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I understand that an electrical appliance will be connected to ground to provide a path back if there is a fault and a live wire touches the case.

However if a person is also touching the case at this point then surely there will be a current through both paths? Because a person is like a parallel resistor to the ground connection. The current won't be as great, but it will be enough to be dangerous.

Do we just hope that the breaker will blow quick enough to prevent the shock happening for too long?

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  • \$\begingroup\$ Why would the current choose to flow through the human touching Earth?? ….short answer it would not, therefore the human is not exposed to a high voltage. \$\endgroup\$ – Jack Creasey May 10 at 1:52
  • \$\begingroup\$ If the case is (properly) grounded then it's at ground potential! So no voltage across the person, and zero current through the person. \$\endgroup\$ – Chu May 10 at 7:22
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    \$\begingroup\$ Its not the circuit breaker but the RCD will trip and save you. If exists. \$\endgroup\$ – user1999 May 10 at 19:11
  • \$\begingroup\$ Here is one of the best explanations in the youtube, Go through the link:youtube.com/watch?v=FOdJmDaarbk \$\endgroup\$ – JuneStar_2918 May 11 at 2:35
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Given the impedance of the live and the ground return path are roughly identical, a person touching the case during the ground fault will face half the live voltage. That's already much better than full live voltage. You could provide a thick ground return path to drop this voltage even lower.

schematic

simulate this circuit – Schematic created using CircuitLab

But yes, the protective effect comes from the fuse on the live wire blowing due to the short circuit.

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    \$\begingroup\$ No, Jack. The current takes all pathes, not only that one with lowest impedance. \$\endgroup\$ – Janka May 10 at 1:54
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    \$\begingroup\$ No, the human isn't touching earth but a case. Given the live and the ground wires are identical, and connected at the case due to the fault, that means the case is at half of the live voltage. It's a simple voltage divider. \$\endgroup\$ – Janka May 10 at 1:58
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    \$\begingroup\$ In addition to grounding the case, to improve safety electrical codes call for the use of Ground Fault Circuit Interrupters (GFCIs) in outlets in kitchens or bathrooms (where there is water involved, what makes a shock potentially more lethal). The idea is that the circuit is interrupted if there is a small imbalance between the live and the neutral currents going to the appliance, what is an indication that current is going somewhere where is not supposed to. In the case of a live wire touching the case directly, the hope is that the GFCI will trip much earlier than the main circuit breaker. \$\endgroup\$ – joribama May 10 at 2:19
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    \$\begingroup\$ @JackCreasey - in addition, in real life you expect a wire to case short to take effect gradually, starting by the wire barely touching the case. In this case the GCFI should trip before the short becomes really dangerous. \$\endgroup\$ – joribama May 10 at 2:51
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    \$\begingroup\$ @Jack: Picking up a live kettle with one hand with the other in the kitchen sink. Earthing has been around a lot longer than GFCI/RCDs. \$\endgroup\$ – Transistor May 10 at 5:53
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The goal of circuit-breakers and grounded cases is to remove any long persisting situations where the metal case was already dangerous when a human touched it.

If you pick up an ancient all-metal electric drill with a 2-prong cord, where the Line conductor has contacted the metal case weeks ago, nothing will happen ...until you step into a pool of water in the basement, or lean on a steel kitchen sink, or grab a metal conduit for balance, etc. Now your body is unexpectedly connected between Line and earth/neutral.

But still you aren't in serious danger, since you'll probably yell YOW!!! and yank your hand away from the earthed surface, or perhaps drop the drill, breaking the circuit.

For less lucky people, the lethal danger is from "clamp on," where the current through your body causes your muscles to push harder onto the electrically-live metal case or grounded object. With the above power-drill, when stepping into a saltwater pool, your hand may grip it hard, and your arm-muscles also freeze, so you cannot shake it free. (For lucky people, their arm muscles contract, violently flinging the power-drill across the room!) During clamp-on, the current may interfere with breathing, so, if you cannot fall to the floor in order to pull loose, you'll slowly die of suffocation. If the path is through your chest, the current may interfere with heartbeat (either immediately for a weak heart, or, since the heart is quite shielded by surrounding tissues, much time may be required before skin conductivity increases at the contact points, the fault current increases, and eventually your heart cannot beat normally.)

Search for electrocution and "clamp on."

The OP question is about a live-enclosure fault which happens while the user is both touching the case, and also is securely connected to Earth (e.g. grasping a faucet with the other hand, etc.,) and ALSO, either the current-path is across the chest and ALSO they have a serious heart condition where a half-second 120VAC shock is lethal rather than just surprising ...THAT, OR they encounter "clamp-on" and suffocate over ten minutes of constant shocking.

Obviously the grounded case and blown breaker removes the last danger.

Heart-attacks caused by brief encounters with 120VAC during the time before the breaker blows: they remain a (rare) instance which could be avoided by that special device sold in the 1970s which detects tens of microamps of ground current, and then "blows the breaker" within a half-cycle of 60Hz, using active electronics.

That 2-prongs world of our ancestors:
thirty electrocutions thirty (from 1930?)

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