I have 2 large outdoor large mercury vapor lights. They have worked for a long time with photocell screwed on top of fixture. Now both are on all the time. I bought 3 different brand of 3 prong twist in photocells b ut the light is on constantly. I measured the voltage in the top receptacle it is 120V, but when the bulb is removed and measuring the voltage in the socket it is 220! What is the problem and how to fix it?
Please help
ehollymd

The high voltage reading in the socket without a lamp is on account of the ballast. Nothing to worry about there. As to your photocells, you do know that they take a minute or two to shut off when you first power them up after replacement. Otherwise I would suspect that you haven't gotten the correct one yet or you have the shutter closed which makes the photocell think it's dark out.

Did you verify the light goes out when photocell is removed, if it doesn't, its in the wiring.

I appreciate ypur answers and understand it.Light goes out when cell is removed. HOW DO YOU OPEN THE SHUTTER?

The shutter is a little metal piece that slides over the photocell and shields it from light to decrease it's sensitivity. If you don't see it, it's not there.

Another thing that occurs to me is that changing these photocells with the power on may not be in their best interests. I don't know if that would destroy them, but with your string of luck I wonder about that. Ballasted loads can show interesting waveforms on the line and maybe when you insert the photocell in the socket under power it is seeing multiple spikes of many times the voltage that it is designed to withstand, hence the failure. Get a good quality photocell and turn the power OFF before changing the next one. Then let us know what the outcome is.

Dear EP Miller. Thank you for your interest and help but
1)All of them recommend to turn off the power before installing, done that,
To the last one I bought is Intermatic made in Mexico costs 19$ no better than the others. No shield on it (120 volts 1800W tungsten 8.3 Ballast).
Thank you
EH

Intermatic is good. Can't understand why 3 photo-controls were DOA. Are you CERTAIN you waited several minutes to let the control get itself set correctly before you condemned them? I've seen it take 3 minutes or so before a new one will shut the light off in broad daylight. I would wait at least 5 before I would start to worry.

Yes, all the lights are on all the time day and night, the bright Florida sun shines on them and they do not turn off! I had a friend who is a full fledged electrical contractor look at it... recommended I get an other one, " they are faulty" but the other on did the same thing...

Where it plugs into socket, look for 120 at hot and neutral(probe until you identify), 3rd slot goes to light, a jumper between the hot and the switched should turn on light, removing should make it go out as you said. Sounds like a bad batch of photocells, as ballast and bulb are working. Not sure the effect of an open neutral to photocell.