AC Circuits: Voltage Changes and Neutral Becoming Live?

[Toffer]

I was just reviewing AC circuits and the diagram shows that the voltage changes in AC,
it goes up and down and on the diagram is shows the current goes one way then the other.

But in AC, does the neutral become live? I thought the red was always
the live wire! but according to the diagram it will flow the opposite way down the neutral.. !

If the polarity inverses:
1. so we should have used a fuse on both wires, as both wires become live
2. We should not have two colored wires..
And how is that possible that the polarity inverses, but only the fuse on the live get burned out, and keeps
the neutral wire Neutral? I'm confused..!

Can someone explain this to me?? thanks in advance..:)

 

[Simon Bridge]

Power to your house is a travelling wave - so the energy arrives at the house on the phase wire and leaves on the neutral. If the dangerous surge arrives, you don't want it ripping through all your electronics before the fuse can blow: so you put the fuse on the incoming wire.

http://en.wikipedia.org/wiki/Transmission_line

 

[Drakkith]

The live AND the neutral wire both have current in them. This is required to have a complete circuit. I think only one fuse is needed because breaking the fuse cuts one leg of the circuit, meaning you no longer have a complete path and no current can flow through either wire.

 

[berkeman]

I was just reviewing AC circuits and the diagram shows that the voltage changes in AC,
it goes up and down and on the diagram is shows the current goes one way then the other.

But in AC, does the neutral become live? I thought the red was always
the live wire! but according to the diagram it will flow the opposite way down the neutral.. !

If the polarity inverses:
1. so we should have used a fuse on both wires, as both wires become live
2. We should not have two colored wires..
And how is that possible that the polarity inverses, but only the fuse on the live get burned out, and keeps
the neutral wire Neutral? I'm confused..!

Can someone explain this to me?? thanks in advance..:)

In the US, by building code, the Neutral wire is Earth grounded at your breaker panel in your house. So the Neutral wires in your house have very little voltage on them (only the IR drop for the return current back to the breaker panel from your lights and appliances).

The fuse is put in the Hot line so that if it blows, there is no dangerous voltage present in your device with respect to Earth ground. If you fused the Neutral line in an appliance, the Hot line would still be connected after the fuse blew, which would present a shock hazard.

 

[Toffer]

Yes, i understood, but what I'm saying is that it's not necessary for the fuse to be in the live, it can be in the neutral, because according to me, voltage inverses in an ac, so, the live becomes neutral, and the neutral becomes the live..
Is it correct what I'm saying? Explain it in details if possible please.. Links and drawings are appreciated..

 

[berkeman]

Yes, i understood, but what I'm saying is that it's not necessary for the fuse to be in the live, it can be in the neutral, because according to me, voltage inverses in an ac, so, the live becomes neutral, and the neutral becomes the live..
Is it correct what I'm saying? Explain it in details if possible please.. Links and drawings are appreciated..

Please read my post above yours (you may not have seen it when you replied). It has to do with the hazardous voltages on Hot (Neutral does not have the hazardous voltages).

 

[Studiot]

Extract from the IEE commentary to the wiring regulations.

If the neutral supply to a water heater with an uninsulated element is lost, current from the phase will return via the water and the earthed metal. Therefore a careful check is necessary to ensure that there is no fuse, breaker or non-linked switch in the neutral conductor

 

[Simon Bridge]

I think only one fuse is needed because breaking the fuse cuts one leg of the circuit, meaning you no longer have a complete path and no current can flow through either wire.

That sort of thinking gets people killed dude!
It's high voltage - it doesn't have to flow through the wire.

The fuse is put in the Hot line so that if it blows, there is no dangerous voltage present in your device with respect to Earth ground. If you fused the Neutral line in an appliance, the Hot line would still be connected after the fuse blew, which would present a shock hazard.

Extract from the IEE commentary to the wiring regulations.

Good find.

If the goal were merely to switch the circuit off then the fuse can be anywhere which breaks the flow. But that is not the goal. The goal is to protect human life and property - in that order.

You got to ask yourself "why did the fuse blow?"
It is because something went wrong.
Putting the fuse in the neutral will leave the, possibly malfunctioning, circuit live when it blows ... does this sound like a good idea?
It is high voltage - it will follow the path of least resistance. If the wire is no longer available, it may find another path through the appliance or a anyone touching it.
A neutral-side fuse does not help protect the appliance from power spikes - the damage has already been done by the time the spike gets to the fuse.

You will notice that we always put the on-off switch on the phase side as well? Now here is a situation where the goal is just to switch the circuit off ... so why not put it on the neutral side?

Much the same reason - when you switch something off you want it to be off.

Lastly, putting the fuse on the phase side means that it is not so important to get it checked and repaired right away should the fuse blow. Whatever went wrong is not getting power anyway.

Now replace "the appliance" and "the circuit" etc. with "your house".
Where do you want the breakers installed?