avinsinanan said:
I was thinking why a AC plugs have a live a neutal wire. We all know AC currents flow in both directions. So on one cycle current would flow from the live wire through the load and then through neutral back to the source. However on the 2nd half of the cycle current will flow from the terminating side of the source back to the originating side of the source. So theoritically the Neutal wire is suppose to be the live wire sometimes.
I know the questions seems silly but it has always confused me.
There's not really any difference, it's not like positive and negative with DC, it's simply how it's connected up.
Mains electricity is usually three phase, you could distribute it down a street using two wires for each phase (lets call them 1 and 2, 3 and 4, 5 and 6) - so we need six thick (and expensive) wires. To save money, and reduce the number of wires, we could join one of each pair together, so we'll join 2, 4, and 6 - and call it 7. So now, the two wires for each phase become 1 and 7, 3 and 7, 5 and 7 - only four wires instead of six, so we've saved 33% of the wire.
As an added bonus, as the three phases are 120 degrees apart, if the current drawn from each phase is exactly the same no current will flow in the common wire - so we can use a thinner wire for the common, as it only has to pass the difference between the other three wires.
For this reason houses are normally connected to alternative phases, so it balances out reasonably down the street. This now has reduced the costs from 6 thick wires, to three thick wires and one thin wire - a lot of money saved!.
This common thin wire is called the neutral, and the three thick wires are called lives (L1, L2, L3) - if you measure from common to any live, you get 230V (in europe), from live to any other live, you get 440V. The common neutral point is usually connected to earth at the substation, so you shouldn't be able to get a shock from the neutral wire, but you could from the live wire (any of the three), and a worse shock between any two of the lives.
It's common practice for servicing to use an isolation transformer, this is a simple 230V in, 230V out transformer - the resultant output is 230V, but neither live nor neutral - just 230V on two wires. A single phase generator is the same, there's no live or neutral, just a voltage.