Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 194

Table 13.2 Maintenance factors covering dirt accumulation on luminaires with different IP codes
in different polluted environments for an operation period of 1 and 3 years; according to CIE ( 2003 )
and UK: Sanders and Scott ( 2008 )
IP code

Pollution category CIE 1 year UK 1 year CIE 3 years UK 3 years

IP2x High 0.53 0.42

IP2x Medium 0.62 0.53

IP2x Low 0.82 0.78

IP5x High 0.89 0.76

IP5x Medium 0.90 0.82

IP5x Low 0.92 0.88

IP6x High 0.91 0.94-0.97 0.83 0.89-0.94

IP6x Medium 0.92 0.87

IP6x Low 0.93 0.98 0.90 0.94

estimated in advance, and the lighting level of the new road-lighting installation
should be increased above the specified lighting level according to this estimation;
only then will there be a guarantee that the lighting will remain within specification
throughout the life of the installation. The longer the interval between maintenance
periods, the lower the maintenance costs will be. But, at the same time, the greater
will be the effect of lamp failures, lamp lumen depreciation and dirt accumulation
on lumen output and thus the higher the lighting level required for the installation
when new. For each situation, therefore, a specific cost calculation has to be made,
based on the actual cost and light-depreciation conditions, in order to determine the
optimum maintenance schedule. Lamps can be replaced individually, as they burn
out, or all the lamps in the installation can be replaced in a group-replacement main-
tenance action at one time, after a reasonable number of burning hours, whether they
have failed or not. It is usual, during the group-replacement operation, to clean the
luminaires and inspect the electrical and mechanical connections. Sometimes a com-
bination of group replacement and individual spot replacement (for failed lamps at
critical locations) is adopted. Group replacement (combined with luminaire cleaning
and inspection) generally offers the most advantageous solution as far as cost and
energy consumption is concerned. The fall in light output of the luminaires due to
lamp-lumen depreciation and luminaire fouling can in this way be kept relatively
low; as a consequence, the required 'initial' lighting level given by the installation
will be comparatively close to the specified level.
Even where systematic maintenance is employed, the lighting level can easily
drop by as much as 20-30 % , or more, by the time lamp replacement is due. A
maintenance factor (MF) corresponding to the drop anticipated (e.g. 0.8 for a 20 %
drop and 0.7 for 30 % drop) should therefore be applied when setting the initial
lighting level to be given by the installation according to the formula:

L initial = L spec / MF