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Old 11-20-2008, 05:42 PM   #1
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Low voltage at switch, unsure what switch is for


I purchased this house a few months ago and haven't figured out what one switch is for. I checked the voltage across the 3-way switch and it was only ~50v. I figured there must be something wrong with the switch and replaced it with another 3-way that I know works. Same low voltage with this unit.

What would be the best way to approach this and find out what the switch is wired to and then diagnose why it doesn't work.

Thanks

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Old 11-20-2008, 06:08 PM   #2
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Low voltage at switch, unsure what switch is for


Where is the switch located? Interior wall? Exterior wall? By an exterior door?
Ron

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Old 11-20-2008, 06:11 PM   #3
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Low voltage at switch, unsure what switch is for


Are you using a digital meter? It could be phantom voltage. Try flipping the switch and measure between the ground and common terminal(usually black) of the switch.
http://www.nema.org/stds/eng-bulleti...ulletin-88.pdf
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Old 11-20-2008, 06:16 PM   #4
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Low voltage at switch, unsure what switch is for


Quote:
Originally Posted by amdspitfire View Post
I checked the voltage across the 3-way switch and it was only ~50v.
3-way switches must have phantom voltages by the way they are wired. A small incandescent lamp loading down your high impedance voltmeter will eliminate the confusion.
You can even use phantom voltages to troubleshoot a 3-way arrangement, eliminating the need to check switches with an ohmmeter.
What gets tricky is that analog voltmeters read a different voltage depending on what scale they're on, because each scale loads the voltage down differently.

Last edited by Yoyizit; 11-20-2008 at 06:19 PM.
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Old 11-20-2008, 09:34 PM   #5
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Low voltage at switch, unsure what switch is for


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Originally Posted by Ron6519 View Post
Where is the switch located? Interior wall? Exterior wall? By an exterior door?
Ron
Switch is on an interior wall, by a door leading to the side of the house.

Quote:
Originally Posted by jerryh3 View Post
Are you using a digital meter? It could be phantom voltage. Try flipping the switch and measure between the ground and common terminal(usually black) of the switch.
http://www.nema.org/stds/eng-bulleti...ulletin-88.pdf
Analog meter Are analog meters a high impedence device?

There is 3 other switches right by this one, all being 3 way also. They all read closer to 110v.

Quote:
Originally Posted by Yoyizit View Post
3-way switches must have phantom voltages by the way they are wired. A small incandescent lamp loading down your high impedance voltmeter will eliminate the confusion.
You can even use phantom voltages to troubleshoot a 3-way arrangement, eliminating the need to check switches with an ohmmeter.
What gets tricky is that analog voltmeters read a different voltage depending on what scale they're on, because each scale loads the voltage down differently.
The scale that the meter is set on should be correct because it reads proper voltage on the other switches.

What can I try? I wish I could track the wires down.
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Old 11-20-2008, 10:05 PM   #6
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Low voltage at switch, unsure what switch is for


50V tells me that the load could possibly be a fluorescent light. They are not low impedance devices like incandescents are, so would not read as high on the voltmeter.
Do you have a garage? Perhaps this fluorescent light fixture is in the garage. Could be that the bulbs or the starter (if it's an older type) burned out.
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Old 11-21-2008, 10:52 AM   #7
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Low voltage at switch, unsure what switch is for


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50V tells me that the load could possibly be a fluorescent light. They are not low impedance devices like incandescents are, so would not read as high on the voltmeter.
Do you have a garage? Perhaps this fluorescent light fixture is in the garage. Could be that the bulbs or the starter (if it's an older type) burned out.
I do have fluorescent lights in the garage, however the switch right next to this one controls them. Reading on that is closer to 110.
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Old 11-21-2008, 11:06 AM   #8
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Low voltage at switch, unsure what switch is for


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Originally Posted by amdspitfire View Post
I do have fluorescent lights in the garage, however the switch right next to this one controls them. Reading on that is closer to 110.
So there goes my theory on fluorescent lighting<g>.
I was thinking more along the lines of CCFL (cold cathode fluorescent lamp), which doesn't have a filament, so probably would present a high resistance to an ohmmeter.

What's next? The basement?
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Old 11-21-2008, 11:18 AM   #9
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Low voltage at switch, unsure what switch is for


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Originally Posted by KE2KB View Post
So there goes my theory on fluorescent lighting<g>.
I was thinking more along the lines of CCFL (cold cathode fluorescent lamp), which doesn't have a filament, so probably would present a high resistance to an ohmmeter.

What's next? The basement?
As a matter of fact there is a basement right around the corner, but all the lights there have switches accounting for them already. There's some weird light boxes outside, way up under the overhang with caps on them. I think next step is open and test all those wires.
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Old 11-21-2008, 02:20 PM   #10
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Low voltage at switch, unsure what switch is for


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 (an incandescent lamp) are good.

Pick one of the two switches and measure the voltage from the wiper (the terminal with a darker color than the other two) 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. Longer traveler conductors with a high impedance meter should give voltages closer to 120v, or to 60v if there is a grounded conductor in the same cable.

Last edited by Yoyizit; 11-21-2008 at 02:33 PM.
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Old 11-21-2008, 02:27 PM   #11
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Low voltage at switch, unsure what switch is for


Christmas light boxes, sounds like a logical place for that switch to control. It does however seem to be a threeway. Is there another switch somewhere that is not controlling anything?
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Old 11-21-2008, 04:24 PM   #12
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Low voltage at switch, unsure what switch is for


Careful when you're way up high investigating those boxes. You would be best using a fiberglass ladder, not an aluminum one, in case you come in contact with electricity. A shock up there will probably throw you off the ladder!
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Old 11-21-2008, 04:29 PM   #13
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Low voltage at switch, unsure what switch is for


Three way switches usually have a mate! Perhaps the mate is defective!
If a mate is not identified, I would wonder if some renovations have taken place and its mate was removed for whatever reason.
If the 'travelers' are showing false voltages, they may not have been protected properly!
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Old 11-22-2008, 11:18 AM   #14
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Low voltage at switch, unsure what switch is for


Maybe you would want to install an AFCI breaker on that branch just in case there's arcing from unprotected wires.
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Old 11-23-2008, 09:54 AM   #15
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Low voltage at switch, unsure what switch is for


For testing whether a circuit is working, measuring voltage between two terminals of a switch is generally not a good method. Or more correctly it takes electrical know how (or a grain of salt) to interpret the results.

Instead you need to be measuring between a switch terminal and neutral. As some switch boxes don't have a neutral in them (a situation called a switch loop) you can also measure between a hot wire and ground (or between a switch terminal and ground).

When all you want to know is whether you have power or not, more positive results for the novice are had by connecting an incandescent lamp (a 7 watt night light will do) as your testing tool from the terminal to ground. If you wish, also measure with your voltmeter across the two lamp terminals. Now you should get the full 110 volts for on and zero volts for off. The light grounds away any phantom or induced currents so as not to give meter readings that might be confusing, although the phantom currents can still be enough to electrocute a person.

Another way of getting a neutral or even a hot wire over to a place where it is needed for test purposes is to string a long wire across the room or between rooms. Of course, be careful about electrocution hazards. Don't touch the wire end to anything at random but instead connect a lamp between the strung wire and the terminal or other wire you want to test.

In a properly wired 3 way switch setup, if the switch is at the line (unswitched) power, the wiper or common terminal will of course always be live. If the switch is at the other (load) end, one of the travelers (connected to light colored screws) will be live and the other dead.

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Last edited by AllanJ; 11-23-2008 at 10:07 AM.
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