How to fix an outlet who's ground voltage is incorrect?
July 14, 2009 5:20 PM   Subscribe

Replaced a 2-prong ungrounded electricl outlet with a 3-prong grounded one, but when testing the voltage, the hot-ground only shows 50v, while the hot-neutral shows 120v. Any advice to fix this?

I am not an electician, but consulted two before doing this and after I got this problem, and they are stumped.

Basically, all the outlets in my living room are 2-pronged without a ground. Needing to plug in electrical things (TV, Computer, etc), I would like a ground, so I shut off power to the outlets at the breaker, and took them out to have a look. Behind each one there are 2 bare copper wires wrapped togethor not connected to anything, the ground wires. I hooked everything up to the new outlets, turned the power back on, and went to test the voltage of the new outlets to make sure they are working properly. When testing hot to nuetral, it reads 120V like it should. But when testing the hot to ground, it reads 50V. Does anyone know what could be causing this, and how I can fix it?
posted by jmg967 to Home & Garden (24 answers total) 2 users marked this as a favorite
That's the result I would have expected, so I'll be interested to hear from someone more knowledgable.
posted by entropic at 5:26 PM on July 14, 2009

The ground wires aren't grounded? Unless you're positive that those ground wires are really grounds maybe you should install a GFCI outlet instead. I had an electrician doing exactly this last week (grounding old two-prong outlets) and in one case where it wasn't easy to get a ground wire in he installed a GFCI outlet which he claimed met code for retrofit work.
posted by GuyZero at 5:32 PM on July 14, 2009

What do you get if you measure the voltage from neutral to ground? It should be zero, or pretty close to it. If it's not, then those ground wires probably don't have continuity all the way back to the fuse/breaker panel, where they should be connected to the neutral.
posted by FishBike at 5:41 PM on July 14, 2009

Response by poster: While searching on here for this answer, I came accross a post saying that GFCI outlets would not protect my electronics from getting messed up.

Here is one I also found saying that.
posted by jmg967 at 5:44 PM on July 14, 2009

My guess also would be that the ground is not grounded, but is "floating" about halfway between ground and hot.

Behind each one there are 2 bare copper wires wrapped togethor not connected to anything, the ground wires.

Not quite sure how to parse this. Are you saying there are only two wires coming into the box?

Can you tell what is coming in from outside the box? For instance, if you have older wiring, you might have steel-jacketed (armored) "BX", for which the jacket (plus the thin steel uninsulated "drain" wire?) acts as ground and SHOULD be connected to the steel box in the wall via a clamp inside the box. Sometimes, though, the clamp connection can get corroded. More likely, there's an intermediate splice somewhere between the box and the fuse box/breaker panel which SHOULD have a box, but may not. If none of this pans out, check (or have someone check) the ground connection inside your fuse box/breaker panel.

If you have very old "post and tube" wiring, there probably is no ground going to any of your boxes, and attaching the outlet's ground wire to the box won't have any effect.

Disclaimer: I am not an electrician.
posted by ZenMasterThis at 5:52 PM on July 14, 2009

Yes, you're right about GFCI outlets not protecting with surge protectors and the like. Sorry for not putting two and two together there.
posted by GuyZero at 6:07 PM on July 14, 2009

Response by poster: If by "post and tube" wiring, you mean that inside the outlet box there are two posts coming down, with a White, Black, and bare copper wire coming from each one, then that is what I have.

So to clarify, the original outlets had the 2 white and 2 black wires connected correctly, and the bare copper wires was just twisted together inside the box, not attached to anything.
posted by jmg967 at 6:09 PM on July 14, 2009

(Post-and-tube is a very old wiring technique from the early days of residential wiring. It typically involved cotton-insulated single wires separated by ceramic insulators nailed into wall studs behind a lath/plaster wall surface.)

Okay. If you have white, black and bare copper wires you're off to a good start.

The bare copper wires, by NEC code, should have a green wire nut with a copper "pigtail" attached via a screw to the box (if it's a steel box). There should also be a pigtail from the green wire nut to the green screw on the 3-prong outlet.

By code, the whites (neutral) and blacks (hot) should each have a wire nut (typically red or gray), each with a pigtail to the outlet. The white pigtail should go to the silver screw on the outlet.

If your box is wired-up like this, you PROBABLY have a defective (missing, corroded, etc.) ground somewhere else. This is a dangerous condition and you should not use the outlet until this is corrected. First try tightening the wire nuts in the box. Then, if you have access to junction boxes between the box in question and your fuse box/breaker panel, open those (or have someone open them) and check them for loose wire nuts, ground screws, etc. Finally open (or have some open) the fuse box/breaker panel) and check for a loose ground. Good idea to shut off the box/panel before doing this.

Again, I am not an electrician.
posted by ZenMasterThis at 6:29 PM on July 14, 2009

"Post and Tube" -- better known as Knob and Tube wiring is a two conductor wiring system where the safety of the system is in the fact that the two conductors are spaced 6" to 12" away from each other. The "Post" or "Knob" are supports that the wire is strung from to keep it away from the wood. There are also ceramic 'tubes' that let the wire pass through joists and whatnot. Either way, insurance companies hate it, so if they ever find out about it, you'll have to replace it. if you have a fire and theres K&T chilling out in there, they may not pay out.

If you have two pronged outlets, and your house was wired before the 1930s, you might have some of that in your house.

But yeah, if your house wasn't rewired in the modern era, then you probably just have the two conductors with no ground. You need to describe exactly where the copper wires you're referring to are coming from. Are they the ground wires that somehow came with the outlet? Do they mysteriously protrude from the wall? Maybe it's time to call an electrician.
posted by Geckwoistmeinauto at 6:30 PM on July 14, 2009

I suspect your ground is open and your hot/ground reading is meaningless. If the ground were sound, then there would be no reason to put 2-prong outlets on the circuit. You should read at most, a few volts on the neutral.

Again, I suspect you are trying to interpret a meaningless reading... a high impedance voltmeter connected between hot and a long, loose wire going nowhere.
posted by FauxScot at 6:49 PM on July 14, 2009

What FauxScot said.
posted by ZenMasterThis at 6:52 PM on July 14, 2009

Response by poster: @ZenMasterThis: The copper wire is coming from the same place as the black and white wires. It is not connected to the box via a green screw as you stated it should be. If I added an extra copper wire by attaching it to the box, twisting it with the current copper wires, and attaching this to the green screw on the outlet, would that maybe create a solid ground and solve the problem?

@Geckwoistmeinauto: The copper wires each come from the tubes that the white and black wires come from, and are twisted togethor inside the box (or were until I connected them to the outlet).

@FauxScot: From what I understand and have been told, the hot-neutral should read 120V and the hot-ground should read 120V as well. If 50V on the hot-ground is fine and won't hurt anything then that works for me too, as long as my electronics are protected and my house won't catch fire.
posted by jmg967 at 7:04 PM on July 14, 2009

I think FauxScott is saying that the 50V you're seeing is actually a bogus open-circuit value because the ground is disconnected. 50V is not OK for hot-ground for any reason.
posted by GuyZero at 7:49 PM on July 14, 2009

Also, per FauxScott's other comment, if you house was built before the late 60's/70's, they only needed two-prong outlets to meet code so they generally didn't run a ground wire. It would be odd to have a two-prong outlet with a working ground in the box. If the ground was good they would have connected it in the first place. Obviously you do have something there but it's not clear that it's actually a ground.
posted by GuyZero at 7:51 PM on July 14, 2009

My interpretation is that you have a bad ground. It is kinda grounded but kinda not. If it was a good ground you would get that 120v reading. If it was not allowing current at all you would read 0v. I don't quite understand what the "tubes" are. Are they metal conduit? If I remember correctly the metal conduit should lead back to ground, so attaching the copper wires to the box that is attached to the conduit should ground your outlet. My dad was an electrical engineer but I am not an electrician.
posted by pointilist at 7:55 PM on July 14, 2009

Best answer: jmg967, that would be true if the ground were actually connected to anything. It probably isn't.

Ground and neutral are supposed to be connected way back at the power entry point of your house. You only need hot and neutral to power a circuit. Normal operation of a circuit sees exactly the same CURRENT on the hot and neutral wires. Ground does not normally carry any current.

Your house won't catch fire, but the presence of a three-prong connector leaves the poor shmuch who moves in later thinking he has a grounded outlet, and he does not. ( A house inspector will check the ground integrity during an inspection and he'll catch this, but it is generally bad form to put a 3-prong outlet in an ungrounded circuit and I am sure it's not to code.)

Anything you plug into your new '3-wire' plug that uses the ground wire as a safety feature will have a floating ground. Not good.

Surge and transient protection will still work, since the suppression is applied between hot and neutral. Your computers won't be destroyed.

On the other hand, a defect in your plugged in products could put a chassis at hot potential and electocute someone under the right circumstances. If you feel like you know what those circumstances are, then you are in a position to assess the risk. It's non-zero.

Get an electrician to run a legit ground lead to the socket you want to ground. It's not do-it-yourself territory. I am an EE and I hire electricians.

( Incidentally, the MD who I bought my house from did DIY wiring and left me with a lethal wiring hazard in the kitchen, actually because he was at a loss for what to do with an extra wire on a 3-way circuit. I found it when troubleshooting why his wiring job didn't alway work properly. )
posted by FauxScot at 7:58 PM on July 14, 2009 [1 favorite]

Best answer: If I added an extra copper wire by attaching it to the box, twisting it with the current copper wires, and attaching this to the green screw on the outlet, would that maybe create a solid ground and solve the problem?

No. It's good practice and a good starting point, but it won't solve the problem. You seem to have a bad ground elsewhere.
posted by ZenMasterThis at 8:15 PM on July 14, 2009

Best answer: It sounds like you have a bad ground.

The resistance between the ground prong in your plug and the ground rod outside your house (or cold water pipe where it enters the ground), should be very low. Ideally it would be zero, but that never really happens.

If the resistance is high, due to a broken ground conductor somewhere (not hard to believe; bare copper wires are fairly fragile) then the tiny current allowed through by your voltage meter — ideal voltage meters pass no current, but in the real world, they all allow a few mA through — decreases the voltage difference from one side to the other.

If you wanted to confirm this, you could go to another outlet with a known-good ground (tested the same way), attach an extension cord to it that you know is good (test continuity of the ground conductor first, obviously), and test from the hot of the questionable outlet to the ground of the extension cord. It should show 120V. If it does, as I suspect it will, then you need to fix your ground.

There may be some easy way to fix the ground in the outlet (maybe running a wire to a nearby cold water pipe), but it's going to depend on local code. It may require re-running the wire from the panel box to the outlet, using modern NM/Romex instead of the BX that I presume is in your walls right now.
posted by Kadin2048 at 8:20 PM on July 14, 2009

You should check the voltage between the common/neutral (white) and the ground. It should be zero. The voltage between the hot (black) and common should be ~120. The voltage between hot and ground might fluctuate depending on what's loaded to ground.

Neutral and ground are the same thing, they are only separate for safety/code reasons. In order for it to be a ground, it has to actually be connected to The Ground.

I guess its possible that due to capacitance, a volt meter would see a disconnected (floating) ground as a bit of a load and show some voltage.

But you should probably complete the job and look in the fusebox/breaker panel and see where the other end of those grounds are connected before you plug anything in. If this seems scary, you really shouldn't be doing electrical work.

And please explain what a hot-ground is....??
posted by gjc at 8:35 PM on July 14, 2009

Unless you are in a building/country that uses two phase 120 volts? IE, two legs of 60vac, 180 degrees out of phase with each other? Rare, but I think it exists.
posted by gjc at 8:38 PM on July 14, 2009

Response by poster: Thanks for all of the help! I have pretty much gathered that this is more than just an easy fix and out of my league to handle. Oh well, I guess I will have to run an extension cord from the other side of the house to my living room for the time being. Thanks everyone!
posted by jmg967 at 8:48 PM on July 14, 2009

Best answer: This is a classic symptom of a floating ground. This is caused by by either bad or non existent connections. Connecting the ground wires to your box isn't going to make any difference unless the box has a separate ground connection (say from metal conduit which may be what your tube are; a picture would help) but it's a good idea for safety. This can cause a problem with computer equipment if the ground experiences transients because the electronics aren't expecting 0V to float around. I've solved many computer crashing problems in commercial settings with an electrician.

First place to start is in your panel. Turn your service breaker off. Check to make sure that the wire leading to the fuse/breaker controlling this circuit has an associated ground and neutral and that the ground is connected to the ground bus. If there isn't that means you can't really depend on the ground wire to provide a ground unless you can figure out where it terminates.

Second you need to retwist all the ground connections in the boxes for that circuit. Despite what the marrette box says wires should be pretwisted before the marrette goes on. The theory is that even if the marrette falls off or breaks the wires should remain twisted together. It would be great if you can figure out which box is first in the run as you can start testing your work right away.

If the above doesn't fix the problem it means that either you have a wire fault (often caused by someone sinking a nail through the cable) or there is another junction that you don't know about. Common culprits are in attic or crawl spaces and sometimes in ceiling boxes. How much that bothers you is up to you; anytime you've got a problem with wiring there is the possibility of an arc causing a fire.

It is possible (and legal in Canada) to run a separate ground to a box which ties into the existing ground and this gives you a known good connection. This is often pretty easy because the bare ground wire can be connected to a grounded metal water or gas pipe or can be run all the way back to your panel in the attic or a crawl space.

However have you tested any other outlets in your house? The outlet on your stove is a good choice because your range is generally on a circuit by itself. It's possible for either the bond connection in your panel to be wrong or the ground outside your house to be missing or ineffective. In which case it doesn't matter what you do to the circuit in question things aren't going to work and you should probably call in an electrician to figure out what is wrong. This can be pretty, uh, interesting as there are some truly scary installations out there and any house could have been hacked on by a former inhabitant.

gjc writes "The voltage between hot and ground might fluctuate depending on what's loaded to ground."

You should always have 120V (give or take depending on your distance from the transformer) volts between a hot and ground. It shouldn't fluctuate to any great degree and nothing should be loaded to ground.

gjc writes "Neutral and ground are the same thing, they are only separate for safety/code reasons."

Neutral and ground are not interchangeable and shouldn't be thought of as the same. Not all circuits even have a neutral conductor (electric water heaters or A/C condensers for example) and GFCI technology depends on a neutral which is why it is a safe way of upgrading two prong outlets to three prong.
posted by Mitheral at 8:59 PM on July 14, 2009

Having a bad outlet would seem to be a warning sign; I'd pick up an outlet tester and check all the outlets in the house.
posted by sebastienbailard at 11:42 PM on July 14, 2009

This is kind of random -- and doesn't negate the end result that you have a floating ground, as advised above, and should seek professional help -- but I suspect the house was wired with pre-wired conduit that contained three wires (including ground) but only had two-prong outlets installed, and so each of those bare ground wires is just running free through the conduit attached to nothing. You may not have a "bad" ground -- you may have none whatsoever, just stretches of disconnected wire intended for a ground.
posted by davejay at 1:53 AM on July 15, 2009

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