How does earthing work

[johnny37]

How does earthing work? Why are do some appliances like lamps and lawnmowers and radios not have an earth wire?

[Jay70]

Basically if something has an exposed conductive surface it must have a connection to earth. This way if the appliance develops a fault where the outer casing becomes live i.e a short cicuit then the connection to earth will cause an excessive current to be drawn i.e much greater than the devices normal operating current, which should blow the fuse fitten in the devices mains plug. This is why it's important to fit the correct rated fuse in a divice an not just go for the standard supplied 13A fuse unless it's required. Otherwise the fuse wont blow as soon as it should.

An appliance which doesn't have a concuctive outer surface doesn't have an earth as if there is a short from the live supply then there is little risk of a shock and the case wouldn't conduct to cause a fuse to blow. This is known as a 'double insulated' device which is indicated by a symbol which consists of two concentric squares - one inside the other somewhere on the appliance.

This is just a simple explination but hop it helps

[tc573]

Good answer Jay70. But! I think I know where Johnny is coming from. Where in the generating cycle does earth come from? If you have a circuit, live-neutral, where does the earth come from and in the case of a TN_C_S system how come neutral is also earth?

[knobbynonuts]

God thats got me thinking back to me college days now, Well the earth comes literally from the earth hence the word grounded i remember when I was in college learning the tricks of the trade (well maybe not the tircks of the trade you learn them on site)you just learn the theory in college our tutor connected a live wire to one side of a lampholder and buired the other side a few feet into the ground and then switched the power on and guess what we had light allbeight a little dim though (bet they cant do that now through health and safety etc).
as tc573 says in the case of a "TN_C_S" system the earth and neutral are linked together and this is done at the mains head coming into your house from your power company, but in the case of a TNS system a TNS system provides a seperate earth either from an armoured cable coming into your home or worse still a very old lead one (and these are the most 2 commen systems used here) the earth and neutral are linked at the sub station or transformer from your power provider so if I have now made myself as clear as mud then bassicly the earth and the neutral of a circuit are more or less the same but it just depends where they are connected together (and please dont think you can use a neutral as an earth or visa versa in your house as it dont work like that and can be dangerous) to be continued as might think that running out of alloted letters on here now

[knobbynonuts]

Right back again now (yawwwn I know) going back to tc573 there is another electrical system where you just hit spikes into the ground and then atttach an earth wire to them to ground caravans, etc.
Anyway to sum up now to prove my point that how earthing works, at the moment I am working on the new A@E 1.1 billion prodgect at whitechapel hospital and a lot of the surroinding footings are in there now and I can see at least 40 earth wires coming out of them that are at least 2 inches thick and buired in the concrete

[johnny37]

Thank Jay, tc and knobby. Sorry guys, but electricity is not my strong point. This doesnt explain to me why people sometimes kill themsleves. I have this idea in my head that appliances are earthed so that if they become live the current passes back to earth, rather than to the person who is touching the appliance and electrocuting themselves (eg an electric shower). It is not obvious to me why this happens. I have heard of people touching microphones and guitars that were not earthed and electrocuting themselves in this way. I have also been known to touch a live wire and although receiving a shock I am still here to post this query. A friend of mine was drilling a wall and I think he hit a wire (could have been a pipe) and received a serious shock but is still with us. Im not sure if the drill was earthed (I think they only have a live and neutral dont they?) but he has now bought a cordless! Why can't electric lamps and lawnmowers become live?

Words of one syllable please !!!

ps I think this sort of thing should be taught a school instead of useless subjects like rugby and swimming (lol).

[johnny37]

Ive just checked my drill. It does have a 'square in square' symbol. I dont know why my friends drill gave him a shock.

[knobbynonuts]

hi johny there is a fairly easily understandable answer for why people get electric shocks when touching just a single live wire, as you know for a current to flow through something like a light etc it needs 2 wires for it to light up as explained earlier in my college days if you disconect the neutral then the lihgt wont work, but if you were to bury the neutral wire from the light into the ground or put it on an earth wire then it will work,the current needs what you call a return path and that is esentually a neutral or an earth (as said earlier the neutral and earth are the same)
Well what happens when you recieve a shock is if you touch a wire that is live and you are standing on the ground then you will get a shock but if say somehow you could manage to float in mid air then you should not get a shock.
what happens is that the live wire needs to dischage its energy to earth so when you touch it the electric litterly takes the shortest path to earth and the shortest path is directly through your body now there a nasty thought.
Thats why people can work on live overhead high voltage cables as they are insulated from the ground so there is no path to the ground for the electric to flow...

[knobbynonuts]

now to the bit why it can kill first of all its very rare for someone to be killed on 230 volts at home but of course it does happen and its not normally the voltage that kills or seriously injures you but the amps that flow through you but of course if you get shocks of higher voltages like 415 upwards then you are more than likely to fry but I have known a few people to get 415 volt shocks with only fairly minor burns.
Also there are 2 types of electric AC ( alternating current) and DC (direct current), why they are called alternating and direct is because alternating (the most commen one and used in every houshold) does what it says it peaks and flows and litteraly goes up and down like a syne (think thats how you spell it) wave does so if you get a shock of AC current then you have a 99.9% chance of fairly easily pulling your hand of.
Now DC current is a tottaly differant thing you would reconise it as in batteries but it is also widely used in industry in motors etc, direct current also does what it says the current always flows directly and does not peak up and down, so if you were to touch a higher voltage DC live wire then you will not really have that much chance of pulling your hand away you would more than likely just stick to it thats why DC is much more dangerous.
Just one more thing to ad if an appliance has a metal outer casing and the outer casing is not earthed and a live wire comes loose and touches that casing and you happen to touch that casing then it would be like you touching a live wire directly and the current will pass through you to earth until you pull away or the fuse blows but if it was earthed then then if a live wire touches it then it will immediatly blow the fuse or mcb.
Hope the mud is a bit more clearer now......