Supply Systems
Supply Systems tests what you know about the electricity you are given before your installation begins: the voltages, the earthing arrangement, the prospective fault current and the limits the supply company imposes. Design answers that ignore the supply characteristics lose marks immediately.
Hong Kong supply characteristics
Hong Kong low-voltage supplies are 50 Hz, 220 V single-phase (phase to neutral) and 380 V three-phase three- or four-wire, provided by CLP Power or HK Electric. Distribution is fed from 11 kV or 33 kV networks; bulk transmission is at 132 kV, 275 kV and 400 kV. A consumer is given a point of supply, a cut-out fuse, the supply company's meter, and an approved loading in amperes.
What you must establish before design
- Nominal voltage, frequency and number of phases.
- Type of earthing arrangement (TN-S, TN-C-S or TT) and the means of earthing at the origin.
- External earth fault loop impedance Ze and prospective short-circuit current at the origin.
- Approved loading, and whether an increase must be applied for.
- Tariff and metering arrangement, plus any supplier restrictions on the position of the intake.
Earthing arrangements in practice
In TN-C-S the supply neutral is combined with the earth and the consumer's main earthing terminal is bonded to it, so the installation must not add a parallel earth electrode as a substitute return path. In TN-S the earth is a separate supplier conductor. In TT the installation relies on its own electrode and an RCD, because loop impedance is too high for overcurrent disconnection within the required time. Earthing arrangement determines disconnection times, bonding conductor sizes and whether an electrode is needed.
Worked example
An existing 100 A single-phase supply serves a shop with a measured maximum demand of 45 A. A new 12 kW instantaneous water heater is proposed: 12 000 / 220 = 54.5 A, bringing the total to 99.5 A with almost no margin and a voltage drop that will breach the 4% limit at the far end. The correct answer is to apply to the supply company for an increased or three-phase supply, then redesign the submain; adding the load to the existing intake would overload the cut-out and meter tails.
Traps that cost marks
Quoting 230/400 V European nominal values; assuming TN-S when the intake is TN-C-S; sizing main bonding from the water pipe rather than from the supply neutral; and ignoring the need to consult the supply company before increasing load.