The transit center parking lot that I go to almost daily is lit by shoebox-style fixtures, two to a pole, 250w HPS. There is a large relay cabinet with a meter-style photocontrol that switches all the lights on & off for the entire lot (I've observed it in proper operation many times). Just last week, I noticed half of the lights dayburning (on most poles, this is one of the two fixtures, but a few poles have both lights on or both off). At night, the same lamps are lit and the others are out. This makes me think there are actually two circuits that are being switched by the photocontrol, or more likely, it is 3-phase and one of each pair of lamps is wired to each phase.

I still am not sure the cause of the failure. If the photocontrol stuck "on", why would only half the lights be dayburning? If the failure was in the relay/contactor, I would expect to see all lights stuck on, all off, or half functioning while the other dayburned. Any ideas?

Whatever fault (maybe a voltage surge) probably welded one set of contacts (maybe separate relays for each phase ?) and took the photo control with it initiating a fail-off fault. I may be totally off track, however...

It's possible that the photocontrol has failed in the "on" position and one phase has somehow become disconnected, either by a failed contactor or a wiring fault somewhere. If the phases have independent overcurrent protection, that could also be to blame.