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0 pointsars6y ago0 comments

The reason for having ground is simply because if an electrician makes a mistake and reverses the hot and neutral, and you plug something in, the body of the device will now be connected to hot, and you'll get a shock.

By having a separate ground, it's harder to mess that up, and devices can connect the body of the device to that.

Inside the electrical box (mains) the neutral and ground are connected together, and are at the same potential.

The only reason they run separate wires is in case of of mistakes.

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4 comments · 2 top-level

mindslight6y ago· 2 in thread

> The only reason they run separate wires is in case of of mistakes.

Erm no.

1. In case the neutral gets disconnected, so the frame isn't at the potential of the hot (no voltage drop across the device)

2. So the return voltage drop in the white wire isn't on the body of the device. ~20 volts between adjacent high-current appliances on different phases probably wouldn't be terribly dangerous to most people, but it is sloppy.

3. For GFCI/RCD, it separates bona fide return currents from "accidental" return currents. With only two wires, dropping a toaster into a (PVC-drain) bathtub wouldn't trip. (Erm, I just realized most toasters are actually only two wires. Well uh, don't do that).

wrycoder6y ago

If that "toaster circuit" is protected by a GFCI, it will indeed trip with only two wires. The GFCI is sensing the imbalance between hot and neutral (the rest went through the bath water). Both currents pass through the same toroid, but they are in opposite directions and normally exactly cancel, so no flux is induced in the toroid. If there is an imbalance, a current is induced in a secondary winding. This will trip the GFCI internal breaker. Note that this does not involve the green wire.

mindslight6y ago

Except there will be no imbalance if there is no other path for the current to travel by...

The geometry of the electric field in water should probably be similar to that in air (mostly contained to within the case), just don't drop both a toaster and a waffle maker in at separate ends.

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wrycoder6y ago

There is also the matter of unanticipated leakage current between hot and the equipment enclosure. The EGC/green wire keeps the enclosure close to ground potential.

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