Street Light Problem

Hi Guys,
In my neighbourhood we don't have big ole strret lamps, there's a post lamp near the end of everyones laneway thats controlled by a photocell at the back of the house. Lets just say mine aint working.
I have removed the cell, measured the voltage at the cell and as expected got 120V. I put a piece of duct tape over the sensor and then went out front and only measured 75V. I went out back, removed the cell entirely and just tied the blacks and whites together so the cell was totally removed from the equation. I still only read 75V at the post.
I understand that there's voltage drop to take into account, but there shouldn't be that much voltage drop. The total run is probably about 60 - 80 feet. The wire is about 14AWG.
(When I moved into the house I found a bunch of fragments from previously exploded bulbs in the fixture. So I think this under voltage problem may have been around for a while)
Anybody have any thoughts? I've considered trying a solar post lamp but can't seem to find one.
Thanks in advance. Matt
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Mattimus wrote:

Two stupid questions that need to be asked, sorry:
1) Somewhere in your home is a switch that also controls the light fixture, allowing it to be powered and then controlled by the light sensor. Are you sure that switch is turned ON?
2) Are you certain that you're reading 75 Volts, not 75 mV?? The little mV indicator is SO easy to miss on DMMs. And 75mV from leakage current on an unpowered circuit is not uncommon (see question 1).
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Well you know what they say.... The only stupid question is the one that isn't asked....
Turns out the mystery switch that I never thought did anything is actually the controller for the street lamp. It seemed counterintuitive to have a photo cell and a switch.... anyhoo, that switch was in the off position and there was a leakage of 75 volts!!!..... when its in the ON position I get a full 120.
Looks like I only have to replace the switch.
Thanks for the slap upside the head.

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The switch does a couple of things. Allows the current to be shut off if you have to do maintenance on the fixture, overrides the photo cell to turn the light off if desired, and maybe allows the light to be turned on over riding the photo cell. Some will go on if you flick the switch a couple of times even if it is light outside. Ed
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I-zheet M'drurz wrote:

Uh, he *did* say he read 120 volts at the cell location. Sounds like that answers that question...

Unlikely he'd read spot on 120 mv at the cell. The "120" is an unlikely coincidence.
I'm betting he used his meter correctly.
Jeff
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    Look for splices near where the line comes out of the house and where it enters the light post. A bad connection can cause a voltage drop and it may be intermittent. If there's nothing bad there start tracing the line until you hit the drop. This doesn't sound like it but if you're using a fluorescent bulb, older photocontrollers can have a hard time with them. It's worth it to buy a new controller with a fluorescent (which are expensive) but you'll get it back in a hurry.
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You left out one key piece of information Mattimus:
Was there a load (bulb) in place in that post lamp when you measured 75 volts?
If there wasn't a bulb there, then the 75 volt reading is probably caused by leakage current around an almost completely open break in the wiring from where the cell is to where you are measuring 75 volts.
If the bulb load wasn't there when you measured 75 volts, try putting it in and measure again. If you get near zero with the bulb in, then you will have to find where wiring break is and fix it.
If you still get 75 volts with the bulb in place you've got a resistive joint somewhere in the circuit, it's probably in the order of 15 to 35 ohms. Check all splices carefully.
BTW, I've never heard of low voltage causing bulbs to shatter, and doubt if anyone else has either.
Good luck, and when you find ourt what's causing the problem let us know,
Jeff
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Are you sure the photocell is directly controlling the light? Many times the photocell is just a sensor for another electronic circuit that actually controlls the lights. If you have a true 75v at the light, it should be glowing dimly.
Bob S.
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Mattimus wrote:

Assuming the a good lamp was not in the circuit when you were measuring the voltage, I'll bet you were using one of those need new digital meters. They are very sensitive. What you likely read was the very low amperage voltage (stray voltage) generated when a dead wire passes close to a live AC wire. An old analog meter would have provided a low enough resistance to so zero voltage, but your new very high resistance meter was able to measure it.

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Matt,
I don't think the undervoltage is responsible for those previously exploded bulb fragments. My lamp post had 4 exploded bulbs due to moisture dripping on hot bulbs when it rained. The lamp area did not appear to be leaking, but water was entering (somehow) a few hours after the light was lit. Finally, I bought a bulb at Lowe's that was specifically for lamp posts and that completely solved the problem. I think it cost about four bucks, but after replacing bulb after bulb, it was well worth it. If I can find the box when I get home, I'll mention the brand.
Ox
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Matt,
I don't think the undervoltage is responsible for those previously exploded bulb fragments. My lamp post had 4 exploded bulbs due to moisture dripping on hot bulbs when it rained. The lamp area did not appear to be leaking, but water was entering (somehow) a few hours after the light was lit. Finally, I bought a bulb at Lowe's that was specifically for lamp posts and that completely solved the problem. I think it cost about four bucks, but after replacing bulb after bulb, it was well worth it. If I can find the box when I get home, I'll mention the brand.
Ox
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Matt,
As your first poster said, make certain the meter is not reading 75 mV. This is a common mistake even us pros make sometimes with autoranging multimeters.
If you are only getting 75 volts AC, did you check this with the lamp disconnected? Is it a standard lamp that takes a standard bulb, or some other variety. I'm asking that to determine if there may be a transformer in there somewhere.
If the thing uses a standard bulb, do you have a ground (green) wire coming out to the light? If so, use your meter to check between the black wire and the ground. Then check between the black wire and a real, honest to goodness ground (if your pole has a exposed bolt or something <no paint>, and it's in the ground, measure there. Make sure you do this after wiring around the photocell so we don't have to worry about it causing problems.
Do you get 120 volts between the black and ground? This means the neutral wire is either open or improperly landed back in your house. A lot of people think ground and neutral are the same in residential circuits, but they are certainly not. The ground circuit is there for protection only and not as a current-carrying conductor. You would not believe the amount of problems this mix-up causes electricians sometimes.
Let us know what you find out.
Jake
Please be aware that 120 volt circuits electrocute more people than any other kind.
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