Electrical power systems and distribution design for buildings
Domain 1: Electrical power systems and distribution design for buildings
This domain tests whether you can turn a building’s loads into a safe, selective and operable distribution system. The exam expects a single-line diagram, voltage hierarchy, fault-level and voltage-drop checks, and a clear earthing arrangement.
Core rules
- Start at the point of supply and define voltage, frequency, source capacity, fault level and metering.
- Separate HV/LV intake, main switchboard, sub-main distribution and final circuits; show isolation and transfer points.
- Size transformers and generators from maximum demand, not connected load, then check motor starting, harmonics and spare capacity.
- Confirm protective-device breaking capacity exceeds prospective fault current at each board.
- Check voltage drop at the furthest load, not only at the board; include upstream drops.
- Use TN-S, TN-C-S or TT deliberately; never mix neutral and protective functions downstream of the origin.
Worked example
A building has 720 kW maximum demand at 0.90 power factor. Apparent demand is 720/0.90 = 800 kVA. Add 20% design spare: 800 × 1.20 = 960 kVA. Select a 1000 kVA transformer, subject to fault-level and motor-starting checks. The transformer does not need to cover every nameplate load because diversity has already been applied.
Traps
- Sizing from the sum of connected loads.
- Omitting fault level at sub-boards.
- Treating voltage drop as a single cable calculation.
- Drawing a neat diagram that lacks source, isolation or metering.