I got 4 Metal Halide 100W floods from Screwfix for our Church recently. One of them has a habit of cutting out after about an hour.
Anyone else experienced this sort of problem - is it likely to be a duff bulb or a duff fitting?
How many hours has it done? Did it do this from the start?
A common cause is the arc tube overheating. This causes it to run at a progressively higher voltage, and eventually there isn't enough voltage headroom across the ballast and the lamp goes out. When it cools enough, it restarts and the cycle repeats over and over. The overheating can be caused by an old lamp with a discoloured arc tube which absorbs too much heat, or by fitting the wrong power rating lamp for the ballast.
It that isn't the cause, try swapping two of the lamps around and see if the fault moves with the lamp or stays with the fitting.
Another possibility could be the ballast overheating, but I don't think it's common for them to have auto-resetting thermal fuses.
PMFJI but can you tell me a bit more about them - dimmable? lamp life? energy efficiency?
Not dimmable. Should be of the order 10,000 hrs, but depends on lamp type. About same efficiency as a good linear fluorescent tube (which is higher efficiency than a compact fluorescent retrofit). Not as good as a high pressure mercury vapour lamp or low/high pressure sodium lamps but better colour output. (I would question if this is really necessary outdoors though, unless you're lighting up a historic monument or similar.)
I have used 4 (non dimmable) to replace 8 500W halogen floods. They give a pretty cold light but it is very good (once they have warmed up) in terms of brightness. I originally used some 70W sodiums which were good as a secondary source but too orange for sole use.
I can't comment on lamp life (yet) but in terms of efficiency, I would rate the light output equivalent to about 750W of halogen flood.
It varies by manufacturer and lamp type, but these are generally available in warm white, white, and cool white. Outdoors for night time use, you usually always want warm white. Even that will look cold if there are any sodium lights around. The fittings don't often come with warm white lamps.
Should have been similar light output from the lamps, but the level of illumination on the ground depends on lots of other things such as efficiency of reflectors, and how well they direct the light where required, and don't waste light where not required, etc.
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