Basic question about AC and its polarity

[ravi.2k17]

hi all,

I have a basic question in electricity.

when there is no polarity in Alternating current then what is the idea of designating phase and neutral wires in AC?
as i know from sine curve AC runs bidirectional from phase side to neutral and then back.

thanks

 

[c_mitra]

You are correct. If you do not have access to any other reference (e.g., a ground potential), the designation of phase and neutral can seem to be arbitrary. On the other hand, there is a polarity in the AC system; it just changes over time at a fixed frequency (say at 50 Hz).

If you can access ground (potential; e.g., for most of us living close to the surface of the earth), you can easily establish the identity of neutral and phase without a problem. This is simply because we can measure only the potential difference and not the actual potential.

It is safer to say that even in AC circuits, current always flows from the higher potential to the lower one.

 

[ravi.2k17]

need more clarification.
>>>>how we designate phase and neutral statically when it keeps on changing ?

you can easily establish the identity of neutral and phase without a problem.
>>> how can we establish the identity of phase and neutral when ground comes into picture?

is static designation of phase and neutral cables running in our home is just a jargon used by various
electricity distribution companies? or it has any scientific significance.

 

[crutschow]

what is the idea of designating phase and neutral wires in AC?

The designation shows which AC wire is at earth ground potential (neutral).
It has not other meaning.

It you measure the voltage from each wire to the earth (safety) ground, the neutral wire will have no significant voltage.

 

[c_mitra]

need more clarification.
>>>>how we designate phase and neutral statically when it keeps on changing ?

What keeps on changing is polarity; the sign of the relative potential difference between the two wires. For a current to flow, you will need two wires (but the direction of the current too will change with the change in polarity).

If you have access to ground (a reference potential), the neutral will have practically zero voltage with respect to the ground. This defines the neutral wire.

With respect to the ground, the potential of the phase wire (simply called line in a single phase circuit) will be a sinusoidal voltage with a fixed (50/60 Hz) frequency. That defines the phase or line wire.

If you do not have access to the ground potential, the designation of line and neutral can be arbitrary.

you can easily establish the identity of neutral and phase without a problem.
>>> how can we establish the identity of phase and neutral when ground comes into picture?

You measure the relative potential of the neutral and phase with respect to the ground.

is static designation of phase and neutral cables running in our home is just a jargon used by various
electricity distribution companies? or it has any scientific significance.

It is not a jargon; it has scientific and engineering significance.

For domestic users, most of the power comes in single phase circuit (one line and one neutral plus one ground connection for safety). For industrial users, the power is delivered in three phase plus one neutral with a local ground connection.

 

[BradtheRad]

The hot wire makes itself known if you touch it while you're installing a dishwasher, and at the same time you touch metal plumbing. You get a shock.

However you don't get a shock from the neutral.

It's by convention that we speak of hot and neutral in house wiring.

 

[KlausST]

Hi,

Indeed the "neutral" problem is not stricktly related to AC, one or three phase, sine or any other waveform.
And the "neutral" problem is a safety issue, but no operation issue.

In floating systems there is no (meaningful) "neutral".
****

Similar to "neutral" there is the "GND" problem on PCB/circuit level.
Imagine two 9V batteries connected in series "+" of one battery is connected to "-" of the other battery. Let's call this node "C" for center.
Now there are two other nodes. The "-" of one battery, let's call it "M". And the "+" of the other battery, let's call it "P".

Now you have three options which node you call "GND". (Mind: this mainly is just a naming problem)
1) let "C" be GND. Then you have "+9V" on "P" and "-9V" on "M". A typical symmetric power supply.
2) let "M" be GND. Then you have "+9V" on "C" and "+18V" on "P". A typical series connected supply.
3) let "P" be GND. Then you have "-9V" on "C" and "-18V" on "M".
It never makes any functional difference, all the time the voltages between the three nodes is the same.
"GND" just defines the reference for your voltage measurements = to which node you connect the black wire of your voltmeter.

Klaus