# How does ac current work?



## RogerS (11 Mar 2008)

This question was asked on AsktheTrades and when I start to think about it the more confused I get and can't quite see how it does work.

In dc there is a clearly defined PLUS and MINUS and so there is a clearly defined potential difference and current flows.

But in ac, sometimes the voltage is UP and sometimes it is DOWN but on balance the two balance each other out....so how does ac work? 

Is it an urban myth? 

Bit like why aeroplanes actually fly...mass belief that it is actually possible?


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## Harbo (11 Mar 2008)

Here's a link:-

http://www.allaboutcircuits.com/vol_2/chpt_1/1.html


Rod


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## CHJ (11 Mar 2008)

RogerS":ywvlyzp3 said:


> But in ac, sometimes the voltage is UP and sometimes it is DOWN but on balance the two balance each other out....so how does ac work?



The same way that a pump handle delivers water or your lungs pump air, it is reciprocating work done over a period of time.

To confuse you even further, all common mechanical DC generators (as opposed to chemical cells) actually produce AC, it is only the fact that a commutator is used to switch the flow direction that it delivers DC at its output. :lol:


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## tnimble (12 Mar 2008)

RogerS":2g8e22gn said:


> This question was asked on AsktheTrades and when I start to think about it the more confused I get and can't quite see how it does work.
> 
> In dc there is a clearly defined PLUS and MINUS and so there is a clearly defined potential difference and current flows.


No there ain't actually.

That is most likely where the confusion starts. One can compare electricity to air. An canister normally contains an amount of air, the preasure is neutral. One can put more air into the canister, the canister is now under preasure. One could also suck air out of the canister, the canister is now vacum. If we make an small hole in the canister either airs flows into or out of the canister.

This is the same for electricity. Normally a piece of material has a given number of electrons in it. We can put more electrons into the material, the material has an negative charge. We can also pull out electrons, the material has a positive charge. If we put two meterials in contact with eachother electrons flow between the materials the equalise the charge.

If we generate electricity we pull electrons out from material A and force them into material B. If we connted both material to gether again through for instance a light bulb the electrons flow back from material B into A untill both materials have back their normal amount of electrons. The flow of elektrons cause the lightbulb to emit light.

The most common source for a DC charge is a batery. A chemical reaction causes a constant amount of electrons to be moved from A to B. When a load is connected to the batery the chemical reaction will move more eletrons from A to B until the chemical reaction ceases.



> But in ac, sometimes the voltage is UP and sometimes it is DOWN but on balance the two balance each other out....so how does ac work?
> 
> Is it an urban myth?
> 
> Bit like why aeroplanes actually fly...mass belief that it is actually possible?



With AC a moving magnet is used to start to transfer electrons from A to B for a while. The charge (of preasure if you think of air) gets higher and higher upto a point the magnet change directions and electrons are moved from B into A again. The charge (or presuare) lowers. It lower that much the oposite occures. This repeats over and over.

For a light bulb it does not matter which way electrons flow. Only the amount of electrons that flow per second matter. Like wise with an air line. To the tube is does not matter is air flow through it from one end to the other the tube gets heated a bit for both directions.

The more electrons per second flow through the bulb the more light is will produce. When about 6.240.000.000 billion electrons flow through a bulb per second we say that there flows an electrical current of 1 Ampere or 1 Coulumb per Second.


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## Big Fat Pig (15 Mar 2008)

Aeroplanes really do fly, it`s not an urban myth. Perhaps you`re thinking of an urban moth ?

So, how do planes fly then ?
Well, the shape of the wings causes a low area of pressure on the upper surface when it moves forward, and the higher pressure underneath pushes the wing up into the low pressure area.
Or to put it another way. The upper surface of the wing is curved so that when it moves forward, the air passing over it has further to travel before it meets up with the air passing beneath the wing at the back edge. This stretches the air molecules apart , ie creates low pressure and the wing is effectively sucked up to try and fill this area of low pressure.
Some wings use different methods of creating low pressure on the upper surface, such as the wings on Concorde for example, but the principle is the same.
And some aircraft rely on this form of lift in a very small way, instead they use their own speed to force the air flowing against the underside of the wing to push the plane up.( like when you put your hand out of a car window and tilt it to go up and down).
Anyone still awake ?
If you want to see the principle in action, take 2 pieces of paper and hold them at one end about 2 inches apart. Now blow down between them .You think they will move apart ? No, they move together. 
I remember this from a Ladybird book i had as a boy.

Want to know how jet engines work ?

ok, 

I`ll get my coat.





Piggy


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## Anonymous (15 Mar 2008)

Roger

Forget analogy's

AC is simply a voltage whose amplitude varies between and maximum and minimum value with time. It crosses through zero volts every half cycle - 1 full cycle is the voltage going positive to peak, through zero and down to a negative peak, then back to zero. UK mains does ths 50 times per second.

It's very simple. AC is usually a sinusoid and described thus:

Vrms = A*sin((2*pi*F *T) + phi) Phi is the phase shift and can be forgotten for now as it is not relevant here

Where:
A is peak amplitude (maximum and minimum voltages)
Vrms is the root mean squared voltage or the one we talk about, so 240V for mains. To find the peak from the RMS, simply use Vrms = Vpeak/(square root of 2). This gives around 340 volts for the maximum voltage for 240Vrms mains.

F is the frequency - 50Hz for mains in UK (60 in US)
T is the time over which we are interested.

So for the mains we get an instantaneous voltage by selecting a time (T) and inserting it into the equation with 50Hz in it:

Vrms = 340*sin(2*3.14159*50*T)

I have knocked up a spreadsheet that plots many instantaneous voltages over time for you to play with here
and the output shows and AC voltage over time is shown below witht the verital axis beiing instantaneous volts ans the horizontal axis time in seconds (actually mS - first point is 0.1mS ):


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## Anonymous (15 Mar 2008)

RogerS":xwsquv1q said:


> But in ac, sometimes the voltage is UP and sometimes it is DOWN but on balance the two balance each other out....so how does ac work?
> 
> Is it an urban myth?



See the post above

The graph shows the voltage following a sinusoidal curve between a maxima and minima. 

If you think about one full cycle, the wave is positive for half the time (0 to maximum to 0) and negative for the second half (0 to minimum to 0), so on average they cancel out. 

However, energy is delivered into a load at any time when the voltage is non zero, so work is done and a machine runs.


When we touch it, we feel the changing instantaneous voltages, not the average and it hurts!!

No urban myth


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## cutting42 (15 Mar 2008)

Tony":ry7uafeh said:


> Roger
> 
> Forget analogy's
> 
> ...




I thought the analogies were extremely well written and clearly explained the concept of AC to anyone without a flair for maths.


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## Waka (15 Mar 2008)

piggy":3tkjeg4l said:


> Want to know how jet engines work ?
> 
> ok,
> 
> ...


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## Anonymous (15 Mar 2008)

cutting42":324c5t7v said:


> Tony":324c5t7v said:
> 
> 
> > Roger
> ...




I don't completely agree. Although well written, I do not agree that they neccesarily clarifed what AC is, and may have made it less clear to some

Analogies don't actually convey what is really happening ( I did not understand the air and canister analogy despite having known AC theory for 28 years), they just convey a conceptual notion. I have nothing against analogies and use then all the time at work, but comprehensive understanding requires more, and Roger asked what AC electricity is, which my post answers.

The maths is pre GCSE level (my daughter has already studied this at age 13) and I would expect most (all) members can follow it fine as they probably already use basic trig in their hobby


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## Fecn (15 Mar 2008)

Tony":35wlszul said:


>



Aside from all the maths and analogies, think about how generators are actually built - it's all coils and magnets. Generalising somewhat, generators work by using some form of mechanical rotational energy (steam, water, wind etc) to spin a coil of wire between a couple of magnets. When a wire moves through a magnetic field a voltage is induced in the wire. The stronger the magnetic field, the more voltage.

Here's a diagram of about the most basic generator imagniable.





The AC sine wave is created as the coil spin between the two different magnetic poles. Magnetic North creates a voltage in one direction and south creates the opposite flow. 





This is the reason we actually have AC.

The reason we have DC is because of electro-chemical reactions (e.g. batteries) which produce all their power in one direction.

BTW - I agree with Tony - The maths really isn't very hard.


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## cutting42 (15 Mar 2008)

Tony":cq2vyrej said:


> I don't completely agree.



Excellent, the foundation of all debate 

I am not denying the basic level of maths here, I am a lapsed IEEE member and studied electronics and thereby some reasonably complex maths to degree level so I understand your explanation perfectly. However, if I am placed in a position of explaining anything with a degree of technicality in it to someone else I will always use analogies (along with their inherent inaccuracies) to describe these processes because it uses common knowledge examples without needing any prior technical aptitude. I would tend not to assume any maths experience however basic it seems to me as many people find it difficult to deal with equations and anything beyond basic +-*/.

There is also a big difference between a 13 year old in full time education and a hobbyist woodworker (and I am going to generalize horribly here) usually some time out of using maths from their O level/GCSE/ School cert (delete as appropriate) or even further education. I am (only) 41 and used to juggle complex numbers and laplace transform for tea and have to think hard about working out my tax return now.

Anyway, as I am in danger of seriously digressing now, in short I liked the air cannister concept, thought it suitable for a non maths savvy audience but do accept it was fairly (intentionally) dumbed down and did not truly explain AC.


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## Losos (16 Mar 2008)

cutting42":3o4a78dj said:


> in short I liked the air cannister concept, thought it suitable for a non maths savvy audience but do accept it was fairly (intentionally) dumbed down and did not truly explain AC.



Gareth, have to agree with you here - I didn't quite make it to IEE status but used to deal with fairly high level maths stuff but nowadays prefer to keep it simple and luckily SWMBO does my tacx return for me :roll: 

There is a place for mathmatical analysis as Tony has shown, there is also a place for analogies.

For someone new to the subject reading this thread should provide them with plenty of food for thought and IMHO a clearer understanding at the end of it than they had before.

Wheteher they choose the analogy or the maths is up to them.


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## Anonymous (16 Mar 2008)

Lovely picture Fecn. That explains it perfectly  









Interesitng that we have a couple more IEE and IEEE members. - should we develop our own handshake for the bashes ? :lol:


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## RogerS (16 Mar 2008)

Tony":25h1kt0c said:


> .....
> 
> Interesitng that we have a couple more IEE and IEEE members. - should we develop our own handshake for the bashes ? :lol:



Something along the lines of 'male' and 'female' sprang immediately to mind...as in plugs and sockets...shortly followed by wondering if the PC brigade had outlawed male and female connectors or whether we couldn't refer to a 'master-slave' arrangement anymore.

Some companies DO carry this to the extreme. I sent an email to a colleague working for a French company and made the innocent sounding comment about seeing a 'chink in the armour'. This was immediately flagged up by their fascistic email monitoring software and he was quietly castigated. Talk about red rag to a bull. We then spent the rest of the morning concocting spurious emails and sending them to him. I do remember that 'everything was spick and span' but that 'there was a nasty nip in the air' that morning.


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## Anonymous (17 Mar 2008)

RogerS":2l6nj799 said:


> Tony":2l6nj799 said:
> 
> 
> > .....
> ...



:lol: 

Instittue of Electical Engineers (UK based) - recently became the IET, Institue of Engineering Technology

Institute of Electrical and Electronic Engineers (US based)


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## Leebrainjitsu (17 Mar 2008)

Hi all, this is my first post on here but I did post the original question on Ask the trades.
To simplify the problem that myself and a few others seem to have with AC theory, please allow me to try. 
If you look at the diagrams of the generator on the previous page you will see that an AC current is formed that goes around a sort of ring backwards and forwards ( the voltage, current, amplitude, frequency etc are unimportant for this). Somehow this backward/forward + - V ends up, say a hundred miles away as current travelling one way, e.g. from the consumer unit via the light switch through the light and back to the ground via 0V neutral.
Another way of looking at it is the power lines which carry the three phases but in one direction (where is the other side of the ring?).
Sorry if I've gone a bit deep but I can't seem to accept that the books just totally ignore this aspect.


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## cutting42 (17 Mar 2008)

Leebrainjitsu":1tzi8qmc said:


> Hi all, this is my first post on here but I did post the original question on Ask the trades.
> To simplify the problem that myself and a few others seem to have with AC theory, please allow me to try.
> If you look at the diagrams of the generator on the previous page you will see that an AC current is formed that goes around a sort of ring backwards and forwards ( the voltage, current, amplitude, frequency etc are unimportant for this). Somehow this backward/forward + - V ends up, say a hundred miles away as current travelling one way, e.g. from the consumer unit via the light switch through the light and back to the ground via 0V neutral.
> Another way of looking at it is the power lines which carry the three phases but in one direction (where is the other side of the ring?).
> Sorry if I've gone a bit deep but I can't seem to accept that the books just totally ignore this aspect.



Hi There

You might find it easiest to forget about positive and negative in terms of AC as the negative part of the cycle does not behave quite the same as the negative on a battery (DC)

I am going to use the terms Live and Return instead of positive and negative which might help. On a DC source the positive is the Live and the Negative is the Return. In AC the whole positive and negative cycle is Live and the return is actually the earth (literally) - usually termed neutral - which is why the power lines only appear to carry in one direction the return is through the ground. If you have every used the old fashioned electrical screwdriver with a bulb in it to test for the the presence of mains current, all you had to do was touch the business end of the screwdiver onto the Live terminal and press your finger on the handle end of the screwdriver which had a neon bulb and a resistor in it and making the circuit to ground would light up the screwdriver. - NB please do not use these as if the resistor has shorted you will get a full blown 240VAC pulsing through you at 50 Hz - most unpleasant.

The current is often referred to a "potential" which is a nice real world word to use as it refers to the work that can "potentially" be done by the current regardless of pos/neg status.

Lightbulbs do not really care about AC or DC they will work just fine whatever direction the current hits them or if is is changing (AC) or not (DC) but most home equipment and electronics do care so the AC is turned into DC by the use of DC Power Supply Units (PSU)which transform the voltage down to 5-12 volts or so and then turn the pos/neg switching into a smoother more DC like supply. Depending on the criticality of the application this can be very crude just using a few Diodes (devices that only let current flow one way) to sophisticated solid state circuitry to smooth and regulate voltage very accurately. The output on these DC PSU's normally has a positive and a negative just like a battery and work the same way as well. 

I have no idea if I have only confused more or missed the point but I hope it helps.


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## Woody Alan (17 Mar 2008)

This is quite interesting, when references are made to DC as if it is in any way relevant to AC. When people refer to DC it's always e.g. +12 and -12v but if it's put correctly it's whatever the volatage is in respect of the 0v or ground or common. 
Cars used to be -12v with respect to chassis of 0v, which was referred to sort of incorrectly as positive earth. This changed when it was realised this configuration caused more corrosion due to electrolytic action and the common +12v with 0v earth took over. both systems had a zero volt rail it was only the way the battery was conected that determined the potential of the opposite to 0v. We have many systems which work with 12- with 0v centre and 12+ so the potential across 12- and 12+ is 24 v but earth or commom is central to both.
My point really is try not to think of DC as a simple reference for AC theory because DC is more complicated than a battery.
I am also well aware that my explanation may be flawed.

Alan


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## Leebrainjitsu (17 Mar 2008)

Cutting 42 (Gareth)

Thanks very much for that sir, that explains a lot to me about that basic bit of + - live neutral etc. 
I don't know if it is just me but you would have thought that some of these electrical installation/physics books or websites would address the problem and explain it in similar terms to how you have done.

Once again, thank you.


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## frugal (18 Mar 2008)

cutting42":3v8yz7fe said:


> I am (only) 41 and used to juggle complex numbers and laplace transform for tea and have to think hard about working out my tax return now.



Tax returns are way harder than Applied Maths. Maths has nice understandable algorithms for calculating the result. The tax return is a lot more... nodeterministic


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## Digit (18 Mar 2008)

Having clarified that Roy now steps in to muddy the waters.  
Many, many years ago the Neutral line was not 0v, but 'floated', usually about 10 volts above true earth. Didn't take too long before consumers, (insulting word that, makes us all sound like Locusts), realised that a 10v-240v transformer could give usable amounts of power without it recording on their meter! 
The companies retaliated by grounding the neutral line! 
My works manager, finding that there was no Neutral line in a machine control cabinet to give a single phase line, simply returned the socket's neutral pin to earth! Boom! Boom! 

Roy.


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## tnimble (18 Mar 2008)

cutting42":xs66f7cv said:


> Hi There
> 
> You might find it easiest to forget about positive and negative in terms of AC as the negative part of the cycle does not behave quite the same as the negative on a battery (DC)
> 
> I am going to use the terms Live and Return instead of positive and negative which might help. On a DC source the positive is the Live and the Negative is the Return. In AC the whole positive and negative cycle is Live and the return is actually the earth (literally) - usually termed neutral - which is why the power lines only appear to carry in one direction the return is through the ground.


This is completely wrong. The power lines contain only 3 conductors (and lightning protection) called Phases (coloured brown, black and grey). These 3 lines are both Live and Return. Which one is the Live and who are the 2 Returns varies over time (for the 50Hz net this is 300 times per second).

Nearby the houses, offices, strores and small factories the high voltage is converted into 230 V +10% −6% 50Hz and a fourth Neutral conductor (coloured blue) is added. The voltage between this added Neutral conductor and the other 3 Phase conductors is 230V. The voltage between the 3 Phase conductors is 400V. (Some regions also have 115V and 200V or other combinations)

For safety reasons this fourth Neutral conductor is connected to the Earth (planetoid) at multiple places (mostly at the generator, transformer and junction box). And also for safety a fifth conductor is added at the home (or some cases ar a juction box or the transformaer) (coloured green and yellow) which is also connector to erath (planetoid).

For simple low power appliances one of the Phase conductors and the Neutral conductor is used. For sush applications the Live is the Phase conductor and the Return is the Neutral conductor.

For appliances that require more power the 3 Phase conductors are used. Again those 3 conductor are both Live and Return.

For appliances the require more power but can have an uneven load all four conductors are used. Again the 3 Phase conductors are both Live and Return. The Neutral conductor evens out any imbalance.

The only current that flows through the earth are fault currents (short circuit) and leak currents (isolation of wiring and equipment is not 100% insulating. Some equipment like a PC power supply produce a very small leak current to get rid of electrical interference signals).



> If you have every used the old fashioned electrical screwdriver with a bulb in it to test for the the presence of mains current, all you had to do was touch the business end of the screwdiver onto the Live terminal and press your finger on the handle end of the screwdriver which had a neon bulb and a resistor in it and making the circuit to ground would light up the screwdriver. - NB please do not use these as if the resistor has shorted you will get a full blown 240VAC pulsing through you at 50 Hz - most unpleasant.
> 
> The current is often referred to a "potential" which is a nice real world word to use as it refers to the work that can "potentially" be done by the current regardless of pos/neg status.


Voltage not current.



> Lightbulbs do not really care about AC or DC they will work just fine whatever direction the current hits them or if is is changing (AC) or not (DC) but most home equipment and electronics do care so the AC is turned into DC by the use of DC Power Supply Units (PSU)which transform the voltage down to 5-12 volts or so and then turn the pos/neg switching into a smoother more DC like supply. Depending on the criticality of the application this can be very crude just using a few Diodes (devices that only let current flow one way) to sophisticated solid state circuitry to smooth and regulate voltage very accurately. The output on these DC PSU's normally has a positive and a negative just like a battery and work the same way as well.
> 
> I have no idea if I have only confused more or missed the point but I hope it helps.


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## cutting42 (18 Mar 2008)

tnimble":f02z8ivd said:


> This is completely wrong.



   

I guess I have been hoisted by my own petard here. I was trying to keep it very simple sticking to single phase where my Live Neutral description works and messed up by referencing 3 phase power distribution and tarring it with the same brush which is completely wrong. In my (feeble) defence never did much 3 phase (mostly fiddled around with electronic electrickery - nothing over 12VDC) and did not think about it enough.

Nice description of three phase distribution though tnimble, I remember it now.


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## Bean (18 Mar 2008)

> Instittue of Electical Engineers (UK based) - recently became the IET, Institue of Engineering Technology
> 
> Institute of Electrical and Electronic Engineers (US based)



Join a proper institution, far too much electrickery going on, long live the IMechE !!


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## tnimble (18 Mar 2008)

cutting42":hv4d1m6i said:


> tnimble":hv4d1m6i said:
> 
> 
> > This is completely wrong.
> ...


 All has been forgiven  The given description could be as stated (with the drawback of huge power losses and environmental issues) Poly phase electricity can be hard to comprehend and explain. Your post was very well explanatory though a bit fictional.


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## Anonymous (19 Mar 2008)

tnimble":2t41hsr3 said:


> This is completely wrong. The power lines contain only 3 conductors (and lightning protection) called Phases (coloured brown, black and grey).



Not in the UK where the original poster orignates. The phases here are coloured Red, Yellow, Blue. No lightening conductor, but there is usually an earth and neutral conductor (neutral not required on a straight 3 phase system as tnimble explained).




> Nearby the houses, offices, strores and small factories the high voltage is converted into 230 V +10% −6% 50Hz and a fourth Neutral conductor (coloured blue) is added. The voltage between this added Neutral conductor and the other 3 Phase conductors is 230V.



Black, and 230V RMS (240V RMS really due to the tolerances allowing us to remain at that potential) .



> The voltage between the 3 Phase conductors is 400V.



415V RMS


The neurtal is the point where all three phases are connected together at the generating point (called a star connection) and the vector sum of the potentials is zero at any instant in time at this point (it is not unknown for some phase imbalance to cause neutral voltages of non-zero). 

Potential between any one of these phases and the neutral point is 240V RMS


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## Anonymous (19 Mar 2008)

Bean":f2irpnae said:


> Join a proper institution, far too much electrickery going on, long live the IMechE !!



Absolutely, I'm a member of that one too :wink:  (electromechanical engineer)


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## DavidE (19 Mar 2008)

Tony":2aqfhz2f said:


> Not in the UK where the original poster orignates. The phases here are coloured Red, Yellow, Blue. No lightening conductor, but there is usually an earth and neutral conductor (neutral not required on a straight 3 phase system as tnimble explained).
> 
> 
> Black, and 230V RMS (240V RMS really due to the tolerances allowing us to remain at that potential) .



Tony - a quick point of note...

Our phase colours are sadly now harmonised with the rest of Europe.
Colour changes from the IET

so we do actually have the delightfully inspiring Brown, Black and Grey as our phase colours. Of course there is a lot of installed Red, Yellow and Blue. Rather brilliantly now we have two schemes where if you have a confused moment can get the neutral and phase mixed up! At least on single phase it is not too bad and follows the flex colours that have been around for a while. Thank goodness we were able to keep the yellow/green for the circuit protective conductor (earth).

There are a lot of these labels springing up on panels now:







Cheers
David (another IET member also considering joining the iMechE too!)


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## tnimble (20 Mar 2008)

Tony":qmrmwypw said:


> Not in the UK where the original poster orignates. The phases here are coloured Red, Yellow, Blue.


As DavidE already said this was until 2004. All colours are now the same. Be thankfull you've only two sets or possible colours that can be used in an instalation. We have new, two old and older colours. Luckely only the colours of the second and third phase differ and low power in the new and 2 old colours.



> No lightening conductor, but there is usually an earth and neutral conductor (neutral not required on a straight 3 phase system as tnimble explained).


That applied only to (hv) power lines not the infeed cable of buildings.



> Black, and 230V RMS (240V RMS really due to the tolerances allowing us to remain at that potential) .
> ...
> 415V RMS
> ...
> Potential between any one of these phases and the neutral point is 240V RMS


Pre 2004 and pre 1995. The voltage is 230V -10% +6% 50Hz and 400V -10% +6% 50Hz. The actually measured voltage various for each location and each country within this range. The voltage in the UK would mostly be on the upper side of 230V and 400V. There is a transitional period of 1995-2008 in which all countries must meet these voltages and tolerances and all equipment must be able to hande.


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## Anonymous (20 Mar 2008)

DavidE":aef4nqmt said:


> Tony":aef4nqmt said:
> 
> 
> > Not in the UK where the original poster orignates. The phases here are coloured Red, Yellow, Blue. No lightening conductor, but there is usually an earth and neutral conductor (neutral not required on a straight 3 phase system as tnimble explained).
> ...



Thanks David - forgot all about that  (and never seen an installation using the new colours yet)


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## Anonymous (20 Mar 2008)

tnimble":2g2ti7wo said:


> Pre 2004 and pre 1995. The voltage is 230V -10% +6% 50Hz and 400V -10% +6% 50Hz. The actually measured voltage various for each location and each country within this range. The voltage in the UK would mostly be on the upper side of 230V and 400V. There is a transitional period of 1995-2008 in which all countries must meet these voltages and tolerances and all equipment must be able to hande.



Whilst the notional change of voltage is true, in reality it just did not happen (+10% means it didn't have to) and in the UK it is 240V RMS and 415V RMS at every site where I have measured


Electrical equipment is forgiving enough not to 'care' if the supply is 230 or 240, particularly domestic equipment with switched mode power supplies.


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## tnimble (20 Mar 2008)

Tony":10ud4p9i said:


> Whilst the notional change of voltage is true, in reality it just did not happen (+10% means it didn't have to)


The change did happen from 240V +- 6% to 230 +10% -6% Before 1995 one could measure upto about 250V



> and in the UK it is 240V RMS and 415V RMS at every site where I have measured


Very doubt full the measured voltage peak, peak/peak or RMS is depended on the current net load, the distance to the transformer and even the distanse from the distribution board.


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## Anonymous (20 Mar 2008)

tnimble":1qtggl4x said:


> Very doubt full the measured voltage peak, peak/peak or RMS is depended on the current net load, the distance to the transformer and even the distanse from the distribution board.



Doubt all you want.

Whilst voltage drops exist in long cable runs with high currents drawn, the view you express here sounds like textbook learning rather than real experience to me.

I have been an electrical engineer for over 20 years and have measured the mains at various sites thousands of times with it mostly sitting close to 240V RMS. I always use Fluke true RMS meters as I usually measure much smaller potentials and require high accuracy from my equipment (I work on electronic circuits more than mains)


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## DavidE (20 Mar 2008)

tnimble - Good point on confusion in other countries. I did see a document justifying why they had gone for those colours and remember seeing with the history others had got a rawer deal!



Tony":1ul4myel said:


> Thanks David - forgot all about that  (and never seen an installation using the new colours yet)



No problems I thought it odd you didn't realise. It's easy to forget - I nearly got stung the other week ordering some single cores. Thought about ordering Brown, Black and Yellow/Green. Fortunately, the brain kicked in and realised that I needed to be ordering the blue!

Believe me you've not missed much seeing an installation in the new colours - they don't exactly jump out at you and make you think 415V!

David


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## tnimble (20 Mar 2008)

Tony":3qxcehen said:


> Doubt all you want.
> 
> Whilst voltage drops exist in long cable runs with high currents drawn, the view you express here sounds like textbook learning rather than real experience to me.
> 
> I have been an electrical engineer for over 20 years and have measured the mains at various sites thousands of times with it mostly sitting close to 240V RMS. I always use Fluke true RMS meters as I usually measure much smaller potentials and require high accuracy from my equipment (I work on electronic circuits more than mains)


Dito here but about half the time. The first job was at the regional electrical net maintainer (do'nt excatly know if that the correct english term, but the people who put up and maintain the mid voltage power lines, end transformer and city switch stations etc.)


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## Stoday (21 Mar 2008)

Tony and tnimble are both right.

The Electricity Safty, Quality and Continuity Regulations 2002, clause 27 (2) says:

_ (2) Unless otherwise agreed in writing between the distributor, the supplier and the consumer (and if necessary between the distributor and any other distributor likely to be affected) the frequency declared pursuant to paragraph (1) shall be 50 hertz and the voltage declared in respect of a low voltage supply shall be 230 volts between the phase and neutral conductors at the supply terminals_

And in (3)

_(b) in the case of a low voltage supply, a variation not exceeding 10 per cent above or 6 per cent below the declared voltage at the declared frequency;_

The voltage tolerance used to be plus or minus 6%.

The allowed voltages used to be 226V to 254V and are now 216V to 253V. This means that Distribution Network Operators can allow the supply voltage to fall another 10V before they do anything about it. *It does not mean *that the average voltage is 230V or 240V.

Electricians, of course, would never see the low voltage because that occurs during the winter peak, which occurs around 17.30. Electricians have packed up by that time and are heading for the pub. :lol:


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## Stoday (22 Mar 2008)

All this talk of alternating voltages, currents and electrons coming & going is confusing. 

What matters is power. Power comes down the lines one way. It comes down the live wire. Touch the neutral and you’ll feel nothing. Touch the live and you can be sure power will flow from that wire. A circuit is necessary for electricity to flow around it but power does not flow around a circuit. Power finishes up in the connected appliance and appears as heat or mechanical power.

Electrical ac power comes in pulses, 100 per second in Europe, 120 in the USA. The most efficient way to generate and distribute power is to do so using three lines. The pulses do not occur at the same time in each of the lines; they are separated evenly from each other in time. That’s why they are called ‘phases’. 

If you use all three phases to power your machines or heaters, you don’t need a neutral because current is the same in each of the phases and the current from one phase is balanced out by the current in the opposite direction in the other two.

You can only use one of the phases for many applications. There are no three phase lamps for instance. Single phase requires a neutral to complete the circuit so that electricity can flow around it. If the circuit includes a lamp it is switched on when electricity can flow around the circuit and through the lamp. The lamp can then convert the power from the live wire into a little light and a lot of heat. Note that the current only has to flow; the direction of flow doesn’t matter. If you stop the current from flowing by opening the switch, the lamp can no longer take power from the live wire.


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