I replaced some old wiring with new 12/2 and 12/3 Romex on a circuit with two 3-way switches for a new light fixture. I wired the circuit to match a simple example from a home wiring book and measure 120 volts when I turn it on. However, when I turn the switches OFF, my volt meter shows either 36 or 38 volts – depending which switch is used – on the wires at the end of the cable run where my fixture will go.
Where is this voltage coming from and how much voltage is acceptable when 3-way switches are turned off?
Replies
Phantom voltage.
You have a digtial voltmeter with very high input impedence. The open wires are acting like antenna's and capacitively coupling to hot wires.
Put in the fixture and screw in a bulb.
You won't see anything them. It will either read 120 or zero.
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Where were all of you guys with your smart answers last week when I called my electrician out just to look at one 32 volt wire that worried me. One service call later...
We where here.Why wheren't your reading this last week?Or asking the question yourself?I wonder if we should do a Karnack bit where we give the answers and then wait and see if any has the question <G>..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I used to install burglar alarms- we were on the roof of a large industrial building that was very close to the KGO (high-powered AM radio station) transmitting towers. We spooled out about 100' of wire. When I stripped the end (the other end was NOT connected to anything), I got a sizeable shock from what I assume was induced voltage. We had a VERY difficult time getting the alarm stuff to work at that location.Bill
Yeah, the electrical engineering building at the University of Louisville sat in the shadow of the WLOU radio antenna. Any sort of sensitive work had to be carried out in a Faraday cage.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I've heard stories -- and of course apply the appropriate crystals of sodium chloride here -- that back in the 1920's when they had 500 kW AM transmitters, there was a farmer who ran lights in his chicken coop with the power he picked up from his fence. ;-)
-- J.S.
>> I wonder if we should do a Karnack bit where we give the answers and then wait
>> and see if any has the question.42
The IQ of some of the people that have disagreed with me in these threads <VBG>..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
What is the meaniong of it all?SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
Perfectly normal.
Where is this voltage coming from and how much voltage is acceptable when 3-way switches are turned off?
Short answer; Radio waves.
Long answer; See posts above.
SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
Not radio -- plain old 60Hz AC.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Radio covers everything from .003 hz to 300Ghz. Or more. If it's electromagnetic, it's "Radio."
And with a digital meter with high enough input impedance, it can measure the signal on your car antenna. Heck, measure the voltage on yourself to an ordinary electrical ground.
Granted, if you're seeing anything over a few volts, it's probably 60 cycle from nearby "Hot" wiring.SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
Well, this is generally capacitive pickup.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Speaking only of Phantom Voltage.
Capacitive transferance is generally large area, close spaced, voltage dependant.
Inductive transferance is generally large length, close spaced, current dependant. Meaning a powered up load somewhere.
Electromagnetic (radio) wave transference is any spacing, power dependant.
All intimately depend on a high input impedance in the measuring unit.
This is why I highly disrecommend the use of digital VOMs by DIYers and other non trained personnel. An analog VOM will not see Phantom Voltages. I myself use a DVOM because it is smaller and sturdier than AVOMs, but I always have an AVOM for whenever I get a wierd reading on the DVOM.
Note: This is not supposed to be a technical treatise, just an explanation for non technically trained people.SamTin a previous life, a highly trained electronics technician, now, sometimes a home wiring dude.
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
An analog VOM reduces the possibilities, but no quarantees.A few people have see phantom voltages on them.http://www.simpsonelectric.com/260-8.htmThe Simpson is 5k/volt on AC and on the 250 scale that is 1.25 meg.A wiggley type won't have this problem.But it won't show the few volt drop that can have with bad connection or over long run.The real answer is to understand the limitation of your tools..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
> An analog VOM will not see Phantom Voltages.
Wrong, stray voltage breath. A **low impedance** meter will not see phantom voltages. There are high-impedance transistor input analog voltmeters (though not nearly as many as there used to be) that will behave pretty much like the digital units, and even a 20,000 ohms/volt electromechanical VOM will read a volt or two or three in cases where a high-impedance unit would read 30-40 volts. There are also low-impedance digital units (though again, not that common).
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Why, you are absolutely correct.
Please, please, please forgive me master.
I have no excuse, no excuse at all.
It completely slipped this ignorant ones mind that some trained technician might actually condecend to read my humble and unworthy effort at helping untrained DIYers.
Perhaps some creature ran in front of this sentence when Master was perusing my meager efforts;
Note: This is not supposed to be a technical treatise, just an explanation for non technically trained people.SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
You are forgiven, grasshopper!!
I taught this c**p many lifetimes ago in Navy Tech school and I didn't fully Grok it then.
And I got the highest score in my class when I was studying it!
That's why I call the source of Phantom Voltage "Radio Waves." It gets picked up like an antenna.SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
AC produces a collapsing field which produces electromagnetic waves which will also produce electricity when the waves are intercepted by a conductor.
SamT:
I thank-you for serving our country as a former "Navy tech dude".All:
Of the technicians I've worked with, the most competent were trained initially in the armed forces. Especially knowledgeable in RF issues, like grounding and shielding, were the Navy techs. I'm thinking especially about the electrical/electronic environment aboard a guided missle cruiser, aircraft carrier, submarine, and other types. What part of the spectrum is not covered on board any one of these ships? From laser to sonar and deep sea long-wave, it is all there, in a very confined space, at very high power, all operating simultaneously 24/7, redundant, with a mission critical 0% downtime expectation over 3 month duty cycles.The laser and radio gear is not only part of the shipboard systems, but also a part of every weapon, excepting bullets and some projectiles.Now add to all that the requirement that the ship be capable of "stealthy" operation. Perhaps the biggest advantage a ship has is its steel and/or aluminum construction, but in any of the weapons and aircraft areas, sparks from static discharge are not allowed.To the readers who make or influence hiring decisions, please remember our armed forces trained and experienced men and women!
VOM designers work long and hard to get their impedance as high as possible, usually several tens of millions of Ohms. That's because in most electronics work, you want to measure what the circuit is doing without changing what it's doing. Any current drawn by the meter would screw things up and give you bad readings.
You can very easily thwart the engineers' hard work and make a low impedance meter out of any VOM. All you have to do is put an incandescent light bulb in parallel with it. Even a 7 1/2 Watt nightlight bulb gets you under a couple thousand Ohms at full operating temperature, and under a couple hundred Ohms cold. That's plenty good enough to get rid of phantom voltages. You could build a little socket into a dual banana plug, and pop it in when you need it.
-- J.S.
Whatever, Dude.
I guess I'll just have to disregard my 4,000 hours of electronics training and all the years of practice in the field of instrument calibration, repair, maintenance, and manufacture.SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
Not really -- the shunting the meter with a light bulb trick should be consistent with your knowledge and experience. Look at it as two resistances in parallel.
-- J.S.
I'ld rather use the proper tool for the job; a 10K ohm/volt VOM.SamT
Anyone who doesn't take truth seriously in small matters cannot be trusted in large ones either. [Einstein] Tks, BossHogg.
... Or you could just use the light bulb by itself. It lights up for real power, but not for phantom voltage.
-- J.S.
Previous posters could be correct, assuming the new romex AFTER the switchleg is next to a live line somewhere, and the wiring was done correctly.
Are your new switches standard 3-way wall switches, and not electronic dimmers? The electronic variety of dimmer definitely leaks voltage when "off". If it is electronic it probably does not have a knob that turns to adjust the light level. Some varieties of lighted switches also leak voltage. These issues are a normal part of their design.
The new switches are standard 3-way wall switches. The new Romex going from the switch to the light runs next to the cable coming from the service panel for 6 feet.
So,induced voltage it is.
induced phantom voltage on your digital - not a problem.
we used to get that on long feeders between ground and neutral when the power was on.
try an analog meter, bet it goes away