Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 11 min read
Electrical installation in buildings
Supply arrangement (typical LV consumer)
Distribution transformer (11 kV/433 V) → LT line → service connection (overhead or underground cable) → cut-out / service fuse (supplier) → energy meter → main switch (with protection, e.g. MCB/RCCB/isolator) → distribution board (DB) → final sub-circuits (lighting, power, AC, etc.).
- Single-phase supply: 230 V (phase and neutral); three-phase: 400 V between lines (4-wire with neutral) — for larger loads.
- Installations must follow applicable electricity safety regulations notified by the Central Electricity Authority (CEA) and relevant Indian Standards (e.g. the code of practice for electrical wiring installations, IS 732, and earthing, IS 3043).
Wiring systems
| System | Description | Advantages | Limitations / uses |
|---|---|---|---|
| Cleat wiring | Insulated wires held by porcelain/plastic cleats on walls/ceilings | Cheap, quick, easy to alter | Poor appearance and protection — temporary installations |
| Batten wiring | Cables (e.g. TRS/PVC sheathed) clipped on wooden battens | Simple, cheap, easy inspection | Not for damp/outdoor locations; limited mechanical protection |
| Casing and capping | Wires run in grooved wooden/PVC casing covered by capping | Reasonable appearance | Fire risk (wood), largely obsolete; PVC channels still used |
| Surface conduit wiring | Wires in PVC or steel conduits fixed on walls | Good mechanical protection, easy rewiring | Visible pipes |
| Concealed conduit wiring | Conduits embedded in walls/slabs during construction | Best appearance, safe, durable, rewiring possible — most common in modern buildings | Higher cost; faults harder to trace; planning needed before plastering |
| Trunking / raceways / cable trays | Metal or PVC channels, cable trays | Large numbers of cables — commercial/industrial | — |
Selection factors: safety, durability, appearance, cost, type of building, environment (dampness, corrosion, fire risk), future extensions.
Wires and cables
- Conductors: copper (higher conductivity, flexible, preferred in buildings) or aluminium (lighter, cheaper, used in larger service/distribution cables).
- Insulation: PVC (general), XLPE (higher temperature rating, power cables), rubber; FR (flame retardant), FRLS (flame retardant low smoke), HFFR/LSZH (halogen free) for public buildings and escape routes.
- Sizes in mm² — commonly 1.0–1.5 mm² for lighting circuits, 2.5 mm² for 6/16 A socket and small power circuits, 4–6 mm² for heavier loads (air-conditioners, geysers), larger for mains — actual size depends on current rating, grouping, ambient temperature, installation method and voltage drop.
- Colour coding — follow the current standard: common practice in India uses red/yellow/blue (or brown/black/grey in harmonised practice) for phases, black or blue for neutral, and green or green-yellow for protective earth. Consistent colours must be used throughout an installation.
FormulaVoltage drop (single-phase, two-wire)
= one-way length; = conductor area. Voltage drop is commonly limited to a few per cent of supply voltage as per the applicable code.
Circuits
- Lighting (light and fan) sub-circuits — limited number of points/load per circuit.
- Power sub-circuits — for socket outlets of higher rating (e.g. 16 A), geysers, air-conditioners — separate circuits.
- Dedicated circuits for heavy appliances; separate DB for each floor/flat in multi-storey buildings.
- Two-way switching (staircase), intermediate switches.
- Load balancing across phases in three-phase installations.
Protective devices
| Device | Function | Notes |
|---|---|---|
| Rewirable (kit-kat) fuse | Fuse wire melts on overcurrent | Cheap; inaccurate rating, can be misused with wrong wire; being replaced |
| HRC (high rupturing capacity) cartridge fuse | Sealed fuse element with filler | Reliable, high breaking capacity; must be replaced after operation |
| MCB (miniature circuit breaker) | Thermal trip (overload) + magnetic trip (short circuit) | Resettable; ratings 6, 10, 16, 20, 32 A etc.; tripping curves B (resistive/lighting), C (general, small motors), D (high inrush loads) |
| MCCB (moulded case circuit breaker) | Higher current ratings with adjustable settings | Main incomers, industrial feeders |
| RCCB / RCD (earlier ELCB) | Trips when residual (leakage) current between phase and neutral exceeds rated sensitivity | 30 mA sensitivity commonly used for protection against electric shock; 100/300 mA for fire protection; does not protect against overload unless combined (RCBO) |
| Isolator / switch-disconnector | Isolates circuits for maintenance | Isolators operate off-load |
| Surge protection device (SPD) | Diverts transient over-voltages (lightning, switching) | Protects electronics |
| Changeover switch | Selects between mains and generator/inverter | Prevents back-feeding |
Principle of discrimination (selectivity): the device nearest the fault should operate first, leaving the rest of the installation energised.
Earthing
Purpose
- Safety of persons — keep exposed metal parts at earth potential so that a fault causes a large fault current that operates protective devices quickly (preventing dangerous touch voltages).
- Protection of equipment and property from fire due to leakage currents.
- Stable system voltage — neutral earthing of transformers/generators.
- Discharge lightning and static charges safely.
Terms
- Earthing (protective) — connecting non-current-carrying metal parts (appliance bodies, conduits, DB enclosures) to earth.
- Neutral grounding — connecting the neutral point of the supply system to earth.
- Earth electrode, earth continuity conductor (ECC), earth lead.
- Neutral and earth are different conductors in the installation; neutral must not be used as earth.
Earthing systems (IEC classification)
| System | Arrangement |
|---|---|
| TN-S | Separate neutral and protective earth conductors throughout from source |
| TN-C-S | Combined PEN conductor in supply, separated at consumer's installation |
| TT | Consumer's exposed parts earthed through their own earth electrode, independent of supply earthing — RCD protection essential because earth fault loop impedance is high |
| IT | Supply isolated or earthed through high impedance — special applications (hospitals, continuous process) |
Methods of earthing (earth electrodes)
| Method | Description |
|---|---|
| Pipe earthing | GI pipe (with holes) driven vertically into ground, surrounded by alternate layers of charcoal and salt (traditional practice) to reduce resistance; watering funnel |
| Plate earthing | Copper or GI plate buried vertically in a pit filled with charcoal and salt |
| Rod earthing | Copper-bonded steel rods driven into the ground |
| Strip/wire earthing | Strips buried horizontally in trenches — rocky or shallow soils |
| Chemical/maintenance-free earthing | Electrodes with conductive backfill compounds |
- Earth resistance depends on soil resistivity (moisture, salts, temperature), electrode size/depth and number of electrodes (in parallel).
- It should be as low as practicable; acceptable values depend on the installation type and the protective devices used (much lower values are required for substations than for small domestic installations) — follow IS 3043 and applicable regulations.
- Measured with an earth tester by the fall-of-potential (three-electrode) method; measured periodically, preferably in dry season.