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I am trying to fully understand the danger of AC voltage but still I have some questions...

I understand that human body has a capacitance that facilitates AC current penetration. I also understand that with an increasing voltage, body resistance is reduced. That's why even if with my multimeter I measure a 1 mega-ohm resistance from left hand to right hand, I just can't use Ohm law and assume that I can put myself in series with a 230V live wire...

But what I am confused about is for a one hand touch on an exposed live wire.

I know I will be shock because electricity will pass through my body to go to earth. But if even my kitchen concrete floor act as a grounding, if I put a live wire on the floor, current should go to earth and my 50 mA differential should switch off.

I have never tried it, but I am pretty sure if I put a live wire on my floor, nothing will happen, right? So why when touching my body the electricity find the earth?

I can understand the shock if I touch a live wire and a grounded object (sink, pipe, etc.) at the same time.

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  • \$\begingroup\$ I'm voting to close this question as off-topic because this has nothing to do with Electrical Engineering \$\endgroup\$
    – Huisman
    Commented Jun 27, 2019 at 19:52
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    \$\begingroup\$ The statement "...with an increasing voltage, body resistance is reduced" is not accurate. Although your body's resistance can change based on a variety of factors (such as humidity and moist or dry skin), applied voltage is not one of them. Higher voltage generates larger current at a given resistance, which is why it poses more danger than lower voltage. \$\endgroup\$
    – JYelton
    Commented Jun 27, 2019 at 19:53
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    \$\begingroup\$ @Huisman I disagree because electrical safety is an inherent part of electrical engineering. Related meta topic. \$\endgroup\$
    – JYelton
    Commented Jun 27, 2019 at 19:55
  • \$\begingroup\$ I disagreed too, let's say you want to try experiencing with a tesla coil, you better know the danger of higher voltage, or make a transformer, ... DIY electronic/electric is really popular and few people really know what they risk. \$\endgroup\$
    – Karnalta
    Commented Jun 27, 2019 at 19:59
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    \$\begingroup\$ Exactly. Safety is a part of engineering... I miss the rest of the engineering. If this topic would have been started with: "I'm designing a SMPS with triple isolation and ... (something about safety related to the design)" than I wouldn't have voted to close. But I'll raise this to meta. \$\endgroup\$
    – Huisman
    Commented Jun 27, 2019 at 19:59

2 Answers 2

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If you touch a 230V live wire with dry hands, the current through your body is roughly 2 mA. This isn't enough to make an RCD switch trip but its actually quite enough to harm you.

Contact surface area matters, too. A wire lying on the ground only has a very small surface where it is actually touching it. It's two hard objects. This is different already when the same wire sinks into a light groove on your skin – the contact area is a hundred times bigger.

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  • \$\begingroup\$ Yeah my two feet on the ground represent way more surface than the tip of a wire. So the more the object I touch is "grounded" the more I get current. Basically, on my kitchen floor it will hurt but no real danger. On a grounded cooper pipe, I will be badly shock and RDC will switch off. (I don't plan to try, of course) \$\endgroup\$
    – Karnalta
    Commented Jun 27, 2019 at 19:56
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    \$\begingroup\$ Honestly, a 30mA RCD (as common) will not help at all with keeping you alive. These are an additional measure against the shaver falling into a water-filled sink. A normal fuse won't trip at such a slight ground short. If you wanted safety at least against lethal fault currents, you needed a 5mA RCD. But you don't want those because you can't use consumer electronics in such a circuit. Their mains filters have ground currents as high as 1mA, which made the RCD trip as soon you plug in the fifth cell phone charger. \$\endgroup\$
    – Janka
    Commented Jun 27, 2019 at 20:26
  • \$\begingroup\$ EU uses much higher current trips for RCD than GFCI's in North America \$\endgroup\$
    – D.A.S.
    Commented Jun 27, 2019 at 20:28
  • \$\begingroup\$ I think 10mA is common in the US, right? \$\endgroup\$
    – Janka
    Commented Jun 27, 2019 at 20:30
  • \$\begingroup\$ I believe it is more like 3 to 5 mA for US GFCI receptacles. \$\endgroup\$
    – user80875
    Commented Jun 27, 2019 at 20:47
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I have never tried it, but I am pretty sure if I put a live wire on my floor, nothing will happen, right ?

Electricity will flow through the floor to ground or whatever object has a different voltage potential than it. The resistance of the floor will probably be high, so not much current. The capacitance of the wire with other objects will also allow current to flow. Any two conductors make a capacitor.

So why when touching my body the electricity find the earth

It travels through the air via capacitance, and essentially turns you into an antenna at 60Hz. It doesn't take much to get a few mA, enough for one to feel it. Your body makes an good antenna/capacitor, because of the partially conductive surface area of the skin.

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  • \$\begingroup\$ You mean that the shock isn't related to the ground but to the antenna effect ? So a RDC switch won't trigger at all ? \$\endgroup\$
    – Karnalta
    Commented Jun 27, 2019 at 19:52

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