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Old 05-31-2011, 07:00 PM   #1
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AC Voltages on the Neutral/Ground


I've measured the AC voltage with my Fluke 77 (peak to peak, not RMS) on several outlets in my house between the neutral and ground and have found they fall into two camps. The first reads .175 VAC or so, the other .5VAC. What can cause this in the home and why am I getting different readings? Is the higher reading acceptable? Is there anything I should do about this, additional measurements perhaps?

Last edited by smata67; 05-31-2011 at 07:03 PM.
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Old 05-31-2011, 07:27 PM   #2
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AC Voltages on the Neutral/Ground


You are getting Phantom voltage because of your meter. To get rid of those, you have to use a load such as a light bulb on the line to with one end to the positive (red) meter lead, the other lead (black) to the line coming from the breaker. Should get zero volts from Black to Neutral, Black to Ground. Measuring White & Ground will always show Phantom on a cheap fluke.
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Old 05-31-2011, 08:32 PM   #3
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AC Voltages on the Neutral/Ground


I have an oscilloscope, I'll check it with that this weekend. I also have a current limiter for audio equipment testing, an outlet in-line with a bulb to directly test what you describe. We'll see if I end up with 0VAC between neutral and ground on that contraption. I certainly hope so.

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Old 05-31-2011, 08:43 PM   #4
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AC Voltages on the Neutral/Ground


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Originally Posted by gregzoll View Post
You are getting Phantom voltage
Quote:
Measuring White & Ground will always show Phantom on a cheap fluke.
The term Phantom voltage appears frequently in this forum but if one doesn't understand what happens when a high impedance meter (a cheap Fluke) is added to a circuit and displays a voltage then they don't really understand electronics.
If a meter is reading a voltage then the voltage isn't false and is present but may not have the ability to do work because of the circuit resistance.
I this example, you are taking a voltage reading between a current carrying conductor and a non current carrying conductor and both of these conductors are tied together at the service panel. So, why the voltage reading?
Copper wire does have resistance and therefore will exhibit the properties of a resister. http://en.wikipedia.org/wiki/American_wire_gauge
So here is a problem:
My grounded receptacle is connected to my main panel and is the only device on the circuit which is wired with 14 AWG wire and it is 50 feet to the panel.
I have a load plugged into this receptacle that draws 10 amps but I have access to the wires to measure voltage.
When I measure from the bare ground to the white wired I read 1.25 volts.
Using the above table you would see that 50 feet of #14 would have a resistance of .125Ω
Ohms law states E=IXR
10 amps X .125Ω = 1.25 volts

BTW, how did you read peak to peak with your 77?
The Fluke Model 77 Series IV is a battery-powered, average
responding-rms indicating multimeter (hereafter "the Meter"), with
a 6000-count, 3 3/4-digit display, and a bar graph.

Last edited by a7ecorsair; 05-31-2011 at 09:15 PM.
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Old 06-01-2011, 02:12 AM   #5
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AC Voltages on the Neutral/Ground


Quote:
Originally Posted by gregzoll View Post
You are getting Phantom voltage because of your meter. To get rid of those, you have to use a load such as a light bulb on the line to with one end to the positive (red) meter lead, the other lead (black) to the line coming from the breaker. Should get zero volts from Black to Neutral, Black to Ground. Measuring White & Ground will always show Phantom on a cheap fluke.
"Cheap Fluke" is an oxymoron. The model 77 is a $260 meter, and Fluke products don't get much cheaper than that. Although the original poster correctly said it's not an RMS meter, it reads the arithmetic average, not peak to peak voltage. The difference from RMS should be negligible with all but the strangest waveforms.

The voltage here is not "phantom voltage", and probably will not change even if a low impedance load is connected across the meter as you suggest. After all, there is ALREADY a very low impedance connection between neutral and ground back at the panel. Instead, the voltage between neutral and ground is the voltage drop that arises from current flow through the neutral (either the circuit neutral or the system neutral after the bonding jumper, depending on how the panel is set up). A fraction of a volt is absolutely normal. With a heavy load on the circuit, a couple to a few volts may be perfectly acceptable. Whatever the voltage drop under load is on the circuit, the voltage from neutral to ground at the receptacle will be half of that.
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Old 06-01-2011, 06:35 AM   #6
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AC Voltages on the Neutral/Ground


Measuring a voltage between neutral and ground at a point up in your house where electricity is being used exemplifies the importance of not connecting neutral and ground together at random locations.

If we use ground at the main disconnect switch as the 0 volt reference, ground anywhere should measure 0 volts. With no current being drawn on the circuit, neutral should also measure 0 volts relative to main disconnect location ground. With current being used, there is some voltage drop in the neutral so neutral to ground out at the receptacle (or neutral at the receptacle to main disconnect location ground) will be some value.

If neutral and ground were bonded out at that receptacle, the voltage between neutral and ground out there would be zero but relative to ground at the disconnect switch, the voltage on the ground has been elevated from zero as opposed to the voltage on the neutral brought to zero. (The actual voltage relative to ground at the disconnect switch would be about halfway in between if the ground path was comparable (wire thickness, routing length, etc.) to the neutral path.
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Last edited by AllanJ; 06-01-2011 at 06:40 AM.
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Old 06-01-2011, 06:51 AM   #7
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AC Voltages on the Neutral/Ground


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I have an oscilloscope, I'll check it with that this weekend. I also have a current limiter for audio equipment testing, an outlet in-line with a bulb to directly test what you describe. We'll see if I end up with 0VAC between neutral and ground on that contraption. I certainly hope so.
And what will that do? Face it, you have some new toys and want to play with them. Really you are over doing it.
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Old 06-01-2011, 07:17 AM   #8
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AC Voltages on the Neutral/Ground


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And what will that do? Face it, you have some new toys and want to play with them. Really you are over doing it.
At least he wouldn't be using a cheap Fluke
An O'Scope is really nothing but a "visual voltmeter" that displays in Peak to Peak. Using an O'Scope to read building wiring can give funny reading unless you realize the scope is connected to the AC you are reading by its own power cord which means the scope signal ground will be building ground.
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Old 06-01-2011, 07:37 AM   #9
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AC Voltages on the Neutral/Ground


Or using a Fluke, and not knowing how to use the tool. As for cheap, really did not mean it the way you took it, meant in the way that the person did not know what they are doing. Hell, when I first got mine back in 86 in the Navy, it took a while to get used to taking readings, due to I was used to using a analog multi-meter.
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Old 06-01-2011, 08:09 AM   #10
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AC Voltages on the Neutral/Ground


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Originally Posted by gregzoll View Post
Or using a Fluke, and not knowing how to use the tool. As for cheap, really did not mean it the way you took it, meant in the way that the person did not know what they are doing. Hell, when I first got mine back in 86 in the Navy, it took a while to get used to taking readings, due to I was used to using a analog multi-meter.
Thanks for clearing that up. I couldn't understand why someone would put down a Fluke. I got my first Fluke, a Blue 8060A, around 1982. It wasn't auto ranging so you had to watch the decimal point and the range button.
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