Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

It's been a while since school and I don't remember the right laws and formulas. So I come here. :)

I've got a power cord extender running from the wall outlet to my washing machine. It's about 3 or 4 meters in length. The outlet supplies 220V, but no grounding. The extender however does have grounding support, as does the washing machine. This means that the extender has three wires in it, and the grounding wire terminates at the outlet.

When plug in one end of the outlet, and measure the AC voltage between the grounding (disconnected) and zero wires, it gives me about 60V. Which happens to be unpleasant enough when accidentally touching the washing machine (even when it's turned off). Considering that the cable is so short, can this really be explained by induction?

To rule out a few other theories: the wall outlet does not connect anything to the grounding wire, it really does terminate there. The grounding wire does not touch any of the other wires, the resistance between it and other wires is infinity (or rather, beyond the range of my cheap multimeter).

share|improve this question
1  
You really should ground your washing machine. It's a hazard. –  Carl Brannen Aug 11 '11 at 17:14
    
@Carl Brannen - I'm aware. But getting a proper ground on 4th floor of an old house that has never seen it isn't simple. :P –  Vilx- Aug 11 '11 at 17:17
    
What are you measuring your 60V with respect to if you don't have a ground??? –  Carl Brannen Aug 11 '11 at 18:02
    
@Carl Brannen - That's just it. I measure it between the "ground" wire (which is really a 3M long wire with the other end hanging in the air), and the zero wire. –  Vilx- Aug 11 '11 at 19:01
1  
Your title says current, but your question says you're not measuring a current, you're measuring a voltage. I would guess that - if the "ground" wire is floating as you say - John McVirgo is correct: it's just some capacitive pickup between that wire and one of the others. The reason you can see a voltage is because the impedance of your voltmeter is bigger than (or comparable to) to the impedance of the capacitive coupling. If you were instead to measure current, you should see extremely little, because the impedance of the capacitive coupling is very large. –  Anonymous Coward Aug 11 '11 at 23:28

1 Answer 1

The effect is mainly down to capacitive coupling between the wires. You have a capacitance $c_1$ between the live and ground wire; and a capacitance $c2$ between the ground and neutral wire. The $220v$ drives current around this series capacitive circuit, and the voltage you measured across $c_2$, the neutral and ground, is found using the capacitive potential divider equation$$60 = 220\frac {c_1} {c_1 + c_2}$$

share|improve this answer
    
When I measured the 60V, the washing machine wasn't connected. That is, I plugged one end of the cable in the wall, and measured at the other end. –  Vilx- Aug 11 '11 at 15:39
    
@Vilx ok, I've modified my answer to account for this. –  John McVirgo Aug 11 '11 at 20:51

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.