Notes On Examination For Registration As A Grade C Electrical Worker

Appendix II

Examination Syllabus for Registration as a Grade C Electrical Worker

Reference: Code of Practice for the Electricity (Wiring) Regulations, IEE Wiring Regulations, local Supply Rules, Electrical Engineering handbooks, relevant international standards and publications in connection with design, installation, commissioning and maintenance of electrical installations.
  1. General Knowledge

    1. Familiarization of relevant local legislative regulations and Electricity Ordinance, such as requirements for periodic testing and certification, obligations of electrical contractors and workers, registration requirements etc.
    2. General understanding of the effect of electromagnetic fields and electric shock. Causes and prevention of electrical related fire incidents. Electrical accident investigation.
    3. General understanding of the need and procedures to assure electrical safety at work, it includes but not be limited to the safety precautions for working on low voltage installations, interlocking systems, circuit identification, safety documentations etc.
  2. Design and Applications

    1. General understanding of various types of earthing systems their merits and demerits (e.g. TT, TN-S, TN-C, TN-C-s etc.), in particular the system adopted in Hong Kong. Selection of protective conductors and earth electrode. Calculation of earth fault loop impedance.
    2. Applications, design principles, principle of operation and characteristics of various types of power transformers and switchgears.
    3. Applications, design principles, principles of operation, characteristics and starting methods of various types of electrical machines such as synchronous machines, induction machines, a.c. commutation machines, d.c. machines etc.
    4. Applications, characteristics, sizing and selection of different types of cables/overload lines with due considerations to load factor, diversity factor, power factor, voltage drop and maximum demand of the installation.
    5. Factors affecting the design of substations, main switchroom, transformer room and emergency generator room such as clearance space, ventilation requirements, layout of electrical equipments, accessories etc.
    6. General understanding of circuit arrangement and segregation of different circuit categories.
    7. General knowledge of the design of power distribution system such as load estimation of an electrical installation, single-line diagram representation of design, sizing and selection of different types of emergency generators to cope with essential load, power factor improvement, Supply Rules of the Power Companies including computation of tariff, conditions of supply etc.
    8. Operating principles, characteristics and application of different types of protection schemes, protective devices on different types of electrical installation. Co-ordination and discrimination of protective devices. Calculation of fault levels of distribution system for deciding on the ratings of the protective devices.
    9. Protection of structures against lightning.
    10. General understanding of the affects caused by non-linear loads in the power system and methods of improvement.
    11. Illumination level calculations and application of different types of lighting equipment.
    12. Requirements for installations under adverse conditions such as the presence of water or high humidity, presence of corrosive or polluting substances, installation exposed to fire or explosion, etc.
  3. Installation, Testing and Commissioning

    1. General understanding of wiring installation enclosures such as steel conduit system, steel trunking system, plastic or PVC conduit or trunking, busbar trunking system etc.
    2. Pre-commissioning and commissioning tests on different types of electrical equipment such as generators, transformers, cables/overhead lines, switchgears, motors, capacitors, protective devices etc.
  4. Maintenance

    1. Corrective maintenance such as fault diagnosis and rectification techniques on electrical installation.
    2. Preventive maintenance and periodic tests such as functional tests on protective and control devices, wiring insulation resistance test, running test on motors, circuit protectors' tripping and operating mechanism check etc.

Sample Examination Questions for Part A

1. The current rating of a protective device is the current which:

a. it will carry continuously without damages.
b. will ause the device to operate.
c. will cause the device to operate within 30 minutes.
d. is equal to the fusing factor.


2. The rationale of sizing a live conductor based on current carrying consideration is to:

a. protect the equipment supplied by the live conductor.
b. protect the live conductor itself.
c. protect the insulation of the live conductor from excessive temperature
d. protect the system from earth fault.


3. The characteristics of overcurrent IDMT relay normally used in Hong Kong to protect the low voltage main incoming circuit breaker is of:

a. standard type.
b. very inverse type.
c. extremely inverse type.
d. any of the above.


4. A circuit protective conductor is mainly used to:

a. bond the exposed conductive parts to the extraneous conductive parts.
b. connect between the exposed conductive parts.
c. bond between the extraneous conductive parts when one of the parts is earthed.
d. connect the exposed conductive parts of equipment to the main earthing terminal.


5. For Class 1 equipment, protection against electric shock depends on:

a. single insulation.
b. double insulation.
c. single insulation and equipotential bonding.
d. single insulation and connection of exposed conductive parts to main earthing terminal.


6. A low voltage is a voltage

a. not exceeding 220 V root mean square (r.m.s.) alternating current (a.c.) but exceeding extra low voltage.
b. not exceeding 380 V r.m.s. a.c. but exceeding extra low voltage.
c. not exceeding 1000 V r.m.s. a.c. but exceeding extra low voltage.
d. not exceeding 3750 V r.m.s. a.c. but exceeding extra low voltage.


7. Overload current is:

(i) an abnormal current caused by insulation breakdown.
(ii) an abnormal current caused by current flowing from live conductor to earth.
(iii) an abnormal current occurring in a circuit which is electrically short-circuited.
(iv) an abnormal current flowing from the live conductor to the protective conductor.

a. all of the above
b. (iv) only
c. (i) and (ii)
d. none of the above


8. The protective conductors used for circuit protection against earth leakage faults cannot take the form of:

(i) a bare copper wire in parallel run with the cable.
(ii) the outer portion of the metallic conduit.
(iii) the steel wire armour of armoured cable.
(iv) the uter portion of the Polyvinylchloride (PVC) conduit
(v) a bare copper wire connected to the water pipe inside a domestic flat.

a. (i) and (v)
b. (iv) and (v)
c. (i), (ii) and (iv)
d. (ii), (iii) and (v)


9. The main advantages of a single-phase shaded-pole induction motor are:

(i) robustness.
(ii) can rotate at either direction easily.
(iii) high efficiency.
(iv) low manufacturing cost compared to split-phase induction motors.
(v) low noise level.

a. (i) and (iv)
b. (iv) and (v)
c. (i), (ii) and (iii)
d. (ii), (iii) and (iv)


10. The conditions for ensuring discrimination between two fuses connected in series in a circuit are:

(i) the pre-arcing I2t of the up-stream fuse is not xceeded by the total let-through I2t of the down-stream fuse in series with it.
(ii) the up-stream fuse should be a slow blowing fuse whereas the down-stream fuse is a quick blow fuse.
(iii) the down-stream fuse must blow within 0.2 s when current equals to 4 times or higher of the current rating of the entire system flow.

a. all of the above
b. (i) only
c. (ii) only
d. (iii) only


11. The correct test sequence for an electrical installation after completion and before energization of the following is:

(i) insulation esistance.
(ii) polarity.
(iii) continuity of ring final circuit conductors.
(iv) continuity of protective conductors, including main and supplementary equipotential bonding.

a. (i), (ii), (iii), (iv)
b. (ii), (iv), (i), (iii)
c. (iii), (iv), (i), (ii)
d. (iv), (iii), (i), (ii)


12. In estimating the size of conductor it is common to limit the current density to below certain values. This implies that:

a. the copper loss per unit surface area of the conductor are below a certain limit.
b. the copper loss per unit volume of the conductor are below a certain limit.
c. the total power loss per unit surface area of the conductor are below a certain limit.
d. the total power loss per unit volume of the conductor are below a certain limit.


13. The output torque of a 3-phase induction motor is proportional to

(i) motor speed
(ii) magnetic flux set up by the stator and linking with rotor conductors.
(iii) rotor current.
(iv) stator current.
(v) input power.

a. (ii) and (iii)
b. (iii) and (v)
c. (i), (ii) and (iv)
d. (ii), (iii) and (v)


14. Induction motors can have a self-starting torque if

(i) there are more than one phase winding.
(ii) the currents flowing in the different phases must have different magnitudes.
(iii) the currents flowing in the different phases must not be cophasal.
(iv) there must be a centrifugal switch to cut off the auxiliary winding.
(v) the phase sequence must be correct.

a. all of the above
b. (i) and (iii)
c. (i), (ii) and (v)
d. (i), (iv) and (v)


15. Two transformers of different ratings and operating in parallel can be arranged to share the loadings in accordance to their ratings if

a. the ohmic internal impedance of both transformers are the same.
b. the per unit internal impedance of both transformers are the same.
c. an qualizing bar is connected in each of the output phases
d. both are made by the same manufacturer.


16. The temperature-rise of tungsten filament lamps depend on:

(i) the positioning of the lamps (whether vertically up/downward, horizontal or inclined at an angle).
(ii) whether there are harmonics in the supply current even if the r.m.s. value of the currents are the same.
(iii) whether there are harmonics in the supply voltage even if the r.m.s. value of the voltages are the same.

a. (i) only
b. (ii) only
c. (iii) only
d. none of the above


17. Induction motors with a larger pole number is generally larger than a motor with smaller pole number of the same power rating because:

(i) the output torque of the motor depends on the motor size.
(ii) a heavy motor could damp out the low frequency vibration much more readily.
(iii) motors with a larger pole number have a higher magnetic flux per pole.
(iv) motors ith a larger pole number need more space for the additional windings.

a. all of the above
b. (i) only
c. (iv) only
d. (i), (ii) and (iv)


18. The advantages of a deep bar induction motor over a standard induction motor are:

(i) low starting current
(ii) high starting torque.
(iii) lower cost.
(iv) easy maintenance.
(v) more stable operating point.

a. all of the above
b. (i) and (ii)
c. (i) and (v)
d. (ii) and (iv)


19. The protective devices and the earthing arrangements for circuits (except separated extra-low voltage (SELV)) in a typical domestic bath room should

a. provide earth-fault disconnection times of less than 5 s.
b. provide earth-fault disconnection times of less than 5 s, and current ratings of less than or equal to 5 A.
c. provide earth-fault disconnection times of less than 0.4 s.
d. provide earth-fault disconnection times of less than 0.4 s, and current ratings f less than or equal to 13 A.


20. For an electric stove of a kitchen that has four cooking plates and its total current rating is 20A, which of the following points are correct?

(i) It should be protected by a 30 mA residual current device (RCD).
(ii) It should be fed from an exclusive radial final circuit.
(iii) It should be controlled by a double pole switch.
(iv) Its control switch should be placed within 2 m of its location.

a. all of the above
b. (i), (ii) and (iii)
c. (i), (ii) and (iv)
d. (ii), (iii) and (iv)


21. According to BS6651, for large building structure over 20 m high, the recommended number of down conductors in the lightning protection systems is:

a. one per 10 m of the perimeter of the building.
b. one per 20 m of the perimeter of the building.
c. one per 10 m2 of the roof area.
d. one per 20 m2 of the roof area.


22. For a circuit protected by a fuse to BS 88 Part 2, which of the following is/are true for the relationship among, the nominal current rating of the protective device of the circuit (In), the design current of the circuit (Ib), the lowest current carrying capacities of any conductor in the circuit (It):

(i) In 3 Ib
(ii) Ib 3 It
(iii) It 3 In
(iv) Ib 3 In

a. (i) only
b. (i) and (ii)
c. (i) and (iii)
d. (ii) and (iii)


23. In the insulation resistance test of a 220 V installation, which of the following statements are true for the phase-to-phase test:

(i) the insulation resistance should not be less than 500 kW.
(ii) the testing voltage should be 1000 V.
(iii) solid state light dimmers should be isolated.
(iv) during the test, a large installation may be divided into sections with groups of outlets, provided that each group contains not less than 100 outlets.

a. (i) and (ii)
b. (i) and (iii)
c. (ii) and (iii)
d. (ii) and (iv)


24. When a cable runs across an expansion joint of the structure of a building,

a. the cable should be marked with a colour code.
b. the cable should form into a loop.
c. the cable should be installed in a conduit.
d. its enclosure should be sealed with internal fire-resistant barrier.


25. According to the Code of Practice for the Electricity (Wiring) Regulations, rigid PVC conduits or trunking should not be used where the expected normal working temperature may exceed

a. 40°C
b. 60°C
c. 80°C
d. 100°C


26. Which one of the following are effective methods to avoid/reduce the effect of heat from external heat sources to a cable?

(i) installing a thermal shielding.
(ii) increasing the separation distance between the cable and the source.
(iii) reducing the effective current carrying capacity of the cable.
(iv) grouping more number of cables in the same enclosure.

a. (i) and (ii)
b. (ii) and (iii)
c. (iii) and (iv)
d. (i), (ii) and (iii)


27. The internal radius of conduit bend for conduits of size up to 32mm should be at least:

a. 2 times of the inside diameter of the conduit.
b. 2 times of the outside diameter of the conduit.
c. 4 times of the inside diameter of the conduit.
d. 4 times of the outside diameter of the conduit.


28. Which of the following recommendation(s) on illumination is(are) reasonable?

(i) 50 lux at floor level of a main switch room.
(ii) 120 lux on the vertical surfaces of equipment in a main switch room.
(iii) 1500 lux on working plane in a security control room with Closed Circuit Television (CCTV).

a. all of the above
b. (i) only
c. (ii) only
d. (iii) only


29. Which of the following point(s) is(are) merits of metal-halide (MBI) lamp over fluorescent tube?

(i) Wider and higher rated power range.
(ii) Higher efficacy.
(iii) Higher general colour rendering index.
(iv) Shorter start-up (ignition) time.
(v) Shorter re-strike time.

a. (ii) only
b. (iii) only
c. (i), (ii) and (iii)
d. (iii), (iv) and (v)


30. Which of the following is(are) measure(s) to cope with harmonic currents in the 3-phase 4-wire circuits of a large bank of discharge lamps ?

(i) Connect power factor correction capacitors across the supply terminals.
(ii) De-rate the current carrying capacity of the cables used.
(iii) Make sure the neutral conductor is at least the same size as the phase conductor.

a. (i) and (ii)
b. (i) and (iii)
c. (ii) and (iii)
d. none f the above

Sample Examination Questions for Part B

Q1 (a) According to earthing arrangement classification of IEE Wiring Regulations, describe the type of earthing arrangements normally applied in Hong Kong under various situations, as required by the Code of Practice for the Electricity (Wiring) Regulations. What considerations should be paid to the earthing arrangement during the design of an electrical installation?
(b) A 220 V, 100 A circuit has an earth fault loop impedance of 1 W. Suggest a protection scheme against earth fault. State your reasons, assumptions and criteria.
(c) Give the reasons for high normal earth leakage current may exist in some electrical equipment (such as computer workstation). What suggestions can be given or special considerations should be allowed in connecting this equipment (particularly in the earthing aspect)?

A consumer is supplied at 380V 3 phase from a 11 kV substation through a 1500 kVA 11kV/380V transformer and a 380V cable as shown in Fig. Q2. An overcurrent relay supplied through a 3000/5 C.T. is installed at the low voltage side of the transformer to protect the cable and the consumer. The fault level of the 11kV substation is 250 MVA. The impedance of the transformer and the 380V cable are shown in Fig. Q2.

  1. Calculate the maximum fault current which the relay will be expected to measure for any type of fault in the 380V system.

  2. Determine the overcurrent plug setting (PS) of the overcurrent relay if the maximum load current which the transformer can tolerate is 1.2 times full load.

  3. Determine the time multiplier setting (TMS) of the overcurrent relay if the relay operating time cannot be longer than 0.6s under maximum fault current conditions.

  4. After the PS and TMS have been determined, calculate the relay operating time for a single phase to earth fault in the 380V consumer switchboard.

The relay rating is 5A. The PS range of the overcurrent relay is from 50% to 200% in steps of 25%. The TMS range is from 0.1 to 1.0 in steps of 0.1. The operating time of the overcurrent relay can be calculated from the following formula:

Fig.1 where t = operating time of the relay in second
If = fault current through the relay
Iset = current setting of the relay

Fig. Q2

Q3 (a)

Four similar single-phase circuits each having Ib = 13 A are to be wired in single-core 70°C polyvinylchloride (PVC)-insulated PVC-sheathed cables to BS 6004 with copper conductors. The cables are enclosed in conduits totally surrounded by a 150 mm long thermally insulating materials.

The ambient temperature is expected to be 45°C and each circuit is protected by a 20 A miniature circuit breaker (MCB) against both overload and short circuit. The supply voltage is 220 V, single-phase.

  1. Based on the ampacity consideration alone, what is the minimum cross-sectional area of the conductor that can be used?
  2. If the circuit run is 10 m and the voltage drop at the MCB board is 2%, determine if the cable size is adequate from the voltage drop considerations.

For a three-bedroom flat with the following data:

  • 12 nos. of lighting points in 2 nos. of 5A lighting circuits.
  • 1 no. of 20 A final socket outlet circuit serving the kitchen individually (in accordance to Code 6E of the Code of Practice for the Electricity (Wiring) Regulations (COP))
  • 1 no. of 20 A final socket outlet circuit serving the living room, dining room, master bedroom, bedrooms and other area (in accordance to Code 6E of COP)
  • 1 no. of 20 A final circuit serving an air-conditioner in the living room with full-load current of 15 A (in accordance to Code 6E of COP)
  • 2 nos. of 20 A final radial circuits serving an air-conditioner in the bedrooms, dining room and master bedroom, each with a full-load current of 8 A (in accordance to Code 6E of COP)

Assume 220 V single-phase supply and allow for 10 % spare capacity, find:

  1. the maximum demand for the flat;
  2. the current rating of the single-phase main switch.

The single line diagram represents part of the main electrical distribution of an industrial building in Hong Kong. The transformers are supplied and installed by the power company.


(a) Advise on the requirements, with supporting reasons in brief, for selecting, installing and interlocking the two section switches in the above system.
(b) Consider that the Circuit F is for fire services equipment and emergency supply that are also connected to a standby generator of the building. Show schematically how the changeover should be arranged.
(c) State, with supporting explanation in brief, the minimum number of earth fault relays that is sufficient to protect the whole system including the standby generator circuit. Indicate their positions in a diagram.
(d) Draw a current transformer (C.T.) connection diagram of a 3-phase earth fault relay. Based on this diagram, illustrate its operation principle.
Q5 (a) With the aid of a practical connection diagram, explain how one could inject direct current (d.c.) into the windings of induction motors in order to inject rated losses into the motor without any shaft load imposed onto the motor. What are the limitations of your proposed scheme?

At its rated speed of 13000 rev/min a permanent magnet dynamometer delivers its rated armature current of 2 A into a 10 W external resistor. When the external resistor is short circuited, the dynamometer delivers 5 A at the same rated field current and 10000 rev/min.

Is this dynamometer suitable for testing a 4-pole 50 Hz induction motor which demands a peak torque of 0.018 Nm? Give a logical explanation to your answer.

(c) As there is no suitable dynamometer to load up a 100 kW, 50 Hz, 4-pole induction motor in order to assess the thermal characteristics of the motor, it is recommended that the motor should be operated with two power supplies, one at 48 Hz and one at 50 Hz as shown in Fig. Q5. By referring to a sketch of the torque/speed characteristics of the machine in response to 50 Hz and 48 Hz on the same diagram, give a brief explanation in simple words to describe the operating principles of this winding connection. Hence deduce the order of magnitudes of these two voltages ?
Fig. Q5
Q6 (a) A building in Hong Kong is to be equipped with a 1500 kVA, 11000/380 V transformer for supplying electricity. The present consumption without any power factor correction capacitor bank is 1200 kVA and 800 kW. Given that 1 kVAr of capacitor bank costs $620 and the depreciation is 8%, find the most economical power factor of the system and hence the kVAr required to achieve such operating condition. Given that the demand charge for each kVA is HK$ 117.


On a 2 kW kettle connected to a 220 V supply, a ault to earth occurs, through the earthed metal casing of the appliance,at a point a quarter along the length of its heating element from the 'live' terminal, as shown in Fig. Q6. The kettle is protected by a 15 A fuse which is designed to blow at a current of 47 A in one second, 33 A in ten seconds, or 28 A in one minute.

Assuming that the resistance of the earth continuity path between the kettle and the main earthing terminal provided by the electricity supplier (R1) is 1 W and the resistance between the earthing terminal and the earth electrode at the supply station(R2) is also 1 W, ignoring the resistance of the circuit wiring and also that between the heating element and the point of fault contact with the earth casing :

  1. calculate the fault current which will flow;
  2. calculate the voltage between the metal casing of the kettle and the earth electrode at the supply station, and further;
  3. elaborate whether the fuse or the heating element will burn out in your opinion.
Q7 (a) Give the definition of total harmonic distortion (THD) of a current waveform.
Sketch a typical current waveform such that it contains only odd harmonics but not even harmonics, and state the main special features in that waveform.
(b) To achieve better energy efficiency, many buildings have been changing most or all filament lighting systems to discharge lighting systems with electronic ballasts. State the positive and negative impacts of this change on the electrical installations of buildings and propose measures to rectify these adverse impacts.

Design a general lighting system for a classroom with a target illuminance 3 600 lux on a working plane which is 0.9 m above floor. Relevant data are listed below:

  • Dimensions of the room: 5 m (L) x 10 m (W) x 3.4 m (H)
  • Location of the blackboard: at the center of the 5 m long wall, it is a full height blackboard.
  • Reflectance: ceiling: 0.7; wall 0.5; floor cavity: 0.2
  • Total light loss factor is estimated to be 0.78.
  • Luminaire (model SDW236) are surface mounted on the ceiling.
  • In each luminaire, two fluorescent tubes are installed, with 2500 average initial lumen per tube.
  • Photometric data of the luminaire are provided (Remarks A).
  1. Sketch the layout of your design, including your suggested orientation of the luminaire. You are NOT required to make a detailed calculation on glare index. However, you have to observe the relative values between crosswise and endwise glare indices.
  2. What will be the expected illumination level on the floor of the classroom under your design?
  3. If all luminaire are turn off except the one nearest to the blackboard, what is the illumination level at the center of the blackboard?

Remarks A: Luminaire data are provided as an appendix of the appropriate examination paper.


Refer to the diagram Fig. Q8, the load has its phase conductors connected to distribution board P and its neutral conductor to distribution board Q.


(a) Calculate the currents in the load.
(b) Using Millman's Theorem or otherwise, determine the voltage of the neutral when the neutral link of the 150A distribution board Q has been isolated.
(c) Point out the problems and potential dangers of this connection.
(d) If both distribution boards are protected by RCDs, describe what will happen.