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 ktulu05 06-22-2008 12:56 PM

3-way illuminated switch - voltage present

Hi,

I'm running three-way switches to a fixture in our breezeway that connects the garage to the house. All new wire/circuits, so no interference from old lines. Power first goes to the switch, then to the fixture, then to the other switch, with the current returning on the common terminal of the second 3-way switch to the fixture.

Here's a picture of how I wired it.

Using a volt meter, I can see that the common wire of the first switch has a voltage of 120 V, as expected. One of the common wires is also 120, also expected. The second common wire, has a voltage of around 60V. When both switches are off, the hot wire for the fixture measures 60V to ground. When one switch is on, the hot wire for the fixture measures 120V. I'm shocked that there is 60 V on the wires when the switches are off, and I can't explain this. Does it have something to do with the fact that it is an illuminated switch, as there has to be some current flow for the light to be lit? I have some non-illuminated switches that I could also try, to see if that makes a difference.

Any thoughts would be appreciated.

Edit: I re-wired the circuit with two other brand new 3-way switches. The illumination was not working, so I wasn't trusting the switches. Now when I re-measure with a volt meter, I see around 20V for the fixture hot wire when both switches are off. It goes to 120V when one switch is on. Is this some residual voltage on the wire, or is it being induced by the current on the common wires?

-Kevin

 joed 06-22-2008 03:36 PM

You are using a digital meter. YOu are measuring a phantom or induced voltage. Use an analog meter(with needle) and you will find the wire has 0 volts.

 micromind 06-22-2008 03:38 PM

If you're measuring this with a digital meter, it's normal to see stray voltages like this. The reason is because the input impedance (resistance) of a digital meter is around 11 megohms. This doesn't load the circuit under test enough to cancel out the voltage induced by the other hot wires contained in the same cable.

The technical term for this is 'capacitive coupling'. A capacitor is nothing more than two conductors separated by an insulator. If one of the conductors has an AC voltage applied to it, and the other one isn't connected to anything (like the neutral or ground), it'll be charged through the effects of capacitive inductance.

In a circuit like yours, the phantom current is extremely low, in a circuit consisting of several miles of overhead power lines, it can be lethal. This is the reason for utility lineman rule #1; 'if it ain't grounded, it ain't dead'. If you have an open section of overhead line, you either work it like it's hot, or you intentionally ground it.

Rob

 ktulu05 06-22-2008 04:26 PM

Thanks for the responses. You're right, I am using a digital multimeter. I'll try out an analog one just to confirm it.

 ntomlin 12-14-2008 12:17 PM

I have the same 60V problem with illuminated 3-way switches, but I know it's a real problem b/c I have an analog voltmeter and I can see the light flickering when it's supposed to be off. I discovered it last night right before I went to bed - the light was just barely flickering at about 1-2 Hz when the switch was off. I unscrewed the light bulb today and when the switch is off, I read 60V between in the light socket. I took out the lights of another 3-way light in my house and it is also 60V when off. I can only see the flickering in the two light fixtures when I low power bulb is in them. I never noticed the flickering light until yesterday after I replaced the old 3-way switches with illuminated ones. Seems like it might be a problem with the illuminated 3-way switches - they must need some power at all time, but 60V seems like a lot. Any ideas?

 AllanJ 12-14-2008 06:32 PM

The illuminated-when-off switch relies on the controlled light bulb or other load to complete the circuit and light up the switch.

When the switch is in the off position, the light in the switch is in series with the controlled light.

When a 3 way switch circuit is in the off condition, one traveler is hot and the other is connected to the controlled load. Except when the controlled load is a very low wattage light, you will measure full line voltage between the travelers.

The light in the switch might draw more than one watt. This may be enough to cause a compact fluorescent being controlled by the switch to emit a glow visible in a dark room.

 Yoyizit 12-14-2008 07:28 PM

For what it's worth, here is a method for troubleshooting three way switches using phantom voltages instead of disconnecting the switch and using an ohmmeter. It assumes the source voltage and load are good.

Pick one of the two switches and measure the voltage from the wiper to either stationary contact terminal.
If you get zero, the switch is closed in this position.
Flip the lever; with a high impedance meter, 10 or 20 megohms, the voltage should go to ~120v or ~60v due to capacitive coupling on the travelers, but there is a wide tolerance on this voltage. The switch is now open with the lever in this position.

You, in effect, used the current supplied by the capacitive coupling, and your voltmeter, as an ohmmeter.

Repeat with the other terminal. If the switch you picked can open and close both terminals, there is close to a 100% chance that the problem is in the other switch or the travelers.

If use a 1000 ohms/volt VOM you might get 10 volts on the 150 v scale with the open switch. Assuming some number of picofarads per foot for Romex, longer travelers with a high impedance meter should give voltages closer to 120v, or to 60v if there is a grounded conductor in the same cable.

 kbsparky 12-14-2008 07:39 PM

Try it with a regular 60 watt bulb. Betcha it works properly with that installed.. :whistling2:

 Silk 12-14-2008 08:02 PM

Quote:
 Originally Posted by Yoyizit (Post 198066) For what it's worth, here is a method for troubleshooting three way switches using phantom voltages instead of disconnecting the switch and using an ohmmeter. It assumes the source voltage and load are good. Pick one of the two switches and measure the voltage from the wiper to either stationary contact terminal. If you get zero, the switch is closed in this position. Flip the lever; with a high impedance meter, 10 or 20 megohms, the voltage should go to ~120v or ~60v due to capacitive coupling on the travelers, but there is a wide tolerance on this voltage. The switch is now open with the lever in this position. You, in effect, used the current supplied by the capacitive coupling, and your voltmeter, as an ohmmeter. Repeat with the other terminal. If the switch you picked can open and close both terminals, there is close to a 100% chance that the problem is in the other switch or the travelers. If use a 1000 ohms/volt VOM you might get 10 volts on the 150 v scale with the open switch. Assuming some number of picofarads per foot for Romex, longer travelers with a high impedance meter should give voltages closer to 120v, or to 60v if there is a grounded conductor in the same cable.
You could go through all that, or you could just check it with solenoid type voltmeter (Wiggy).

KISS

 Yoyizit 12-15-2008 01:15 PM

Quote:
 Originally Posted by Silk (Post 198083) You could go through all that, or you could just check it with solenoid type voltmeter (Wiggy). KISS
What does a Wiggy read with phantom voltages?

 InPhase277 12-15-2008 01:17 PM

Quote:
 Originally Posted by Yoyizit (Post 198345) What does a Wiggy read with phantom voltages?

 Yoyizit 12-15-2008 02:20 PM

Quote:
 Originally Posted by InPhase277 (Post 198348) Nada.
Perdón, señor.
http://ets.freetranslation.com/

In that case you can't use this method to troubleshoot 3-way switches. :(

 Silk 12-15-2008 04:06 PM

Quote:
 Originally Posted by Yoyizit (Post 198374) Perdón, señor. http://ets.freetranslation.com/ In that case you can't use this method to troubleshoot 3-way switches. :(

I can, why can't you?

 Yoyizit 12-15-2008 04:36 PM

Quote:
 Originally Posted by Silk (Post 198426) I can, why can't you?
Cause I'm. . .

Lazy?
Incompetent?
Used to know how to do this stuff?
Never knew how to do this stuff?
Duhhhh. . .

:laughing:

 Silk 12-15-2008 05:47 PM

Can I choose "All the Above"?

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