Volume 8 – Mechanical and Electrical Services - Malaysia Geoportal

Chapter 7 ELECTRICAL SERVICES
The distance between luminaries and the wall should not exceed one-half the distance between
luminaries. Where desks or benches might be located along the wall, the distance between
luminaires and the wall should not exceed 750mm.
Average Illumination is the measure of the average concentration of light on a surface. The unit of
illumination is the lux as mentioned above. Thus if 50% of the light output of (2) nos. 36W
fluorescent lamps ultimately fall on a working plane measuring 3m by 3m. What is the average
illumination
The lighting design lumens of the fluorescent lamps are given by the manufacturer to be 3650
lumens.
Total light output
= 2 x 3650 lm
Light reaching surface = 50/100 x 2 x 3650
=3650 lm
Area = 3x 3 m 2
Therefore illumination, Eav = incident light (7.1)
Area
= 3650
3 x 3
= 405.56
= 406 lux (lm per m 2 )
Installed Flux is the product of the light output of the luminaire and their number installed.
A room is lit by 4 nos. of 1200mm, 36W fluorescent lamps. What is the Installed flux
Installed flux = 4 x 3650
= 14,600 lm.
If working plane =36 m 2 , then
Installed flux per unit area = 14,600
36
= 405.56
=406 lux
Co-efficient of Utilization (CU) is the ratio of the actual flux received on a working plane to the
installed flux. It is a measure of the degree to which the installed lamps has been usefully applied.
This ratio depends on the proportions of the room, the design of the fitting, and the reflection
factors of the rooms’ surfaces. Illumination thus can be expressed as:
E = CU x installed flux per unit area. (7.2)
(provided if the lighting installation is perfectly clean)
March 2009 7-3