Page 166 - Offshore Electrical Engineering Manual

Calculation Procedure 153

The CIBSE technical memorandum TM5 provides a table of effective reflec-
tances R , but an approximate value can be obtained from:
e
CI × R a
R e = ,
CI × 2 (1 − R a )
where R is the area-weighted average reflectance of the surfaces of the cavity and CI
a
is the cavity index, defined as
LW
CI =
(L + W) d
where d is the depth of the cavity.
The ratio of the spacing between luminaires and their height above the horizontal
reference plane, the spacing-to-height ratio (SHR), affects the uniformity of illumi-
nance on that plane. The nominal and maximum values of SHR should be quoted in
the UF tables (Table 2.14.1).

CALCULATION PROCEDURE
AVERAGE ILLUMINANCE
The following sequence of calculations is recommended in order to calculate the
number of luminaires required to achieve a given average reflectance:

1. Calculate the room index RI, the floor cavity index CI(F), and the ceiling cavity
index CI(C).
2. Calculate the effective reflectances of the ceiling cavity, walls and floor cavity.
This must include the effect of furniture and/or equipment.
3. Determine the UF(F) value from the manufacturer’s tables, using the room
index from the effective reflectances calculated.
4. Calculate the maintenance factor (MF) by multiplying together the various
factors obtained from manufacturer’s curves which reduce effective luminaire
output.
5. Use the lumen method formula to obtain the number of luminaires required.
6. Arrange the lamps in a suitable configuration for the room. If the furniture or
equipment in the room requires variations in spacing, refer to the method given
in the CIBSE Technical Memorandum No.5 and recalculate the number of lumi-
naires using the modified UF(F) obtained.
7. Check that the proposed layout does not exceed the maximum SHR.
8. Finally, calculate the illuminance that will be achieved by the final layout. The
values of illuminance required should be above the following minimum values
for offshore locations:-(Table 2.14.2).