Ship Con/Electrical Flashcards
Q1
(a) State FOUR different causes of earth faults. (8)
(b) For EACH cause stated in (a), explain how the fault is rectified. (8)
a) FOUR causes of earth faults are:
A breakdown of insulation.
Excessive Ambient Humidity.
Excessive heat destroying insulation.
Sea water ingress into the equipment.
(b)
A breakdown of insulation (motor): clean and spray varnish or carry out rewinding.
Excessive Ambient Humidity: Ventilate. In motors ensure the heater is operational.
Excessive heat destroying insulation: Remove the heat source and protect the insulation. Add additional ventilation.
Sea water ingress into the equipment: Wash with distilled water and dry thoroughly.
- With reference to electrical distribution systems
a. State the meaning of the term earth fault (4)
An earth fault is when electrical current starts flowing in a direction where it is not intended to flow. This happens due
to weakness in insulation whereby insulation can no longer contain the electrical current to electrical conductor
b. State four possible causes of earth faults to occur in an electric motor • Dampness in motor
• Isolation weakened
• High Temperature
• If internal connection box is dirty
(8)
c. State three undesirable consequences of earth faults (6)
If an earth fault is not rectified it may lead to failure such as short circuit which can lead to fire and explosion or even
sever injuries and death
Q2
Explain the procedure to be taken in EACH of the following circumstances in order to prepare a large electrical generator set for maintenance:
(a) Taking the a.c. generator off electrical load; (6)
(b) Isolating the machine in order that work may be safely carried out. (10)
- Explain the procedure to be taken in each of the following circumstances in order to prepare a large electrical generator set for maintenance:
a. Taking the A.C. generator off electrical load (6)
• Ensure that sufficient capacity remains on other generator before any load shifting takes place
• Decrease the load of out-going machine using the governor control, increasing the incoming machine maintaining
constant frequency and voltage
• With only a fraction of the load remaining 50-100KW open the circuit breaker of the out-going machine
b. Isolating the machine in order that work may be safely carried out (10)
• Lock and tag the Main Circuit Breaker keeping keys on your person
• Ensure that the generator controls are placed into OFF or MANUAL on the main switchboard and place signage
• Shut down lube oil priming pump, cooling water, fuel and air supply. Lock off and tag the valves and controls. Drain
the pressure of all systems
• Open all indicator cocks ensure that the fuel rack is in the OFF, CLOSED or STOP position
Ensure that the excitation equipment is isolated and all electrical connections proved “dead
Q2
Explain the procedure to be taken in EACH of the following circumstances in order to prepare a large electrical generator set for maintenance:
(a) Taking the a.c. generator off electrical load; (6)
(b) Isolating the machine in order that work may be safely carried out. (10)
(a) Procedure for taking generator Offload:
Prepare and start another generator (incoming generator);
Synchronise and parallel the oncoming generator with the running generator;
Transfer the load from the running generator to the oncoming generator - Turn the “speed adjustor” of both generators 1⁄4 clockwise (fast – for the incoming generator) and anticlockwise (slow – for the running generator). “Turning is to be effected at the same time”, otherwise the frequency settings will be affected.
Open the circuit breaker of the running generator when its load share starts approaching zero.
(b) The procedure to isolate the machine for work to be carried out is as follows:
Define the scope of work
Place “Do Not Start” label on the control panel, isolate the start mechanism
Switch to local start, put down the start lever to the stop position and Engage the turning gear
Open indicator cocks, Shut off fuel, isolate the alternator from the main switchboard feed system
Confirm DEAD using suitable test equipment before work starts
Lock out and tag out any electrical connections specific to the generator Mechanically isolate the pieces of equipment relevant to the work being carried out (e.g. pre-lube pump connections; air and water pipes; etc.)
Ensure a safe working zone
Q1
(a) Describe, with the aid of a sketch, a freeing-port. (8)
(b) Explain how freeing-ports assist in maintaining the stability of a ship. (8)
a) A freeing port assists in maintaining the ship’s stability by allowing quick draining of the accumulated green (sea) water from the decks. Seawater can present a risk to stability if not drained promptly, as it can be of considerable weight and induce a free surface effect.
(b) The freeing port maintains stability by quickly allowing large volumes of water to drain away which would otherwise be able to gather on the weather deck. The effect of this water would increase the mass of the vessel which would increase displacement and draft and improve gravity a little. Having this water collecting on the upper deck would raise the center of gravity, therefore decreasing GM, consequently, the righting lever GZ would decrease. This would adversely affect stability. The free surface effect could further decrease stability due to the large volume of water accumulating on the deck, which could cause a permanent list.
Q1
(a), with the aid of a sketch, a freeing-port. (8)
(a) Describe, with the aid of a sketch, a freeing-port. (8)
(b) Explain how freeing-ports assist in maintaining the stability of a ship. (8)
(ii) Explain how freeing ports assist in maintaining the stability of a ship (16)
Freeing ports remove water from the deck, thereby reducing the free surface effect. If water was left to accumulate this could cause a free surface effect which is a sideways movement of the ships centre of gravity which can therefore cause the ship to list and possibly capsize. The additional weight causes a shift in G towards M, thus reducing stability and this is why freeing ports are used to remove any water collected on the deck
