|
PECUs (Photocells) are light operated switches. They switch the supply to a load ON when the light level falls beneath a given value (usually at dusk), and switch the supply OFF when it rises above another level (usually at dawn). The ratio between the two light levels is known as the switching ratio. Part Night PECUs an Part Night Dimming PECUs are specific types of photocells.
Positive ratio:
When the ON level is lower than the OFF level (e.g. if the load switched ON at 70Lux and switched OFF again at 105Lux, the unit has a ratio of 1:1.5). As can be seen from Table 1 below , positive ratio units have significantly longer annual burning hours than negative ratio units.
Unity ratio:
When the ON and OFF levels are the same (1:1 ratio).
Negative ratio:
Most street lighting loads have a warm-up time during which the lamp achieves full brightness. This is typically several minutes. The idea of negative ratio units is that the ON level needs to predict when the lamp will achieve sufficient output, whereas the OFF level is when the light is no longer needed. Typically a 1:0.5 ratio is ideal for this (e.g. turning ON at 70Lux and OFF at 35Lux).
Part Night PECUs (Photocells)
Using advanced functionality; PECUs (Photocells) can be designed to predict the time of day by measuring the day length. They can, for instance turn OFF at midnight and back ON at 5:30am
Use of such systems can dramatically reduce annual burning hours. An example of such a product is the Zodion SS9.
As the time is determined from cumulative light measurements at dusk and dawn over a number of nights it is important that the PECU (Photocell) remains continuously powered. If the power is removed, it will take the PECU (Photocell) some days to re-synchronise and re-establish the correct timing. Please note that the timing is not exact and does not compensate for changes in the time due to daylight saving.
Part Night Dimming PECU (Photo Electric Control Unit)
A further variant is a ‘part night dimming PECU' (Photocell). The operation is similar to the above, but instead of extinguishing the light during the early morning hours, the lamp is operated at reduced output. Dimming is achieved by use of suitable control gear in the lantern.
While this does not affect burning hours, it reduces the average consumption as the light is operating at reduced consumption for several thousand hours annually.
Zodion Sontek is an example of a part night dimming PECU.
Also see
To find out more about the Zodion Lighting Controls range of Photo Electric Control Units, please browse our catalogue pages.
Our PECUs range includes:
If you have any questions, please contact us on 01422 317 317 or fill in our contact form we will be delighted to answer any questions you might have on PECUs.
|
