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This is in reference to "Why three prongs?".

Why does this hold true: "Curious kids might still stick their finger in a light socket and receive a shock, but the unwanted current was directed through the length of their finger and caused no danger of heart-stoppage."

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  • \$\begingroup\$ what a great article for lay-people, definitely worth a read. Notice that the context of the statement in the question is floating electrical systems. \$\endgroup\$
    – vicatcu
    Commented Jun 3, 2012 at 18:56
  • \$\begingroup\$ In power systems that most people will use, a shock to the fingers from a light socket could easily be fatal. \$\endgroup\$
    – Russell McMahon
    Commented Jun 5, 2012 at 17:45
  • \$\begingroup\$ After reading this discussion, I posit that the word "dangerous" should be replaced with "guaranteed fatal". IMHO, something that burns you but does not kill you should still be referred to as "dangerous", and saying it is "not dangerous" has a high risk of misinterpretation. \$\endgroup\$ Commented Jun 6, 2012 at 3:13
  • \$\begingroup\$ @MikeDeSimone The important thing is that people actually read the whole article and not assume anything based on an excerpt from it... \$\endgroup\$
    – vicatcu
    Commented Jun 6, 2012 at 17:08
  • \$\begingroup\$ It holds true for UNGROUNDED systems, as my article discusses. With no electrical system grounding, you don't get shocked by standing in flooded basements while touching a "live" wire. But if you stick your finger in a light socket, and touch both the center contact and the threaded part, your finger really hurts, and your autonomic nervous system yanks your arm back in a fraction of a second. (Of course the article also mentions that touching either AC wire during a thunderstorm is ...bad.) \$\endgroup\$
    – wbeaty
    Commented May 18, 2013 at 6:28

4 Answers 4

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The context of the statement in the question is the important thing. In simple terms, current takes the path of least resistance (actually current is inversely proportional to resistance, so a less resistive path will draw more current for a given potential). The statement in question was made in the context of an ungrounded (i.e. no earth connection, totally floating) AC power system. As the article explains, you an loosely think of your feet as being at Earth potential. You can model a child's finger as a resistor. The potential difference between the two sides of the child's finger is much much less than the potential difference between either side of the child's finger and the Earth potential, so current will simply flow through the child's finger rather than its heart.

This is NOT the case in modern (or even not so modern) wiring, as is explained further in the article, so please don't take this statement out of context, as you can easily DIE these days by sticking your finger in a light bulb socket that is not GFI protected. The more likely hazards are better protected though, it's a trade-off. Read the whole article.

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  • \$\begingroup\$ Even in a floating system, you can easily contact both live and neutral in a bayonet socket. You might not die but it will take the end of your finger off. \$\endgroup\$ Commented Jun 3, 2012 at 19:06
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    \$\begingroup\$ Note - current does not take the path of least resistance - that is a convenient way of attempting to describe what happens but is incorrect enough to be dangerous at worst and misleading at best. Current divides in the ratio of the inverse resistances of any paths available. A high resistance path gets relativelt little and a low resistance path gets relatively much, but all paths get some. Simply apply I = V/R to each path. V is common for all paths and 1/R varies per path. Big R = small I. Small R = big I. \$\endgroup\$
    – Russell McMahon
    Commented Jun 4, 2012 at 0:39
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    \$\begingroup\$ @vicatcu - I do pedantry as an art form, BUT in this case I think it matters. When dealing with mains the concept of "path of least resistance" could easily assist somebody to think they are safe if a lower or much lower or much much lower path was available as well as them. In practice a "solid" wire ground MAY not save you from shock* - unusual but possible. Certainly, fingertip phase to neutral PLUS fingertip phase and neutral via feet does not stop you getting current via the feet and, along the way, the heart. Mains will happily power both paths at once. \$\endgroup\$
    – Russell McMahon
    Commented Jun 4, 2012 at 11:29
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    \$\begingroup\$ @vicatcu - * Test: Stand barefoot with feet in grounded metal pan of salt water (we'll make that part obvious). Take a say 1 foot / 300mm piece of hookup wire with insulation stripped off it so wire is bare. Push one end into mains power neutral outlet. Hold other end of wire with say 1" protruding from your bare fingers. Push wire end into phase outlet [don't try this at home]. Will you get a shock? Will you die? Answers may need statistical content. Would you try this experiment? Answer is unlikely to need statistical content :-). \$\endgroup\$
    – Russell McMahon
    Commented Jun 4, 2012 at 11:33
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    \$\begingroup\$ @vicatcu: The purpose of fuses and circuit breakers is to prevent enough energy from moving to start a fire due to heat dissipation from the building wiring. They are not sized to prevent damage at the load, or accidental arc-welding of body parts. \$\endgroup\$ Commented Jun 4, 2012 at 14:32
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My father received a 33 kV shock and survived due to this effect. He was working at an electrical substation and climbed up a ladder to work on a transformer. Unfortunately the wrong transformer was switched off and he climbed up a live one instead. As he reached up, a spark went into his palm and out through his forearm to the aluminium ladder. The spark set his overalls on fire which put him in the burns unit, but from the elbow up he had no ill effects from the electrical shock.

Note that you should not rely on this to survive a high voltage shock. He was very lucky to survive.

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I heard it explained to me that when working with power in the walls, it is best to keep one hand in your pocket. That way, electricity flows down your side and to the ground instead of across your heart, to the other arm, then to whatever else you were holding to get to the ground. The danger of heart stoppage is reduced because the voltage goes down to the ground and little or no amperage crosses the heart.

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  • \$\begingroup\$ as an aside - you definitely used to have to do this when working with televisions - the voltage from the flyback transformer in a colour CRT TV could throw you across the room, so TV engineers would never wear a tie, always wear rubber soled shoes, always use their right hand, and use plastic tools where possible. \$\endgroup\$
    – Rory Alsop
    Commented Jun 3, 2012 at 19:18
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    \$\begingroup\$ While "throwing you across the room" is a bit of an exaggeration, it should be noted that even one-handed, rubber shoes etc aren't enough. You can get pretty effective capacitive coupling to a concrete floor under the right circumstances. I've personally experienced this (in addition to the flyback transformer voltage in a television and my own HV experiments). Electricity needs to be respected, period. \$\endgroup\$
    – akohlsmith
    Commented Jun 3, 2012 at 19:28
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    \$\begingroup\$ @gobernador - " ... no chance of heart stoppage ..." is very very wrong and potentially fatal if relied on. BUT " ... less chance of heart stoppage ..." is probably correct. The advice is potentially good - but only as an added safety measure after all other possible ones have been followed - and it may not save your life in some cases. \$\endgroup\$
    – Russell McMahon
    Commented Jun 4, 2012 at 11:36
  • \$\begingroup\$ @gobernador - That's a start. Still highly misleading. Really does suggest a significant safety factor increase. If you faced a say 1:10 chance of dying activity how would you feel about a device that changed this to 1:30. Sure, it's an improvement 0 but still in the "highly likely to be fatal realm. There is NO REASON that electric current given any arm to foot path will not involve the heart along the way. \$\endgroup\$
    – Russell McMahon
    Commented Jun 4, 2012 at 15:11
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Reiterating what's been said, and adding some clarification:

Electric current across the heart is really what is lethal. Current as low as 1mA directly through your heart is enough to be lethal (see "Ventricular Fibrillation" here). If current doesn't travel through your heart, though, electric shock is most dangerous with its indirect effects (such as Optimal Cynic's father, who received severe burns from the heat created in the shock).

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