Electrical services principles, wiring, safety

General principles of electrical services

• Safety: protect people and property from shock and fire (earthing, fuses, MCB, RCD/ELCB).
• Adequacy: the number of points, wire capacity and load capacity must be enough for present and future use.
• Economy and simple design: short runs, balanced loads on the phases, minimum voltage drop (not more than 3-5%).
• Reliability and continuity: separate circuits for lighting (5 A) and power (15 A); a fault on one circuit should not stop the others.
• Accessibility: distribution board at an accessible central location; switches at about 1.2-1.4 m height.
• Standards: follow the Nepal electricity rules and standards (NBC 207 / IS 732 / IEC); use ISI/NS-marked materials.
• Appearance and flexibility: concealed wiring where possible; provision for extension; good lighting levels.

Wiring systems

1. Cleat wiring

PVC or VIR cables held in porcelain/wooden cleats on the wall. Cheap and temporary; used on construction sites and at exhibitions. Poor appearance and not safe for permanent use.

2. Casing-capping wiring

Cables laid in the grooves of a wooden casing and covered by a capping. Cheap but old-fashioned; a fire risk and affected by moisture and termites; rarely used now.

3. Batten wiring

Sheathed cables (TRS/CTS or PVC-sheathed) are clipped onto teak wood battens fixed to the wall. It looks neat and is cheap, but is poor against moisture and fire; used in dry places.

4. Conduit wiring

Cables drawn inside PVC or GI pipes (conduits).

• Surface conduit: fixed on the wall surface; easy to repair; used in workshops.
• Concealed conduit: buried in the plaster or slab; best appearance, safe against fire, moisture and mechanical damage. Most used in modern buildings, but costly and difficult to alter.

5. Sheathed (TRS/CTS) and lead-sheathed wiring

Cables with a tough-rubber or cab-tyre sheath (TRS/CTS), or with a lead cover, clipped directly to the surface. Resists moisture; used in damp areas; lead-sheathed wiring is costly.

6. Others

Metal-sheathed cable, bus-duct/trunking (large buildings and industry), underground cable.

Comparison: concealed conduit is the safest and neatest, cleat wiring is the cheapest (temporary work), batten is the cheapest permanent system, and conduit/trunking is used in most multistorey buildings.

Systems of distribution: the tree (branch) system and the ring/loop system are the two layouts; the ring system gives better voltage and reliability.

Safety precautions in using electricity and appliances

• Earthing: connect all metal parts of appliances and the DB to earth (earth pit with a GI plate/pipe, resistance less than 5 ohm for buildings) to avoid shock.
• Protective devices: fuses, MCB, MCCB and ELCB/RCD (30 mA) to cut the supply on overload, short circuit and leakage; a lightning arrester on the roof.
• Proper materials: use standard-quality wires, switches and fittings of the correct current rating, and the proper conductor size for the load.
• Switch only the live (phase) wire: switches and fuses on the phase; neutral kept solid; use 3-pin plugs.
• Insulation and joints: all joints properly made and insulated; no bare wire; use junction boxes.
• Avoid overloading: do not connect many appliances to one socket; use separate circuits for heavy loads (AC, geyser).
• Safe location: keep wiring away from water pipes, gas lines and hot surfaces; place sockets and switches out of reach of children and away from bathrooms (or use waterproof types), at sufficient height.
• During work: switch off the main supply before repair; use insulated tools, rubber gloves and mats, and "danger/men at work" notices; avoid working in wet conditions; employ qualified electricians only.
• Fire precautions: keep extinguishers (CO2/dry powder) nearby; never use water on an electrical fire.
• Testing and maintenance: test insulation resistance (megger) and earth continuity after installation and periodically.