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I would like to replace this old light switch with a newer one, but as all newer electrical items it has a grounding screw to be connected and there is no obvious ground connection available in the box.

This house was built in 1963 (Colorado, USA), and it has several 3-prong outlets around the house that look newer, but not on this wall.

My investigations in this box actually make me worried if any of our 3-prong outlets have proper ground, as I read that the standard socket checking devices could be fooled with ground-neutral connections?

Here's why I'm worried:

  • In the switch box photo one can see 2 neutrals connected via wire nut
  • In the back of the box (which is metal) one can see a blank wire looking like its intention is to "ground" the box? It is screwed to the box at the back.
  • I confirmed that the neutrals and the box are electrically connected (does that equal a bootleg ground?)

Adding to the confusion is the cabling of the switch:

  • one red going in above,
  • two blacks at the bottom, both on the same screw (but this should be a 2-way switch)

Hypothesis regarding the last point: Half of the rooms in this house have fans installed, is the double-wired black maybe a provision in case one wants to install a fan in this room as well?

So, the new switch of course wants a hot, a neutral and a ground wire, what am I supposed to do?

For additional background I add a photo of the 2-prong socket that is on the same wall. It also has a blank copper wire connected to the back of the box (interestingly, in both cases the screw connection was rather weak, I tightened it, hope that wasn't a special technique in older houses for some reason?)

My background, in case you wonder what level of detail you can load your answer with: I'm a physicist with lab experience (so give me all your details! ;), but no formal education as electrician; meaning, I'm confident enough when I understand things, but also reasonable enough to concede to the professionals when it just gets too complicated.

Switch box

2-prong outlet on same wall

EDIT: Adding layout graphic how it is now. Layout before

EDIT2: Attaching a graphic implementing the solutions given below. enter image description here

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I agree, the /3 Romex is certainly intentional future-proofing in case you add a ceiling fan later. Unused wires should be capped, not wired to hot.

It looks like the Romex they used does indeed have a ground wire in it, and they have grounded the steel junction boxes, assuming corrosion has not damaged that.

However, note how the yokes (metal ears) of the switch and receptacle attach to the metal boxes, i.e. Only by the screws. The yoke does not bottom out onto the metal box, and where it makes contact is painted. The screw threads are not a legitimate grounding path.

So. Buy a couple feet of 12/2 NM cable and tear off the sheath and grab the ground wire. If you need more ground wire, strip the insulation off the other wires. Cut about a 6" length and pigtail it off the ground screw of the switch or receptacle. Then, unhook the ground wire(s) screwed onto the back of the box, and replace that with about a 6-8" pigtail. Now join both pigtails, plus the wire(s) you took off the screw, with a wire nut (red or green should be the best size). Shove all that into the back of the box.

Are those ground wires any good? The simple way is to use a 3-lamp tester and look for 2 yellow lights, or measure 120V between hot and ground.

The more conclusive way is to deliberately connect a duplex receptacle so the hot and neutral screws are actually connected to hot and ground wires. Then hook up a big load like a hair dryer or 1500W heater, measure the voltage across hot to ground, turn on the load momentarily, and measure the voltage again. Don't leave it on. It should sag only a few volts, about the same as it sags when wired correctly. Ground is not neutral and is not a substitute for neutral, but it is intentionally bonded at the panel, and this tests that continuity under load.

Almost any test between hot and ground will trip a GFCI device, since it is in fact a ground fault.

  • And what do I connect to neutral on the switch? Or do I have to connect the red (e.g.) to hot and pigtail the two blacks and put them onto neutral and then leave the box neutrals the way they are? – K.-Michael Aye Feb 27 '17 at 7:35
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    Switches don't take neutral, unless they are smart devices of some kind: dimmer, motion sensor, day/night sensor, etc. (smart switches SHOULD get neutral; because if they don't, they power themselves via a hokey, obsolete method that doesn't play well with CFL, LED and any modern lamp.) – Harper - Reinstate Monica Feb 27 '17 at 17:42
  • Ah, I see, sorry, I must have remembered wrong what my new switch has. I also have a receptacle with some USB ports, maybe that one wanted neutral? But in that case I better make a new question. I also just learned that neutral should actually be never switched, anyway, apparently. – K.-Michael Aye Feb 27 '17 at 18:07
  • Something like this?dropbox.com/s/3lo3doj9hjkfr7b/switch_layout_new.png?dl=0 – K.-Michael Aye Feb 27 '17 at 22:42
  • Yeah, that looks right. Every receptacle needs neutral, USB or not. – Harper - Reinstate Monica Feb 28 '17 at 0:21
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It appears the NM in your house is of a peculiar 60's type that has an undersized ground wire in it. This is no longer legal as a means of grounding for new construction, but is OK in a grandfathered situation such as yours (it's certainly better than nothing!). It's also not a bootleg ground situation, as ground and neutral are bonded together at the main panel (and only at the main panel) to allow ground to serve as a "safety drain" for errant current to get back to its source, hence the reading you got that they're connected.

However, they goofed originally when connecting the ground wires to the box -- twisting two wires together and clamping them under a screw does not a legal splice make. Fixing this is simple -- get a pre-made ground pigtail with a 10-32 screw. With the power off, fit it into a matching hole in the box then use a wirenut to attach the two NM grounds to the box ground and to a bare or green pigtail (you can buy bare wire or green THHN at the hardware store if you don't want to shuck NM for pigtails) that can then attach to the receptacle or switch ground (green) screw. Once you have everything buttoned up, turn the power back on.

I'd repeat that procedure for all the wiring devices in your house, by the way -- it's not hard to do, and the ground pigtails come in a nice, big bag, so you'll have plenty anyway.

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