I have a question about those MH can lights which include an backup halogen bulb.  My school has several of these which run 24/7.  I have noticed that periodically the lamp will go out, and the backup lamp will come up.  Once the lamp has cooled for a little while, it will restart.  Is this because the arc on the lamp extinguishes itself, or does the fixture do this as a safety device since the lamp is running 24/7.  Just wondered as this never has happened on the lights that are shut off every night.

I have a question about those MH can lights which include an backup halogen bulb.  My school has several of these which run 24/7.  I have noticed that periodically the lamp will go out, and the backup lamp will come up.  Once the lamp has cooled for a little while, it will restart.  Is this because the arc on the lamp extinguishes itself, or does the fixture do this as a safety device since the lamp is running 24/7.  Just wondered as this never has happened on the lights that are shut off every night.

It was cycling, so it extinguished itself, remember seeing some high bay MH lamps do that and it def did make the halogen go on too!

Well MH lamps are supposed to be shut off every 15mins a week if run 24/7 to decrease the chance of non-passive failure.  I suppose there is a device out there that does let the lamp cool every once in a while.  It could also be a slight power dip that extinguishes the lamp too.

how often does it cycle?

Yeah cycling frequency would tell us a lot.  I know when pulse start MH lamps age and wear out they start cycling, some fixtures with the halogen standby lamp will light the halogen when the MH cycles off due to lamp age.