Strange voltage measurements?
#1
Member
Thread Starter
Join Date: Aug 2001
Location: Los Angeles County, California
Posts: 238
Upvotes: 0
Received 0 Upvotes
on
0 Posts
Strange voltage measurements?
I have a ceiling receptable box with the light fixture removed. There are two pairs of wires going into the box, currently unattached. One pair has 120v on one side and 0 v on the other. But the second pair has 14v on one side and 14.6v on the other. The wall switch that controls this fixture has 14v on one side and 14.6 on the other.
Can someone suggest what this means...is there a short in the wall somewhere or is there a normal explanation for these voltages.
John
Can someone suggest what this means...is there a short in the wall somewhere or is there a normal explanation for these voltages.
John
#2
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,245
Upvotes: 0
Received 0 Upvotes
on
0 Posts
It suggests that you are using a digital voltmeter. Ignore the readings. You are reading phantom voltage. Put away your digital voltmeter. Use an analog meter or none at all.
#3
Member
Thread Starter
Join Date: Aug 2001
Location: Los Angeles County, California
Posts: 238
Upvotes: 0
Received 0 Upvotes
on
0 Posts
Is this true with all digital meters? I'm using a good one, a Fluke. I'll scout around and see if I have an analog meter that still works, but this is a surprise.
John
John
#4
#5
Member
It suggests you have a switch loop.
Connect the hot black the 14 volt white.
Connect the fixture to the other black and the white that is in the same cable as the hot black.
The switch should have a black and white connected to it.
Connect the hot black the 14 volt white.
Connect the fixture to the other black and the white that is in the same cable as the hot black.
The switch should have a black and white connected to it.
#6
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,104
Upvotes: 0
Received 0 Upvotes
on
0 Posts
The better the digital meter, the worse the problem with 'phantom voltage'.
The reason is that 'phantom voltage' is very real; it is actual voltage imposed on the wire by various inductive, capacitive, and leakage effects.
All voltage sources are characterized by how the voltage changes when current flows. When you are looking at a properly connected circuit, the 'voltage drop' is slight, and many amps have to flow before the voltage drops much.
But with a 'phantom voltage' source, there are only microamps available, or less. The current flow is so small that as soon as you place any sort of load on it, the voltage will drop right to zero.
The better the digital meter, the less the current it draws to make a measurement. Old analog meters draw significant current to make a measurement, and thus 'drain' away the phantom voltage. But a _very_ good analog meter would show phantom voltage, just like a digital meter.
In industrial environments with high voltages and long control lines, 'phantom voltage' can be enough to cause a noticeable shock.
-Jon
The reason is that 'phantom voltage' is very real; it is actual voltage imposed on the wire by various inductive, capacitive, and leakage effects.
All voltage sources are characterized by how the voltage changes when current flows. When you are looking at a properly connected circuit, the 'voltage drop' is slight, and many amps have to flow before the voltage drops much.
But with a 'phantom voltage' source, there are only microamps available, or less. The current flow is so small that as soon as you place any sort of load on it, the voltage will drop right to zero.
The better the digital meter, the less the current it draws to make a measurement. Old analog meters draw significant current to make a measurement, and thus 'drain' away the phantom voltage. But a _very_ good analog meter would show phantom voltage, just like a digital meter.
In industrial environments with high voltages and long control lines, 'phantom voltage' can be enough to cause a noticeable shock.
-Jon
#7
Member
Join Date: Oct 2005
Location: north dakota/montana
Posts: 130
Upvotes: 0
Received 0 Upvotes
on
0 Posts
I work on center pivots and use a fluke meter (does 187 sound right). Common problem solving for these systems is continuity testing. I have found that in a quarter mile machine you can’t trouble shoot the system looking for power because you will get 120 volts of induced or phantom voltage. I have never tried an analog meter and won’t because my trouble shooting is better done with the power off anyway.
However, we often bury a guidance wire that is a couple of miles long and a five dollar analog meter works better to tests its continuity than my three hundred and fifty dollar fluke.
Anyway, not really on subject other than I have also seen induced or phantom voltage.
Keep in mind though that a cheap meter probably does not carry the same safety rating as the fluke. I have seen videos of people who have tested live circuits and had a meter blow up in their faces. These people where severely disfigured and some even lost limbs to the burns. This is pretty important to me as most of my work in on 460 volts, but I would bet that is important on any voltage. My meter and leads are cat three and 1000 volt certified.
However, we often bury a guidance wire that is a couple of miles long and a five dollar analog meter works better to tests its continuity than my three hundred and fifty dollar fluke.
Anyway, not really on subject other than I have also seen induced or phantom voltage.
Keep in mind though that a cheap meter probably does not carry the same safety rating as the fluke. I have seen videos of people who have tested live circuits and had a meter blow up in their faces. These people where severely disfigured and some even lost limbs to the burns. This is pretty important to me as most of my work in on 460 volts, but I would bet that is important on any voltage. My meter and leads are cat three and 1000 volt certified.
#8
Member
Thread Starter
Join Date: Aug 2001
Location: Los Angeles County, California
Posts: 238
Upvotes: 0
Received 0 Upvotes
on
0 Posts
Thanks everyone for the explanations, I understand what phantom voltage is now. Unfortunately I have two brand new looking analog meters(Radio Shack) and neither of them work.
Joed, talking about the switch loop...I think you left out a crucial word in your second line but if I understand what you said its to: 1) connect the 120v black to the 14v white, 2) connect the fixture to the 14v black and the 0v white, and 3) make sure the switch has a black and a white going to it. This being a very old house I hadn't noticed anything but black wires in these pairs, but I'll look closely and see if I can distinguish one from the other.
John
Joed, talking about the switch loop...I think you left out a crucial word in your second line but if I understand what you said its to: 1) connect the 120v black to the 14v white, 2) connect the fixture to the 14v black and the 0v white, and 3) make sure the switch has a black and a white going to it. This being a very old house I hadn't noticed anything but black wires in these pairs, but I'll look closely and see if I can distinguish one from the other.
John