Electric shock

[sparkie710]

i know we do but shouldnt get lil tingles where we getcomplacent or it gets late, but if you held onto tails at the board i understand you wont be able to let go so how long will it take before that took your life ? seeinghow theres no protection ? regards

 

[1shortcircuit]

I imagine it would be different for each and every person.

If you would like to do the holding I will happily do the timing lol

 

[isitnow]

the shock is worse the bigger the load on the supply, i suppose holding tails shouldnt be as bad as say holding on to a busbar in a db under full load conditions, but it does depend on the person,

 

[GLENNSPARK]

From what i have been told it depends on the amperage with an AC supply...and if i remember that amperege value was quite low n all (to make you grip it) that is.....the real demon here is DC.....nasty stuff that is and to be treated as such.........:evil:

 

[Rockingit]

As I understand it, any current value of 230v @ 50Hz starts to do nasty things to the body. 50Hz is a particulary awkward frequency as it's roughly one of the body's resonant frequencies and therefore very disruptivwe to the normal heart pattern. Anything in excess of 50mA is bad news, will cause the muscles to contract and hence grip hold onto the source. The real killer is heat; eventually causing the blood to boil, the internal organs to melt and ultimately set you on fire.

Given that the human body is mostly water, we each have a natural resistance of not very much, and also a natural capacitance. Larger people tend to have a lower resistance due to increased water (fat) versus surface area (resistance in parallel).

A little bit of Ohms law tells us: 230v/100A = 2.3Ohms or less as a 'load' would be needed, instantaneously, to blow a 100A cut-out fuse (table 3A3 ignored, for the purists before I get shot) but in reality it actually needs several hundred amps over several seconds before it will even think about giving up the ghost. By which time, you'll most likely be one too.

 

[Infamous]

The point at where you make contact with a live conductor; whether it be tails, fuseboard, socket is often irrelivent with AC. What matters is your own personal circumstances at the exact moment you come into contact with that conductor.

What are you standing on?

Is any part of your body in contact with something that would give you a good earth path?

Exactly what path or direction is that current going to take? Is it accross my chest or head?

If so large earth fault currents are likely to flow right through you which can be fatal. Arm to arm or arm to foot often spells trouble as the current can pass through your heart on its journey, interfering with your bodys own personal harmonics and signalling. Remember your brain is telling your heart to beat via electrical signals and impulses. A foreign current can completely mess this up.

Small currents, as little as only 35 mA-60 mA are enough to cause cardiac arrest. The current that flows through you will be the earth fault current caused by you, so in theory the loading of a supply is also irrelivent.

As for not being able to let go, with AC you should in theory be able to let go of any conductor you touch by accident because the sin wave of an alternating current supply passes through 0 degrees twice a sinewave therefore twice a sinewave no net power is transfered allowing your body to let go. That is if you survive the intitial shock of course.

This is where DC can be a deadly as direct current doesnt pass through 0 at anytime so if the voltage is high enough to penetrate your skin (>100V DC rouhgly) youll be holding onto the beast untill circuit protection operates or somebody hits you with a wooden broom.

Although with DC youll need to put yourself accross both parts of the supply unless one of those conductors is bonded to earth.

 

[Rockingit]

As for not being able to let go, with AC you should in theory be able to let go of any conductor you touch by accident because the sin wave of an alternating current supply passes through 0 degrees twice a second therefore twice a second no net power is transfered allowing your body to let go. That is if you survive the intitial shock of course.

50 cycles a second that start at 0 and has 0 in the mid cycle = 100 times per second. Which is four times the body's natural harmonic. And of course if it's 3ph then it's never at zero (as pd).

 

[Infamous]

50 cycles a second that start at 0 and has 0 in the mid cycle = 100 times per second. Which is four times the body's natural harmonic. And of course if it's 3ph then it's never at zero (as pd).

Thanks for noticing that i actually meant to put twice a sinewave not second. And yes for 3 phase all though your more likely to get a single phase electric shock to earth unless you put yourself accross phases

 

[Richard Burns]

I remember receiving an significant electric shock when I was a student many years ago, luckily for me it was only passing through my thumb, however it took serious effort of will to make the movement to remove my thumb from the contacts and felt like a very long time, although it was probably a few seconds at most.

I think the original intent was for ac to be safer than dc as the muscles would spasm rather than clench and so throw you away. normally causing more damage! I was taught! to touch live parts with the back of the hand rather than the front so the hand would move closed, away from the source, if shocked.

But certainly the resistance of the body to earth is a critical factor when receiving a shock as is the path of least resistance for the current. people also react in slightly different ways to the shock.
I do remember pretty pictures of shock victims at my training course that certainly encouraged you not to receive any sort of shock.

 

[Rockingit]

Just as a sobering moment: According to the Wiki I've just been reading, the electric chairs used in the U.S. for deliberate electrocution have a design spec of 10A at 2,450V AC, for between 20-30 sec bursts.

And interestingly, that article talks about how AC won over DC as being the most efficient delivery of fatal charge.

sweet dreams, chaps.

 

[Knobhead]

THE HISTORY OF THE ELECTRIC CHAIR

 

[FAA Greenie]

As above, depends on the circumstances. If you just touch the live and you are stood on a wooden floor in rubber boots, odds are that you wouldn't even notice it. live in one hand and neutral/earth in the other, you are in for the ride of your life without the protection of an RCD, the grid would not even notice your passing....

 

[isitnow]

Just as a sobering moment: According to the Wiki I've just been reading, the electric chairs used in the U.S. for deliberate electrocution have a design spec of 10A at 2,450V AC, for between 20-30 sec bursts.

And interestingly, that article talks about how AC won over DC as being the most efficient delivery of fatal charge.

sweet dreams, chaps.

ye that makes sense we know voltage is the "pressure" that drives current as i was taught in college, so a high voltage with a relatively highish current will defo do the job to kill you, hence the reason for voltage optimisers as the voltage is reduced the current is less and saves money (random)

 

[Rockingit]

THE HISTORY OF THE ELECTRIC CHAIR

Cheers Tony, a fascinating insight into both the origins of the chair but also the battle for DC/AC supremacy in the early days. It's particularly interesting to read how Westinghouse (developing the AC technology corner) refused to supply the transformers etc, and even funded the first few appeals on the basis of electrocution being cruel and painful.

Also of course, back in Blighty at around the same period our forefathers were writing the 1st edition, designed to keep people safe.

Nice to know it's the stuff we work with everyday.

 

[GLENNSPARK]

Just as a sobering moment: According to the Wiki I've just been reading, the electric chairs used in the U.S. for deliberate electrocution have a design spec of 10A at 2,450V AC, for between 20-30 sec bursts.

And interestingly, that article talks about how AC won over DC as being the most efficient delivery of fatal charge.

sweet dreams, chaps.

I have read a bit about this on the net.....the chap who was trying to push this form of execution forward (back in the 1890s) was experimenting on dogs and cattle.............