AC theory

[Silly Sausage]

One of my crazy lecturers at uni said one day, 'Go home and impress your girfriends by sticking a neutral on your tongue'

Like a drunken idiot I did it after getting home from the pub. Thank goodness my wiring was in order.

 

[johnboy6083]

Discharge lighting gives a hell of a belt on the neutral!

 

[johnboy6083]

think of a neutral as a return. Your live is the feed, and the neutral is the return. Forget polarity with the sine wave, as you receive power from both halves of the cycle.

The idea of a neutral Is to tie it at as near earth potential as possible, in order to create the highest PD in order that current flows.

 

[Mark42]

Hi La Poste. That’s an intelligent, thoughtful question well worth asking and exploring.

I’m not a professional electrician but I do like to understand how things work.

My understanding is as follows:

The neutral wire is simply a return path, at, as you say, earth potential. When an AC current is in the positive part of its cycle (WRT earth), energy flows TO earth, when in the negative part of its cycle, the energy flows back FROM earth. Back and forth, as you say. From each of these flows, work can be extracted.

So when you touch the live wire you get a shock both times, but not as the AC cycle crosses the zero point. That by the way is why AC switches are easy to build, and your muscles can let go of a live AC conductor.

So, in principle, in a balanced circuit where the neutral wire offers a low impedance path back to earth, touching it has no ill effect, since you and the wire are always at the same potential – therefore no current can flow, and no shock results.

This is all out of my head, not any book, so it may be rubbish, but it’s how I understand it

BTW if you want to think of ‘electrons moving back and forth down a wire’ they go a lot further than a ‘few cm’ in 1/50 of a second! I suggest you look up the speed of light…

 

[Silly Sausage]

BTW if you want to think of ‘electrons moving back and forth down a wire’ they go a lot further than a ‘few cm’ in 1/50 of a second! I suggest you look up the speed of light…

I'm going have to contend that one!

The effect is approx c but the wee eeees don't. Check out electon drift.

 

[Mark42]

But break the neutral wire that you’re holding’s connection with earth and you DO get a shock. The current flows back and forth to and from earth still, but this time through you rather than through the wire. That’s why ‘borrowed neutrals’ are dangerous. Most electricians have I’m sure been surprised at getting a belt off black wires occasionally (that dates me, I know) when working on installations where only one or two single pole fuses have been pulled, rather than a whole installation isolated.

The current is flowing from live, through some switched-on load somewhere and then to earth through you when you break two neutrals in, for example, a ceiling rose. It can be very annoying and cause you to fall off a step ladder. I know this.

A good analogy for AC is to think of it as water in pipes. Someone on here mentioned this in a thread on PV metering, and it was very helpful.

Imagine the current being a flow of liquid in a hosepipe. The generator is a push-pull pump at one end. Some poor bloke is employed to work the pump. He and the pump gets hot. That’s the inefficiency and losses in the generator plant.

The other end of the hose is in a huge swimming pool. You are in the pool. That’s the earth potential. You feel nothing.

Now fit a little turbine halfway along the pipe. It rotates as the water is pumped back and forth. Work is extracted. Bloke has to pump harder. That’s a load.

BTW, bigger-bore pipe, water moves easier, smaller pipe, harder to push a given amount of water through. When the pipe’s too small, only a dribble comes out from the end. That’s volt drop.

Bloke pumps faster, uses more energy and gets tired quicker, but more water flows. That’s high voltage.

You’re in the pool, you’re effectively holding the neutral wire. You feel nothing. Now cut the pipe and point it at your chest. Bloody hell, you’re knocked flat! You’ve just got a shock.

Does that make sense? It does to me but I just made it all up.

And another thing – the whole idea of ‘electrons’ as little lumps of stuff moving about is complete rubbish. It’s only a model of the quantum world that our little primate brains can use to try to get some kind of handle of how things work. I prefer to think about water, I know what that is.

 

[Silly Sausage]

But break the neutral wire that you’re holding’s connection with earth and you DO get a shock.

True, when I did it I got lucky, I think was a spur, I'd be in the mad house or worse by now if it hadn't.


And another thing – the whole idea of ‘electrons’ as little lumps of stuff moving about is complete rubbish. It’s only a model of the quantum world that our little primate brains can use to try to get some kind of handle of how things work. I prefer to think about water, I know what that is.

Electrons are waves...until you look at them!
I've just read 'In Search of Schrodingers Cats', it's all mental!!!

Next book, 'Fabric of the Universe'
I think I need a job digging holes to keep me occupied.

 

[Des 56]

Next book, 'Fabric of the Universe'
I think I need a job digging holes to keep me occupied.

Now we are talking Branes and I aint got the Brains for electrons never mind M theory

 

[Silly Sausage]

Now we are talking Branes and I aint got the Brains for electrons never mind M theory

You're just being modest now!

I need to decide if I read it or not and go off in a perpendicular universe. They might still be on the 1st edition?

 

[Mark42]

Now we are talking Branes and I aint got the Brains for electrons never mind M theory

Yeah, and now Higgs' wotsit hasn't turned up (or has it?) the Standard Model is probably out of the window, we have super-luminary neutrinos flying across Italy by either breaking the laws of physics or going via other dimensions (ie the shortest distance between two points in no longer a straight line) and so it goes on.

All that amazing stuff happening and we're expected to think some trivial, ephemeral, little regulations dreamed up by Brussels bureaucrats really matter in the overall scheme of things? Pah!

BTW Archy, I think Fabric and parts of Cat are superceded now. There was only ONE cat by the way, that's the whole point! (Superposition of states)

 

[Darkwood]

As mentioned already its a big misbelieve that electrons wizz around the circuit and also that current flows positive to negative (DC is negative to positive) ... i read up on quantum theory alot and to explain in simple terms the electrons in A/C dont really move more vibrate left and right where as in DC the migrate at a very slow pace in the order of several meters per minute, what is actually happening is the propergation of the electron field cloud which will be close to the speed of light and its this that does the business end of the circuit, f your interested i could explain how this works as it can get all confusing very quickly but using everyday ref' it can be understood.

 

[Richard Burns]

Just my thoughts on the matter:
A current will flow if there is a potential difference between two points connected by a conductor.
Whilst a current flow is speed of light (ish) it is the transfer of a charge along the conductor, this is not the same as movement of electrons,
In a DC circuit you have a positive with a potential difference from the negative so current will flow.

Here you are considering a circuit that is connected from the line supply to the neutral.
In an AC circuit the line conductor has a sinusoidal value of potential difference with respect to the neutral.
The peak voltage is 325V (ish), the actual nominal sign (+ or -) of the potential difference is immaterial it is still a potential difference with relation to our nominal (approximate zero value) of the neutral.
At the start of a cycle there is no potential difference and no current flows, as the potential difference rises up the first peak current increases to maximum when the potential difference reaches 325V, and then starts to decrease as the peak drops back to zero. The potential difference (a difference not a value) then starts to increase in the opposite direction and current flows in the opposite direction rising to a new maximum at (-ve)325V. Repeat 50 times per second.


Where you have a resistance in a circuit then there is a voltage drop across that resistance that is proportional to the resistance.

If you think of a line and neutral supply connected by cable to a lamp.
There is a very low resistance in the line conductor, a relatively high resistance in the lamp and a very low resistance in the neutral conductor.
The voltage drop from the start of the supply (230V rms) to the end of the neutral (approx 0V) is of course 230V rms.
The voltage drop in the line conductor will be very small (for example say 4V) as the resistance is very low, the voltage received at the lamp will now be 226V rms. Assuming that the neutral conductor is the same csa and length as the line conductor this will also drop 4Vrms, so therefore the lamp will drop 222V rms to give a total volt drop of 230V rms.
This means that on exiting the lamp the voltage is at a maximum 4V, which you will not feel, so you will not get a (noticeable) shock.
If you disconnect the neutral from its connection to earth and hold the neutral cable as it exits the lamp, you are now a very high resistance connected in the circuit (if you are connected to earth in any way). The total volt drop to earth must still be 230V rms, however you will provide the maximum volt drop as the largest resistance in the circuit so at the exit from the lamp it will be much closer to 230V and you can definitely feel this!

 

[Knobhead]

Here’s one for the more intrepid (bonkers) members to try. (I take No responsibility for death or injury if you try it).

View attachment 10351
This system is used in photographic studio flash units. I calculated one of my 1000 joule units could punch out 650V at 400A for < 100[SUP]th[/SUP] second

 

[Andy 7671]

Fascinating read,

electron field cloud..gonna throw that one in at work tomorrow..

Dash..was the cat a ghost or did it live in the end...?

La Poste is now trawling Amazon looking for more books

 

[i=p/u]

la poste maybe opened his lamp and touched the line conductor..... but this shows you how little i knw