I been using a cooper motion sensor to turn on a fluorescent light for around 5 years and it stopped switching the light off. The light will go off by pressing the button until its off. It has 3 setting, autio, on and manual. So I assumed something went wrong with it. I replaced it with a Lutron MS-OPS5MH because it didn’t require a neutral wire.

The switch is wired as a switch loop so there are only 2 wires and a ground. The Lutron is doing the same thing, the light comes on but nothing else works. It won’t respond to any pressing of the buttons. It does require a ground to work, not really sure the old cooper actually needed a ground but it was hooked up.

So I check some voltages and kind of confused about something. I got 122 volts between the black hot wire to ground. But when the Lutron switch is hooked up there is only about 3 or 4 volts between the hot wire and ground. I don’t understand why the voltage drops when the switch is hooked up and light is on, between the hot wire and ground.

I thought maybe there was something wrong with the ground but really not sure now. The switch won’t work without a ground. I tested the switch just rigging up a test light on a bench and it works fine. But if I removed the ground the lights goes off.

I called Lutron and the guy first said a faulty ground can cause the light to stay on, but had no answer to why the voltage goes away. If the switch loop was wired wrong and using the neutral wire then far as I know there wouldn’t be 122 volts between one wire and ground?

Anyway, I’m kind of confused on what to check but can take the light apart and look at the wiring. I know there is a GFCI before the light, maybe something weird happen to it?

Anyone got any ideas about this ?

 

Therein lies your problem.You know there is 120v on the black wire because the light is working. So you should see 120v from hot to ground AND hot to neutral. Your ground is not grounded.

Check from that same black wire to neutral. My guess is you'll see 120V like you should.
Now check from neutral to ground. You should see nothing or 0 v but I'll bet you see +/- 110v on it.

Note: A lot of switch devices use the ground to power them. They can do that as long as their current consumption is under a few microamps.

 

Well, you're probably right. There is no neutral in the switch box so I'd have to try it another way to check that. I did test for continuity between the ground in the switch box to a receptacle near by and it did read, so it must be kind of grounded.

I did find out the old switch didn't use a ground and I cannot seem to find another one, everyone out of stock. I'll just have to take the light down and see if it the ground is hooked up there. Maybe I should just run a neutral into the switch box so I can use the switches that need one, since that appears to be what most want to build these days. But its more trouble.

 

That would make the switch box compliant with NEC 2011 code. Nothing wrong with that. Assuming a switch loop with xx-2 cable you would replace it with xx-3 cable. You can not run a neutral outside of a raceway/sheath.

 

Then the black hot wire and the ground are both fine.

BTW, in a switch loop, the white wire should be marked with black or red tape or permanent marker at each end and used as the hot wire, with the power going back to the light on the black wire, but that won't change the fact that your conductors are working properly.

That's typical, and a good result. The light is on. The power is going to it, and has a better path to ground that way. You would only see 120V to ground if the switch was off or had failed.

This may be the problem. Fluorescent ballasts won't work with most photocells, unless they're dimmable ballasts, because they require full power at startup. The same appears to be true of some motion sensors. However, the fact that the light comes on and that you see a lower reading when the light is on would argue against that. Plus, you say that the switch does turn the light on. JOC, how are you turning the light off in order to verify that the switch will reliably turn it on?

If you can replace the fluorescent lamp(s) with an incandescent one, including a halogen one, that would be a way to determine if this is the problem.

You can plug an extension cord into a nearby receptacle and then use the neutral slot in the female end of the cord for the test.

If you want to do that, great. But it won't help you resolve the problem with the new switch not shutting off.

How much time have you set the switch to stay on for? Have you tried changing that to see if it makes a difference?