Wiring Systems, Installation and Protection
Core Concepts
Understand wiring systems: cleat, casing and capping, conduit (PVC, metal), trunking, cable tray, and busbar. Know installation methods (surface, concealed, buried) and their impact on current-carrying capacity. Understand protection devices: fuses, MCBs (miniature circuit breakers), RCDs, RCBOs, and their coordination. Know earthing systems (TN-S, TN-C-S, TT) and their characteristics. Understand diversity factor, maximum demand, and balanced loading in three-phase installations. Know the requirements for bathroom and kitchen zones (IP ratings, supplementary bonding).
Traps in Questions
- Confusing MCB types: Type B trips at 3-5× rated current, Type C at 5-10×, Type D at 10-20×.
- RCDs do not protect against overload; they protect against earth leakage.
- Earthing systems: TN-S has separate earth and neutral; TN-C-S has combined PEN split into earth and neutral; TT has local earth electrode.
- IP ratings: first digit = solids, second = liquids; IP44 is splash-proof.
- Diversity: not all loads operate simultaneously; apply diversity factor for maximum demand.
Worked Example / Rule of Thumb
Example: A domestic installation has a 10 kW shower, 3 kW cooker, and 2 kW of lighting and sockets. Apply diversity: shower 100% = 10 kW; cooker 10 kW + 30% of remaining (but cooker is 3 kW, so 3 kW); lighting 66% of 2 kW = 1.32 kW. Total ≈ 14.32 kW. At 230 V, current ≈ 62 A. Select a 63 A main switch and appropriate tails. Rule of thumb: For domestic, allow 100% of largest load plus 40% of others for quick estimate.