And who said mains voltage it 240V LoL

[#1]

Sorry I disagree and your original posting stated

Quote

It's known as equi-potential earthing

Which backs what I have posted.
Edit: We could be disagreeing because of terminology. Earth bonding is necessary because of leakage current to metal work. The source of leakage current is not just confined to faulty appliances.

[#2]

Sounds like it, I thought you were making a wider point about what earthing is for when I read this ...

Quote from: itsme

The main reason for earth bonding is that ...

But to stoke up the argument up a bit:  I've always understood that the equi-potential part of the earthing equation is simply to do with ensuring miscellaneous bits of metalwork are all connected to a known reliable electrical system common earth point, in order to comply with Institute of Electrical Engineers regulations, which require that any metal parts of a building that could become live should are earthed. (Rather than relying on the natural earthing of the building structure and the land it stands upon, which may be of variable quality.)
In a nutshell, it's no longer enough to ensure your appliances are all earthed, you practically have to ensure that your entire building is earthed to the electrical distribution network. 

[#3]

Recently I had a new boiler fitted. The gas board insisted on earth-bonding the gas mains to my electricity distribution board. Seems a bit scary really.

[#4]

It's an interesting topic that I've delved into on another forum. There's a lot of ways of earthing that I never knew about.
Anyway, my house has a 2-cable overhead supply which is of course live and neutral.
We had some "questions" over earthing a few years back - had been the usual clamp on the water main job.
Anyway, it turned out earthing was actually OK but the local "wiremen" came round and just connected a sodding great cable between my earth and the incoming Neutral.
As proved by an earth loop test this was perfect, so I asked for an explanation. They said on the 2-phase supply live was live but neutral was earthed at every alternate telegraph pole by a very big long spike!

[#5]

That'll be the technique that goes by the exciting name of 'Combined Neutral and Earth'. 
Its failure modes require additional precautions like those you've described. They may also have used special co-axial supply cable designed to make sure that the live wire breaks before the neutral wire if your supply cable gets damaged.

[#6]

okrzynska wrote 'Both wires are switching at 50Hz from +240V to -240V' (sorry have not worked out the quote system yet.
Going back to my 'Electro Tech' days if I remeber correctly, power is generated in this country as a 3 phase. For heavy duty machinery, keeping things simple, a 3 phase supply is used allowing connection across the phases. For domestic use,  a connection is made between line and the centre of the 3 phase star as it is known. However, although the nominal 'voltage is 240 (230v) phase to neutral, that is the RMS value. i.e. the equvalent DC voltage. In fact the voltage rises well above 240v but also falls to 0 so the effectve voltage is 240v. If you measure the peak it rises to about 340v peak if I have done my maths right. We use alternating voltage for, debateably, 2 main reasons. Firstly is allows easy conversion from 1 voltage level to another - a transformer - and secondly it gives 100 chances a second to let go if you connect yourself across the potentil. A direct current would cause your muscles to seize locking you to the live thing you are touching.

[#7]

your answer is completely wrong, it is virtually impossible to let go of a live wire.
As has been said - the neutral is at a constant  zero (or nearly) to earth.
The live does go from - to +ve
Three phase is 440 volt

[#8]

Quote from: pierre_pierre

your answer is completely wrong, it is virtually impossible to let go of a live wire.

Not true - if the current causes your muscles to contract, so that contact is broken, it is virtually impossible to hold on to a live wire. 

[#9]

Ooooer briston, some of your post is more than a little misleading.

Quote from: briston

However, although the nominal 'voltage is 240 (230v) phase to neutral, that is the RMS value. i.e. the equvalent DC voltage.

The RMS value is not an equivalent DC voltage. Strictly, it's an average voltage that causes equivalent average power consumption when feeding a purely resistive load.
Apart from that it's nothing like a DC voltage. Its instantaneous voltage is still going from +240 RMS to -240 RMS relative to neutral/earth at 50 times a second for a start. But you are right in mentioning that the peak voltage of each transition of the sine wave is about 340 Volts (+340 to -340 that is). My ancient copy of Scroggie's Foundations of Wireless and Electronics helpfully explains that peak AC voltage is 1.414 times the RMS value.

Quote from: briston

We use alternating voltage for, debateably, 2 main reasons. Firstly is allows easy conversion from 1 voltage level to another - a transformer - and secondly it gives 100 chances a second to let go if you connect yourself across the potentil. A direct current would cause your muscles to seize locking you to the live thing you are touching.

I'd say that whoever told you this was winding you up. There's no way your brain and and muscles would react quickly enough during the millisecond or so around each zero-transition.
We use AC power because it's more efficient to transmit electricity over very long distances at extremely high AC voltages. But as you suggest, using AC does then make it easier the convert this high voltage to lower voltages for consumption. A secondary benefit of AC was to provide a time-standard for time-keeping. It's less important these days because better time-standards are available, but in the past the mains frequency was maintained very accurately at 50 Hz for this purpose. For example, years ago most of the street lamps would have had an electric time-switch that depended on the mains frequency. They use dusk and dawn light sensors these days, I believe.

[#10]

Quote from: itsme

Also BT 'live' is -50v

Yep. It is to increase longevity of their wires.
You may remember from Chemistry lessons something called REDOX (No not for a soapy bathtime experience!), during any chemical reaction there will be some atoms that loose electrons and some that gain electrons, the loss of or 'reduction' in electrons is called oxidation. A REDuction in electrons if OXidation. We commonly call this rusting and corrosion. By charging a wire with 'negative volts' you can reduce the amount of oxidation.
On big metal hulled ships, they do the same thing with a large piece of zinc strapped to the hull called a 'sacrificial anode'.
SW.

[#11]

I think it's more likely, as telephone systems are approximately 100 years old, that they used conventional flow. This is when they throught the electricity flowed from positive to negative. This is also when cars positive terminals use to be attached to the bodywork. It was then discovered the electricity flowed from negative to positive, electron flow. So in the 70's earthing of cars was changed to negative which cause fun and games installing older radios in these cars.

[#12]

Quote from: axisofevil

Quote from: pierre_pierre your answer is completely wrong, it is virtually impossible to let go of a live wire. Not true - if the current causes your muscles to contract, so that contact is broken, it is virtually impossible to hold on to a live wire.

Also with heat hense the reason that you are taught when feeling your way out of a burning building to use the back of your hand.

[#13]

I've always found that DC will hold you but AC will throw you off. Having had numerous shocks from both over the years from the insides of old TVs it seems to be true.

[#14]

Had a couple of shocks from the mains in my more youthful days - but times up one finger and back down another. Never got to find out if I could move my fingers away from the current as each time I instinctively pulled my arm away.
Not too painful in the circumstances - but still not something I'd recommend.

[#15]

Anyone seen "The Green Mile"?