Earthing questions

Excuse me, but I think I'll avoid your hotel.

john2

If it's a 13 amp fuse there *will* be a fuse unless someone is trying to apply Darwin's law.

The fuse will have blown.

All consumer units have fuses. etc. If no earth connection to the plug you'll get a shock - and rightly so for being a pratt.

You're body doesn't provide enough resistance to blow a 5 amp fuse so you'll die, Mr bond.

Makes no difference. With no safety earth you'll get a shock.

With a decent short and on a 30 amp ring main the flex would likely fry for a bit before the fuse in the CU went.

Safety earth missing *anywhere* in the chain and you have problems.

Probably the lowest value fuse will blow.

If it's not earthed it will make little difference. If earthed it will almost certainly kill you as the current flow from arm to arm is across the heart.

Firstly, don't make the mistake of believing fuses are there to protect you from a shock. With even a low rating fuse, you could be dead long before it got warm.

Secondly, looking at the scenerio of a "live" metal case and you touching it. If you don't also touch (including standing on something) that completes the circuit you'll get a shock. If you don't complete the circuit you won't.

The principle of "earthing" metal cased items so to ensure there is a low resistance path (other than through you) to complete the circuit.

Brian

Sorry, silly typo in there! If you don't complete the circuit, no shock.

If you do complete the circuit, you will get a shock.

Brian

There must be a fuse somewhere. The fuse is the weakest link in the circuit. If you seriously had no fuse, or no fuse beyond the 100A service fuse, then the flex to the table lamp would be the weakest link and would probably have burst into flames.

Current would flow from the live lamp casing to earth through you. If enough current flowed, you would be dead.

Ditto as 1. If enough current flowed, the 5A fuse would blow. This might be before or after you were dead.

Current would flow from the lamp casing to earth through the circuit protective conductor. If enought current flowed, the CU fuse would blow. This might be before or after you were dead.

Ditto as 5. Plug fuse would blow.

Ditto as 5. Plug or CU fuse would blow.

a) if the water pipe isn't earthed/earthy, wouldn't make a difference.

b) if the water pipe is earthed/earthy, this will lower your resistance to earth and increase the current flowing through you. More importantly, this will case a hand-to-hand current flow across the heart, significantly increasing your liklihood of death.

"Charlie says, don't play with electricity boys and girls. You might get hurt."

Owain

Large snip

I appreciate all the "replies", but can anyone actually "answer" the questions?

Before anyone gets there "knickers in a twist", I do not own a hotel, nor have I not got any fuses in my CU or plugs, I also do not go around sticking my fingers in live sockets. I am just trying to understand how earthing and electrocution works, and thought that if someone could answer the original post it would help to clairify a few grey areas.

TIA

Jon

That's assuming you are hanging on to the lamp when the short occurs. The 'what ifs' are getting more far fetched.

On Wed, 03 May 2006 13:06:18 GMT someone who may be "jon" wrote this:-

You will have been extremely stupid to make such hard wired connections.

When you say "no fuse" do you mean that the fuse has been removed, leaving an open circuit (= no power, low risk), or do you mean that there is/are no fuse(s) in the wiring, just direct connections? If you mean the latter then the wiring itself will if, necessary, act as a fuse by burning out, probably setting fire to the insulation in the process.

how does one explain technical details in non technical terms? The only non technical way to put it is that to not have the right fuses mcb and earth connection would be a bit stupid.

those are simple, you either let go extremely quickly and mumble mixed obscenities, or you fry.

What happens next depends on the plug fuse. With plug fuse, the appliance is now disconnected. With no plug fuse, its not, and you'll repeat the cycle until you give up, possibly causing another personal fryup in the meantime.

this is less straightforward, as it depends on the earth and leakage protection arrangements. In very old wiring with a local rod and no protective device, the fuse should blow, but not quickly, and while its thinking about blowing everything earthed in the whole house will be live at 240v. Anyone touching it during that slow blow time would wake up very fast.

Now, if this old system used a water pipe earth, as they normally do, and the water main had been replaced with plastic, nothing would blow, and everything would remain live indefinately until a good earth connection were established somewhere.

If OTOH the TT install is modern enough to have an ELCB or RCD, that will pop (probably for the 43rd time). Again, until it does, everything is live, but that would be for a very short moment of time.

in both cases fuse pops, and while its heating up to popping point, you get bit. What voltage you get bit by depends on the installation details.

How about if you touch it with your tongue versus your toes. Why dont you do a practical and pee on it.

NT

in case theres a genuine retard here, never pee on live electrics, nor touch them with your tongue. Or with anything else.

Are we back in the 1950's? Are there no MCB/RCD's?

Old fashioned fuses mostly provide protection against overloading wiring, not protection against electrocution.

Appliance earthing mostly protects against elctrocution if metal cased equipment becomes live - by causing a to fuse blow.

Clearly a live-to-earth fault with no fuse present is likely to result in a wire burning through somewhere, and possibly a fire.

For the same reason other metal surfaces in the home are usually also earthed.

it goes up the arm, round the skull and back down the other arm you live and become a little wiser dont put it to the test though

Yes, left foot to left arm is worse.

Christian.

Right. But I'll continue to believe it'll kill me. Keeps me on my toes, safety wise.

Cheers Owain,

yes. don't worry I have all the right fuses in the right places, just wondered what would happen in the different scenarios below.

I was under the impression that at 240V a current as little as 50mA is enough to kill you?

I was under the impression that the min resistance of the body is 1000ohms, therefore the max current will be 240/1000 = 240mA which means that the fuse won't blow and ill be frying until I die?

Surely the leccy takes the path of least resistance therefore I wouldn't get any shock, or very little, ie enough to not make me want to touch the lamp again?

I thought all living people knew "It's the volts that jolts, it's the mills (iamps) that kills.

Irrespective of voltage only a few milliamps need to pass through your body to upset heart rhythms. However voltages quite a bit higher than domestic mains voltages are required UNLESS you've made special arrangements* to make a bit more current flow.

It's not so simple, the resistance of the human body is quite high mostly due to skin resistance - but break that down* and the blood conducts very nicely.

The current passed along each path will be in proportion to the conductivity.

What you thinking about ways to kill the wife??

"Surely the leccy takes the path of least resistance therefore I wouldn't get any shock, or very little, ie enough to not make me want to touch the lamp again? "

You'll still get a shock. It's true that more current will flow through a path of lower resistance, but *some* current will still flow through any alternative parallel path (i.e. you). The value of the current depends on the impedance of each available path. It only takes a few thousandths of an amp through the heart muscle to stop it beating or to make it go into fibrillation - in both scenarios you are likely to die.

And you could never send a post that says "I told you I was right"

Adam