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Ac Confusion


templeofseven

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templeofseven

So, I'm not an electrician and have been pondering this for some time, unable to find the answer.

 

From physics I learnt there are 3 current types,direct, alternating and static.  However I'm somewhat confused with home wiring and AC.

 

I remember being taught that DC flows in one direction, conventionally from the positive terminal to the negative and that AC flow both ways at a particular frequency usually 50 or 60 cycles per second.

 

I've recently been installing new breakers into my consumer unit along with an RCD as it didn't have one.

 

My question is, why is only the line or live isolated with a breaker and all the neutrals common.

 

Surely if this is true AC then current should flow equally between the line and neutrals back and forth all the way back to the power station isolated along the way by step down transformers.

 

From discussions with electricians at work they believe current only flows on the line in a sinusoidal wave and not back and forth between the live and neutral. And that the neutral is bonded to earth at the customer side of the last transformer on route to your home.

 

In my mind this is just a variable voltage on the line with a voltage of zero to 240. In effect a varying DC voltage.

 

Please help

 

 

 

 

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Andy™

 

From discussions with electricians at work they believe current only flows on the line in a sinusoidal wave and not back and forth between the live and neutral.

 

If thats what they think, then they have no right to call themselves 'electricians'

 

power does flow between live & neutral. neutral is... well... neutral... so doesnt always need to be switched

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Sidewinder

So, I'm not an electrician and have been pondering this for some time, unable to find the answer.

 

From physics I learnt there are 3 current types,direct, alternating and static.  However I'm somewhat confused with home wiring and AC.

 

I remember being taught that DC flows in one direction, conventionally from the positive terminal to the negative and that AC flow both ways at a particular frequency usually 50 or 60 cycles per second.

 

I've recently been installing new breakers into my consumer unit along with an RCD as it didn't have one.

 

My question is, why is only the line or live isolated with a breaker and all the neutrals common.

 

Surely if this is true AC then current should flow equally between the line and neutrals back and forth all the way back to the power station isolated along the way by step down transformers.

 

From discussions with electricians at work they believe current only flows on the line in a sinusoidal wave and not back and forth between the live and neutral. And that the neutral is bonded to earth at the customer side of the last transformer on route to your home.

 

In my mind this is just a variable voltage on the line with a voltage of zero to 240. In effect a varying DC voltage.

 

Please help

 

ac current in a UK distribution system varies between +/- 230V.

It's NOT dc between 0 (earth) voltage & + 230V.

It DOES flow "back and fore" between line & neutral.

Yes, neutral is connected to earth at the local Tx, and several other places possibly, or not depending on the design of the supply network.

 

As Andy says if your "electricians" really believe exactly what you have related in your post, then if you employ them, please sack them all tomorrow as they are incompetent!  :o

 

Remember, neural is a LIVE conductor as it carries the same current as the phase or line conductor.

Thus any current flowing in the line is equal to that in the neutral.

Thus a breaker is only required in the line.

If the circuit is overloaded for some reason disconnecting the line conductor will make the circuit safer than disconnecting the neutral conductor even though both carry the same currents.

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ProDave

The reason we can use single pole switching is because neutral is connected to earth at the substation.  So even by the time it gets to your house, it will be very close to earth potential.  So assuming you are standing on the floor (earth) and you touch neutral, .you won't normally get a shock.

 

Because neutral is "earthed" then L will be alternating between +230V and -230V 50 times  a second (engineers bear with me I know that's not strictly correct) So if you touch that, it's going to hurt. So you have to isolate that before it is safe to touch.

 

If the neutral was not referenced to earth, then you would indeed need 2 pole switching and 2 pole isolation.

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ok officially electrons do move along line conductors towards neutral at about the speed of a snail apparently. Oscillating the energy backward and forwards means you get lots of energy without the heating effect on cables that DC (non-oscillating eenrgy) causes, in which case you would have a very fast snail. It's a bit difficult to get your head around and even harder to explain. This is why we like to use the water flowing through a pipe analogy, here goes:-

 

you have a 2 reservoirs of water, one at a higher level than the other. A pipe at the top reservoir runs off to a water wheel and returns to the bottom reservoir. If you turn on a tap water will flow from the top along the pipe and return to the lower reservoir. Consider the top reservoir as Line (live as we used to call it). When the tap is open it supplies water at a force to turn the wheel. When the tap is closed, the wheel stops, but the return pipe is still full of water, consider the return pipe as neutral. When the wheel is in use the return pipe will have energy flowing through it, dangerous, with the tap shut it still has water in it, but no force behind it, so isn't dangerous anymore..

 

Only trouble with that analogy is it doesn't really explain the AC bit. That's more like a fish tank with a garden spade in the middle. If you move the spade backwards and forwards you can create waves on both sides, the water goes nowhere because it's in a tank (apart from splashing over the sides), but waves can transmit energy, its just a case of having something that converts linear wave energy into something you can use, which in our world is the myriad of electrical appliances we all own around the house. Take the water wheel, convert into a totally contained system with pipe in and out of a single contained tank of water with an anti-backflow valve in the neutral pipe so energy is effectively is only transmitted down the line, and that is about as close as I can get to explaining AC electrics at 11pm when I'm tired.

 

Incidentally if you don't understand electrics, why are you messing with MCBs and RCDs

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steptoe
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apprentice87

Hi all,

 

Some VERY well put explanations there...

 

john...

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