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I today confused the entire company (we work on batteries for trains) when I stumbeled about the following fact:

When you touch the negative pole of a battery having electromagnetically isolated shoes, without touching its other end, nothing happens to you.

I thought: No.

The battery has an surplus of (negatively charged) electrons. These electrons will flood into my body creating a measurable electric current until a charge balance is created (charging of a plate capacitator). Seemingly, they do not. Why?

I then continued after my work to do the following expirement:

I took a battery out of my computer mouse. I measured the voltage between its ends (one end at the time) and the ground. 0 Volt. I also measured the current. 0 Ampere. My assumption was the battery would push the electrons through the cable into the ground. It does not. Why? Why would I need isolated shoes in the first place?

I wonder if this is true for all capacities, especially plate capacitors.

EDIT: In production we use Voltages up to 3kV.The battery I used for the grounding test only has 1.5V

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  • $\begingroup$ Could you edit your question with the voltage of your company's traction batteries? $\endgroup$
    – TimWescott
    Jul 5, 2023 at 16:44
  • $\begingroup$ Thanks, I did it. $\endgroup$
    – Niclas
    Jul 5, 2023 at 17:33

1 Answer 1

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From Wikipedia: "The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΩ resistor."

From Desco Industries: "Most people do not start feeling the effect of an ESD event until the discharge is at least 2000 volts. It typically takes a ESD discharge of greater than 2,000 or 3,000 volts for a person to feel the 'zap'."

If you are isolated from ground and touch a conductor (such as your battery terminal), current will flow for about half a microsecond (three time constants of that 1500 ohms and 100pF). Technically this is measurable, even with a 1.5V battery -- but it would be a very difficult measurement to make.

Two conditions would have to be met for you to feel a zap from your traction batteries: first, there would need to be a potential difference. Over time things inside that railcar will end up having potentials close to the car's frame. If the battery is grounded to the car via its negative terminal, then when you touch it, you'd already be close to its potential. Even if you touched a terminal that was a few hundred volts different, you probably wouldn't feel anything (see the comment from the Desco site).

So, all in all -- yes, a current will flow. However, if you're truly isolated from ground then you won't notice it unless your batteries run to thousands of volts -- in which case you're violating all sorts of safety precautions by playing with them.

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    $\begingroup$ The battery ends don't have an absolute voltage (relative to ground) of 1.5V unless the negative terminal is shorted to ground. They have a voltage between the anode and the cathode of 1.5V. The absolute voltage of either end (and your own absolute voltage before touching it) is completely uncertain, and can fluctuate wildly if it is, for example, charged up by having been rubbed against something shortly after being electrically isolated from the ground. Your answer has some valuable considerations, but I think you missed some as well. $\endgroup$
    – AXensen
    Jul 5, 2023 at 17:02
  • $\begingroup$ I was trying to answer the question as asked -- "why don't I notice this voltage". I tried to stick with that. Yes, if it's a dry day and you scuff your feet on the carpet walking up to a 1.5V battery, you're going to notice some electricity, but it's going to be (A) way more than 1.5V, (B) unrelated to the battery, and (C) unrelated to the question. $\endgroup$
    – TimWescott
    Jul 5, 2023 at 17:19
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    $\begingroup$ I guess my point is - undefined voltages relative to ground do have everything to do with the question. In my mind, the battery doesn't shock you for the same reason touching a stone sitting on some rubber doesn't shock you. Unless you complete the circuit the fact that it's a battery, and the 1.5V figure, is entirely irrelevant. $\endgroup$
    – AXensen
    Jul 5, 2023 at 17:24
  • $\begingroup$ Thank you for the good answer! The other question might remain; Why do I measure 0V to the ground with a normal 1.5 V Battery, when I do not close the circuit. $\endgroup$
    – Niclas
    Jul 5, 2023 at 17:37
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    $\begingroup$ @Niclas because your voltmeter has a finite resistance. Any voltage difference between the battery terminal and ground discharges through the meter. That only requires moving a tiny amount of charge, and by the time it's been connected long enough to make a measurement, the voltage is zero to within the meter's resolution. $\endgroup$
    – hobbs
    Jul 5, 2023 at 17:48

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