Strange home wiring question

[JHZR2]

Hi,

Im trying to take out a dimmer switch that the PO had in my house. The switch has two hot wires and another wire. The PO had the neutral wire attached to the ground spot on the switch (this is only for a kitchen light, and I think it is the original knob and tube in there).

The strange part is this: if I put my multimeter from the hot in to the neutral/ground, I get 121V. If I put it between the other lead (that the switch closes to complete, and ground, with the dimmer switch open (off), I get 36V.

If I look at the potential between the two leads that the switch completes when closed, and the switch is open, I get 16V.

This seems odd to me. Any ideas???

Thanks,

JMH

 

[Crashbox]

If you have a digital multimeter you may actually be reading parasitic induction between the two wires being evaluated (or a very close wire to one and ground or the other and ground). If it's an analog needle-type multimeter then ??? possibly the same but higher current?

I remember seeing something like this some years back at a friend's house- never did truly solve it. I pointed it out to him and he didn't seem to sweat it.

Both scenarios are still a puzzler to me though.

 

[eljefino]

Are you doing this with the wiring hooked up and the bulb in?

Might be the dimmer "leaks" a little current through the bulb filament to allow itself to work regardless of a neutral. That you can pick it up with a DVOM is a sign that the dimmer itself is a real high impedance.

Try unscrewing the bulb, I would learn from this too.

 

[Master ACiD]

its been my expierence that most switches and dimmers are installed using the neutral and hot both as a hot.

pay no attention to the actual colours.

 

[eljefino]

Master Acid, are you sure you're not just seeing a "ceiling drop" switch install?

The "real" wire leads to the ceiling outlet box then the black (hot) wire is attached to the "drop" white wire that goes down to the switch. Then from the other switch terminal the black wire comes back to the light fixture and is attached to the bulb. Finally the circuit is completed through the "real" white neutral. If the switch is on, both insulated wires in the switchbox will have 120v potential to the bare copper ground.

And OP, as long as you're experimenting, try looking for voltage in the socket with the switch off.

 

[JHZR2]

OK, Im using a high quality digital mm - fluke 179 true rms

I first was testing wires with the switch off, and found what I did.

There was a dimmer attached, but I took the dimmer out to put in a regular switch. how the thing works is that I have a two-pole switch in the kitchen that controls the whole wired setup, and then a three-pole switch at the top of the stairs that controls the light on and off from up there. I dont know why it was wired this way, but I guess its an artifact from the 1930s.

Anyway, I tested the bulb socket, with the switch powered on and off - 0V and 121V.

I then went upstairs and replaced the 3-way, as it was very old. I checked it out, and got the following (switch does not have a ground, and I doubt the box is really grounded).
-between two poles, 121V
-between next two poles, 5V
-Between next two poles, 31V
-between hot and metal on switch itself, 139V (!)

I dont get that either. I replaced both switches and everything seems fine, but I just don't get it... the 139V part is crazy - I dont feel a shock when I touch it, nothing gets hot or anything, and the whole circuit works fine.

Strange...

JMH

 

[JHZR2]

I recall from reading somewhere that knob and tube doesnt really have a hot and neutral, but rather it is just a 'delta' of 120V or so... that somehow it is not the same as more current wiring... Could that be the reason for my strange phenomena?

ANy thoughts would be appreciated.

Thanks!

JMH

 

[eljefino]

Knob and tube used to run a "common neutral" which to me sounds almost like a "loose neutral" in your house or at least somewhere where the phases aren't quite equal.

They make dimmers that integrate into 3-way lighting systems. I still think the dimmer powers itself by leaking through the bulb filament to neutral.

 

[JHZR2]

well, now there is no dimmer, and still the same thing...

hmmmm... this is odd, but looking at google groups, it appears that others note similar phenomena!

JMH

 

[Master ACiD]

man i wish i could make a wiring diagramn for you guys. its really simple once you realise that the colour of the wiring has no meaning with a switch or dimmer.

 

[Bob Woods]

Its called ghost capacitance, caused by two wires laying close to each other being at different potentials. With an old analog voltmeter you very likly would not see it as the meter drains the voltage and equalizes the two conductors. With a high impedence digital volt meter the input resistance is in the mega-ohm range and the time to discharge is equal to or longer than the time to charge and you see this phenomena. Years ago when we switched from old analog to digital meters we scared the #@$%! out of us until we figured it out.

 

[JHZR2]

Quote:

---

man i wish i could make a wiring diagramn for you guys. its really simple once you realise that the colour of the wiring has no meaning with a switch or dimmer.

---

That's fine, it really is simple... especially in 3-way switch setups, often they use the white and black conductors, which may mean something in a breakerbox or whatnot, as the "to and from" (for lack of better words) from the other switches or the fixture, etc. Ive seen this before.

But the post directly above is the real answer... Glad I learned something today!

JMH