Earthing: Questions on Circuit Completion & Current Flow

[Odlaw]

Hi all, I'm new here, hopefully I've posted this in the right section (it's not homework related).

Ive got a question that has been baffling me. I understand that current travels from high potential to lower potential, generally taking the easiest path (s/c path if it exists). However, from my understanding, for current to travel, as well as requiring a potential difference, there also needs to be a complete/closed circuit.

How is it then that if a single live wire were to connect to the ground, it would allow current to pass when the circuit is not complete?

I've tried to sketch this in a diagram to make it clearer.
http://img706.imageshack.us/img706/6145/shockf.png

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Thank You

 

[0xDEADBEEF]

What is the definition of a live wire?

One wire is earthed at the generator. The other one is called the "live wire". So you can close the loop through the Earth and you don't necessarily need the return wire to get zapped.

 

[Odlaw]

Hi OxDEADBEEF, thanks for the quick reply. I was hoping someone would say that as that was exactly what i was thinking before but people i asked would say otherwise.

So then to confirm, if the generator was not earthed from the generating side (so return path is just neutral wire), touching a live wire to ground should not do anything?

Then how does one explain lighting strikes to ground? We have one path of current travel (charged cloud to earth) but what is the return path that closes the loop?

 

[0xDEADBEEF]

Ok now we are making things a bit complicated. Why do we Earth at all? For short distances your thoughts are correct, but for large distances things turn ugly. Earth is not such a great conductor. Through charges coming from weather effects a line of a couple of kilometers can charge up to fairly high voltages. To remedy this you want to give the system a point of zero voltage. Floating lines mean that you could potentially be zapped by both lines. Unless you would get them perfectly isolated and shielded.

The second type of effects are dynamic ones. A return path is only necessary when there are no suitable charge reservoirs. If you make current flow and the loop is broken, then charges collect at the ends of the wires. How much charge depends on the capacity which is generally very tiny. A large capacitor has no current path between the terminals. But if you put one end on the power line and the other one in your hand you get zapped nonetheless. The charges flow in and out of the charge reservoirs at the line frequency (50 or 60 times a second) producing enough current to zap you.

In thunderstorms charges get separated by friction effects. Areas kilometers wide get charged up like large capacitors. When these charges find a path of low resistance they recombine in a powerful current.

 

[Odlaw]

Ok now we are making things a bit complicated. Why do we Earth at all? For short distances your thoughts are correct, but for large distances things turn ugly. Earth is not such a great conductor. Through charges coming from weather effects a line of a couple of kilometers can charge up to fairly high voltages. To remedy this you want to give the system a point of zero voltage. Floating lines mean that you could potentially be zapped by both lines. Unless you would get them perfectly isolated and shielded.

Hi 0XDEADBEEF, sorry to be such a simpleton but to make sure i understand, in order give a system a point of zero reference (such as earth), you must ground both ends of the system (including for large distances)? I am assuming here perfect conditions (i.e. perfect isolation, not charge buildup due to weather effects, no s/c etc...). Now with these perfect conditions in mind, if the generating end is not grounded, touching live to ground should have no effect at all?

A return path is only necessary when there are no suitable charge reservoirs. If you make current flow and the loop is broken, then charges collect at the ends of the wires. How much charge depends on the capacity which is generally very tiny. A large capacitor has no current path between the terminals. But if you put one end on the power line and the other one in your hand you get zapped nonetheless. The charges flow in and out of the charge reservoirs at the line frequency (50 or 60 times a second) producing enough current to zap you.

I understand how a capacitor works but I'm not sure if i understand how it relates to the question of grounding both ends? Do you mean that the ground path between the two grounded ends acts as a capacitor?

 

[sophiecentaur]

The point of having an Earth is that, if a fault in equipment provides a path to an exposed part from the live side then the exposed part will be maintained at zero potential - protecting you from getting a shock.

The Earth only needs to operate 'locally' as that the only thing that counts is the PD between you and the metal that you happen to be touching. Keep that at zero and you are safe. Most supply systems have a Live supply wire, which is at the nominal supply volts, and a Neutral wire. This is held near Earth potential with a connection to an Earth, somewhere in the distribution system. Depending on the details of the supply, the conductivity of the soil, whether or not the supply has its own Earth conductor and the load balance between phases in some distribution systems, the Neutral can be several volts away from Earth.

If the resistance path through the exposed, live, metal part is low enough to pass a high current (and you can't rely on that, always) then a fuse will blow and isolate the problem. Many faults will pass a few hundred milliAmps without blowing a fuse and your equipment will not kill you. However, this is not a good situation because it is requires the Earth to be there constantly. Internal piping can supply an Earth connection and it was common for plumbers to get shocks whist disconnecting pipes and removing the Earth path (it went through them!). Nowadays, all internal metalwork has to be connected to Earth with wires, which avoids the problem.
Residual Current Circuit Breakers take care of this problem by detecting the difference between Live and Neutral currents (to within about 10mA, which won't kill you) and cutting off the supply.

Odlaw, you say you are confused by the apparent difference between the ideas of potential difference and a complete circuit. A complete circuit is necessary for a steady current to flow and a current will flow to Earth from a fault because there is, in fact, a path from the Earth conductor to the Neutral - however tortuous. A possibly lethal current can, however flow, briefly, between two objects which are charged to different (static) potentials yet are not parts of a circuit, until they acquire the same potential. But there needs to be quite a lot of charge - which requires either a high PD or a large capacity, which is not really relevant to situations involving mains (current) electricity, where lots of charge is constantly being supplied by the generators.
Also "taking the easiest path" is an oversimplification of how current flows. The current is always shared between paths - it's just that the majority will flow through through the lowest resistance path. The current will be inversely proportional to the resistances so a 0.5Ohm Earth path will take 200000 times as much current as your 100000Ohm body path.