Wiring Systems, Cables, and Protection Devices
Wiring systems, cables and protection devices cover selection and erection: which wiring method suits the building, which cable construction suits the environment, and which device protects the circuit without nuisance tripping.
Wiring methods and cables
- Conduit embedded in concrete or plaster, surface conduit, trunking, cable tray and ladder, and direct burial with armoured or sheathed cable. Each method fixes the tabulated current capacity of the cable.
- PVC insulation (70 °C conductor) against XLPE (90 °C); halogen-free and fire-resistant cables for escape routes and life-safety circuits; armoured cable or steel conduit where mechanical protection is needed.
Protection devices
- MCB magnetic trip ranges: type B 3–5×In, type C 5–10×In, type D 10–20×In. Breaking capacity Icn must exceed the prospective fault current at the point of installation.
- MCCBs for larger currents with adjustable settings; RCDs (30 mA for socket-outlets and additional protection); RCBOs combine overcurrent and residual protection.
- Discrimination: the upstream device must not operate for a fault that the downstream device can clear.
Worked example
A 4 mm² PVC cable clipped direct is tabulated at 32 A. Ambient 40 °C gives Ca = 0.87; three grouped circuits give Cg = 0.70. Iz = 32 × 0.87 × 0.70 = 19.5 A. For a 15 A load a 16 A MCB satisfies 15 ≤ 16 ≤ 19.5; a 20 A device would not.
Traps
- Fitting a B-curve MCB to a motor and treating the inrush trip as a fault.
- Ignoring the derating for cables run inside thermal insulation.
- Using an RCD as the sole protection against overload.
- Paralleling cables of unequal length or cross-section, which unbalances the current.