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I have a building originally wired for electricity in 1952, with other sections done more piecemeal within the last thirty years or so. This old wiring is a two-wire, cloth-sheathed wire that feeds into metal electrical boxes. The receptacles are two-prong outlets.

I know that one cannot simply replace these plugs with grounded three-plug outlets without ensuring a proper ground. After doing some research, I've seen that sometimes the box itself can be grounded with either AC or a grounding wire outside the box. Using a test plug and my multimeter, I measured right around 120V between hot and the metal box. I then switched out the two-plug with a three-plug, attached a pigtail ground wire to the back of the box, and tested it again with the ground plug. Again, I get right around 120V. I tried most of the other plugs around the building with similar results.

I know that in a properly grounded, modern (for lack of a better word) system, this is normal. My question here is this: does this reading indicate that the line is, in fact grounded back to the box? Is there any way that the reading on my multimeter is "lying" to me? I am assuming, if it is grounded, that there is AC line behind the wall or a grounding wire run somewhere. If this is the case, is attaching the plugs to the box through a pigtail ground wire a reliable ground? I know code allows for conduit or AC cable to act as a ground.

Thanks for your help and advice!

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    you can get a three-LED outlet tester for under $10 that will tell you that and more. – dandavis Jun 27 at 22:50
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    Can you post photos of the insides of the boxes involved please? – ThreePhaseEel Jun 28 at 0:47
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    Is the wire encased in conduit? Flex or rigid? – whatsisname Jun 28 at 5:22
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Well it is possible this home is a expanded metal stucco home and just happens to have some pipe touching the metal looking like a ground, it can also be possible that the boxes are grounded from the back side, your location in the world would be helpful but with cloth covered wiring I would suggest you add a grounding electrode other than your water pipe (the most common method for the decade prior and after your home was built) it has been legal to add a ground separately to these circuits.

I would not trust the chance there is a ground but adding a GFCI at the breaker or first outlet on the string or chain you can then install 3 pronged receptacles and just put the sticker that comes with the GFCI on the new 3 prong that’s states GFCI protected no equipment ground. Yes the stickers fall off but this is a legal way to update a 2 wire system, if you add a ground that is the best way but also the most expensive and usually only done when remodeling/ redecorating (big difference between the 2 names and code requirements BTW.

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  • Will a plug in tester distinguish between an intentional, high current capacity ground and an incidental, unreliable ground? – Jim Stewart Jun 29 at 11:51
  • No incidental contact may Cary many amps but can not be relied on. most 3 light testers use neon lamps these only draw 3-4 watts per lamp The electronic version maybe less. – Ed Beal Jun 29 at 13:54
  • Was "cloth-sheathed 2-wire" of this time period installed as NM cable is today or would it be in conduit? – Jim Stewart Jul 2 at 0:55
  • Cloth was Very much like today infact The first “plastic” nm was 2 wire also so there is that out there also. All open, staples , bored through studs of corse there were more metal boxes until the “non metallic” started hitting the market. – Ed Beal Jul 2 at 13:16
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Plug a long extension cord into the new grounded receptacle and get the other end near your panel. Using your meter on resistance setting measure the resistance between the GROUND in the extension cord and one of the screws holding the front of the panel. You should get a low value if the receptacle is grounded.

You don't have to use a grounding pigtail. Since you have metal boxes the end tabs of the receptacles would contact the tab on the box if you remove at least one paper keeper on the screws. (Clean off any deposit to bare metal.) Some fancy receptacles have a wire keeper on one end. You might have to break off the ears on the receptacle to achieve contact. Notice the scoring to break the ears off. Use the ears as conducting shims if the box is inset too far. The ears are intended for this use.

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    Jim I have done a lot of electrical that had the exact issue a very low resistance only a few ohms and there was no intentional bond , we usually found where the pipes came through the walls for sinks or water heated the pipes touching the metal. Just having a low resistance is not sufficient because when the worst happens the inadvertent contact may not be enough to dump the fault to ground. – Ed Beal Jun 28 at 16:08

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