Phantom voltages helpful?

[Lxnxjxhx]

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.

I don't like disconnecting brittle wires in someone else's house.

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.

Thanks for the promotion from junior member to member.

 

[e57]

For what it's worth - I do not beleive in "Phantoms" of any kind...

It is a potential - not a usable potential - but a potential none the less....

Either way - nicely put. Haven't thought about going about it like that before, but in your head it becomes one of those 'givens' so to speak - recongnizing a nominal voltage vs capacitive or inductive reading and/or readings through a load. Which hopefully someone wouldn't confuse with a capacitive or inductive effect.