Dangerous electricity

cast_iron

I'm not sure, perhaps Victor did some of his own research since the OP due to the "incoherence" of the replies here.

In any case, I think he sparked an interesting, healthy debate. I hope this forum can enter into these sort of topics and not be confined to " how do I connect a socket" type questions only.

I suppose, if we took Victor's question extremely literally, I proportional to sqrt(P) would have corrected his assumption that they are not exponentially related.

RoundyMooney

Would type a longer reply, but I have to run.

The OP has generated a nice bit of traffic, which is no bad thing.

I freely admit I'm learning one or two things here myself (and I considered myself knowleadgeable enough.

Keep it coming, I say

Stoner

cast_iron wrote: »

I hope this forum can enter into these sort of topics and not be confined to " how do I connect a socket" type questions only.
.

I agree cast_iron, it is the intention to keep this broad and develop it as much as possible, the forum is now 5 days old and has generated a fair bit of traffic.

Victor

Apologies if the original question was overly simple. I didn't know the answer was this complicated.

Copper wrote: »

1) 50mA is the internationally accepted minimum level of lethal current. So to avoid risk of death, the current flowing through you should be kept under 50mA. This is why the standard rated current of RCD's is 30mA, and the whole point of electrical personal protection is to ensure this level of current is never delivered to a person.

So on a typical domestic 220V electrical supply, even a very modest current can kill.

* Typical fuse 3 to 13 Amps
* Probable risk 0.05 Amps

With other installations and battery powered equipment there are many variables.

Without current , there is no electrical power, the magnitude of the current determines the magnitude of the power, therefore as we have all answered, current kills.

This and similar statements create the impression to the layman* that current, measured in amps is the only factor involved.

In otherwise identical circumstances, would 3 Amps at 220V be more dangerous than 2.99 Amps at 110kV?


* I did Leaving Cert physics and Building Services as part of Construction Studies in college and neither deals with the point adequately. In fact, at the time, the school teacher over stressed the use of the phrase 'electrical tension' which had us all thinking in terms of stress and strain. :rolleyes:

cast_iron

Victor wrote: »

Apologies if the original question was overly simple. I didn't know the answer was this complicated.So on a typical domestic 220V electrical supply, even a very modest current can kill.

Indeed. I know a lad who had a live fluorescent light fitting fall on him a few months back. The frame became live and the ballast stuck to his arm. He was holding the side of a metal ladder with his other hand and couldn't let go for a number of seconds. The MCB did not trip out; he was quite lucky he only escaped with a couple of scars that still remain today.

This and similar statements create the impression to the layman* that current, measured in amps is the only factor involved.

In otherwise identical circumstances, would 3 Amps at 220V be more dangerous than 2.99 Amps at 110kV?

Again, that question is a little confusing. Assuming you are referring to "dangerous to a person/body", and referring to a real world example, the question doesn't make sense.

Back to basics: V=R*I
For a given person/body, the resistance (R) is assumed (for simplicity) constant. So you can't get 3 Amps @220V and 3 Amps @ 110kV - by definition!
If the voltage increases by 500 (as above), from 220V to 110kV, the current will also increase by the same factor - leaving you with a current of 1500 Amps.


To answer the actual question you asked, we must have 2 different bodies - one with 500 times the resistance of the other. If we assume all other factors are the same (very unrealistic in practice), I don't see how one voltage is more dangerous than the other.

Stoner

Because the bodies internal resistance is very unpredictable but in your question it is assumed to be constant.

Because the resitance is the same, we cant have those combinations of Voltage and current

Lets say I have an internal resistance R

Current = Voltage/Resistance, so the more Voltage we have available the higher the amount of potential current, thats why High Voltage is dangerous, as it can produce higher currents.


Your question does not take into account time, so lets consider it to be constant


The examples questioned do not stack up IMO
the constant R cant occur
as

R=V/I

R= 220/3 = 73Ohms

and

R = 110000/2.99 = 36789 Ohms.

Does that answer your question Victor ?.

Edit..
just to add that it would not be possible for 220V to cause more or less the same current in someone as 110000V.

If R was 100 Ohms

110000V is 500 times greater then 220V and would therefore accross a constant resistance produce 500 times more current,
i.e.
V/R=I

220V / 100 ohms = 2.2 Amps
110000V / 100 Ohms = 1100 Amps.

2 stroke

I think Victor is asking about these voltages through a 3 amp fuse. A 3 amp fuse dosn't keep current below 3 amps, it will allow a current more than 3 amps to flow until the fuse blows.
If you did set up a 2 circuits limiting current through your body to the parameters mentioned, then 2.99 Amps at 110kV (328.9kw) would fry you more than 3 Amps at 220V(.66kw). Unless you were thrown clear which I think is more likely at the higher voltage.
However the resistance of the human body would not allow 3 amps to flow through the circuit.

2 stroke

Victor wrote: »

I'm disturbed by the lack of coherence of the answers here.

You may find this helpful. http://en.wikipedia.org/wiki/Electric_shock

Stoner

2 stroke wrote: »

If you did set up a 2 circuits limiting current through your body to the parameters mentioned, then 2.99 Amps at 110kV (328.9kw) would fry you more than 3 Amps at 220V(.66kw). .

The parameters mentioned don't add up they dont make sense for this to be the one person, it has to be two different poeple with different values of R.
It is very simple we have three variables in an equation, one is considered a constant as per these parameters (R), therefore it is not possible for the other variables (I and V) to be arranged as per Victors example.

There is no way that in the same person that 220V would produce 3 Amps and 110KV would produce only 2.99 Amps.
IMO if the voltage is 500 times greater accross a constant resistance (the person) it will produce a current that is 500 greater accross that same constant resistance.

For them to balance out you would need two different people, one with an internal resistance that is approx 500 times greater then the other person.

If someones R allowed 3 Amps at 220V, that same person would have 1500Amps going through them if the voltage was increased to 110KV.

High voltage is dangerous as it allows higher current.
Anyway they will both kill you, and its very difficult to measure your internal resistance as the resistance of a body.

2 stroke

There is no way that in the same person that 220V would produce 3 Amps and 110KV would produce only 2.99 Amps.

Except in a hypotechical situation where only approx 3 amps supply is available at 110KV.

Copper

Anyway they will both kill you, and its very difficult to measure your internal resistance as the resistance of a body.

Just stick a megger across yourself:D

So on a typical domestic 220V electrical supply, even a very modest current can kill.

* Typical fuse 3 to 13 Amps
* Probable risk 0.05 Amps

Even the smallest fuses are not going to stop you receiving a lethal current. The fuse isn't there to stop the current flowing through you when you're receiving a shock, its not going to help you at all. Its much too slow to act and the current through you would already be at many multiples of the lethal current before it acted. Thats why we have RCD's.

The fuse is only there to protect the cable, and to provide automatic disconnection of supply in the event of an earth fault - in other words the fuse prevents you receiving a shock by killing the supply when a live cable touches the frame of an appliance for example. Once you are receiving a shock the fuse is of no use to you at all.

In otherwise identical circumstances, would 3 Amps at 220V be more dangerous than 2.99 Amps at 110kV?

3 Amps is a crazy amount of current, 60 times more than the minimum lethal current. If 3A was passed through a human body, I doubt anybody could survive it. But yeah, all other factors being identical I'd imagine say 0.05A at 230V would be more dangerous than 0.049A at 110kV- in other words its the current passed through the body and the time it is passed through that are the critical factors. At 110kV, you're likely to be thrown back and end up with serious burns (because the power developed is higher).

As I understand it though, its not the power that does the real damage, its the fact that even miniscule current levels passing through the body are misinterpreted by the body's nervous system as being strong electrical signals from the brain. These signals cause muscles controlling vital bodily functions (pumping blood, breathing) to contract and stop working. The actual energy imparted to the body when receiving a shock is tiny. Its the level of current and its duration which disrupts bodily functions that is the danger. I think.

Stoner

Copper wrote: »

As I understand it though, its not the power that does the real damage, its the fact that even miniscule current levels passing through the body are misinterpreted by the body's nervous system as being strong electrical signals from the brain. These signals cause muscles controlling vital bodily functions (pumping blood, breathing) to contract and stop working. The actual energy imparted to the body when receiving a shock is tiny. Its the level of current and its duration which disrupts bodily functions that is the danger. I think.

I agree, it is the current as we have been saying all along.

2 stroke

I have to admit I'm learning something here. I've always been inclined to think of the number of watts flowing. However I've just remembered a fuse dosn't care what voltage is involved, a 3 amp fuse will blow at pretty much the same amps no matter what the voltage is.
One thing I think is worth mentioning is, the danger of arcing at higher voltages. I've never had to deal with 110KV so maybe someone can advise.

Victor

Stoner wrote: »

Because the bodies internal resistance is very unpredictable but in your question it is assumed to be constant.

Because the resitance is the same, we cant have those combinations of Voltage and current

I suppose I'm thing of the supply available as opposed to the supply that actually passes through the person.

Capt'n Midnight

Victor wrote: »

Which bit is dangerous - total current or total power?

cba reading the whole thread

It's volts that jolts, but mils that kills

20V can give you a shock
anything less than 50V probably isn't lethal

9mA across the heart will probably kill you ( CSI said 7mA if you want to believe TV) - even an earth leakage trip won't save you as they are usually 30mA

a weak blast might get your heart fibrillating and so be worse than a good strong shock

DC you get one spasm , secondary injuries caused by muscle spasms - where you fly off the chair or knock your head against soemthing - are probably more dangerous than the electricity in cases of non-lethal shocks

AC you get multiple spasm's so if you touch the wrong way you stick to it - always check cables with the back of your hand (not always check cables, I mean always the back of the hand)so you don't "grab" the cable when your muscles are triggered.

it varies but when I measure the resistance between my hands it's about 300 K ohms , if I was to get 1mA thorough me, the current would cause some changes in me, like sweating which would lower resistance and allow more current through

Capt'n Midnight

2 stroke wrote: »

I know that is a typo but in case someone is realy stupid, its not supposed to be a l instead of a h.

:eek:

Stoner

Victor wrote: »

I suppose I'm thinking of the supply available as opposed to the supply that actually passes through the person.

that's a good way of putting it, same as a fuse, current and the resistance makes it blow,

Sparky

Simple rule as I've been told.


Volts shock
Amps kill

Capt'n Midnight

Spankeh wrote: »

Simple rule as I've been told.


Volts shock
Amps kill

Volts Jolt , Mils kills
( Milliamps )