The Case of the Shocking-Hot TV Cables

By: 
June 24, 2011

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.

Tell us your experience in solving a knotty engineering problem. Send examples to Rob Spiegel for Sherlock Ohms.

Comments (0)

Please log in or to post comments.
  • Oldest First
  • Newest First
Loading Comments...