8.9 Make sure not to replace losses with losses

Still, these considerations do not yet include the following circumstance:

Dimmed operation of fluorescent lamps represents permanent cathode heating operation. The position »Lights off« is usually identical with the position »Dimmed down to 0«. Unless care is taken that the supply voltage to the lighting installation is shut off after work and on weekends, the lamps continue to be operated in a »Dimmed down to 0« state. This sabotages the underlying endeavours to save energy. E. g. with the following assumptions:

• On a T8 lamp rated 58 W (whose systems power is 59 W in class A3 or A1, respectively) a power saving of 55.8 W be possible (»Dimmed down to 0« with a residual consumption of 3.2 W – see Fig. 8.18),
• an average office be in operation for 3000 h/a,
• the light be in operation for about ²/₃ of this time, yielding 2000 h/a,
• during half of this time, say, 1000 h/a,
• half of this power level be enough, i. e. 500 h/a savings potential, converted to full-load hours,
• the stand-by consumption, however, remaining active during all of the 8760 h/a yield the following calculation for the energy saved:

The basically useless additional consumption calculates as:

Thereby a savings potential does no longer exist. In some favourable exceptions this is taken into regard and installed accordingly , so that the user does not deplete the daily savings at night, but it remains to be doubted that this practical approach is the rule among specifiers and designers.

Fig. 8.18: Behaviour of a dimmable electronic ballast according to manufacturer's documents

If installed as a refurbishment, each sensor and each actuator of such a monitoring system will need its own power supply from the mains. The net DC requirement may be as low as some very few milliwatts each, but each single one of them employs a mains adaptor including a small transformer. However, the smallest commercially feasible transformer is a unit rated around 1 VA and has about 1 W of no-load loss. Load loss may be negligible on account of a very low loading factor – but the multitude of such power supply units form the major constituent of the standby consumption in the entire lighting arrangement. Advanced control systems like EIB, which are easy and not too costly to install if the cabling has been prewired right during the construction phase of a building, employ one central AC adaptor for all connected units. Signals and the SELV DC supply share a common line. This technique provides the potential to cut the gross standby consumption down to a fraction. Therefore it remains to be considered in each individual case whether the use of high-efficiency magnetic ballasts plus some less sophisticated control technique, simply shutting off parts or all of the lamps completely while not needed, could be both the cheaper and the more effective approach.