This mystery begins right after my girlfriend and I moved into a new (to us) house. When it came time to hook up a TV in the bedroom, I had three cables runs to choose from. One came from one corner, one from another, and one from the closet. I don’t know why there were three, but consolidating them was a project for another day. After trying them all and receiving no signal, I went to the basement to see if any of the drops tied into the main run (which entered at the end of the basement).
I spotted my first candidate -– a hanging cable wire that was not hooked up to anything. I grabbed the end to begin tracing its path. ZAP! The instant I touched it I received an energizing shock. It took me a few seconds to shake off the shock. It certainly didn’t feel like a static shock.
I got my multimeter out, and to my astonishment the shield read 120V ac when grounded to the nearby copper pipe. How could this be? My first action was to unplug and disconnect the TV, in case it was attached to this particular run of coax. My initial thought was that this could have been due to a poorly placed nail that lanced a power run to coax. Or maybe someone ran the coax through a junction box?
The source of this problem could have been anywhere. I went through the house checking for voltage on the other cable jacks. They were all fine. I returned to the basement and found that the voltage was no longer present on the hanging cable. Baffled, I returned upstairs and reconnected the bedroom TV. As I connected the cable wires I got zapped again! The multimeter confirmed the voltage source was the TV! A short circuit inside the TV, or an issue with the power wiring, perhaps?
I pulled out my receptacle tester. The receptacle to which the TV was connected read “neutral/hot reversed.” OK, I was getting somewhere. I checked all of the outlets on the circuit, and they all showed the same. I began checking each outlet for a wire reversal and found none. I checked at the breaker, and it was correct.
After an hour or so of head scratching and frustration, I tested the area between the neutral and the receptacle box and got 120V ac! The outlets in this room were added post-construction and consisted of four externally-mounted receptacles with a painted metal conduit connecting each. The conduit and boxes were hot! Apparently my receptacle tester indicates a hot/neutral reversal when there is power at the ground plug and hot lead.
With a systematic connection and disconnection of receptacles, I was able to locate the problem area between receptacles two and three. I removed the wire and found there was chafing where it made the 90-degree turn in the corner. That was where the hot decided to share itself with the conduit and boxes.
To recap: The chafed hot wire made the conduit and boxes hot, which made the ground plug hot, which made the chassis ground of the TV hot, which made the coax shield hot, which made my finger tips hot. The reason the breaker never tripped was because whoever wired this circuit didn’t bother to connect the ground from receptacle one back to the breaker.
The problem was solved when I replaced the run of wires between the two receptacles and fixed the missing ground. Somehow, after all of this, the TV still works, but I still find myself briefly hesitating before grabbing hold of cable wires.
Chris Hill is a mechanical engineer specializing in metal fabrication, machine design, and ASME code pressure vessels.
This entry was submitted by Chris Hill and edited by Rob Spiegel.
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The key sentence here is, "...whoever wired this circuit didn't bother to connect the ground from receptacle one back to the breaker." Having owned three old houses, I can attest to the fact that older homes often get worked on by "handymen" who aren't quite as handy as they think they are.
I had installed additional lighting in a very old store as well as replacing the light switch. I made the final connections live. After completion, I turned the switch on and some of the original flourescent lights dimmed dramatically while the newly installed lights lit up at same dim level. Turn the swith off and original lights brighted again, while new installs went out.
Now store owner now wondering if I had a clue as to what I was doing. Not a fun position to be in. The switch conduit box had the hot pass through wire, so naturally I had connected the ground wires to the metal box. As luck would have it, the battery had died in my multimeter, but as this was not rocket science,I did not believe that I really needed a meter to verify the problem, so no need to run to the store for another battery. So, I removed switch and made the contact closure by just touching the lead together. All lights came on as they were supposed to. As rediculous as this seemed, I replaced the light switch, put the cover back on, flipped the swith and problem was there again.
Ok, so what is the difference between making the light power contact closure by using a light swith or by touching the leads together manually? Nothing! So what else changed? Only ground point for the ground wire was one of the screws that also held on the metal cover. That ground to the switch box was lost when I removed the switch. So, I replaced the switch, left the ground off, and Wholla, lights illuminated! Issue is when gound placed onto fixture, problem appears.
Someone has the neutral and hot leads reversed somewhere down the line. Customer was advised. As he was not even sure where the fuses were for the store, he decided to let this go for a future time. Eventually I heard that several years later, another contactor had found the same problem when they installed additional fixtures and pull their wiring off the same branch circuit. This has now been corrected with new runs from a new load center.
Not a good idea for the customer to skip finding the fuses while the lights are still on. Gets harder in the dark...
Hope you informed them that this was a serious safety hazard and pressed them a bit to have it fixed. While I understand that the customer doesn't know electricity and might back away from doing anything because they're uncomfortable, don't really understand and don't want to spend the money, it will be a lot less comfortable and convenient if they put off fixing it until after someone gets a fatal shock.
BTW, I'll guess that "whollah" is "voila", pronounced "vwah-lah", a French word.
That is quite lucky that your TV did not short out or fry your video card. The new LCD and plasma TV's are great, but they are significantly more fragile than the old tube models. During a move, one of our 19" tube TV's dropped about 6 feet to the ground and only had some casing damage. The picture was fine. I do not think that my 42" plasma would survive the same drop.
In our many Sherlock Ohms mysteries, one of the most common challenges is finding the solution to a home or building's electrical peculiarities. Almost invariably, the problem traces back to some off-reservation electrical work that wasn't properly grounded. In one case, the problem resulted in the loss of life: The Case of the Floating Dock.
At a tanning plant, while raising a load, the mast of a forklift smashed into the socket of a 220Vac fluorescent overhead lamp. Unbeknowst to the driver, the forklift frame was now energized by the 220V line. The forklift driver dismounted the lift to dump his load of hides into the tank before lowering the forks. While doing so, he touched both the forklift frame and the tank. The driver became the ground path: lamp fixture to forklift to driver to the caustic tank. The tank had a circulating pump that was (correctly) grounded. He received a fatal shock. The story is told in more detail in "The Case of the Lethal Lamp," in the May 18, 2009 Design News Magazine.
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