Hi,

how can mercury vapor lamps periodically switch on and off Normaly I know that from HPS lamps but I never saw MV bulbs! But in my village is one MV lantern that swith on and off periodically!

Is the lantern on photocell control? If it is, then what could happen is that the photocell could come loose and hang beside the lantern and makes it switch on and off. Alternatively lantern on the column could be beside a white wall which could be reflecting light towards the photocell.

This happened to a city once that the photocell was on top and the lantern was beside a white wall and the wall managed to reflect light to the photocell causing it to think that it is daylight and nightime etc.

That is all I could think of.

It could also be a bulb ballast mismatch

No, the lantern has not a photocel. It's group switched.

It also could be that there is a problem with the ballast. Becuase the lanterns are more than 30 years old.

It sounds most likely the ballast. Probably even there are power surges that causes the arc to disappear. It then takes time to cool down and the pressure to reduce in the arc tube.

Poor connections, that cause a momentary voltage dip, the lamp is then off till it cools down. Most likely if old fittings/installation, where the cycling loosens screws, and the aging loosens the lamp in the holder.

Oh yes, that also could be the reason.

I have seen several incidences of mercury vapor lamps that cycle like this due to lamp problems (i.e., poor quality lamp, or lamp at end of life). It's rare though. It can also happen if a mercury lamp is run on an HPS ballast, particularly the U.S. low-voltage (55v) type used for 150w and smaller HPS lamps.
The rarest behavior I ever saw was a 175w mercury lamp in a security light fixture that was flickering wildly and randomly on and off. I've only seen that behavior (and rarely, at that) in 100w & smaller medium base lamps.

Yes, he lamp is on the end of the life. It's very dimmed.

I knew 2 old shovel shaped mercury fixtures in my town on a firestation (which was old but was remodeled to a new small shopping place and firestation was moved...

but anyway...one of the light would sometimes go on and off and on and off and on and off slowly but not warming up....just dimming and brightining slowly and completely off then slowly going back on...they were mercs......the one that kept doing it every few seconds...it sometimes would work completely.....it also had an incandescent-like color.....

the other one was more creamy greenish....man how I miss those lights

I also like this old mercs..........beautiful.