OK, My school just forked out A LOT of coin to install motion sensor switches in our school.
My question is: I was always taught that florescent lights take the most energy when turning on, so don't turn them off unless they were going to be off for over 30 min. Also I thought the light bulbs burnt out faster when turned on and off vrs just leaving them on.
The installer actually said: "the energy savings will pay off these devices in 10 years :o :o :o"
Oh by the way, the installers came from Arizona.....I'm in NE Kansas!!! That chaps my hide that the board hired someone outside of town.
Enough rant for now!!!
Woodsteach
It's not particularly easy to measure the energy required to turn on fluorescent lights, but by making a few WAGs you can do some back-of-the-envelope calcs to see if your installer is blowing smoke.
For example, suppose your lights are on a 15A 120V circuit and it takes about 2 seconds at rated current to turn on the lights. Suppose also, that current over 15 amps will trip the breaker instantly (not true, but we're just trying to get within the right order of magnitude here).
15A(120V) = 1800W = 1800 J/s (Joules of energy per second)
(1800 J/s)(2s) = 3600J of energy to turn on the lights.
Suppose you have two four foot tubes, totaling about 80W (= 80J/s).
(3600J)/(80J/s) = 45 s
So if our assumptions are correct, turning on the lights takes enough energy to run them steady state for 45 seconds. Even if we're off by a factor of 10, 450 seconds is only 7 1/2 minutes. If it takes less energy to turn on the lights than we assumed, the 45s number gets smaller.
So it might be a good idea to turn the lights off when leaving the area. It might be true, though, that cycling them will cause them to fail sooner. YMMV
Tim
The rule of thumb we use in the energy manager world is 15 minutes. We usually set the timers on these sensors to turn off after 15 minutes. That's what we do in government buildings anyways.
Bruce
We've installed motion detectors throughout our 105,000 square foot factory, and they've made a noticeable difference. Also, we get a tax break of some kind, which is probably the real motivating factor. I think today's fluorescent fixtures use a lot less power to start than in the old days. That's what the salesman said, anyhow. :D
Just don't do like what the electrician did at my work. He wired 4 outside lights to one photo eye. When operating, the lights draw 13 amps. That doesn't even cover the startup draw from kicking the ballasts on. The problem was that the photo-eye was only rated for 2.5 amps- when another electrician came in, he showed us the old eye- melted almost in half.
Charles
Thanks guys for the replies; I was probably just ranting...
from my principal: "no money for a new table saw" current one doesn't have 'T' grooves in the top for miter gauges and was purchased 1960's.. but we can spend at least 3x's a new sawstop for motion sensors!! go figure I thought education was about the students but again I digress...
Woodsteach