Voltage at fixture after disconnecting switch leg

[TheWantonElectrician]

I'm feeling like a chump.

Troubleshooting a bathroom light that has a dim glow when the switch is off. I disconnected the switch and am getting 6V AC on my digital multimeter (so not a bad switch). Homeowner mentioned another light had the same issue so they used a halogen to make the glow disappear. I found 6V AC there as well. I'm getting 124V at circuit breaker (to neutral bar). GEC to water at house intake is present, but I haven't confirmed rods.

"All" knob and tube was supposedly replaced by romex a while back, but the wiring I see looks like homeowner work everywhere (neutrals landed on ground bar, loose wires all over attic).

I have a sneaking suspicion that this particular circuit still has some knob and tube as the J-box in the attic that ties the power to the switch leg for the fixture had all the grounds loose - that is, not tied together (something I might do if replacing a section of knob and tube with romex and protecting with a gfci somewhere upstream). Still can't see how that would be related to 6v between disconnected switch leg and ground. I know a floating neutral gives me 30-80V AC on my ddm sometimes, so I'm thinking this is current on neutral somehow and a "floating" switch leg (when the switch is off). Again, feeling like a chump and really hoping someone can help.

 

[qcroanoke]

I'm feeling like a chump.

Troubleshooting a bathroom light that has a dim glow when the switch is off. I disconnected the switch and am getting 6V AC on my digital multimeter (so not a bad switch). Homeowner mentioned another light had the same issue so they used a halogen to make the glow disappear. I found 6V AC there as well. I'm getting 124V at circuit breaker (to neutral bar). GEC to water at house intake is present, but I haven't confirmed rods.

"All" knob and tube was supposedly replaced by romex a while back, but the wiring I see looks like homeowner work everywhere (neutrals landed on ground bar, loose wires all over attic).

I have a sneaking suspicion that this particular circuit still has some knob and tube as the J-box in the attic that ties the power to the switch leg for the fixture had all the grounds loose - that is, not tied together (something I might do if replacing a section of knob and tube with romex and protecting with a gfci somewhere upstream). Still can't see how that would be related to 6v between disconnected switch leg and ground. I know a floating neutral gives me 30-80V AC on my ddm sometimes, so I'm thinking this is current on neutral somehow and a "floating" switch leg (when the switch is off). Again, feeling like a chump and really hoping someone can help.

Is the service bonded?

 

[ramsy]

dim glow when the switch is off. I disconnected the switch and am getting 6V AC ..another light had the same issue

How can a small current flow thru bulbs if switch-leg is de-energized?

* Geometry of other circuits under load crossing de-energized switch leg

* Conductive salts or debris in screw shell, between bulb electrode and other terminals.

* Other energized circuits making partial contact with de-energized switch leg

 

[ramsy]

another light had the same issue

When multiple lights are glowing in different rooms with switches off

* Switched neutrals wired with constantly-energized electrodes, are leaking current thru bulbs.

* Resistive or open neutral upstream places small current on fixture neutrals that leaks thru lamps

* Radio transmitter inducing a voltage on de-energized wiring

 

[ptonsparky]

I replaced our bath fan night light with an LED vs the incandescent. It emits light when all switches are Off. Yes, there is a few volts of what would normally be considered a ghost voltage, but it is enough.

Add a small load to the circuit and your LED will go out.

 

[ramsy]

Add a small load to the circuit and your LED will go out.

Some of those modern bath fans run 24/7 by design, to maintain a negative pressure in the room, or to prevent mold.

 

[ptonsparky]

Some of those modern bath fans run 24/7 by design, to maintain a negative pressure in the room, or to prevent mold.

The humidity sensing circuitry in mine is still active even though the fan is off. The old incandescent had no issue. LED, yup.

Eta: 14 years old.

 

[TheWantonElectrician]

Pretty sure it's bonded at service, but i can double check when i return tomorrow. In either case, this circuit's neutral is connected to the actual neutral bar.

This is only a light, no fan in this fixture and no humidity sensor.

Would my voltage tester indicate a hot wire at the switch if it was switched neutral?

I don't follow the geometry of other circuits and load crossing... perhaps shared neutral?

There are other fan/light combos on the same circuit that are control the speed of fan, as well as lighting, using a special wall switch that just has two black wires (presumably power and switch leg), so it is either sending a signal on the switch leg or perhaps I'm mistaken about how it works or is wired and may be sending rf signal.

 

[Little Bill]

In a nutshell, it's the LED bulbs that are the culprit. No danger there, just a tickle of voltage causing them to glow.

 

[TheWantonElectrician]

But if the bulb is removed and I'm getting 6v, it seems like that can't be the answer.

 

[ptonsparky]

But if the bulb is removed and I'm getting 6v, it seems like that can't be the answer.

A high impedance meter will read 6 volts. Put an incandescent lamp in the circuit. Measure the voltage. What's the voltage with the LED in the circuit?

 

[ramsy]

Would my voltage tester indicate a hot wire at the switch if it was switched neutral?

Screw shell electrode has line voltage with switch turned off.

I don't follow the geometry of other circuits and load crossing...

Moving magnets or currents can induce small voltages on adjacent wires.

There are other fan/light combos on same circuit

Lighting circuit junctions may induce small voltages in adjacent wires, or switch legs.

..using a special wall switch that just has two black wires

Many dimmer switches will produce a measureable voltage in the off position.

 

[Jraef]

Parallel runs of the cables in the ceiling and walls is picking up an induced voltage. It can be read by high impedance DMMs and can sometimes be enough to make an LED glow slightly, but if it was an incandescent or CFL, you wouldn't see anything and if you measured it with a Wiggy, it would read zero volts..