Matter of interest...

[Big_Spark]

But isn't the reason for shorter disconn times on rings because of the risk of muscle spasm if encountering faulty hand-held equipment??

The disconnection time for all circuits at 230V is 0.4 seconds.

Muslce spasm is not a significant factor, well not spasms in the arm/hand/leg musles anyway. The only musle spasm to be concerned about is in your chest, and this causes ventricular phibrilation, this is were the heart attempts to beat at the same speed as the electrical impulse it recieves, and at 50 times a second it is fatal as the heart walls collapse caused by dynamic stress. You have a heartattack basically!

 

[ban-all-sheds]

The disconnection time for all circuits at 230V is 0.4 seconds.

I thought that for final circuits supplying only stationary equipment the requirement was relaxed to 5s?

 

[Big_Spark]

The disconnection time for all circuits at 230V is 0.4 seconds.

I thought that for final circuits supplying only stationary equipment the requirement was relaxed to 5s?

It used to be, but they seem to have quietly gone back to only stating 0.4s for any circuit rated at 230V

 

[waran]

Although I agree in terms of damage that can be caused by a given voltage AC is worse as it usually contains more energy that statement is inaccurate, DC is far worse than AC for one very simple reason..

DC contracts muscles and therefore PREVENTS the victim from releasing any grip they may have had on the source of energy.

AC can and does cause muscle spasms, but usually this results in the person being disconnected from the source of energy.

Whilst I would bow to your superior experience in this matter FWL_Engineer, physiologically and historically I think my original statement is usually correct.

DC usually causes a single spasm of skeletal muscle and this tends to move the victim away from contact to the source. This does however increase the likelihood of blunt injury.

AC causes tetany (which is repeated contraction of muscle) tending to hold the victim to the source.

In the early days of electricity, there was considerable debate between the use of AC and DC on the safety versus efficiency grounds. Edison (who developed and popularised DC) and Westinghouse (who developed AC) were both advocating their system and to illustrate the efficient killing nature of AC, Edison proposed using it for execution.

This would seem to correlate well with electrical trauma care experience from the US. In their studies exposure to AC is about 3 times worse than DC of the same voltage.

Whilst what breezer said may well be good advice for fireman, I am not sure that is the natural reaction. Whilst intuitively it sounds logical I'll have to check and see if there have been any studies looking particularly at what muscles contract when shocked. Normally your relex arc would cause your whole hand and arm to be withdrawn (similar to what would happen if say you burnt your hand on a hot pan) rather than to close around the handle. This is why AC is inherently more dangerous as it would cause repeated contraction of all muscle not allowing any one to predominate and withdraw your arm/body.

I must stress that most of this is academic and the underlying point is that prevention is always better than cure!

 

[AdamW]

If you get a shock through your hand the muscles in your hand and forearm will cause your hand to close (humans can grip with more force than they can open their hand, so in an antagonistic pair the close muscles will win out).

The fireman point is that if you walk palm forwards, you hit a bare live wire and your hand closes tightly around the wire. If you walk palm backwards then your hand will close tightly, but is more likely to be NOT around the wire.

Comparing electric shock with pain reflexes is not entirely correct. Touching a hot pan causes a reflex action, induced by the nervous system. Grabbing a live wire might cause the nervous system to attempt a reflex action, but it will be swamped by the electric shock.

 

[sterose]

Whilst I would bow to your superior experience in this matter FWL_Engineer, physiologically and historically I think my original statement is usually correct.

DC usually causes a single spasm of skeletal muscle and this tends to move the victim away from contact to the source. This does however increase the likelihood of blunt injury.

AC causes tetany (which is repeated contraction of muscle) tending to hold the victim to the source.

In the early days of electricity, there was considerable debate between the use of AC and DC on the safety versus efficiency grounds. Edison (who developed and popularised DC) and Westinghouse (who developed AC) were both advocating their system and to illustrate the efficient killing nature of AC, Edison proposed using it for execution.

This would seem to correlate well with electrical trauma care experience from the US. In their studies exposure to AC is about 3 times worse than DC of the same voltage.

I would think that the electrical effect on your muscles, by either AC or DC will be roughly the same.

The determining factor on which muscle(s) contract would surely be dependent on the path the current takes through your arm. If it flows through the muscles that contract the fingers, your hand will grip, if it flows through the muscles that extend the fingers, your hand will release. Obviously it may flow through both, and produce a 'tensing' effect, but the above still applies.

The reason AC was chosen to electrocute people was because it flowed better through the body.

 

[ban-all-sheds]

..The reason AC was chosen to electrocute people was because it flowed better through the body.

This is a fairly grisly topic, but are you sure that AC isn't used because it is much easier to step up a standard commercial supply voltage to the couple of thousand needed for an electric chair if it's AC rather than DC?

 

[waran]

Comparing electric shock with pain reflexes is not entirely correct. Touching a hot pan causes a reflex action, induced by the nervous system. Grabbing a live wire might cause the nervous system to attempt a reflex action, but it will be swamped by the electric shock.

Fair point AdamW. You're right they are not the same. The shock would cause direct contraction of the muscles independent of the nervous system. However the point I was trying to make was that your whole arm/shoulder/back (being much more powerful muscles) would act to withdraw your hand rather than close round a cable. Still I'm guessing that this is what happens as I don't know if anyone's experimented with this in the lab.

 

[Big_Spark]

DC usually causes a single spasm of skeletal muscle and this tends to move the victim away from contact to the source. This does however increase the likelihood of blunt injury.

This would seem to correlate well with electrical trauma care experience from the US. In their studies exposure to AC is about 3 times worse than DC of the same voltage.

DC DOES NOT OPEN MUSCLES AS YOU SUGGEST, it is a physiological fact that DC causes muscles to CONTRACT. Due to the nature of human muscles this may appear to be what you suggest in certain circumstances, however muscles do contract under a positive DC charge.

Step on a train line and see what happens. (Only Joking)

The reason that AC is worse than DC in the US is a complete Red Herring. DC circuits in the States are all below Nominal Distributed Supply Voltage, therefore the effects will be worse.

If you look at European figures they tell a different story. In the UK and in Europe, trains are powered by 650V DC and this kills some 98% of the time. AC does kill, but in reality in only a very small percentage of cases.

There are industries in the UK that have, historically, used large DC supplies for machinery, these supplies tend to be 200V DC up to 1200V DC, and these do kill on almost every contact. Electrical Safety around this equipment is the strictest anywhere.

 

[sterose]

..The reason AC was chosen to electrocute people was because it flowed better through the body.

This is a fairly grisly topic, but are you sure that AC isn't used because it is much easier to step up a standard commercial supply voltage to the couple of thousand needed for an electric chair if it's AC rather than DC?

Seems reasonable... either way I'll take your word for it.

DC DOES NOT OPEN MUSCLES AS YOU SUGGEST, it is a physiological fact that DC causes muscles to CONTRACT. Due to the nature of human muscles this may appear to be what you suggest in certain circumstances, however muscles do contract under a positive DC charge.

I agree. Muscles have no 'expand' function, they can only contract, or not contract. Any expansion or stretch is due to opposite muscles streching the muscle. That is for example, the tricep extends the arm and steches the bicep.

However FWL, I'm fairly sure that any muscle subjected to electrical charge of ANY sort causes contraction of that muscle. AC and DC. Positive and Negative charges.

As said, the action of the muscles (i.e. whether they pull the arm away, or grasp the live cable) is purely due to the path the electrical charge takes through the body (or in this example, the path through the arm).

 

[Big_Spark]

However FWL, I'm fairly sure that any muscle subjected to electrical charge of ANY sort causes contraction of that muscle. AC and DC. Positive and Negative charges.

In theory this would be correct, however the fact is that AC appears to cause the opposite as it causes muscles to contract and expand, I know you say muscles cannot expand, but they do in principle at least.

AC is a strange thing in relation to the body. It has effects which DC can only dream about, it is this that causes some of the misconceptions that exist.

 

[plugwash]

The reason AC was chosen to electrocute people was because it flowed better through the body.

no the reason AC was choosen to electricute people (and animals for demonstrations!) was a FUD campaign by edison against AC power.

 

[Pensdown]

Has that been bugging you for 2 years

 

[plugwash]

sorry i think i followed a link here from another topic and didn't pay attention to the dates.