How is an occupancy sensor switch supplied ?

Thread Starter

Externet

Joined Nov 29, 2005
1,879
Regards.
There is a mechanical relay in the device printed circuit board that turns on/off the lights in the room. It shows "No neutral wire needed" (But there is a wire for ground that needs connection)

ACphase------------------A-------------relaycontacts--------------B-----------------Lightbulb---------------------neutral.
Ground------------------------------------------G

The red portion is in the device that mounts in a wall. When the lamp in the room is on, between A and B there is no voltage to run the microcontrolled sensing circuitry. When the lamp in the room is off; there is plenty of voltage needed to energize the sensing and operation circuitry.

Is ground being used as a supply conductor ? What happens if there is a GFI ?
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LowQCab

Joined Nov 6, 2012
2,315
"" Is ground being used as a supply conductor ? What happens if there is a GFI ? ""

Yes the tiny current required flows on the Ground-Wire.
GFCI-Circuit-Breakers are not generally used, nor required by Code,
to be installed on Lighting-Circuits, so it should be a moot point.

Other possibilities ................
A Handyman could have installed a GFCI-Circuit-Breaker on your Lighting Circuit,
or, a newly installed Light could have been tapped into an Outlet-Circuit,
or, You could be dealing with a "Switched-Outlet" which is on a GFCI-Breaker,
or, You could be in a very old House that was not up-graded properly.

If You live in a Mobile-Home, DON'T TOUCH THE WIRING,
it's garbage, and it's dangerous to mess with it.
.
.
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MisterBill2

Joined Jan 23, 2018
12,690
The warning in post #2 about mobile homes is probably in reference to aluminum wire, which has always been a stupid choice for inside wiring. because the connections will always develop a high resistance and overheat enough to start fires.
NOTE that I did not say "May overheat", because they WILL overheat, not right away, but eventually.

An occupancy switch may only require a minimum of power, which it can obtain when it is ON by having a small voltage drop that iis usually not noticed. Thus no ground or neutral connection is required for operation. The ground wire is there to ground the exposed metal part of the switch.
 

Tonyr1084

Joined Sep 24, 2015
6,954
I've run into similar issues. Some of the devices I've wanted to install could not be used because the house was originally wired with "Interrupt" circuit wiring. Just two wires in the switch box that interrupted neutral from the lamp back to the panel. One would assume that a grounded box would be there. In that case you could use the grounded box. But in my 1961 home grounds were few and far between. In many places I've added grounds, but not in light switch boxes. And that has presented a problem. I've had to run ground wires to those boxes so I could use things like occupancy sensors and light dimmers.

Bottom line, if you don't have a ground (or neutral) you'll need one. If you have just the two wires in the box likely yours is an interrupt circuit setup as well. New homes run power to the switch box. In a properly (modern) wired switch box you'll see the neutral wires nutted together and the black (hot) wires from the panel and going to the light fixture connected to the switch. In a "three way" setup you'll have an additional (US) red wire between the two switches.
 

Tonyr1084

Joined Sep 24, 2015
6,954
Older homes may not have this ground in the switch box in which case you simple don't connect it.
Tried installing a remote dimmer unit in my bedroom that didn't have a ground. The unit would not work without the ground. Had to put one in to make it work. Fortunately there was a grounded outlet just below the switch, so running a ground wasn't too much of a bother. I still had to open the wall to run the wire thought. Drywall patch work may be necessary. Just saying that's my experience with it.
 

Ya’akov

Joined Jan 27, 2019
6,260
The warning in post #2 about mobile homes is probably in reference to aluminum wire, which has always been a stupid choice for inside wiring. because the connections will always develop a high resistance and overheat enough to start fires.
NOTE that I did not say "May overheat", because they WILL overheat, not right away, but eventually.

An occupancy switch may only require a minimum of power, which it can obtain when it is ON by having a small voltage drop that iis usually not noticed. Thus no ground or neutral connection is required for operation. The ground wire is there to ground the exposed metal part of the switch.
Aluminum wiring is perfectly safe and does not cause fires when properly terminated, or spliced to copper with an appropriate connector like AlumiConns. Aluminum wiring is heavily used in industry, and for residential service drops.

If you use the proper terminating hardware, it’s as safe as copper.
 

MisterBill2

Joined Jan 23, 2018
12,690
I HAVE USED an occupancy sensor that used ONLY the two wires. So certainly some of them WILL work very well without any third connection. Consider if the internal drop is five volts: you might be able to detect the drop in light level with a good instrument but you will not notice it. If the load were a 60 watt bulb, just consider that five volts at half an amp is 2.5 watts, plenty of power to operate quite a bit of circuitry.
My second point is that on many occasions the reality is not quite so obvious.
 
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