Is 9 volts on disconnected wire normal?

[mike33]

I disconnected the red wire from MWBC at breaker. I found the 12/3 in a junction box and disconnected the red and black from everything. I turned on breaker with the black wire only connected. Now I read 9 volts from red wire to ground. Is this normal when using 12/3? I did not get the same results when I reversed the wires (energized the red and measured the black to ground - no voltage)

 

[adelle]

Yes this is normal for a digital meter. These meters have very high input impedance, 10megohms is not uncommen, and it will sense very, very, low currents.

The sure way is to connect a 7w test lamp from the wire in question to ground. Don't use a neon tester as these too can light on very low current stray voltage.

 

[gar]

mike33:

What is the real question here?

We will assume a high input impedance DVM, like a Fluke 27 or 87, and a typical residential environment.

Try these experiments:

1. Select the normal AC voltage range. Plug in the two test leads. Hold them up in the air with your arms spread apart, and without touching the probe tips. What voltage do you read?

2. Put one lead in the neutral of an outlet, and hold the other probe in the air. What do you read?

3. Same as 2 but put the one lead in the hot side of the outlet. What do you read?

4. Roll out a 50 ft or so extension cord on the floor. Connect the extension cord to nothing, except one wire to the meter. Connect the other meter lead to neutral or ground. What do you read?

All of these voltages read are a result of capacitive coupling.

If you put a moderately low resistance, like a 25 W incandescent bulb, across the meter input, then the voltage will drop to a low level.

.

 

[LarryFine]

If you put a moderately low resistance, like a 25 W incandescent bulb, across the meter input, then the voltage will drop to a low level.

Or, more simply, use a "wiggy", aka a solenoid-type tester, which has a low-enough impedance to eliminate phantom-voltage readings.

 

[ATSman]

Phantom Voltages

Phantom Voltages

Or, more simply, use a "wiggy", aka a solenoid-type tester, which has a low-enough impedance to eliminate phantom-voltage readings.

Yeah, the wiggy for all the oldtimers out there, will do the trick. I have a little story to tell on the subject. Years back when I was doing startup on 5 substations at an airport. It was Cutler-Hammer DS double-ended switchgear. That was when they used a line of red and green LEDs next to the breaker control switch to indicate closed and open. The control cables that were several miles long (mucho capactance!) were run in the same raceway as the power cables for the guideways of the trains. During very foggy days in the bay area both the red and green LEDs were lit. Was the breaker open or closed????It really bugged the inspectors and cust reps when they saw this.
Turns out the induced (fake) voltage measured at 70VAC was lighting the open lights when the breaker was closed. I came up with a capacitor in series with a resistor fix (snubber) to bleed off the voltage, sent it to C-H for approval and was able to modify all the control circuits to correct the problem. Of course, the response from C-H was "that's the first we heard of that problem,....Right!!!!:roll:
Anyone interrested in the spec for the fix, contact me.
Happy Hollidays All!
Tony

 

[mike33]

Thanks for the tips. I have now done some reading to try to get at least a basic understanding of capacitive coupling (still learning). My boiled down question really was "can someone get shocked or can equipment be damaged from capacitive coupling?"

I use a Fluke T5 600. There is no adjustable range. The guy I usually work with (oldtimer) uses a wiggy. When he checks for power at a light fixture to be sure the power is switched off, his tester won't warn him that there is still just enough to shock him from the dimmer switch night-light.

 

[iwire]

Check this link out for some more info

http://www.nema.org/stds/eng-bulletins/upload/Bulletin-88.pdf

 

[gar]

101204-1717 EST

mike33:

A current of 0.1 MA or 0.000,1 will flow with a source voltage of 120 V thru a 1,200,000 megohms impedance. At 60 Hz this will result with a capacitor of about 2200 pfd, or in the ballpark of 100 ft of #12 or #14 Romex. At 1 MA the length would be about 1000 ft. You may not feel 0.1 MA, but you will feel 1 MA. At 5 MA you get a substantial kick.

A wiggy or other low impedance meter would not detect 1 MA of current. A 1 MA current would result from a 120,000 ohms leakage impedance and a source voltage of 120 V.

Yes you could get a shock or damage some equipment from a capacitive coupled current. The leakage current from capacitive coupling is proportional the the cable length and is determined from the capacitance per foot of the wiring, and the source voltage.

.

 

[glene77is]

Or, more simply, use a "wiggy", aka a solenoid-type tester, which has a low-enough impedance to eliminate phantom-voltage readings.

Larry,
Mine has 4KOhms Impedance, pulling some 40 mA.
I prefer my little LED 3W bulb.