Difficult electrical questions (well for me, anyway:-)

[lorraine]

Hi everyone,

as you may know. I am in the process of having my house rewired and after I posted, Ban all sheds kindly pointed me to a section on the oDPM website and after much wrangling with my LABC. They are coming round to certify mine and my dad's work!

anyway I have a few questions regarding Alternating Current just to get my head round it.

1. To start off, the live wire carries the phase (sine wave if graphed) and the neutral wire carries the ground (0v). Am I right in thinking that the live is the only one that alternates (hence sine graph)? Giving momentary +230V and -230V?

2. this comes on to my next question. when the live is on +230V does that mean the current flows from the live to the neutral? And when the live is on -230V the current flows from the neutral to the live? Hence if the live is disconnected then the circuit is safe?

3. And my final question is related to the above, I am thinking about changing my old crappy fuse box for a shiny new one with MCB's and was thinking about getting it done at the same time as I rewire (my local electric board keep giving me the runaround). However as my service sealed unit (100A fuse) has no seals?? I was thinking about pulling it myself, is there only 1 fuse (live)? or two (live and neutral). and also any tips you could give me for doing this if I decide

Sorry if these questions sound simple but I really want to get my head around the concept. Hopefully one day I will be as sharp as you lot as I have enrolled at my local college for the City and GUilds electrical installation and plumbing courses

thanks in advance

Lorraine.

 

[andy]

1. To start off, the live wire carries the phase (sine wave if graphed) and the neutral wire carries the ground (0v). Am I right in thinking that the live is the only one that alternates (hence sine graph)? Giving momentary +230V and -230V?

not quite there. there are 2 conductors. they both alternate and there is a voltage between them. RMS value, its 230V. altho they peak to over 300V. both conductors have to alternate so that power can flow. if you had a single phase transformer feeding your supply, they both flow opposite. i.e live flows out and neutral flows in, then the other way.

2. this comes on to my next question. when the live is on +230V does that mean the current flows from the live to the neutral? And when the live is on -230V the current flows from the neutral to the live? Hence if the live is disconnected then the circuit is safe?

AC doesnt have a polarity as such. both go between positive and negative 50 times per second. but neutral is grounded, so you dont get a shock from it. live isnt grounded so you can, which is why the live is always isolated.

3. And my final question is related to the above, I am thinking about changing my old rubishy fuse box for a shiny new one with MCB's and was thinking about getting it done at the same time as I rewire (my local electric board keep giving me the runaround). However as my service sealed unit (100A fuse) has no seals?? I was thinking about pulling it myself, is there only 1 fuse (live)? or two (live and neutral). and also any tips you could give me for doing this if I decide

pulling the service fuse is NOT for beginners/people with no experiance. altho if your wiring the house id imagine that you do have some experiance and can unerstand the risks. there is only 1 fuse. neutral is not fused because if it was, they are both likely to blow and if the neutral goes first, it still stops power flowing out but it doesnt isolate faults circuit (live is still connected to it may appear off, but the risk of shock is still there)

if you do go ahead with removing the fuse, make sure you have the main isolator turned off before touching the fuse (if its on, it could arc and do damage). after you get it out, take the fuse outta the holder and put the holder back in so you cant accidently touch the live terminal

Sorry if these questions sound simple but I really want to get my head around the concept. Hopefully one day I will be as sharp as you lot as I have enrolled at my local college for the City and GUilds electrical installation and plumbing courses

plumbing?! thats only for people who failed all their exams! sparky is what you want to be!

 

[Adam_151]

1. To start off, the live wire carries the phase (sine wave if graphed) and the neutral wire carries the ground (0v). Am I right in thinking that the live is the only one that alternates (hence sine graph)? Giving momentary +230V and -230V?

Volatage is also called potential difference, its the electric potential between two conductors, you can't say that x is at y volts, it would be that x is at y volts with respect to z. In terms of mains electricity we meaure things relative to the physical mass that is the earth on which we stand, and we make neutral 0v by connecting it to it (in reality it might deviate by a few volts), and you are right, the live alternatives between either side of the neutral in a sine curve (because of the nature of electromagnetic induction - the formula has the cosine of the angle between the magnetic flux lines and the coils), but it is actually between +340v, and -340v, 230v is the rms value 340 / root 2 = 230. Live is also called phase, and there are three phases called red, yellow, and blue the sine curves for these are 120 degrees out of sinc, rms phase to phase volatge is 400v, houses only tend to have one of the phases (there will be some houses on red, some on yellow and some on blue) factorys, etc tend to have all three, they balence their circuits between them all, some circuits for things like big motors take all three, it means motors can be smoother running, if phases are perfectly balenced, eg if I put a 3kw on each phase, as it would be perfectly balenced, the neutral (which carries the imbalence) would carry no current. I'll skip power factors and third harmonics for now

2. this comes on to my next question. when the live is on +230V does that mean the current flows from the live to the neutral? And when the live is on -230V the current flows from the neutral to the live?

Tricky one this, technically when the live is above the neutral current flows from the netral to the live and vice verse, which is the opposite way from what you thought. Though people often picture it the way you have in much the way as they picture current flowing from positive to negative on a battery when in reality its the other way round. The incorrect way is called convential current, and it was because physics was wrong for a number of years, this for some reason is still taught in the younger years of school to children, so its not supprising the confussion that surrounds it! Then we have the issue of whether its correct to refer to flow of current, current is a flow of electrons, so flow of current is flow of flow of electrons.

3. And my final question is related to the above, I am thinking about changing my old rubishy fuse box for a shiny new one with MCB's and was thinking about getting it done at the same time as I rewire (my local electric board keep giving me the runaround). However as my service cable has no seals?? I was thinking about pulling it myself, is there only 1 fuse (live)? or two (live and neutral)

I don't think you should be replaceing the fuse box yourself without more knowledge, however when you have a sparky do it, if the service fuse lacks a seal, I'm sure he will be more than happy to just pull it without messing around contacting the electricity company

Sorry if these questions sound simple but I really want to get my head around the concept. Hopefully one day I will be as sharp as you lot as I have enrolled at my local college for the City and GUilds electrical installation and plumbing courses

Good on you, you are obviously eager to learn, its also important to remember that everyone of us is still learning, there are members on this site who know much, much more than me, and I'm sure they are still learning too, I guess they would probably make a better job of explaining these points to you as well

thanks in advance

Lorraine.

 

[pdcelec]

and there are three phases called red, yellow, and blue
Lorraine.

The harmonised colours are Brown Black Grey. L1 L2 L3 is my prefered method since it can be used for both colour schemes

 

[Adam_151]

The harmonised colours are Brown Black Grey. L1 L2 L3 is my prefered method since it can be used for both colour schemes

I'm alright explaining it like this until april next year

 

[Steve]

2. this comes on to my next question. when the live is on +230V does that mean the current flows from the live to the neutral? And when the live is on -230V the current flows from the neutral to the live?

Tricky one this, technically when the live is above the neutral current flows from the netral to the live and vice verse, which is the opposite way from what you thought. Though people often picture it the way you have in much the way as they picture current flowing from positive to negative on a battery when in reality its the other way round. The incorrect way is called convential current, and it was because physics was wrong for a number of years, this for some reason is still taught in the younger years of school to children, so its not supprising the confussion that surrounds it! Then we have the issue of whether its correct to refer to flow of current, current is a flow of electrons, so flow of current is flow of flow of electrons.

I will fill in where you left off, adam :

electrons have a negative charge, therefore flow from negative to positive (opposite charges attract).

Therefore the actual flow of electrons in a circuit is from neutral to live (DC circuits), and in a battery circuit, negative to positive.

But dont confuse this with "conventional" thinking and start connecting live wires to neutral terminals etc live is still positive and neutral is still negative.

Perhaps the conventional thinking is still used to teach children because it is easier for them to grasp, without going into too much detail, and it still teaches them the dangers of touching live conductors.

 

[slippyr4]

factorys, etc tend to have all three, they balence their circuits between them all, some circuits for things like big motors take all three, it means motors can be smoother running, if phases are perfectly balenced, eg if I put a 3kw on each phase, as it would be perfectly balenced, the neutral (which carries the imbalence) would carry no current

It's not so much about "balance".

Since Power is proportional to the square of voltage, then in a single phase supply the instantaneous power varies with the phase.

With 3 phases, your available power (using all 3) remains perfectly constant.

High power demands may need 3 phases for continuous power delivery.

 

[Qedelec]

Lorraine, may I applogise for my fellow sparkies.
I am sure you did not expect to be taken through the classroom on a forum such as this.
By the way, glad to see the fairer sex taking on technical roles.

 

[ban-all-sheds]

By the way, glad to see the fairer sex taking on technical roles.

Maybe not - remember Barbara the taxi driver?

 

[Qedelec]

Pink fluffy cables with tassles??

 

[ban-all-sheds]

Don't think that was Barbara's image...

 

[lorraine]

Hi everyone

thanks for the replies.

I just have one further question about Root mean square.

Does RMS mean that if I integrate the area of the sine wave (I know the total is zero) but where it is positive and divide by the time limit then I get 230V.

Basically if I can do maths on here RMS = Integral (sin(x) between limits of say 0sec and 1/100th sec divide by 1/100th sec

thanks again

Lorraine

 

[andy]

Hi everyone

thanks for the replies.

I just have one further question about Root mean square.

Does RMS mean that if I integrate the area of the sine wave (I know the total is zero) but where it is positive and divide by the time limit then I get 230V.

Basically if I can do maths on here RMS = Integral (sin(x) between limits of say 0sec and 1/100th sec divide by 1/100th sec

thanks again

Lorraine

to get RMS, times the peak value by 0.7671

 

[pdcelec]

In electrical engineering the "x" value is omega t
where omega = 314 radians per second or
2 pi f where t is time in seconds and f is frequency

the general expression for an alternating voltage is given by

v=Vmax sin(omega t +/- theta)

Andrew do you mean 7071?

 

[andy]

In electrical engineering the "x" value is omega t
where omega = 314 radians per second or
2 pi f where t is time in seconds and f is frequency

the general expression for an alternating voltage is given by

v=Vmax sin(omega t +/- theta)

Andrew do you mean 7071?

possibly, altho i was told at college it was 7671 (altho my lecturer does make a lot of mistakes... i.e 'its impossible for a light to work if you switch the neutral')