Grounded by faith
October 26, 2011 4:35 PM   Subscribe

How the heck is this outlet 'grounded'?

My house is really old. Most of the outlets on the upper floors aren't grounded. (I had an electrician ground the ones downstairs.) There's a single three-prong outlet upstairs. It appears to have been installed by a moron (there's a giant hammer hole in the wall around it).

I plugged in my outlet tester and it said "hot/neutral reversed." So I flipped the breaker, switched the polarity, and turned it back on. I plugged the tester in again and it said "correct."

The thing is -- while I was looking at the back of the outlet, I noticed there wasn't any ground wire! So why doesn't the tester say "open ground"?

I confirmed that the box itself is grounded, using my multimeter and touching it to the screw that holds in the outlet plate. That can't be a good enough connection to be a real ground, can it?
posted by zvs to Home & Garden (20 answers total) 1 user marked this as a favorite
Response by poster: (Oh, and the tester isn't defective -- other outlets in the house I know to be ungrounded give the correct "open ground" reading.)
posted by zvs at 4:39 PM on October 26, 2011

That's plenty for your tester to work. It's not like it's looking for wire of the proper gauge, merely for the right electrical connection.
posted by mollymayhem at 4:57 PM on October 26, 2011

Are any other outlets on that circuit? How do they test? What does the wiring look like going into breaker - does it have a ground wire?
posted by mightshould at 5:19 PM on October 26, 2011

Response by poster: There are a couple other outlets on the circuit (it appears to be the ONLY circuit on the entire 3rd floor), but none of them are 3-prong so my tester doesn't fit.

The panel installation obscures the wiring to the individual breakers, so I'm not sure what it looks like. At the outlet, it's two cloth-wrapped wires. At the panel, there are a few pairs of cloth-wrapped wires going in and out; most of the circuits appear to be wired using flexible conduit. It seems extremely unlikely somebody ran a ground up that far.

What I really don't understand is how there's any ground at all... unless the ground screw is rubbing up against the outlet or something.
posted by zvs at 5:24 PM on October 26, 2011

Response by poster: I say "It seems extremely unlikely somebody ran a ground up that far." because the walls and wallpaper definitely haven't been modified in the last few dozen years (LAYERS of wallpaper) -- but the outlet is pretty new looking (it has a barcode).
posted by zvs at 5:25 PM on October 26, 2011

Is it grounded via metal conduit?
posted by Midnight Skulker at 5:27 PM on October 26, 2011

Response by poster: I'm not sure how to tell.

The idea that the box is grounded makes sense. Kinda. It's not insane, anyway. What I don't get is how the outlet can be connected to ground without any actual wire connecting it to the box.
posted by zvs at 5:33 PM on October 26, 2011

The metal tab on the outlet is attached to the ground terminal - when you screw the outlet to the metal box you're making contact.
posted by Kid Charlemagne at 5:37 PM on October 26, 2011 [1 favorite]

There's nothing "magical" about wire. Electrical connections can be made by screws, nails, accidentally touching pieces of metal, wet skin, and in the case of many circuits in your car, attachment to the frame, which is presumed to be attached to the negative pole of your battery.

The box is typically grounded by attaching the third wire of the supply romex (sheathed wire), but can also be grounded by strapping to a cold water pipe (which, barring PVC supply lines, runs into the physical earth at some point).
posted by IAmBroom at 5:41 PM on October 26, 2011

Response by poster: Looking at some pictures I'm beginning to think this is BX rather than Romex. I guess I understand now. In the meantime I'm going to treat this as "not a safe ground".
posted by zvs at 5:51 PM on October 26, 2011 [1 favorite]

It's not that uncommon to find wacky grounds in old work. In my house they screwed the loose steel BX sheathing to the back of the box and connected the other end to ground at the breaker panel.
"Safe" is probably debatable - the primary reason for the ground is to prevent exposed metal on a device from accidentally connecting to the hot wire (since it's already hard-wired to ground, so hot touching too would blow the breaker). You could in theory measure the resistance from the ground pin to ground at the panel to see how good the ground connection is - worst case is that it will see a few dozen amps for a few milliseconds before the breaker blows; the danger is if you have a high reaiatan
posted by range at 6:00 PM on October 26, 2011

(stupid posting from stupid phone with stupid fat fingers, sorry)

... The danger is if you have a high resistance in some spot, that spot will heat for those few milliseconds, possibly catching on fire. I'd guess that outlet-screwed-into-box isn't bad contact (it's not just the screw, there's a lot of contact area between the two steel tabs too) but god only knows what's going on in the next few links in the chain.
posted by range at 6:11 PM on October 26, 2011

If you fit residual-current detectors at the breaker box, then even quite high-resistance grounding will protect you without causing fires; the RCD will trip as soon as it senses even very small leakage currents to ground.
posted by flabdablet at 6:21 PM on October 26, 2011

Sounds like your outlet is grounded, but... I would take the multimeter and measure the voltage and resistance between the outlet ground pin and a cold water pipe, just out of curiosity and to double-check your outlet tester.
posted by exphysicist345 at 7:37 PM on October 26, 2011

I've only ever seen it done on purpose, but are you positive that you don't have some form of bootleg ground?

Also: If I were you I'd install a GFCI outlet as soon as you can to make the outlet less dangerous ASAP.

(I say less dangerous because if NOTHING goes wrong the configuration isn't inherently dangerous.)
posted by ooklala at 7:49 PM on October 26, 2011 [1 favorite]

The last apartment building I lived in was built in 1927. Eight years after I moved in (around 1999, say) new management had the place rewired. The electrician told me that prior to that, literally every single outlet in my unit had been grounded through the water pipes.
posted by trip and a half at 8:54 PM on October 26, 2011

Response by poster: Didn't look like any bootleg ground to me, unless I'm severely misinterpreting the two-wire configuration.

RCDs (GFCIs in my lingo) are fine, but I want to connect a power strip. My understanding is that won't correctly protect from a real surge... that is, my equipment won't be grounded, although personal safety would improve.
posted by zvs at 8:55 PM on October 26, 2011

As I understand it it's kosher to use the metallic flexible (or rigid) conduit as the ground conductor in lots of situations, and "self-grounding receptacles" exist and are designed to be grounded to a metallic box via their mounting screws. You'd have to check the code to know the details, but the setup you describe— metallic conduit as ground, connected to metal box by appropriate hardware, connected to the outlet by the mounting screws— might be 100% OK. (Of course, more likely, it's totally screwed up; that's my default assumption wrt old house wiring.)
posted by hattifattener at 10:15 PM on October 26, 2011

I want to connect a power strip. My understanding is that won't correctly protect from a real surge... that is, my equipment won't be grounded, although personal safety would improve.

As long as your power strip has a three-pin plug and three-pin outlets, it should be fine.

Surge protection in power strips is generally provided by three MOVs, one each between each pair of the three wires from the power socket. The way a MOV works is that it flips to a very low resistance state when its breakover voltage is exceeded; hopefully that will mean that most of the extra current draw caused by the voltage surge gets diverted through the MOV instead of the equipment plugged in downstream of it. But a MOV can't absorb much surge energy without destroying itself, so they're really only good against quite short surge spikes.

Putting in an RCD (GFCI) upstream of a power strip with MOV surge suppressors is a good idea, because one of those MOVs will be connected between hot and ground and if a surge causes this to conduct it will trip the RCD, protecting both your equipment and (to some extent) the MOV itself. This will still work even with quite dodgy conduit-based grounding.
posted by flabdablet at 11:34 PM on October 26, 2011

I think the metal ears on the outlet and the ground prongs are both connected to the grounding lug on the outlet. In this case, the ground path is flowing though the ears to the box to the conduit.

According to wikipedia, gfci devices can protect any downstream devices in the circuit if installed correctly. So if you can ascertain which outlet is the first one on your circuit, you can install one gfci there and protect the rest of the outlets on the circuit.
posted by gjc at 6:31 AM on October 27, 2011

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