Question 1
(a) Explain each of the following terms used in electrical installation:
(i) open circuit voltage;
(ii) closed circuit voltage.
(b) State three causes of each of the following faults in a d. c. generator:
(i) no output;
(ii) low output
(iii) noisy generator
(c) List five materials used for surface wiring.
(d) (i) Outline three ways of maintaining a lead-acid battery.
(ii) State two methods of measuring the efficiency of a battery.
(e) State two ways of rescuing a victim of electric shock.
Observation
1(a): It is evident that the candidates possess a good understanding of the concept of open and closed circuit voltage, as most of them were able to define it accurately.
1(b): The candidates' performance was deemed fair, as the majority of them were unable to identify the causes of faults in d.c. generators.
1(c): Most candidates were able to list materials used for surface wiring.
1(d): Candidates performed well in (i) but poorly in (ii) as most of them could not state the methods of measuring efficiency of a battery.
Expected response:
- EXPLANATION OF TERMS USED IN ELECTRICAL INSTALLATION
- Open circuit voltage
It is the voltage when there is no load/offload voltage
OR It is the voltage when current is NOT flowing through the load
- Closed circuit voltage;
It is the voltage when there is load /is the voltage on load
OR It is the voltage when current is flowing
- CAUSES OF FAULTS IN A D.C GENERATOR
- NO OUTPUT
- Sticking brushes
- Corroded commutator
- Shorted/open armature
- Shorted/open field
- Dirty commutator
- Defective AVR
- Tripped breaker
- Blocked/Faulty excitation circuit
- Loss of residual magnetism
- Broken conductor
- LOW OUTPUT
- Loose or worn drive belts
- Low brush spring tension
- Dirty commutators
- Corroded commutator
- Worn commutator
- Low voltage on the alternator
- Loss of residual magnetism on the alternator
- NOISY GENERATOR
- Loose mounting
- Loose pulley
- Worn or dirty bearings
- Weak brush spring
- Faulty/broken radiator fan
- Broken/loose exhaust pipe/silencer
- MATERIALS USED FOR SURFACE WIRING
- Cables
- Clips
- Nails (screw, brass, concrete)
- Insulation tape
- Rawl plug
- Wooden block
- Boxes
- Consumer unit
- Connector
- Switch
- Socket
- Ceiling rose
- Joint box
- Lamp holder
- (i) WAYS OF MAINTAINING A LEAD-ACID BATTERY
- Re-charge battery when voltage and specific gravity fall
- Recharge battery regularly even if not in use
- Electrolyte should not be added to a battery unless there is a spillage
- Battery should be kept topped up with distilled water above plate level
- Do not overcharge, boost charge or over discharge a battery
- Always allow battery to charge fully
- Always check cable, plugs and chargers regularly
- Disconnect cable from battery terminals when not in use
- METHODS OF MEASURING THE EFFICIENCY OF A BATTERY
- The ampere-hour efficiency
- The watt-hour efficiency
- Columbic efficiency
- Voltage efficiency
- PROCEDURES FOR RESCUING A VICTIM OF ELECTRIC SHOCK
- Switch off supply
- Remove victim from the danger with an insulated material
- Remove any obstructions to breathing
- Commence artificial respiration
- Call a medical personnel