Find the Answer
This group of photos was sent in to me by Russ LeBlanc, a master electrician and Code instructor in Massachusetts. Here's the background story as relayed to me by LeBlanc. A maintenance manager of an apartment unit called me to check out a problem that was reported to him by one of his tenants. The tenant said they felt a "tingling sensation" whenever they came out of the bathroom (door on the right) after taking a shower and went to get a towel from the linen closet (door on the left).
Behind the left door is the linen closet. Behind the right door is the bathroom. The light switch is for the light in the bathroom. The walls are constructed with metal studs. Although the doors are made of wood, the door frames are constructed of metal.
I initially took some voltage readings with my multimeter, but couldn't come up with a reason why this might be happening. I checked every metal surface I could find in the area, but couldn't pinpoint the problem. Finally, I caught a break. After I closed the bathroom door and turned the light switch on, I suddenly was able to measure 120V from doorknob to doorknob!
Whenever I shut the switch off, the voltage dropped to zero. When I turned the switch back on the door knobs were energized again. I just couldn't believe it. This is when I went to my truck and got a temporary light socket and some wire. After connecting one wire to each doorknob and turning the light switch on, the light lit up as shown in the photo. Now I had to figure out what was causing the problem.
I assumed that during construction the wires for the bathroom light were run from the switch, up the left side of the closet wall, and then through the metal studs in the wall over the top of the closet door, as the ceiling in this location was cement. I decided to cut the wall open above the closet door at the corner where I thought the wires may have gotten damaged. And there it was. Whomever installed the door frames did so with screws that were much too long. These long screws penetrated the insulation of the wire and made contact with the energized conductor of this circuit.
So, what was happening? When the switch was turned on it energized the cable to the bathroom light, which in turn energized the sheetrock screw and the metal door frame. In fact, all of the studs in the entire wall became energized. Whenever the door was closed, this also energized the doorknob. The breaker never tripped because the studs were not bonded to the equipment ground of the circuit, and plastic boxes were used.
Why was there a potential difference between the two doors? This happened because the metal studs in the other wall had grounded metal water pipes routed through them and the studs in that wall never touched the studs in the energized wall.
Section 300.4(B) of the NEC outlines the protection requirements for nonmetallic-sheathed cables and electrical nonmetallic tubing running through metal framing members. More specifically, 300.4(B)(1) states, "In both exposed and concealed locations where nonmetallic-sheathed cables pass through either factory- or field-punched, cut, or drilled slots or holes in metal members, the cable shall be protected by listed bushings or listed grommets covering all metal edges that are securely fastened in the opening prior to installation of the cable."