Purpose of "earth" ground

[iamsand]

I am trying to understand the purpose for earth ground. Imagine a world where the power company never attached the neutral conductor to ground, anywhere, not at the generation source, not along the transmission route and not at the destination. The neutral would be isolated from the earth, the same as the hot conductors are now. In that world, if either a neutral, or hot conductor were to come in contact with a metal equipment case and a person were standing in water with bare feet and touched it, they would not get shocked would they, since there would be no path back to either the neutral or hot side through the earth?

How would the above scenario be any different than the real life situation where I touch a live conductor while having good insulated shoes on? The reason I don't get shocked is that there is no path back to a live conductor. If the earth were not attached to a conductor (neutral) the way it is now, then there would also not be a path back to a live conductor, even if I were standing barefoot on the ground. How is earthing the neutral making things safer? It seems like we would have been better off if they never would have done it.

[Tomi Engdahl]

I am trying to understand the purpose for earth ground. Imagine a world where the power company never attached the neutral conductor to ground, anywhere, not at the generation source, not along the transmission route and not at the destination. The neutral would be isolated from the earth, the same as the hot conductors are now. In that world, if either a neutral, or hot conductor were to come in contact with a metal equipment case and a person were standing in water with bare feet and touched it, they would not get shocked would they, since there would be no path back to either the neutral or hot side through the earth?

This works as you described on small scale systems well.
There are completely isolated electrical distribution systems in use on some application fields, for example in hospitals.

The compete isolated system has some problems:

If there is a single fault somewhere form either power carrying wire to grounded case, the whole system becomes as dangerous as the grounded system without anybody knowing that this happened. So this single fault situation could not be easily detected. When you have lots of equipment on the system, sooner or later there will be a fault sooner or later.
(on grounded system live wire touching a grounded case will immediately burn the fuse, and thus putting system to safe state and tell there was a problem)

When the system is completely isolated the common mode voltage on both power wires can easily float to different values. When you have world where there is static electricity, lightning strikes, power harmonics/resonances, you will end up that there can be cases where there are quite high common voltages on those both power wires. This high voltage can put the insulation on wires, transformers and in equipment on quite high stress (those fail more often than in grounded systems).

If the isolated power system is large, there will be considerable capacitance between the ground and the power carrying wires. This capacitance will be caused by wire capacitances, capacitance on equipment (for example filtering capacitors on power supplies etc..), etc..
When you have a quite large capacitance, the system is no longer completely isolated in the sense that you will not be shocked if you touch one wire. In case you touch one of the power carrying wires, there will be always some shock current that leaks through those capacitances to ground. This current can be significant on large systems, it can shock you.




So completely isolated system will add safety when used on suitable application (small systems). But it has it's limitations and problems, this it is not applicable everywhere. For general electrical distribution application the grounded neutral system is generally better choice for overall system reliabity and safety perspective.

[iamsand]

Thank you for your answer. I would like to ask one question though. You describe at the end of your comment where one would still be in danger of a shock in an isolated system due to "capacitances to ground". Why would the current be seeking "ground" even through capacitance, if there were no connections of the live conductors to ground anywhere? Where is the current seeking? I thought that it is always seeking a return path back to the negative side.

[Tomi Engdahl]

You describe at the end of your comment where one would still be in danger of a shock in an isolated system due to "capacitances to ground". Why would the current be seeking "ground" even through capacitance, if there were no connections of the live conductors to ground anywhere? Where is the current seeking?

The current will go though this circuit:

You touch neutral wire and ground.
There is capacitance between live and ground.

So this forms an electrical circuit that consists of your resistance in series with live-ground leakage capacitance between the live and neutral wire. Here is where the current flows.

The leakage capacitance can be formed of cable capacitance (large especially on cables with shields, installed on grounded metal tubing or underground) and the capacitances in equipment (RF filter capacitors live-ground and neutral-ground).



Some more details
http://www.touchbriefings.com/pdf/13/ho ... fheinz.PDF