Electrical Discipline Core Subject Areas (Scheme "A" Training)
What this domain tests
The Electrical Discipline paper samples the technical breadth defined by the HKIE Scheme "A" training objectives for electrical engineers, applied to Hong Kong codes and site practice. Questions are calculation-bearing and design-oriented: power systems, LV installation design, building services electrical systems, protection and control, and testing and commissioning.
Core areas to command
- Power systems: per-unit working, short-circuit and fault-level calculation, protection grading and discrimination margins, motor starting and voltage dip, transformer and generator operation, system earthing arrangements.
- LV installation design: the IEE Wiring Regulations as applied in Hong Kong together with the local Code of Practice for the Electricity (Wiring) Regulations; current-carrying capacity with derating factors; voltage drop; protective conductor sizing; discrimination and back-up protection; RCD selection and earth fault loop impedance.
- Statutory framework: Electricity Ordinance (Cap. 406) and its subsidiary regulations, the Supply Rules, registered electrical worker grades and the scope of work each permits, Buildings Ordinance interfaces, and EMSD energy efficiency codes.
- Systems: standby generation and automatic transfer, UPS topologies, harmonic distortion and power factor correction, lightning protection to BS EN 62305, fire alarm and ELV interfaces, lift supplies.
- Delivery: testing and commissioning (insulation resistance, continuity, loop impedance, polarity, phase sequence, interlocks and functional tests), documentation and handover.
Worked example
A 200 A final circuit uses a 4-core XLPE copper cable whose tabulated rating is 319 A. Ambient 40 °C gives Ca = 0.87; grouping of three circuits gives Cg = 0.80. Then Iz = 319 × 0.87 × 0.80 = 222 A. Because Ib (200 A) ≤ In (200 A) ≤ Iz (222 A), the cable is adequately protected. The second check is voltage drop: at 200 A over 80 m, the drop from the origin - including source impedance, not the cable alone - must sit within the design limit (commonly 4% of nominal for the installation). Only after that, plus earth fault loop impedance and thermal withstand (k²S² ≥ I²t), is the design complete. Candidates who stop at Iz lose roughly half the marks.
Traps that cost marks
- Using tabulated ratings without applying Ca, Cg, Ci and thermal/soil factors, or applying them in the wrong order.
- Checking voltage drop across the cable only, ignoring transformer and source impedance.
- Confusing registered electrical worker grade scope with an engineer's responsibilities.
- Quoting a British Standard clause without the Hong Kong amendment or the local Code of Practice.