Street lighting in general is going to be a 3 phase supply, with a control photocell in the cabinet doing the group switching, and this then allows you to have either shorting photocells or regular light switches in the individual street light luminaires.
I would guess all the failed lights are group switched, and do not have a local photocontrol, as those also include as part of the unit a really big, really beefy VDR, and a built in thermal disconnect to allow it to fail safe when subject to overvoltage events of long duration. Shorting caps ( here they are yellow, to tell them apart easily from the ground) do not have the VDR and thermal fuse, just a 30A fusible link inside ( actually just the brass strip, which will blow at around that), and the luminaire has to provide it's own overvoltage protection.
Thus a filed neutral, or an intermittently open one, will result in a segment having a floating neutral, and this will be an issue at stub ends or split sections, as they will likely have a non multiple of 3 luminiares on them, so one phase will be dragged down, one will be slightly high and one will be very high, just from the imbalance. Made worse if the install team did not do a correct phase rotation, as they might have more lamps in one phase than the other 2, and you will find the lost neutral made the lowest load phase the loser in the voltage stakes.
Lost neutrals are very common, as they generally do not show up easily as failed sections, just long term short lamp life, or well cooked ballasts in magnetic fixtures. Remember the ballast in a regular MV, MH or HPS fixture is designed to operate long term without failure into a shorted lamp, as the failure mode of the lamp is either to be open circuit for MV, or for MH and HPS to have the starter fail short circuit trying to start an EOL lamp without success. Thus the design is to allow up to 130C above ambient of 50C for the insulation, while in normal use they typically run at 130C alone. I have seen regular lost neutral events with the lights mostly working, just either large segments cycling for a half hour after sunset before the lot is running stably, or with a regular sequence of cyclers showing the low phase.
Treez, that your lamps are failing from overvoltage says you need to look at either increasing input voltage allowed range to include running phase to phase ( 400VAC input range) ot having an overvoltage trip out that is resettable, and which indicated that input line voltage is poor. Something like a replaceable VDR and thermal fuse unit like a regular HPS starter unit, or a fuse or circuit breaker that can be tripped on overvoltage with a label saying overvoltage tripped this, though the replaceable MOV probably is cheaper than having 1500V rated silicon and 800V rated caps in the supply, along with making the startup supply be capable of long term running on 600VDC main bus rails. however making a fitting that runs phase to phase will be a marketing advantage, as you save in using a 3 core plus PE cable over a 4 core plus PE cable, and only need to make the units group only or supply a matching NEMA twistlock standard photocell that will operate perfectly from 150VAC to 440VAC supplies.
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