Grounding wire in household circuitry?

[rude man]

No, I believe you are wrong here.

The OP is correct in asserting that if the two wires supplying electricity to your house were completely isolated from ground, you would need to touch both of them to get shocked.

If you touch one side of a battery to the Earth, will it drain the battery?

The topic of this thread is the practicality and tradeoffs of maintaining this isolation.

Appliances are connected one way or another to ground whether intentional or not. It may be by sitting on ground making Ohmic contact, or it may be capacitive - but a floating power system would have enough imbalance to ground to present a potentially lethal hazard to anyone making ground contact to either hot side of the floating source.

The idea of a perfectly floating source is science fiction.

 

[gt 5]

Ahh, yes. I see your point, Jim. You are correct. But keeping the system isolated is the problem. The idea, here, is to assume that the worst could happen. If a tree were to make contact with with the transmission lines, or any other form of ground fault should occurr, the system would not fail safe. It would then be grounded waiting for you to make the circuit complete (again referring to the loose wire touching the housing of the wash machine). If the housing were also grounded, you couldn't complete the circuit.

 

[sophiecentaur]

You, being of higher resistance are, then, not the path of least resistance.

I have to point out that "least resistance" doesn't mean 'all the current'. The current is shared between the Earth path and the person holding the faulty equipment. There will be a finite but small current through the body (often detectable as a mild tingle). The presence of the Earth conductor means that the two parallel paths present a very low resistance so the voltage drop to Earth will be safely low.
Kirchoff 1 always applies and the naive statement that "current takes the easiest path" is not to an appropriate standard for PF discussions.

 

[jim hardy]

Maybe this will help cement the concept:

but a floating power system would have enough imbalance to ground to present a potentially lethal hazard to anyone making ground contact to either hot side of the floating source.

The idea of a perfectly floating source is science fiction.

it's a matter of scale. Large system yes; small system, well, maybe.
When a system gets large enough that its surface area creates capacitance sufficient to pass a good part of an amp, maybe 1/50th, it can deliver a painful shock through that capacitance as rude man stated.

That capacitance has some impedance at line frequency.
In industrial systems it is recommended(IEEE142) that the grounding impedance be a smaller number of resistive ohms, so that Q remains low.
In residential systems that number is zero.

Everything powered by one of these

is powered from a "floating" supply. The system may or may not be earthed. The laptop I'm typing this on is not.

There is minute capacitance between primary and secondary windings, as mentioned.
That's minimized by split-bobbin winding, which also assures that an insulation failure won't let their two wires touch.

Here is a little transformer using a divided (or split) bobbin. This is very practical, because it completely separates the primary from the secondary winding, making it much easier to achieve the degree of insulation required for safety.

http://ludens.cl/Electron/trafos/trafos.html

What's important for beginners to grasp is that "electricity" has no magical affinity for "ground", current only goes there in obedience of Kirchoff's laws.
Ground is simply another wire that happens to go most everyplace. We have this discussion a lot here.

 

[dlgoff]

Ground is simply another wire that happens to go most everyplace. We have this discussion a lot here.

With that in mind, I'll post this image again from http://www.rapidtables.com/electric/electrical_symbols.htm#ground symbols.

 

[the_emi_guy]

If houwever there is no ground wire, you MAY become the conduit to the Earth and complete the circuit into the Earth's infinite 'sink' or 'supply' of electrons..

I should also point out that the Earth does not have an infinite number of electrons. It may be a large number, but then again the number of electrons in a short piece of wire is also a large number. When dealing with quantitative topics it is usually not good to invoke infinity.

From quick Google search:
Number of electrons in earth: order of 10^50
Number of electrons in 1m long piece of wire: order of 10^23.