Volume 8 – Mechanical and Electrical Services - Malaysia Geoportal

Chapter 7 ELECTRICAL SERVICES
Room Index , K is given as:
L x W (7.3)
Hm( L+W)
Where:
L = length of room
W = width of room
Hm = mounting height of fitting above working plane.
Maintenance factor (MF) is estimated for the effective delivery of average illumination reaching the
working surface. Dirt on the fitting has the effect of reducing its light output from it. The
conventional assumption is that on average, a lighting installation delivers 80% of the light it would
do if it were perfectly clean. Thus the average maintenance factor, MF= 0.8. A higher maintenance
factor, say 0.9, can be assumed if the fittings are cleaned regularly, or it could be as low as 0.5 in a
foundry. Taking dirt into account, the modified illumination formula is
E = CU x MF x installed flux per unit area. (7.4)
Practical Design Example by considering a classroom of floor dimension 7.5m by 9m and ceiling
height of 3m. The lumunaire chosen is 1.2m, 36W fluorescent lamps. IES standards recommended
an illumination of 300 lux for reading room.
To determine how many lamps are needed to achieve this illumination level
Assume the table top is 0.76 m high, The mounting height is 2.1m, then L= 9m, W= 7.5m,
Hm=2.1m.
Room Index, K = L x W
Hm( L+W)
= 9 x7.5
2.1x (9+7.5)
= 1.95
= 2 (approximately)
As the ceiling and walls are painted white, reflection factor of ceiling of 70% and walls of 50% can
be reasonably assumed. From technical data obtainable from manufacturer, or IES guide book,
Co-efficient of utilization, CU = 0.6
Take maintenance factor, MF = 0.8
Therefore, installed flux required = illumination flux x area
CU x MF
= 300(lux) x (9 x7.5)
0.6 x 0.8
= 42,187.5 lm
The lighting design lumens (LDL) = 3650 lm
of 1 x36W fluorescent lamp
Therefore, number of fluorescent = 42,187.5
lamp required. 3650
= 11.6
= 12 nos.(approximately)
7-4 March 2009