2 Prong Outlets-Sanity Check

[bunnspecial]

Our house was built in the 1950s, and is mostly 2 prong outlets. A few circuits(garage, kitchen, bathroom) have GFCI installed with "No Equipment Ground" stickers on the plates(as per code, as I understand it). When I've probed those circuits, they do indeed test out as the ground being "dead"(completely open circuit).

Some of the 2 prong outlets are at the end of their useable life(won't retain plugs properly), and I know that often times in older wiring an earth ground is present through the metal fixture box. With those, as I understand, a 3 prong can be installed as long as you properly ground it(either through the outlet screws to the box, or by a short pigtail screwed into the box, or both).

I went investigating in our living room today to see if there's an available earth ground, and what I found I think should have me concerned but I'd appreciate a second look at it, and yes I know that this probably warrants a call to an electrician.

In any case, when I use a multimeter across the two prong, I get 120V as I'd expect

I'd originally checked this with the plate on the outlet by just checking against the plate screw, but the numbers seemed crazy so I pulled the plate and probed a few places around the box on this fixture. What I saw was consistent no matter where I checked.

As I understand it, if the metal receptacle box were grounded, I should measure 0V from the wide plug to the outlet box, and 120V from the narrow one to ground.

This is...not what I get...

Wide to box(or any other metal part of the receptacle, including the plate cover screw) measures ~35V

And narrow to box measures ~95V.

I checked other outlets in the living room(all the same circuit) and they all give this same set of readings. For the others I just checked off center screw.

Obviously I have a problem here that needs to be fixed, but does anyone have any thoughts on what could cause these sort of voltage readings?

 

[Rmay635703]

Hopefully that’s just a bad meter and not real

Unless somebody ran the ground to an unbonded ground wire…

Yeah that’s not great either having a floating area that should be an NC instead with voltage. That can happen in wet areas with some electrolytes or in the case that the insulation is deteriorated somewhere or there is something stabled/crimped that shouldn’t be.

I would want to test how much resistance there is from hot to your floating ground because a charged up floating ground can result in certain appliances and other devices getting a charged up chassis.

 

[bunnspecial]

Meter is old but quite a good one...literally no reason to doubt it(and I've checked it recently against good HP bench meters). Just to be sure, I just checked with another meter-this one a much newer higher end Extech, and got more or less the same readings.

I think this is definitely in call an electrician territory. I killed power to this circuit ran a wire to the ground rod outside. Checking continuity to a known good ground, I get around ~10Ω on one prong, and around .2Ω on the other to ground. I think there's a wiring issue here that's definitely outside my ability to assess and handle...

This is the inside of the breaker box if anyone is wondering, or if there's anything to be learned from this

 

[Wireman1987]

Our house was built in the 1950s, and is mostly 2 prong outlets. A few circuits(garage, kitchen, bathroom) have GFCI installed with "No Equipment Ground" stickers on the plates(as per code, as I understand it). When I've probed those circuits, they do indeed test out as the ground being "dead"(completely open circuit).

Some of the 2 prong outlets are at the end of their useable life(won't retain plugs properly), and I know that often times in older wiring an earth ground is present through the metal fixture box. With those, as I understand, a 3 prong can be installed as long as you properly ground it(either through the outlet screws to the box, or by a short pigtail screwed into the box, or both).

I went investigating in our living room today to see if there's an available earth ground, and what I found I think should have me concerned but I'd appreciate a second look at it, and yes I know that this probably warrants a call to an electrician.

In any case, when I use a multimeter across the two prong, I get 120V as I'd expect
View attachment 237758

I'd originally checked this with the plate on the outlet by just checking against the plate screw, but the numbers seemed crazy so I pulled the plate and probed a few places around the box on this fixture. What I saw was consistent no matter where I checked.

As I understand it, if the metal receptacle box were grounded, I should measure 0V from the wide plug to the outlet box, and 120V from the narrow one to ground.

This is...not what I get...

Wide to box(or any other metal part of the receptacle, including the plate cover screw) measures ~35V

View attachment 237759

And narrow to box measures ~95V.

I checked other outlets in the living room(all the same circuit) and they all give this same set of readings. For the others I just checked off center screw.

Obviously I have a problem here that needs to be fixed, but does anyone have any thoughts on what could cause these sort of voltage readings?

View attachment 237757

Is that by chance a switched receptacle. Aka half hot?

 

[Dave Sherman]

Your box is not grounded, and you are picking up phantom voltages. There are meters that have a low impedance (low-Z) mode that prevents capacitively coupled phantom voltages from registering, which your average digital multimeter will show. I'd get with an electrician on what your options are for adding grounded outlets.

 

[Hall]

and I know that often times in older wiring an earth ground is present through the metal fixture box

I thought that was only valid if there was metal conduit or BX cable going to the box. The conduit or armored cable carried the ground, then you could tie into it.

That is an old-school meter though ! Is that from the 70s ? It looks a lot like the handheld Mattel Football, Basketball, etc games !

 

[nomas]

I thought that was only valid if there was metal conduit or BX cable going to the box. The conduit or armored cable carried the ground, then you could tie into it.

That is an old-school meter though ! Is that from the 70s ? It looks a lot like the handheld Mattel Football, Basketball, etc games !

I used a Meter like that back in the late 80's . It's a Beckman . Very good meter as I recall .

 

[jhellwig]

Ghost voltage does weird things on cheap meters.

 

[bunnspecial]

Your box is not grounded, and you are picking up phantom voltages. There are meters that have a low impedance (low-Z) mode that prevents capacitively coupled phantom voltages from registering, which your average digital multimeter will show. I'd get with an electrician on what your options are for adding grounded outlets.

Thanks, I think you nailed it.

I dug out an analogue meter that I know is 10KΩ, and it read zero. I know most DMMs are in the MΩ range, and this one is specced at 2.2MΩ.

Just to be sure too, I also measured again with this meter, and it reads zero on either prong to the box with a 20KΩ resistor in parallel.

Thanks for the help, and sounds like this is actually a non-issue.

 

[bunnspecial]

I used a Meter like that back in the late 80's . It's a Beckman . Very good meter as I recall .

Correct, it's an early 80s Beckman 330. I think Beckman's metering division has died off, but it was not a cheap meter when new and is probably about equal to a Fluke 8060 or so in both features and quality. This one is a true RMS meter-it's not quite as good as my HP bench meters if you start throwing really weird wave shapes at it, but for the common squares, triangles, shark fins, etc it is every bit as good. It's old but it works great-I have a fair few meters around here, but this one is one of my go-tos becuase it's simple, easy to use and I know I can trust it. The only thing I really miss compared to some of my newer meters are things like the continuity beeper and auto-ranging, although on a lot of portable meters auto-ranging seems to take forever to pick a range, and they need way too many button presses to get out of auto-ranging and manually select your range.

 

[nomas]

Correct, it's an early 80s Beckman 330. I think Beckman's metering division has died off, but it was not a cheap meter when new and is probably about equal to a Fluke 8060 or so in both features and quality. This one is a true RMS meter-it's not quite as good as my HP bench meters if you start throwing really weird wave shapes at it, but for the common squares, triangles, shark fins, etc it is every bit as good. It's old but it works great-I have a fair few meters around here, but this one is one of my go-tos becuase it's simple, easy to use and I know I can trust it. The only thing I really miss compared to some of my newer meters are things like the continuity beeper and auto-ranging, although on a lot of portable meters auto-ranging seems to take forever to pick a range, and they need way too many button presses to get out of auto-ranging and manually select your range.

I replaced the Beckman with a Fluke 8060 . Good meter but way too many buttons to push . Ended up with a Fluke 87V by the time I retired .

 

[eljefino]

You've got a little paint or corrosion connecting your metal box to its conduit or at a joint somewhere on the way to the main house ground, and your high impedance meter doesn't like it.

I'd install a grounded outlet as you initially planned, and get one of those circuit testers that plug in and show neon lights if everything's wired up correctly. I bet they will work. If you get something like an air conditioner with built in GFI plug, try that, too.

I've gotta ask, is the outlet box you show in the pictures metal or plastic? It looks like a plastic "Carlon" style box which IDK if they used in the 1950s.

 

[Hall]

I've gotta ask, is the outlet box you show in the pictures metal or plastic? It looks like a plastic "Carlon" style box which IDK if they used in the 1950s.

Looks like metal that's been painted over countless times. Notice the side plates extend slightly above ? Plastic boxes won't have that.

 

[bunnspecial]

You've got a little paint or corrosion connecting your metal box to its conduit or at a joint somewhere on the way to the main house ground, and your high impedance meter doesn't like it.

I'd install a grounded outlet as you initially planned, and get one of those circuit testers that plug in and show neon lights if everything's wired up correctly. I bet they will work. If you get something like an air conditioner with built in GFI plug, try that, too.

I've gotta ask, is the outlet box you show in the pictures metal or plastic? It looks like a plastic "Carlon" style box which IDK if they used in the 1950s.

Looks like metal that's been painted over countless times. Notice the side plates extend slightly above ? Plastic boxes won't have that.

Yes, definitely metal, although it threw me off at first too and I thought gray plastic.

Repairs can sometimes be almost an archeological dig in this house. A couple of years ago, I replaced the likely original(and well past its prime) 5 gallon, 4 bolt toilet with a nice new American standard 1.3 gallon(that's unstoppable). That one was a bigger project than I expected, and among other things got a nice look at the very 70s looking dark brown-green/cream striped lineoleum that was under the current laminate, and the probably original to the house alternating black and white tiles under that. Even more fun with the toilet project(and relevant to the post I'm responding to) was that the old tank sat flush against the wall, and the new tank sits an inch out. It was bare plaster(yes, plaster) behind the tank, but I also could see a full history of every color the room had ever been painted. There were probably 5 or 6 layers, and all of them so stereotypical of different decades that you could pretty well guess when it had been painted. There was an avocado green, light pink, darker green, a yellowish buttercream color, and one or two others mixed in with that. The extra fun when I did paint it came from that many layers being so thick compared to the bare plaster that it took several coats of paint along with some strategic sanding to not have that spot quite literally stand out(or would sink in be more appropriate?).

 

[SHOZ]

Pull the outlet out of the box and see what you get. Maybe an issue with the wiring behind it or the outlet could be ghosting.

My house was built 1960 and still has a lot of the original two slot outlets. When they ran switches the ran hot to the device and switched the neutral. Took a while to figure that out.....

 

[Hall]

When they ran switches the ran hot to the device and switched the neutral. Took a while to figure that out.....

That's a very standard way of wiring lights. You avoid running (2) lines back and forth to the light plus you're not daisy-chaining light switches.

 

[Hall]

Repairs can sometimes be almost an archeological dig in this house.

Our previous house was built in 1913 and many things were renovated but there were still some interesting things. There is an outlet on the side of the fireplace that was literally multiple pieces of copper strips in a cut-out hole in the wood mantle, covered by a wood outlet cover.

One of the bedrooms had many layers of flooring under the carpet, including newspaper sheets from WWII, all of this on top of the original wood floors.

 

[SHOZ]

That's a very standard way of wiring lights. You avoid running (2) lines back and forth to the light plus you're not daisy-chaining light switches.

Sinking source switching they call it. Used a lot in electronic control. But there is no wire savings whether the hot or neutral is switched. Less load usually on the inrush. But a always hot wire at a device switched off is no good. Especially is the wires get mixed up somewhere along the line.

 

[Wireman1987]

Pull the outlet out of the box and see what you get. Maybe an issue with the wiring behind it or the outlet could be ghosting.

My house was built 1960 and still has a lot of the original two slot outlets. When they ran switches the ran hot to the device and switched the neutral. Took a while to figure that out.....

My old home had a receptacle that was switched and when in the off position showed voltage similar to that 90 or so volts.

 

[bunnspecial]

I replaced the Beckman with a Fluke 8060 . Good meter but way too many buttons to push . Ended up with a Fluke 87V by the time I retired .

I have a real affinity for older good quality test and measuring equipment, and a lot of it is really inexpensive now. No-name Chinese stuff is plentiful on Amazon and elsewhere, and a lot of hobbiests seem to prefer that over hunting down older quality stuff(which use to be the realistic option).

This is my stack in service now, although I really need to rearrange and get the scope in a better spot. I have two of these scopes-they're first gen HP Digital scopes, and are quite good units with what I find to be a really intuitive UI(although I've been using 80s-90s HP lab equipment since I was in college, and there's a lot of commonality in UI whether dealing with a gas chromatograph or an oscilloscope of the same age, so that may be a bit of my bias). I have the meters and scope all tied to my computer through HP-IB. I can't do a ton over HPIB with the scope-Keysight does provide a free program that can do screen captures from it but that's about it-but the meters I have some open source/freeware programs that can control them and do data logging. If I really wanted to, there's an HP 300 emulator out there that can do a lot of stuff with all of these through HP-BASIC programming, but I haven't encountered anything where I really NEEDED that. I have another Tek 2235 scope that I don't currently have out, but have had it for years and in a lot of ways I actually prefer a good analogue scope for tasks where it will work.

And as of now, since my office also does not have earth grounds, I'm a bit nervous that the scope and most of this other stuff is currently floating. Almost everything I look at on the scope is either fully isolated(battery powered) or maybe powered from the bench PSU, but I still know that can be a dangerous situation.