Unit IIIConcept of Building Services

Electrical Systems & Illumination

Power safely in — and the right amount of light on the desk.

≈ 40 min + worked example

Two halves of the same unit. First the supply that must arrive safely — single- and three-phase power, the breakers that protect against fire and shock, and the earthing that gives a fault somewhere safe to go. Then the light that supply produces — measured in lumens, candela and lux, and sized for a room by the lumen method.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Concept of Building Services:

CO3 · Understand

Distinguish single- and three-phase supply and the role of earthing.

CO3 · Understand

Identify protective devices — fuse, MCB and ELCB/RCCB — and what each protects against.

CO3 · Apply

Use the photometric quantities (lumen, candela, lux) and the laws of illumination.

CO6 · Apply

Apply the lumen method to size the lighting for a room.

Power in

The supply, safely

Single-phase 230 V serves small loads, three-phase 415 V the heavy ones. MCBs guard against overcurrent and ELCBs against earth leakage (shock); earthing bonds every metal body to ground.^{[1, 8, 9]}

DiagramOne sine wave for single-phase 230 V and three offset sine waves for three-phase 415 V

DiagramA pipe-earthing pit with the GI electrode in charcoal and salt, and the earth wire bonding an appliance

230 V and 415 V

A single-phase 2-wire supply (phase + neutral) at 230 V serves homes and small loads; a three-phase 4-wire supply (three phases + neutral) at 415 V line-to-line serves heavy and motor loads — lifts, HVAC, pumps. One phase to neutral still gives 230 V.^{[1, 8]}

PhotoAn earthing electrode pit with its inspection chamber — the safe path to ground for fault current.Unknown author · CC BY-SA 3.0 · via Wikimedia Commons

Illumination

Light, measured and designed

A lamp's output is lumens, its intensity candela, and the light landing on a desk lux. Illumination falls by the inverse-square and cosine laws, and the lumen method turns a target lux into a count of fittings.^{[10, 11]}

DiagramA lamp emitting flux in lumens, a cone of intensity in candela, illuminance in lux, and the inverse-square law

The photometric quantities

Luminous flux is total light output in lumens (lm); luminous intensity is flux per solid angle in candela (cd = lm/sr); illuminance is light landing on a surface in lux (lx = lm/m²). They are different things — a lamp is rated in lumens, a desk in lux.^{[10, 11]}

PhotoRecessed LED ceiling panels lighting an interior — the kind of luminaire the lumen method counts.Party Lin · CC BY 2.0 · via Wikimedia Commons

Live calculator

Light a room

A 6 × 4 m office at 300 lux with 4000-lumen panels (UF 0.5, MF 0.8) needs five fittings. Change the room, the target lux and the fitting and watch the count — always rounded up.

Lumen method · how many fittings

Room length6 mRoom width4 mRequired illuminance E300 luxLumens per fitting Φ4000 lmUtilization factor UF0.5 Maintenance factor MF0.8

N = (E × A) / (Φ × UF × MF), rounded up. Target lux: ~150–300 living, 300–500 office, 500–750 drawing office.

0.0 m²

Area A

Fittings needed

0.00

Exact (before rounding)

0 lux

Achieved illuminance

Always round up so the room meets or exceeds the target lux.

At a glance

The distinctions that matter

Aspect	One	The other
Wires / voltage	Single-phase: 2-wire, 230 V	Three-phase: 4-wire, 415 V
Protects against	MCB: overcurrent	ELCB/RCCB: earth leakage (shock)
Light quantity	Lumen: total from source	Lux: landing on the surface
Illumination law	Inverse-square: distance E = I/d²	Cosine: angle E = I·cosθ/d²
Colour synthesis	Additive (RGB light) → white	Subtractive (CMY pigment) → black

Vocabulary

Key terms

Single-phase supply

2-wire 230 V AC for homes and small loads.

Three-phase supply

4-wire 415 V (line-to-line) for motors and heavy loads.

MCB

Miniature Circuit Breaker — trips on overcurrent (not earth leakage).

ELCB / RCCB

Earth-leakage breaker — trips on leakage to earth for shock protection.

Earthing

Bonding metal parts to an earth electrode for a safe fault path (IS 3043).

Luminous flux / candela / lux

Total light (lumen), light per direction (candela), light on a surface (lux).

Utilization factor (UF)

Fraction of luminaire flux reaching the working plane.

Maintenance factor (MF)

Allowance (<1) for dirt and lamp ageing.

Apply it

Worked example

For a 6 m × 4 m office (A = 24 m²) needing 300 lux, with 4000 lm panels, UF 0.5 and MF 0.8: N = (300 × 24) / (4000 × 0.5 × 0.8) = 7200 / 1600 = 4.5 → 5 fittings. Drop the maintenance factor and you would wrongly need fewer — and the room would dim below target as it ages.

Check your understanding

Self-assessment

1. Which device protects against earth leakage (electric shock)?

2. The SI unit of illuminance (light landing on a surface) is the —

3. By the lumen method, a 24 m² room needing 300 lux with 4000 lm fittings (UF 0.5, MF 0.8) needs —

In a nutshell

Recap

Single-phase 230 V serves small loads; three-phase 415 V serves motors and heavy loads.

MCBs handle overcurrent; ELCB/RCCB handle earth leakage (shock); earthing gives faults a safe path to ground.

Light is measured as lumens (output), candela (intensity) and lux (on a surface); illumination falls by E = I/d² and the cosine law.

The lumen method sizes lighting: N = (E × A) / (Φ × UF × MF), always rounded up.

The evidence

References & further reading

[1]NBC 2016 — National Building Code of India, Part 8: Building Services, Section 2 (Electrical and Allied Installations). Bureau of Indian Standards.
[8]IS 732 — Code of Practice for Electrical Wiring Installations. Bureau of Indian Standards.
[9]IS 3043:2018 — Code of Practice for Earthing. Bureau of Indian Standards.
[10]IS 3646 (Part 1):1992 — Code of Practice for Interior Illumination. Bureau of Indian Standards.
[11]CIBSE / IES — Lighting design references (lumen method, recommended illuminance schedules).