knob and tube switch

[Noah A]

I recently got a call to trouble shoot a light. No big deal, until I got there. The house is old and has knob and tube wiring in the attic. The light that I am to trouble shoot they are not sure when it quit working, but they think that it used to work. I found romex going to a single gang double switch from the porch light that was not hooked up with two black wires in it and no neutral. I don't know where the neutral goes or comes from obviously bad news. After some investigation and frustration I came to the realization that I need to figure out a new plan of attack. I was going to get power from the switch that is already in the box, this is the knob and tube. When the light is off I have power to the switch and non off the switch leg, when I turn the light on there is no power at the switch at all. All the switches in the house do this (there are only four). As I don't have many years under my belt and most all of it is comercial/genset this is my first knob and tube experience and I am lost. Any help would be great.

 

[augie47]

I don't follow your voltage situation, but keep in mind on some knob & tube jobs the neutral was switched.

 

[kwired]

I don't know why but most knob and tube I have ever run into the neutral was the switched side of the light. You have voltage when the switch is open because the circuit is open. The voltage goes through the load and to the open switch and has no place to go until the switch is closed. Voltage to ground goes away when the switch is closed because it is a grounded conductor and there is no voltage drop across the switch. (There is some small resistance but it can be ignored for the theory of what is being asked)

 

[K8MHZ]

I don't know why but most knob and tube I have ever run into the neutral was the switched side of the light. You have voltage when the switch is open because the circuit is open. The voltage goes through the load and to the open switch and has no place to go until the switch is closed. Voltage to ground goes away when the switch is closed because it is a grounded conductor and there is no voltage drop across the switch. (There is some small resistance but it can be ignored for the theory of what is being asked)

I have worked with a lot of K&T and many times the neutral was switched. Not always, but enough to be aware of the possibility.

Switched neutrals play major games with tick tracers. If you are working with K&T, don't trust your tracer. Take some wire and bring it back to the panel and connect it to the neutral for a reference and then use a solenoid tester. You will be glad you did.

 

[al hildenbrand]

I agree with the others that your description sounds like the switch is in the neutral from the light.

I routinely use Marky's suggestion in the form of a good three-wire extension cord plugged into a good grounded receptacle (here, usually the bath or kitchen receptacle). I use a low input impedance voltage tester (non digital, moving needle or a Wiggy) and a continuity tester with the extension cord equipment ground conductor.

However you do this, use a "known ground" extended on a conductor of some kind. Without that certainty of the measurement, its easy to make the wrong assumption when diagnosing K&T in the beginning.

 

[Noah A]

Thanks guys. Looking at it now in the attic it makes more sense. So much for the easy route grabbing power at the switch.:blink: Thanks again!

 

[kwired]

Thanks guys. Looking at it now in the attic it makes more sense. So much for the easy route grabbing power at the switch.:blink: Thanks again!

Even if the hot conductor was switched you can't get power from a two wire switch loop - there will be no neutral for the load you are adding.