MEO Orals on Marine Electro Technology Function 5- Part 2

What is essential load ?

• Essential services are those required for the safety of personal and for the safe navigation and propulsion of the ship.
• They include certain supplies to navigation aids, machinery spaces, control stations and steering gear.

What is non-essential load ?

• It is a load that has no effects on the safe navigation and main propulsion unit when they are cut off.
• They are air conditioning system, some blower fan except E/R blowers, galley power, refrigerated cargo plant, deck equipment, purifier, air compressor, cabin power source except lighting

What is dash pot, where fitted ?

• It is a mechanical device which makes time lag of trip with different viscosity of oil.
• It is fitted in overload trip, preferential trip, under voltage release / trip

What is dash pot ?

• Dashpots are fitted for overload trip to get time delay action, so that breaker will not be opened, due to momentary current surge
• When load current is in excess, it attracts plunger of the solenoid
• Plunger or piston moves up against the displacement of viscose oil or silicone fluid, through a small hole on the piston
• Time lag depends upon hole size, and viscosity of oil
• Load current setting for trip is about 25% above maximum, but should not exceed 50%

What is preferential trip ?

• It is a generator protection device which is designed to disconnect nonessential load from the main switchboard in the event of generator over load or partial failure of the supply.
• Operate after a fixed time delay, causing non-essential loads to be shed
• Usual setting for overload trip is 150% load (50% overload)
• When generator load reaches 110%, preferential Trip comes into operation as follows

First tripping at 5 sec

• Shut down non-essential loads (air-condition, entertainment, accommodation fans, cargo hold fans, amplifiers, etc.) to reduce the generator load

Second tripping at 10 sec

• Shut down essential loads (service required for running the ship properly, leaving the loads of top priority services to maintain propulsion and navigation) if generator load is still high

Third tripping at 15 sec

• Shut down the main generator as last action, if the load is still too high, it may be due to short circuit or insulation breaking.

What is the purpose of reverse power relay ?

• If prime mover failure occurred, the generator would act as a motor; the reverse power relay detects this fault and acts to trip the generator circuit breaker.

What is fuse ?

• Fuse is a protecting device in the circuit against damage from excessive current.
• It is fitted in a circuit to protect the circuit from short circuit and over load.
• Once a fuse is blown, it must be replaced.

Difference between Circuit Breaker and Fuse ?

Circuit breaker

1. Has switching actions to close the circuit or to open the circuit, and has a trip circuit if load current exceeds the set value
2. After tripping, circuit breaker can be reused without replacing any part

Fuse

1. Have only breaking function, and fitted in the circuit to protect the circuit from damaging effect of high current flow
2. It breaks the circuit by melting the fuse metal itself
3. After breaking, the blown fuse must be renewed

 What is an intrinsically safe circuit or equipment ?

1. An electrical circuit or part of a circuit is intrinsically safe, if any spark or thermal effect produced normally (e.g. by breaking or closing the circuit) or accidentally (e.g. by short circuit or earth fault), is incapable of igniting a prescribed gas mixture, under prescribed test condition.
2. An equipment, which cannot released sufficient electrical or thermal energy, under any condition to ignite a particular flammable vapour in its vicinity.

 What is primary cell ?

• It is a chemical cell in which it is possible to transform chemical energy into electrical energy.
• It cannot be recharged.
• The internal resistance of a primary cell is usually high
• Capacity of cell low
• Voltage per cell is also low
• Simple cell consists of copper plate (positive plate) and zinc plate ( negative electrode) and dilute H2 SO4 acid.

What is secondary cell ?

• It is a chemical cell (accumulator) which store up electric energy converting chemical substance into another form while charging.
• The stored electric charge in chemical form transforms back electrical energy.
• It has two types [lead acid and alkaline (Nickel Cadmium battery) type].
• It has low internal resistance
• Capacity and voltage per cell is high. 

Reference:

Oral Guide by – MIN ZAR TAR

MEO Orals on Marine Electro Technology Function 5- Part 2