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I'm a first time home buyer and through the inspection found out that some of the receptacles in the house were ungrounded. The house is in Denver, CO and was built in 1954. The house itself is grounded and all the receptacles in the finished basement are showing as grounded/normal using a simple receptacle tester. I hired an electrician to come in a ground the ungrounded receptacles. He told me that the boxes were metal and that he could ground the outlets by bonding a wire from the outlet straight to the box. After he completed the work I tested the outlets with the receptacle tester and they still showed open ground. I asked him if that was to be expected and he said he can't explain why they'd show that way on the tester but they are definitely grounded. Is it possible for them to be grounded and still show as open ground on the tester?

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    They might be grounded to the box, but is the box grounded to anything? If the boxes aren't bonded to the service entrance ground, connecting them to the receptacle's ground screw won't do anything.
    – Chris O
    Feb 23, 2023 at 18:50
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    Either the outlets are bad/defective or the box grounding is bad. A voltage meter should read ~120v between the hot screw/wire of the outlet and the box if good grounding.
    – crip659
    Feb 23, 2023 at 18:51
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    Are the metal boxes connected back to the panel via conduit? Unlikely in a house built in Denver in the 50s, so that's likely why you're not getting any indication of ground. Connecting a ground wire from the receptacle to the box is pointless if the box itself isn't ground to anything. It's like putting a leash on your dog, then dropping the leash on the ground - isn't going to hold him back when the mailman comes, so why bother...
    – FreeMan
    Feb 23, 2023 at 18:54
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    .... And either way a competent electrician should know what to do. Which might be to install GFCI outlets, which provide similar safety even without ground.
    – keshlam
    Feb 23, 2023 at 18:55

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Three pieces are needed:

  • A grounded/three-prong receptacle - Some are better than others...
  • Connecting the receptacle ground to the metal box
  • Connecting the house ground to the metal box

The first and the second are related. The cheapest receptacles require a ground wire from a screw on the receptacle to the box. The better receptacles ("spec. grade" or marked as "self grounding") automatically connect the receptacle ground to the metal box simply by screwing the receptacle into the box, provided there is clean metal-to-metal contact between the yoke of the receptacle and the metal box. An actual wire doesn't hurt, but just isn't required with the better receptacles.

So assuming the electrician did what he claims he did (and which is perfectly normal) then the problem is the third part - connecting the house ground to the metal box.

In my house (Maryland, 1950s) every metal box with a two-prong ungrounded receptacle where I have replaced it with a grounded receptacle already had a ground wire connected to the metal box. Maybe I got lucky, or maybe Maryland tended to have more ground wires than Colorado in the 1950s. However, the electrician should not have assumed that was the case - he should have tested each one to make sure. There are two simple ways to do that:

  • A three-light tester - which showed "Open Ground".
  • A multimeter - it should show ~ 120V when checking hot to ground (or the surface of the metal box) and ~ 0V (and very low resistance) when checking neutral to ground (or the surface of the metal box).

I would check it with a multimeter. If you find no connection between hot and ground or between neutral and ground then you do not have ground.

If there is no ground and there is no "almost ground" (a ground wire in the cable that was never connected but can be connected easily) then you have two basic options:

  • Retrofit ground

Run a ground wire from each box to another grounded location (e.g., a grounded receptacle, the main panel, etc.)

  • GFCI in lieu of ground

Due to the way GFCI works, (a) despite the name it doesn't actually need ground to do its job and (b) in almost all situations where ground would be useful, the GFCI will trip, which makes things safe.

GFCI in lieu of ground is installed just like a regular GFCI/receptacle. However, that does mean that if a receptacle has multiple cables (typically one from the panel or previous receptacle and one going on to the next receptacle) then you have to figure out which is which as only the cable that is coming from the panel (or previous receptacle) is connected to "line" and the other is connected to "load". Only the first receptacle in a circuit needs to have GFCI if it is installed properly. But you must put labels on all of the GFCI protected receptacles (including the GFCI/receptacle itself) to make it clear that there is "No ground".

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    I would try with a test instrument or a 25 watt light bulb and probe from the ground to the hot side of the receptacle, also check the neutral which will show true. I have a voltage tester with a solenoid that shows the approximate voltage.
    – Gil
    Feb 23, 2023 at 21:18

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