Electric shock

[Jabbajaws]

Hi All, l have been pondering this in my head for absolutely ages and need an answer, if possible, to help me grasp this subject.

I understand that a person receives an electric shock when touching both a live conductor 230V, and an earth/neutral conductor 0V at the same time, because the live conductor is chasing 0V to continue the electrical path.

Now as far as im aware, a high enough current 80-100mA would cause a person's nerves to contract, with them not being able to let go.

Now if a person was to touch a live conductor, with no earth/neutral return path, would that person recieve an electrical shock? The person touches the live conductor of 230V, yet has themself a potential of 0V, so in theory, some electric current should be able to pass between these two points.

WOULD THIS CAUSE THE PERSON TO RECEIVE AN ELECTRIC SHOCK, AND 'THROW THEM OFF' AS THERE IS NO EARTH/NEUTRAL RETURN PATH?

ALSO WITH THE SAME PRINCIPLE, WHY DOES ELECTRIC SEPARATION PROTECT A PERSON WHEN THE LIVE CONDUCTOR IS 230V, YET THE PERSON IS 0V?

All feedback welcome ppl.. .

 

[Des 56]

If the generated supplied power is via two insulated conductors(as they are)where lies the fault path to fry those testicles ?

It exists only because of earth connections made deliberately or accidentally

If the supply was kept well insulated from terra firma,no earth fault could exist

The possibility of the double fault problem is a main reason for making that earth shock hassard available to us,without the double fault, the shock hassard (live neutral excepted)would not exist

 

[Jabbajaws]

Thanks for all your input everyone. Things seem alot clearer...

 

[Jim The Fish]

We earth fittings to provide a fault path which is low enough to create a high fault current needed to operate the protective device.

Standing on a carpet or not, if you touch live some current will flow, enough to kill maybe not but then again maybe.
Now can we guarantee that everyone remains fully insulated all the time - no, so we need to provide an alternative to disconnect the supply.

I can second that, I managed to inadvertently re-energize a cooker circuit whilst replacing the cooker switch, i was wearing well insulated work boots and had one hand on the underside of a wooden stair case (hardly a decent return path!) when i grabbed hold of the 10 mm t&e supplying the cooker and promptly ended up on my sorry arse. Luckily no damage done, well not physical any how! But just proves that current will find way its way home even through the highest resistance paths.

 

[Jabbajaws]

This was my theory all along with the 'hold on' effect, where the earth return is completed and the 'throw off' effect where like with yourself, you were thrown because the live conductor was at 230V, you were at 0V, and there was no where for the current to go so it threw you off.

That's my theory anyway... Please tell me if im wrong...

 

[Jim The Fish]

Well i'm not sure if i was thrown off it exactly, my theory is that i didn't take enough current to lock me up so i was still able to let go but then when i did let go i just dropped to floor, it happened so quick and was such a shock ('cuse the pun) i don't really know what the hell happened all i know is that i've never felt anything that strong before, as there was no RCD on the board i have no idea what sort of current i was pulling (well it was defiantly less than 40A, thankfully), i'll rig up an ammeter next time i change the switch LOL

 

[Jabbajaws]

Cheers for the response Jim. A close shave eh? And l thought a belt from the megger was bad...lol

 

[Adam W]

It's not so much the mistake, it's the fact that you've called him "Jerk off"!!

I find things are easier to remember if they are a little bit amusing, unfortunately there is always the danger of forgetting the correct terms for things, eg that orange plastic thing that covers the MI pot isn't actually called a 'carrot' (or the other thing we called it at college), it's a 'shroud'.

I apologise to any members of the KIRCHHOFF family that may have been offended.

 

[telepath]

Now as far as im aware, a high enough current 80-100mA would cause a person's nerves to contract, with them not being able to let go.

Your current is a bit off:
5 - 10 mA would throw you off with a painful sensation,
10 - 15 mA would cause your muscles to contract and you can't let go,
20 - 30 mA would impair your breathing and
50 mA would cause ventricular fibrillation and death.

This is why RCD's have a 30mA rating.

Now if a person was to touch a live conductor, with no earth/neutral return path, would that person recieve an electrical shock? The person touches the live conductor of 230V, yet has themself a potential of 0V, so in theory, some electric current should be able to pass between these two points.

WOULD THIS CAUSE THE PERSON TO RECEIVE AN ELECTRIC SHOCK, AND 'THROW THEM OFF' AS THERE IS NO EARTH/NEUTRAL RETURN PATH?

ALSO WITH THE SAME PRINCIPLE, WHY DOES ELECTRIC SEPARATION PROTECT A PERSON WHEN THE LIVE CONDUCTOR IS 230V, YET THE PERSON IS 0V?

If there is no return path then there would be no current flow so the person wouldn't get an electric shock.

You can see this using a light bulb connected to the positive with a switch later in the circuit. While the switch is open (no return path) no current will flow and the bulb will not light. If the switch is closed (a return path) current will flow and the bulb will light.

 

[J Sabo]

Your current is a bit off:
5 - 10 mA would throw you off with a painful sensation,
10 - 15 mA would cause your muscles to contract and you can't let go,
20 - 30 mA would impair your breathing and
50 mA would cause ventricular fibrillation and death.

These are the values i have always been taught.

Also had quite a few shocks, lucky never got stuck to a cable (LV jointer) Was taught to hit person stuck to cable with the nearest insulated item!

 

[shedbuilder]

This was my theory all along with the 'hold on' effect, where the earth return is completed and the 'throw off' effect where like with yourself, you were thrown because the live conductor was at 230V, you were at 0V, and there was no where for the current to go so it threw you off.

That's my theory anyway... Please tell me if im wrong...

Hi Jabbajaws,

There's no distinction in 'hold on' or 'hold off' effects. A shock is a shock whatever way you look at it. Your percieved difference between the two is probably down to what you were doing at the time. For example if you had your hand grasped around something, then the contraction of your finger muscles during the shock would probably make your grip tighten, and you perceive 'hold on'.

Forgive me if I'm teaching my grandma to such eggs with this next bit, but I think you may be thinking that you're always at 0V. Think of your body as a low value resistor (it's full of salty water that conducts electricity quite well), and the soles of your shoes as a high value resistor. Think of them as being in series, and think of the bottom of the 'boots' resistor being connected to earth, and the top of the body resistor being connected to 230V (e.g. by your finger touching a line wire). You're in trouble if the current through this series resistance hits much more than 20mA as has been said. So, ohms law gives us R = V/I, which comes out at around 12,000 ohms. So, if that series resistance less than that, things get hairy.

Well insulated boots may well give you a much higher resistance than that (let's say 100,000 ohms). The body's resistance however is very much lower (typically tens or hundreds of ohms, but let's say 100 ohms to keep the numbers simple). So, for a 100,000 and a 100 ohm resistance in series we get a current of 2.2977mA, which is reasonalbly insignificant and well below RCD trip levels. We can also calculate the voltage drop across those two resistors using ohms law (V = IR). Across the body it's 0.23V, and across the boot sole it's 229.77V. Since you're finger is still touching line and at 230V, this means that (to a close approximation), your entire body is at 230V.

Hope this helps

Paul

 

[tomhop]

hello Guys,

just wondering then, considering you can touch a plastic plug which has a live contact inside at 240v , then that should mean that if you stand on a piece of plastic like a plug and touch a live wire you wont get shocked?

Tom

 

[Bromd123]

if you stood on an insulated matt you wouldnt be touching earth and therefore would have no return path, therefore no shock at all!! not sure i would want to chance it myself though lol

 

[Traineenoob]

Ok here it is lol i was standing on a handrail of a staircase all woodent spindles ect!!! changing a light pendant that was still on...This was my 7th one i was doing now and was bored. I accidently grabed the live conductor with my fingers!!!! owwwwww i had no return path that i can think of but still recived one hell of a belt!

Health and safety first

 

[Bromd123]

Ok here it is lol i was standing on a handrail of a staircase all woodent spindles ect!!! changing a light pendant that was still on...This was my 7th one i was doing now and was bored. I accidently grabed the live conductor with my fingers!!!! owwwwww i had no return path that i can think of but still recived one hell of a belt!

Health and safety first

the wood would still be a path too earth even though it would bbe high resistance

 

[pushrod]

I think some of the hold on / throw off effects and degree of burning etc are due to due to the the nature of the current, ac or dc , and not just the value of the current. I do have it all in my notes somewhere, but am too tired to go and check

A piece of useless info for you all is that the very first electricians often did not have meters to check for lives so they would carry talcum powder to dry out their skin and so increase resistance before touching a suspect wire so that it was more of a tingle than a belt !!! You wouldn't have wanted to be one of them if you had sweaty hands