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I replaced a standard single pole switch with an Intermatic ST01 timer switch, into a metal electrical box with three other switches. The switch I was replacing had no ground connection, but some of the others do.

When installing the timer switch, I attached the green ground wire from the switch to a screw at the back of the electrical box. This is my first time doing this, so I wanted to find out if this is properly/safely grounded, or if I missed something.

enter image description here

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Is there room in that box for the timer switch with all those wires and wire nuts behind it? The specs for the ST01 series say that it is quite deep (i.e. 2.75 in. H x 1.73 in. W x 1.63 in.D). –  Michael Karas Mar 4 '13 at 0:44
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2 Answers

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That screw looks like it's for the wire clamp that secures wiring going through the back of the junction box. Instead, you should connect the green wire with a wire nut to the exposed copper wires that I believe are visible in the bottom of the picture. Make sure to use the proper size wire nut, which is based on the wire size and number of wires being joined together.

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Those copper wires are already connected to other things in the box. There is a hole in the back of the box (obscured by the wires), without a screw. Is that where a grounding screw goes? Would using that be safer? –  Joe Attardi Mar 4 '13 at 0:27
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With a metal box, you'd ground the box, but you don't use the box to ground your switch. All you're doing is adding another wire to the grounding bundle, just like all the neutrals are likely bundled together in other places (you may or may not see that inside this box depending on the power source). –  BMitch Mar 4 '13 at 0:40
    
It doesn't look like much of the wire is under the screw either –  Steven Mar 4 '13 at 1:44
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Yeah, I think I may be in a little over my head (and skill level). I am going to call an electrician tomorrow to come re-do it properly. –  Joe Attardi Mar 4 '13 at 1:55
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BMitch is absolutely right. The green wire needs to be connected to the existing bare grounds, and at least one bare ground from the ground bundle bonded to the metal box using one of the green screws. –  shirlock homes Mar 4 '13 at 12:27
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Each switch should be grounded with bare copper wires coming from the green screws on the switches, to a bare copper wire screwed onto one of the wire clamp screws of the metal box, to the bare copper wires of all incoming cables.

All of these wires should be interconnected in a sound and professionally clean way.

To test that everything is properly grounded you can use a standard multimeter tool to test resistance or continuity between a grounded source and the neutral wire feeding the box.

enter image description here

You can set your voltimeter to continuity or to a low resistance setting as shown above. Touching one end to neutral and another to ground should show near zero resistance on the display because the neutral and ground bars are connected at the breaker box. Near zero resistance means that they are continuous.

A high resistance number or a display of 1 essentially means that there is infinite resistance between ground and neutral, and that means that the receptacle or switch you are testing is not grounded.

  • Put one end on the green screw of one of the switches and the other on the neutral to test each switch.

  • Next place one end on the metal box itself and the other to the neutral wire to verify the box is grounded.

  • Next test all the other ground wires from all other cables to verify that they are all grounded as well.

You will be able to perform this test safely with the breaker(s) shut off as you won't need a live power source to test continuity.

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Depending on the wiring, you may not have a neutral at a switch to test continuity. And from the first paragraph, it's not clear whether or not you're suggesting the OP wire the switch directly to the box or to ground wiring bundle. –  BMitch Mar 4 '13 at 19:58
    
@BMitch I can tell the load is to the box because you can see the white neutral wires in the picture. On your other point, I am saying that the ground wire bundle should also contain a ground wire that is clamped to the box, as it is a metal box and should be grounded as well. –  maple_shaft Mar 4 '13 at 20:09
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The white wires can be the switched hots going back to the fixture where the line may be located. The presence of a red wire makes me think you're right, but I just can't be sure. Maybe I haven't had enough sleep, but the first paragraph is confusing even if the advice is good. It reads like you're suggesting they wire the switch to the box and then to the line. –  BMitch Mar 4 '13 at 20:47
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