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.. .

 

[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

 

[martysparky]

DC makes you freeze like in the old films. AC alternates the current and the wave of alternation is what throws you off if it passes through your body. Probably why that type of current is used in a house etc.

If you touch the live conductor and get a belt it does not have to travel to earth via finger to foot. why because it can pass through part of your body. eg. up one finger down another or up one arm across your chest back to neutral at the ceiling rose/ cpc/ earthed/extraneous conductive par. think of the Megger tester safety test button to keep one arm away from the hazzard / danger.

basic stuff??

 

[Jabbajaws]

Thanx, everyone,

I must say im well chuffed with the response. I have learned so much about electric shock, merely by asking these questions and receiving advice from the experienced.

I will continue to monitor this topic, incase of any more feedback.

Many Thanks from JABBAJAWS...

 

[pushrod]

DC makes you freeze like in the old films. AC alternates the current and the wave of alternation is what throws you off if it passes through your body. Probably why that type of current is used in a house etc.

AC is widely used because of the ease with which it can be transformed from one voltage to another, so that it can be transmitted at very high voltages and small currents with consequent smaller power losses. It can then be stepped down easily to safer voltages for other uses, eg domestic etc.

 

[ezzzekiel]

also it comes quicker in the alphabet

 

[martysparky]

AC is widely used because of the ease with which it can be transformed from one voltage to another, so that it can be transmitted at very high voltages and small currents with consequent smaller power losses. It can then be stepped down easily to safer voltages for other uses, eg domestic etc.

Not so basic stuff

 

[kniteforce]

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.



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.

20 odd years in the trade nd this subject still fascinates me and i can never conclusively answer it when asked!!

what confuses me even further is if 30ma rcds were inented to protct against ultimately,death,then why were 100 ma rcds ever fitted in domestic properties?? far too many questions!!

 

[Deleted member 26818]

killed to death.

As opposed to being killed almost to death?

 

[kniteforce]

As opposed to being killed almost to death?

loved that term as well!

 

[snowhead]

what confuses me even further is if 30ma rcds were inented to protct against ultimately,death,then why were 100 ma rcds ever made?? far too many questions!!

Or 300ma or 500ma or Rising rate or time delayed.

 

[pushrod]

20 odd years in the trade nd this subject still fascinates me and i can never conclusively answer it when asked!!

what confuses me even further is if 30ma rcds were inented to protct against ultimately,death,then why were 100 ma rcds ever fitted in domestic properties?? far too many questions!!

Or 300ma or 500ma or Rising rate or time delayed.

Larger rcds will help prevent the risk of fire through earth faults and time delayed ones allow discrimination when two are in series( I think, but happy to be corrected)