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AC Electric Neutral Current Flow

by mjcguest
Tags: alternating, current, electric, ground, neutral
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mjcguest
#1
Jun25-13, 07:23 AM
P: 22
Hi everyone

Can someone point me in the right direction here please. Happy to do the digging, but so far I can't find anything that helps!

In an AC alternating-current electrical circuit, the Neutral wire is ultimately connected to "mother earth" / "big stake the ground" - sometimes at the household (e.g. UK), sometimes at a distribution point (e.g. europe). I've used "mother earth" as this is NOT a question about the "earth" wire you find in a plug...!

So... Into my simplistic circuit (let's say from the left) comes my Live 240v / 120v wire, goes into my appliance (toaster / light bulb), out from the right of the appliance comes a Neutral wire that somewhere way over to the right ends up buried 6 feet in the ground.

So far so good. The bit that is doing my noggin in, is the neutral line has a zero voltage. I can not get my head around the How & the Why?
- How can I have 240v going into a light bulb and 0v coming out?
- Why does the neutral need to be grounded if it's Zero volts (what part DOES it play in the circuit?)

I know I've picked up a wrong concept in here somewhere... any help in finding it would be most appreciated!

Thanks
Matt
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nasu
#2
Jun25-13, 08:12 AM
P: 1,969
Yes, the voltage is the concept that you picked somehow wrong.
It does not even make sense to say "240 V going into" and so on.
The voltage does not circulate or move through the circuit.

In order to have a current through your load you need to have a potential difference across this load. So one "end" of the load must have a higher voltage than the other.
The actual values associated with each end are quite arbitrary. May be 240 and 0 or 120 and -120 etc. It depends on the reference.
A common convention is to attribute zero to the ground or earth potential, in problems with circuits.

The analogy with water flow was mentioned so many times...
CWatters
#3
Jun26-13, 04:07 AM
P: 3,092
What nasu said.

How can I have 240v going into a light bulb and 0v coming out
I know people use the expression "you have 240V going into...." but it's best to think of is as..

Current goes through things.
Voltages appear across things.

The power station produces 240V between Live and Neutral. The load is connected to Live and Neutral so 240V appears across the load.


Aside: At the risk of confusing you... Not all systems connect one of the power wires to earth. In the UK house builders use 110V power tools on building sites for safety reasons. A transformer is used to generate 110V from 240V. The transformer has two wires coming out. If you measure the voltage between them you will see 110V AC. However the transformer also has a centre tap which is connected to earth. So if you measure between either wire and earth you see 55V AC. The power tools are connected between the two 55V lives so they get 110V AC. However Bob the Builder (who walks around with bare wet feet) will only get a 55V shock if he touches either wire. His feet are at earth, his hand at 55V. To get a 110V shock he would have to touch both wires which is unlikely.


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