Voltage at fixture after disconnecting switch leg

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TheWantonElectrician

Member
Location
Madison, WI
Occupation
Master Electrician (Residential and small-scale commercial)
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.
 
TheWantonElectrician said:
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.
Click to expand...
Is the service bonded?
 
TheWantonElectrician said:
dim glow when the switch is off. I disconnected the switch and am getting 6V AC ..another light had the same issue
Click to expand...
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
 
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TheWantonElectrician said:
another light had the same issue
Click to expand...
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
 
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.
 
TheWantonElectrician said:
Would my voltage tester indicate a hot wire at the switch if it was switched neutral?
Click to expand...
Screw shell electrode has line voltage with switch turned off.
TheWantonElectrician said:
I don't follow the geometry of other circuits and load crossing...
Click to expand...
Moving magnets or currents can induce small voltages on adjacent wires.
TheWantonElectrician said:
There are other fan/light combos on same circuit
Click to expand...
Lighting circuit junctions may induce small voltages in adjacent wires, or switch legs.
TheWantonElectrician said:
..using a special wall switch that just has two black wires
Click to expand...
Many dimmer switches will produce a measureable voltage in the off position.
 
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..
 
TheWantonElectrician said:
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.
Click to expand...
Check ground to neutral voltage
 
TheWantonElectrician said:
But if the bulb is removed and I'm getting 6v, it seems like that can't be the answer.
Click to expand...
The 6 volts is what is making the LED glow. Most likely capacitance coupling between the switch leg and another ungrounded conductor. As others have said, a small load on the circuit will eliminate the problem.
 
It takes under a microamp to make a bare neon or the most basic of L.E.D. type circuit produce light that is visible in the darkness.
In the past, it was common back when L.E.D. lamp bulbs started to hit the shelves. When you pair it with a solid state motion activated switch, the slightest of the leakage current was enough to cause them to do a faint glow, and sometimes cause the bulk storage capacitor in the LED ballast to slowly build up and cause it to flash at some interval.

Modern LED lamps have the LED ballast designed in such a way that a microamp level current flowing through the socket won't pass through the LED elements themselves to prevent the nuisance glow.

Try checking the hot and neutral wires at the fixture using non-contact voltmeter. If the open circuit voltage is truly a few volts, it won't trigger. If it's near line voltage, it will notice. You're only seeing six volt, because the 10 MOhm input of the meter is enough to pull it down to that level.
 
Five 100 kohm resistors in parallel between hot and neutral would probably be enough to pull it down and eliminate the glow. When the circuit is on only 6 mA would flow in the resistance and each resistor would only dissipate 0.14 watts. I'd use 1/2 watt resistors to give some overhead.
 
Where is your voltage measured? At fixture hot to neutral or hot to ground or neutral to ground? this would tell you the source direction of the voltages. Is "Glow" a short term glow or will go away after a period of time? Seen most LED glow for a few minutes after shutting off light, this from the internal drivers, some more some less duration. Don't think 6 volt should not be enough to come through the driver to light the LED.
 
Fred B said:
Where is your voltage measured? At fixture hot to neutral or hot to ground or neutral to ground? this would tell you the source direction of the voltages. Is "Glow" a short term glow or will go away after a period of time? Seen most LED glow for a few minutes after shutting off light, this from the internal drivers, some more some less duration. Don't think 6 volt should not be enough to come through the driver to light the LED.
Click to expand...
Not sure what you mean here or if it says what you thought you were saying.
 
ptonsparky said:
Not sure what you mean here or if it says what you thought you were saying.
Click to expand...
How much does the driver itself use? the driver has some level of impedance in converting the AC to DC for the LED. Question is, Is 6 volts enough to cause perceptible light from the LED and give even 1-6 V DC when you drop the AC source from the nominal 120V AC to 6 volt AC?
 
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