Not RV issue...but could be

[tlmgcamp]

Replacing bathroom fan/light/night light. Shut off breaker marked for the fan/light/night light. In junction box that housed the fan/light/night light switch I found a 3-wire (black, red, white, ground) coming in. The red wire when to the old fan/light/night light switch and the black went through the box to power the circuit for the GFI outlet in the bathroom. after removing the switch and pulling the wires out so I could see what I was working with, I checked the wires with a "Non-Contact Voltage Detector". This is not a meter. It is a gadget that detects voltage without needing to strip wire. It told me that both the red and black were still hot. After some investigation, I found that as long as either the red wire breaker OR the the black wire breaker was on, the voltage detector would show voltage in BOTH black and red, whether tested at the junction box or the panel. I am very sure that I know the route of the 3-wire and can see it for the majority of the length from the breaker panel to the junction box. It was installed 20 years ago during a bathroom renovation that i contracted out to do. The only thing that I could think of is that somebody accidentally put a nail or screw through the 3-wire and hit both the red and the black, making them act in unison. Of course, the chances of that happening without also hitting the white (common) or the bare ground seemed pretty slim. I went ahead and tested for continuity between the red and the black (breakers off) and found none, which seemed to squash the errant screw theory.

While pondering it over a beer last night, I decided to actually put a regular tester on the wires. At the panel with the red breaker off and the black breaker on, I found 110V in the black breaker wire, and 2V in the breaker red wire. Same result with red on (110V) and black off (2V). Can anybody suggest what might be going on here? Note the red and black wires at the junction box were not connected to anything during all these tests. Only the white (common) wires and bare ground wires for everything in the junction box were still connected.

 

[Gary RV_Wizard]

Induced voltage? With the wires lying close together along the entire route, the magnetic field around the powered wire can induce a modest current in the other.

If there is a nail bridging the wires, you can detect that with an ohm meter (continuity check).

 

[Kirk]

If you are saying that the red wire has 120V when it's breaker is closed and 2V when open, with black closed and that the situation is reversed when you close the breaker for black and open the one for red so that the 120V & 2V just change locations, there are quite a few possibilities and induced voltage is one of them. I have never heard of a nail that would transmit 2V only from a 120V source and so feel quite confident that there isn't one. Much more likely is the possibility that there is something that is connected between the black and red wires that you have not yet found. Have you checked the voltages with both breakers open?
If you are saying that you have a cable with red, black, white, and bare to supply separate circuits, you have some very unconventional wiring to start with. I am not current on electrical codes, but am quite confident in the opinion that whoever you hired to do your electrical work ignored codes. To really know what is happening you need to trace that cable all of the way between.

I checked the wires with a "Non-Contact Voltage Detector".

And there is the reason that I don't generally use one as they see no difference between 2V and 120V or even more. The light turns on if there is enough voltage for it to detect one and it really doesn't matter what that voltage is. You can use it to trace a specific wire if you know that it is the only one with power but that is about all that it is reliable for.

 

[tlmgcamp]

Continuity check found nothing. I had considered the magnetic field effect but drawing that kind of conclusion is pushing the limit of my comfort zone regarding electrical. The original installation was 20 years ago with no problems, but that doesn't guarantee that it's right

 

[tlmgcamp]

If you are saying that the red wire has 120V when it's breaker is closed and 2V when open, with black closed and that the situation is reversed when you close the breaker for black and open the one for red so that the 120V & 2V just change locations, there are quite a few possibilities and induced voltage is one of them. I have never heard of a nail that would transmit 2V only from a 120V source and so feel quite confident that there isn't one. Much more likely is the possibility that there is something that is connected between the black and red wires that you have not yet found. Have you checked the voltages with both breakers open?
If you are saying that you have a cable with red, black, white, and bare to supply separate circuits, you have some very unconventional wiring to start with. I am not current on electrical codes, but am quite confident in the opinion that whoever you hired to do your electrical work ignored codes. To really know what is happening you need to trace that cable all of the way between.

And there is the reason that I don't generally use one as they see no difference between 2V and 120V or even more. The light turns on if there is enough voltage for it to detect one and it really doesn't matter what that voltage is. You can use it to trace a specific wire if you know that it is the only one with power but that is about all that it is reliable for.

"On" breaker shows 110v while neighboring "off" breaker shows 2v. No voltage anywhere when both are off

 

[Lou Schneider]

Continuity check found nothing. I had considered the magnetic field effect but drawing that kind of conclusion is pushing the limit of my comfort zone regarding electrical. The original installation was 20 years ago with no problems, but that doesn't guarantee that it's right

Actually, it's probably capacitive coupling between two wires that run in proximity to each other for some distance. Magnetic fields are generated by current flow through a wire, capacitive fields are generated by voltage on a wire. A capacitor is simply two conductors (usually foil plates) in close proximity to each other seperated by an insulator. This is how the no-touch voltage sensors work even though there's no physical contact between the tip and the voltage carrying wire.

 

[Domo]

Non contact testers are not to be trusted - especially with your life.