Motion Detector Switch: What's the deal with the bare metal ground wire?

Recently purchased this motion detecting light switch:

If you notice in those pictures, in addition to 2 black wires, a blue wire, and a green ground wire, there’s a bare metal wire which is attached to the plastic housing at the front top of the switch.

The instructions tell you to connect this bare metal wire along with the green ground to your house’s ground. Here is an installation video:

You can see the bare metal wire in question starting at about 7:53. He just calls it one of 2 ground wires.

What is this wire for and how does it work? As mentioned, it doesn’t appear to be connected to the internals of the switch at all, it’s only attached to the external plastic housing.

Ground wires are common in all electrical outlets and switches.

I must be a worse writer than I thought. Does the OP really make it seem like I don’t know what ground is for?

You tell me,

Alright then, let me try to ask it this way.

Why would a device, that already has a normal green ground wire, need a SECOND bare metal wire acting as a second ground, which is not even attached to the internals of the device but rather the plastic housing?

Maybe for static?

Makes no sense to me, unless it’s supposed to be for some sort of static discharge.

My guess would be that after they had the thing designed and into production, they found that it was susceptible to being damaged by static discharge, so they retrofitted the faceplate grounding wire.

Looking at the product sheet, that switch comes with a white sleeve to cover the green. Looks like its a ground when no nuetral is available, and a nuetral if it is.

I suspect that the bare wire is connected to ths yolk or is routed in the way tbat is is so that you can wrap it around the mounting screw if its to be used for grounding in ths case that the green wire is connected to nuetral.

Ground **or **Neutral Wire required.

Yep, that’s what it is. If no neutral is available in the box, and you use the ground instead, current may flow down that ground. A tiny amount of current, and maybe that’s legal by the UL. Somehow this device has to be able to cycle current or it can’t function. (the current is needed so it can run the sensor). So the reason for the separate ground wiring for the screw plate is so no current is flowing in the metal of that screw plate. (the current flowing from the switch’s power supply is downstream).

As for why the screw plate has to be grounded - again, electrical codes. This is what happens if you don’t :

Yep, I believe this is more or less true.

However, this:

I don’t really get. Even if you wrapped the bare wire around the mounting screw, the mounting screw is not connected to anything metal on the switch itself. I guess you are saying basically the same thing as SamuelA here:

I suppose you are saying the bare metal wire would come in contact with the screw once the switch is installed (which does seem to be the case now that i look at it more closely). HOWEVER, if the switch’s green ground wire is used for neutral instead of actual ground, then connecting the bare wire to the house ground wouldn’t seem do anything. You are just connecting the electrical box in the wall to the house’s own ground (box -> screw -> bare metal wire -> house ground: switch itself not in play). The bare metal wire in this scenario never connects to anything inside the switch.

Unless I am misunderstanding you. (That reddit post is pretty funny though.)

The static electricity idea makes some sense.

The only thing on a wall switch that would require geounding would be the yoke, if thats plastic then idk then.

There are metal screws on basically all wall switches. The screws screw into a metal plate inside the box. For whatever reason, because the screws are metal and exposed to touch, they must be grounded. (I don’t know the code reason for this)

If you buy a new cheap light switch, you’ll notice that new switches have a ground screw, while the old ones didn’t.

I think the reason is that since modern electrical boxes are often plastic, and the wires inside the box might have nicked insulation or an energized strand sticking out of a wire nut (if stranded wire is used), there is a chance that one of the wires in the box could be pressed against the metal screw plate. If it isn’t grounded, the plate will be energized and it won’t trip the breaker. Oh, if a fault causes excessive current flow, wires can melt through their own insulation, so it can happen that way as well.

Ergo, so you don’t get shocked if your hand brushes against the metal screw while flipping the switch, you ground it.


Unlike a normal switch, the internal circuitry for this switch requires 120 VAC in order to operate. 120 VAC power is supplied to the switch’s internal circuitry using two wires:

  1. Either of the black wires. One of the black wires is guaranteed to be hot all the time, and the circuitry is “smart” enough to be able to be powered from either wire.

  2. The green wire. Ideally this wire should be connected to a neutral wire since the circuitry is an electrical load. But many light switches are not mounted to an electrical box that contains a neutral wire. Therefore it is apparently acceptable to connect the green wire to the ground wire inside the electrical box. (Normally ground wires are not supposed to carry any load current. Perhaps UL or the NEC has made an exception for certain devices that draw very little current?)

And then there is the bare wire. I am guessing it is simply connected to the metal hardware on the switch. This wire should be connected to the ground wire inside the electrical box.

Pretty much any metal likely to become energized is required to be bonded. Including the metal backstrap, interpreted their response to mean the mounting bracket was plastic, if tbat were tbe case I’ve yet to meet an ispector that would require the screws tbemselves to bs bonded, nor do i see tbe need to.

Mounting bracket is not plastic and I’ve never seen it be plastic. I don’t think plastic can take the tensile stress. I’ve installed that exact Lutron switch, I’ve installed 6 other motion sensitive switches, replaced some regular switches, and replaced a whole buncha outlets. All use metal metal brackets.

I just had an issue with switches at my house. The bare ground wire was touching one of the terminals of the hot wire. This was causing a breaker to trip. The electricians who installed these switches neglected to tape the body of the switch and cover the 3-4 live terminals on each switch.

It took about an hour to figure out what the fault actually was.

Neglect was involved but it wasn’t due to a failure to tape. Bare wire should be pushed into the back of the box, with only the pigtail to the device coming foward.
I never was a fan of taping the terminals on plugs and switches. It makes a sticky mess when you go in after the fact. Also, about half the time the tape was wrapped to tightly and slid back off the terminals into a usless glob around the wire.

I’ve seen a lot of switches myself possibly even this one. Ive handled hundreds, maybe thousands of them. I’m pretty sure I’ve seen one with plastic tabs but I cant be sure, regardless, on second reading it seems I misinterpreted arseNal’s post to mean that the mounting tabs were other than metal. I was mistaken.

No pigtails were used. Just a bunch of bare ground wire that could possibly hit one of 4 hot terminals. How does a pigtail ensure no contact?

No, it is plastic, if you are referring to the part of the switch I think you mean. Look at the pictures in the first link in my OP, you can see the white part that takes the screw for mounting into the box in the wall, that’s all plastic (top and bottom). And that’s what the bare metal wire is attached to.