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Question about AC currents and Live and neutral wires.

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avinsinanan

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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.

I know I am totally confusing this and would be grateful if someone could clear this issue for me.

Thanks for your time.

Yours Respectfully
Avin Sinanan
 
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.
 
Phase voltage is 120 or 240 Volts?

Hello,

Thanks for the response above. However I though that the phase to neutral voltage in Europe at a home was 110V or 120V and not 230 or 240V.

However i heard you can connect 120 phase voltages to get 240. Or do tou take a 240V and slip it in 2 to get 120 V ?

Which way is it?

Thanks

Yours Respectfully
Avin Sinanan
 
Nigel, that would be the most complicated answer I have seen in a long time :wink: , well done :D

Re the active/ neutral question: Look at it this way, the current reverses 50 (or 60) times a second, so at one instant it is flowing out, the next instant back in, at the active terminal. You need another terminal to complete a circuit, this is the neutral terminal. That applies to single phase circuits, for 3 phase, see Nigel's answer above.
Klaus
 
Re: Phase voltage is 120 or 240 Volts?

avinsinanan said:
Thanks for the response above. However I though that the phase to neutral voltage in Europe at a home was 110V or 120V and not 230 or 240V.

In the UK the mains used to be 240V, and in mainland Europe it was 220V, but a few years ago all EU countries were standardised at 230V. As far as I know it's never been 110/120V.

230V per phase gives around 440V three phase.

I notice you don't have your location filled in ( I could be getting obsessive about this! ), so I don't know where you are.
 
Re: Phase voltage is 120 or 240 Volts?

Nigel Goodwin said:
I notice you don't have your location filled in ( I could be getting obsessive about this! ), so I don't know where you are.

ROFLMAO :) One of my pet peeves! It's really hard to give an accurate applicable response to some questions without knowing the geographic locale of the person asking the question!

On a world-wide board such as this, that information should almost be mandatory! :twisted:
 
Re: Phase voltage is 120 or 240 Volts?

ChrisP said:
On a world-wide board such as this, that information should almost be mandatory! :twisted:

Bring back hanging :twisted:

NO! - that's too gentle, how about hung, drawn and quartered - with the head stuck on a spike as a reminder 8)
 
No torture ?


To get a simpler understanding of neutral think of it as the center-tap of a transformer.
 
Re: Phase voltage is 120 or 240 Volts?

Nigel Goodwin said:
ChrisP said:
On a world-wide board such as this, that information should almost be mandatory! :twisted:

Bring back hanging :twisted:

NO! - that's too gentle, how about hung, drawn and quartered - with the head stuck on a spike as a reminder 8)

and what send the quarters to the far reaches of the british empire - oh wait we havn't got on any more. hehe
 
Live vs Neutral lines

Bleh, just realized I resurrected an ancient thread. Sorry!

IANAL disclaimer: This is just what I know. Please seek a more reliable source before doing anything reliant on it - 120V is dangerous!

From what I am to understand, the distinction of neutral vs. live stems from the third path electricity can flow through: the ground. Remember that voltage is all differentials, not absolutes. 120V actually means nothing more than the difference between the two lines oscillates with an amplitude of 120V RMS. We can pick any conductor and declare it to be a ground merely by declaring it thus.

In the case of modern wiring, we all declare that the potential at the physical earthen ground is "0V". This is also the potential of the third conductor, if you have a three prong AC connector. Now we could set up the voltages on the two lines to be anything we please, so long as they are 120V RMS apart. For safety reasons, modern wiring declares the "neutral" line to be at the same potential as ground. Thus, if you touched a ground line and a neutral line, or a neutral line and a puddle of water at your feet, nothing would happen because everything is at the same potential (IANAL... no.. nevermind. No laweyer references... just don't do stupid stunts because a forumgoer said you could). Now if you did the same stunt and touched the "live" or "hot" wire by accident, you're going to get 120V RMS coursing through your body and generally have a bad day.

If I recall, this whole setup is actually accomplished by connecting a metal pipe or stake burried into the ground to the "ground" line going through the house. There is then a small conductor joining the "ground" line (and the physical earth) to the "neutral" line from the power station. This sets all three physical entities to the same voltage, and life is good!

This whole setup is important when dealing with metal bodied devices with 2-prong connectors. It would be VERY dangerous to leave the metal body "floating" without connecting it to anything. If a hot wire came loose, it could make contact with the metal body and now the whole thing is at 120V RMS from ground (which you're usually standing on, completing a circuit). For safety, the metal body is connected to the "neutral" line. That way, if a hot wire comes loose and makes contact, it immediately shorts the device out and flips the circuit breaker before killing you. Of course, everyone has to agree what side is neutral, and which is hot, or touching two metal things at the same time could be dangerous. This is why plug have one prong that is wider than the other, and filing down that prong to fit into an old symmetrical outlet is very dangerous (50% of the time)!
 
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Let's try to make this simple if you even can.

The point about "Neutral" is that it's the same potential as ground. It is the reference. It is current carrying usually.

A confusing issue is that residential power is called 120/240 single phase. The term is actually a misnomer. The high voltage that feeds the 240/120 transformer is indeed single phase, but the power being supplied to the house is actually better described as bi-phase.

The secondary of a 120-0-120 Volt transformer, the 0 point or the center tap is tied to both ground and neutral at one point only. Neutral can carry currents. Ground carries fault current and acts as a reference.

In a 240 4-wire outlet, you have L1, L2, N and Ground. If the load is purely 240 V, there will be no neutral current.

If the load on L1 is 10 A and the load on L2 is 15A, then the neutral will carry the difference current.
It's sign can change depending on whether the L1 current is bigger or less than the L2 current.

Neutral is also the same potential as your kitchen faucet and the earth locally around you.
 
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I agree the neutral is just that 'neutral', there isnt sposed to be any voltage present on it, the live voltage during the first half of the cycle is positive with respect to neutral, and then on the second half of the cycle its negative, so current does go both ways.

The neutral is usually tied to earth back at the substation, some countries tie them in the consumer unit.
 
I agree the neutral is just that 'neutral', there isnt sposed to be any voltage present on it, the live voltage during the first half of the cycle is positive with respect to neutral, and then on the second half of the cycle its negative, so current does go both ways.

The neutral is usually tied to earth back at the substation, some countries tie them in the consumer unit.

My understanding is that each leg goes from 0 to roughly +or- 169 Peak. Not +85 Peak to 0 continuing to -85 peak.
 
Yep, +- √2*120 or about +-169 V for 120 VAC.

Where it starts is anyone's guess. It's a continuous 50 or 60 Hz sine wave. To make life easier, we asuume it starts at 0 voltage and 0 current. Throw a switch and the input to say a light bulb could actually start at the peak voltage.
 
The system might be different overseas, in the uk neutral is sposed to be zero volts, in the plant where I work neutral varies, when all the heaters on our extrusion machine are on because they are 240v and use the neutral, and take a few hundred amps, there is a voltage on neutral, but its only because of the resistive volt drop on the cables from the substation.
 
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