Hi,

In an AC 240V supply , there is Live and Neutral .
When we touch the Neutral , we will not get electric shock.

Why？ Is it the electricity company already connect the Neutral to Earth?

Thank you,

 

You don't state where you are located, but generally speaking the neutral wire is connected to earth out on each of the poles lining your street. So the neutral is typically less than 10V above ground potential and that's why you don't get a shock from it.

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Correct. In most English countries the MEN system is used ( Multiple Earth Neutral ) The Neutral and Earth are linked at the point of supply switchboard, distribution transformers and pillars.

In European countries the Neutral and Earth are separated and not linked as above and under certain conditions the Neutral can have a voltage on it.

NOTE !!
In older 127 / 220 Volt systems both poles of the socket are alive with respect to earth

This also applies to the 120/240 Volt American sockets for Ranges , Dryers etc.

For you safety, test first.

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Electricity, Electric clocks, Meters and Trains are great.

 

And in the U.S the 220V is generated from a transformer with a center-tap that is the neutral/ground. You get 220V by going across both hot leads. You get 110V by going from either hot lead to neutral/ground. This is a safety factor since, if you are grounded and touch either lead of a 220V outlet, the voltage will only be 110V.

 

This is a very smart design, I shall consider this in my lab power supply or for those repairman.

If use 1:1, 240V:240V transformer, will it get shock if touch the either lead at the secondary ?

 

With an isolation transformer, you will only get a shock if you touch both of the secondary leads because there's no path to earth.

Here in the UK the earth and neutral share the same conductor untill it enters your house, therefor the neutral is normally pretty close to 0V.

I some areas, like building sites or workshops, a 110VAC reduced voltage supply is provided for powering tools from. This is normally supplied from a transformer which is earthed at the centre tap so the maximum voltage from earth is 55V, although in some cases a three phase transformer is used in which case the maximum voltage from earth is 62.5V.

The neutral is normally earthed to prevent it from floating at any voltage, for example if you connected a 5kV auto-transformer to the mains and the neutral wasn't earthed, if an earth fault occurs on the 5kV output the mains would float at 5kV which would cause insulation breakdown and a fire.

In some cases an isolated system is used (i.e. in hospitals), in which case the use of auto-transformers is not permitted.

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[quote=Hero999]
if an earth fault occurs on the 5kV output the mains would float at 5kV which would cause insulation breakdown and a fire.

What does an earth fault mean ? when neutral has lifted for some reason on the secondary or the primary.

 

An earth fault is when a line conductor is shorted to earth.

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VictorPS

That is true IF the MEN strap is in place at the entry point in Australia (strapped Earth bar to Neutral bar)

I am probably stating the obvious here, but the fact that there is no voltage relative to Earth does not mean it does not carry current. You cannot open the Neutral and expect not to get a shock. The moment you open the neutral there can be line voltage on the neutral side of any appliance that is switched on.

As an aside, the neutral in an office block may carry almost 1.7 times the current that is present in any single phase due to triple N harmonics currents generated by non-linear devices such as SMPS , Fluoros and general sloppy balancing of phasing loads . Early installations (before smps and other computer loads) had a neutral wire that was only as big as the phase wire. I have been in installations where the Neutral Conductor was running quite warm in the cable and switchboards.

I think that modern installations now require a larger diameter Neutral .

The main point here being (use a meter to check before you touch any bare conductor) Especially in dual or triple switched lighting ccts (the convenient ccts where you turn on and off the lights at different switches)

Cheers
RH

 

Agreed, to many deaths caused by ignorance, a licensed sparkie should be engaged to do this work. Besides if you kill yourself and the house burns down, insurance companies will deny any payout on life or properties insured.

You and your family lose everything.

Rebuild the house that's no biggie but once you are declared dead, there is no returning!