Using plaster as a neutral

[Les]

This is the scenario:

I own a 1927 bungalow that is immaculate except the "second" Floor still has knob and tube wiring. It feeds one outlet, one overhead light and a ceiling light on the first floor small hallway. The light on second floor is fed with k&t thru a three way switch and works just fine. The hall ceiling light never worked and is switched with a wall switch.

I thought I would fix it today so went upstairs and pulled up some floor boards to expose the wires and ceiling box. The box is a 1" typical k&t round box. The k&t is connected to the dining room ceiling light abt 12' away and I can see it. No voltage on either wire. The switch wire in the wall is a cloth covered piece of "romex" and has continuity thru switch. The supposed hot k&t is the switched wire, neutral goes into ceiling box. Seeings how it is dead, I pull the wires out of the box up into the joist bay. The hot wire arcs like crazy and blows breaker.

I pull the box out of ceiling and found it was originally mounted on a wallpapered ceiling and had a 1" wide expanded metal strap between original ceiling and box that just disappears between old ceiling and new layer of plaster. So the ceiling is originally wood lathe and plaster with wall paper surface and another layer of plaster over that with this metal strap between them. Strap is about mesh size of a Choreboy and must be a ground, as only one of the k&t wires produces the fireworks and voltage.

I'm in this business 30years and never seen anything like this.

This house is a rental that I had upgraded to 100 amps abt 5yrs ago and left the upper floor alone.

If one of you does not know the answer, maybe you could refer me to a good insurance agency![:-hot]

[Jim Katen]

Originally posted by Les

This is the scenario:

I own a 1927 bungalow that is immaculate except the "second" Floor still has knob and tube wiring. It feeds one outlet, one overhead light and a ceiling light on the first floor small hallway. The light on second floor is fed with k&t thru a three way switch and works just fine. The hall ceiling light never worked and is switched with a wall switch.

I thought I would fix it today so went upstairs and pulled up some floor boards to expose the wires and ceiling box. The box is a 1" typical k&t round box. The k&t is connected to the dining room ceiling light abt 12' away and I can see it. No voltage on either wire. The switch wire in the wall is a cloth covered piece of "romex" and has continuity thru switch. The supposed hot k&t is the switched wire, neutral goes into ceiling box. Seeings how it is dead, I pull the wires out of the box up into the joist bay. The hot wire arcs like crazy and blows breaker.

I pull the box out of ceiling and found it was originally mounted on a wallpapered ceiling and had a 1" wide expanded metal strap between original ceiling and box that just disappears between old ceiling and new layer of plaster. So the ceiling is originally wood lathe and plaster with wall paper surface and another layer of plaster over that with this metal strap between them. Strap is about mesh size of a Choreboy and must be a ground, as only one of the k&t wires produces the fireworks and voltage.

I'm in this business 30years and never seen anything like this.

This house is a rental that I had upgraded to 100 amps abt 5yrs ago and left the upper floor alone.

If one of you does not know the answer, maybe you could refer me to a good insurance agency![:-hot]

I don't understand. If there were no voltage on either wire, there would have been no arcing. So there must have been voltage on one of them. Did you test them with a multi-meter or just one of those stinking, lying, SOB volt sticks?

I doubt that the ceiling box is grounded, at least not intentionally. Wiring systems from that era weren't grounded. No one would have gone to the trouble to ground a ceiling box anyway, even in later years. The mesh strap is probably something else. Perhaps some kind of plaster attachement method? Have you tried using a multi-meter to read the voltage between a known power outlet and the box once it's been isolated from its wires? If so, what was the voltage?

Why don't you think it was simply a hot-to-neutral short across the box?

Was the switch loop really romex or just k&t in loom? It can look like romex.

- Jim Katen, Oregon

[Les]

Jim

It is really romex, actually labeled Mitchell Wire from the late fifties.

I used a digital multi-meter and did not believe it, so went back with my trusty Snap 8. The box is completely isolated, except for contact with this mesh strip. When box is not touching mesh, then no arc. No arc when touching two k&t wires together either. Nothing when neutral to mesh, only when "hot" to mesh. So I am thinking the mesh is hot.

I will try and get back today and get picture(s). Jeeze, I hope no one is thinking I'm crazy.

I have seen radiant resistance wires in plaster before, but never a mesh. Could this be a re-work in the hall with radiant ceiling heat cable because of the double ceiling plaster?

[Terence McCann]

Originally posted by Les

Nothing when neutral to mesh, only when "hot" to mesh. So I am thinking the mesh is hot.

Hey Les:

When you're using your VOM you measure voltage between the mesh and the K&T neutral?

[Les]

Terry, at that point I'm looking for this voltage. I'm trying anything. Shoot - I was one step away from the lick test! I'll try to get back there today and post some photos.

[Les]

No photos, but found out what it was!!!

When the tradesperson re-plastered over original, he installed a strip of this mesh from box to box as a ground connection. It came in 3/4" wide rolls, like tape. He just nailed it on the surface of old plaster, across ceiling, down the wall and folded it over the edge of the existing metal box. So if you look inside the wall box you see the end folded over. I checked other boxes in the house and found the same thing. According to an old fart in the coffee shop, this was commonly done in the late 30's until mid 50's in this area.

This goes to show you that even after 30+yrs of inspecting houses, you can learn something. And to think I considered myself an expert on old methods and materials!!!

[blunsford]

Good find. That's a heckuva shock hazard.

[Bradd Judd]

Hey Les,

If you supply the EMT, Wire, and Fixtures.....I'll bring my bender, meters and beer!!!!!