ratio between the weight of the water and of the liquid in question.

Each cell voltage = 2VOne block of 6 cell battery = 12V

Off load voltage = 25V ( when battery is disconnected from the load i.e.GMDSS console . Mains ON means battery is Off load).

On load voltage = 24V (when battery is connected to the load GMDSS console. Mains Off means battery is on load).

The voltage should not drop below 22V.

AH = Ampere x Hour, is the Current delivering capacity of a battery over a period of time.

Depending upon the requirement it may vary from 70AH to 240AH.

Requirement depends on total power consumption and working hours.

Discharging or charging should be limited to 10 PCT of the total AH.

If a battery has 70 AH, the charging / discharging current should not be more than the rate of 7 ampere per hour.

That means we can draw a maximum of 7A current for 10 hours from a 70AH battery.

Similarly, in a 200AH battery, we can draw 20 A for 10 hours

Defects of Lead Acid battery :

1. Buckling effect:If charged or discharged above the recommended rate of current, large amount of heat will be produced inside the battery, causing the electrodes to bend.Due to this bending, the plates short each other, resulting in reduction in the Voltage.

To overcome this effect, do not over charge or discharge.

2.Sulphation :

If the battery is left idle without charging or discharging for long time, the sulphate from the sulphuric acid starts settling on the electrodes,thus reducing the area of conduction, thereby reducing the voltage.

To overcome this, use trickle charging.

Trickle Charging :

After the battery is fully charged and if the battery is not required for operation and kept in idle condition, keep it charging with a mild current i.e. low rate of charging, say at the rate of about 1A or 2A.

Cells in series increase the voltage Cells in parallel increase the AH.

If two blocks of 12V / 200 AH batteries are connected in series, then the total is 24V / 200 AH. (voltage increased & AH remains same)

If two blocks of 12V / 200 AH batteries are connected in parallel,then the total is 12V / 400 AH.(AH increased & voltage remains same)

ELECTROLITEDiluted Sulphuric acid in Lead Acid cell is 65 percent of water and 35 percent of acid.

Specific Gravity is measured by Hydrometer. Full charge = 1280Completer discharge = 1230

While taking the Specific Gravity readings of each cell, large variations between cells usually mean that one or more cells no longer retain charge and so it is a warning of imminent failure of the battery.

BATTERY MAINTENANCE

1.Daily: On load and Off Load voltage check.

Monthly:

Check Specific Gravity, top up if required and charge fully.(During charge /discharge, the water level will come down from the required marked level and current delivering capacity will be less).

Keep cell terminals clean and dry and coat them with Vaseline or petroleum jelly, (otherwise, sulphate formation on the terminals will restrict the current flow)

Check vent holes, (vent cap of each cell will have number of vent holes to facilitate venting out hydrogen gas during charging).

Tighten terminals with lug connectors to prevent sparking.

Topping up

If electrolyte level is less, then add distilled water up to the marker line or 1 cm above plates, but do not overfill.

Never use or keep concentrated sulphuric acid in the battery room.

Always top up with distilled water only. (if Distilled water not available you may use tap water).

Annually: capacity check.

After topping up, charge the battery fully. (the correct method of finding out the condition of the battery is only Specific Gravity reading and not the voltmeter reading). Note down the Specific Gravity. Then disconnect the battery from normal load. Connect the battery to few bulbs in parallel so that it discharges 10 percent of the total AH.

Example : 200 AH battery means, we can draw 20A current for 10 hours.

This means 20 A x 24 V = 480 Watts (W=IV) (rounded up as 500 watts ).

So, connect, 500w or 2x250w or 5 x 100w bulbs in parallel. The bulb/s should light for 10 hours or a minimum of 8 hours.

Less than 8 hours means the battery is losing its capacity.

SAFETY PRECAUTIONS:

When working on batteries, following effective safety precautions must be taken:

Keep all ventilations open before entering battery room. (Doors & Port holes)Wear protective goggles, rubber gloves and protective clothing.Never use naked flame inside the battery room.Do not wear loose metal ornaments such as neck chains or bracelets.( it may come in contact with the terminals and sparking may take place ).

Exercise extreme care when using metal tools. ( special non conductive tools are recommended ).

Use funnel and beakers to pour distilled water inside each cell.

Never leave any rags or waste cloths inside the battery room.

MANDATORY TEST of GMDSS equipments1.Daily tests :

DSC self test (no radiation)Batteries ON/OFF load (voltage) checkPrinters ( DSC, Navtex, telex, satcom ). Check for sufficient paper.

2.Weekly Tests :MF/HF DSC live test to coast radio station. (or with a ship station, if no CRS replies).VHF DSC live test to be carried out only with duplication DSC on board or nearby ship. But never to a CRS.Reserve source of energy : emergency generator.Survival craft VHF portable not on Ch.16 ( on Ch.06 with wheel house )

3.MONTHLY TESTS :

EPIRB self test (no live transmission)SART using the test facility.

4.YEARLY TESTS :

EPIRB live test by shore technician in the presence of MMD surveyor.HRU test by shore technician