How Can Electricity Possibly Be Dangerous?

[tobyw]

I salvaged some motors from a printer and they don't run very fast off a 5 volt battery.... so I thought hey maybe I should connect them up using the mains adapter which came with the printer - which is marked as 32V outputting 1300mA

Well, anything above a 9V battery makes me think I will probably electrocute myself, so I stick to AA's mostly. But then I thought maybe I should actually try to learn enough to know what is dangerous and why.

So I found some sites explaining about the conductivity of human skin, and the thresholds in terms of current that will cause death. For example

http://electronics.stackexchange.com/questions/19103/how-much-voltage-is-dangerous


Also I understand that if the current passes through your heart then it is more dangerous, whereas if it enters and exits your finger, then it is less so. But still it doesn't quite make sense for me...

There is no obvious census on this - some sites say 10mA through the heart would be fatal and some say 500mA without mentioning the path. Big difference. But some of the resistance values given are quite large - even wet skin has a resistance of 1kohm according to a few sites I have seen. Well, 240/1000 = 0.24? - which is nowhere near the 500mA quotes on the site I linked to.

I don't plan to start grabbing hold of live 240v wires - I know empirically that this is a B.A.D.I.D.E.A. But I am just interested to know how you actually prove theoretically the maths on whether something will harm you, which would make me feel more confident about knowing what is what.

For example, I guess if I grab a single live 240V wire, with skin resistance of 1k - then this will allow 240mA to pass through my body to ground through my feet, and if it decides to travel the scenic route, no wonder it would be curtains for me. But if I grab the earth wire first, and then touch the live wire to the same hand....would this theoretically mean only a portion of my hand is conducting, and I would just get a burn rather than a heart seizure?

But then again, according to that link, dry skin has a much much higher resistance of around 1M. But if I were to allow someone to touch a live 240V wire to the back of my dry hand, this theoretically means the current the skin would allow would be less than a milliamp - 240/1000000 = 0.00024? But that implies I would be getting current which is theoretically less than the perceptible amount?! I must be calculating this wrong surely, because somehow I doubt I would be chatting about the weather while they did that.

And then 32V. So if I grab a live 32V wire, and ground it via my body - if my skin resistance when wet is really 1k, then this gives a current of 32/1000 = 0.32 If that goes via my heart it could kill me, right? But what would make it go via my heart - because according to that link, if it DOESN'T, then I would just get some controllable muscle cramps. Does that mean working with 32V off the mains is essentially just as dangerous as working with a 240V wire, or only likely to cause real danger if you are unlucky?

I don't fancy connecting myself up to the mains and trying to write notes and observations at the same time to work this out...so could anyone enlighten me about how you would go about calculating the results of touching different wires, in different ways? It might make me feel a bit more confident about doing things with relays or wall plugs when I need to.

Thanks

 

[JimPerry]

You are correct about grabbing Earth and live in one hand - sort of (DO'NT TRY IT).

How do you think birds manage to sit on electricity cables? No Earth so only a circuit through legs and a minimal voltage difference. Same thing with people in ultra-high voltage areas being hit by lightening bolts ....

Current kills - voltage tickles?

 

[Jeremy Harris]

There is a massive variation between one person and another, and in one individual under different conditions, in terms of skin resistance, so it's really hard to generalise. There's also a fairly big difference between individuals in terms of how much current is needed to stop the heart - the most gruesome illustration of this being that in order to ensure death by electric shock the electric chair uses over 2000 V and passes this through several metal straps to wetted skin.

I once helped with a rewiring job where one of the electricians had high resistance skin. His party trick was touching wires to see if they were live. He could just detect 240V when he poked his fingers into a light socket, something that would throw most people right across the room.

It's generally considered safe to work on live circuits up to 50V AC, 75V DC, because the chance of getting an electric shock that's severe enough to kill is considered to be very remote at these voltages. You may be able to feel 50V, but for the majority of people this wouldn't be either particularly unpleasant or harmful.

 

[bfgstew]

If in doubt, don't touch it!!!!! Simple.

My old work mates trick was to pull the HT leads off with his hands to see which spark plug was faulty, I thought it was safe! I can still feel the jolt in my arm from 20 years ago!
I did chuckle though, when our 'so called' senior technician, said "It's ok, it's only 24 Volts, it won't kill you", and like a fool, my mate touched it, then duly picked himself off the floor were he had been unceramoniously dumped on his a**e by the 240 Volts!

If in doubt, test it, discharge it or switch it off, before touching.

As Jim has said, Current kills, Voltage tickles (most of the time), underlying health problems, standing bare foot in a puddle of water, holding onto bare metal, all these other things have a factor in wether you live or die.

 

[John West]

It sounds like you have a pretty good understanding of the basics. I'll just add that as the frequency of the voltage goes up it will tend more to flow on the surface of the skin, rather than penetrate, causing what's known as "RF burns."

I've been zapped by so many voltages and currents in so many different ways that I'd rather not contemplate it, and the fact that I'm still here is due to little more than dumb luck. You're well advised to use caution at ALL times. Also, don't forget about the dangers of low voltages and high currents. While it's reasonably safe to work with your hands around the terminals of a car battery, it isn't safe to do the same thing with a screwdriver or a spanner. You're well advised to wrap them up with tape first. A car battery can vaporize or blow bits of metal from a metal bar that inadvertently brushes both battery terminals. Even a small 3.3 or 3.6 Volt Lithium Ion battery can do something similar due to its very low internal resistance.

On a damp day, I've been jolted by touching the 24 V of a pair of automotive batteries used to start a US Army tank. BTW, if you think your tiny 9V battery has no capacity to bite you, touch the terminals to your tongue. (That's how I tell if they are still good.) Then imagine that same sensation across your heart.

Electricity is power. Treat it with care and respect.

 

[Captain Haddock]

I've worked on a ups battery bank at the local power station, with 240volts and 100Ah DC and no breaker you tend to be very careful with the battery enclosure open, that will do a bit more than tickle, and of course with DC you just hang on till the batteries are flat, no chance of being thrown off.

 

[fernando_g]

Batteries have the added danger that, when short-circuited, can burst and throw REALLY NASTY chemicals over your hands and face.

 

[BeanieBots]

As you have already found out, it very much depends....

There are three basic mechanisms with electricity that can do harm.
The most sensitive and the biggest killer is disruption of your internal system functions such as the signals telling your heart to beat. The natural internal voltages in the order of 10's of mV. Any voltages which penetrates your skin and disrupts these CAN KILL. It's a question of how much voltage externally is required to do that and that depends on resistance and path.

The second cause of damage is eletrolysis causing the break down of tissue. This can happen at only a few volts. Try dropping a 1.5v into a glass of water and watch it break the water down into hydrogen and oxygen.
This where I disagree with an eralier comment about 50v being safe. The worst electric shock I ever had was from 60vDC. At the time of contact I felt a slight tingle which I thought was caused by the machine vibrating. However, because the contact was prolonged, the damage internally was severe and the PAIN did not kick in until several days after the event. There was no permanent damage but my arm was painful (and I mean painful) for about three months.

The remaining damage is caused by simple burning. This is straightforward I^2.R damage.
To get enough current to cause such damage requires very high voltage usually in the order of 10's or 100's of kV such as a lightning strike. If it doesn't kill, the victim will enevitably have severe burns. Such high voltages will usually travel down the outside of the skin which is why death does not always happen but the entry and exit points can become badly burned.

A quick last note:-
It's only got a couple of AA batteries, it must be safe...
Tasers as used by the police are powered by 4 off AA bateries!

 

[jedynakiewicz]

I'll just add that as the frequency of the voltage goes up it will tend more to flow on the surface of the skin, rather than penetrate, causing what's known as "RF burns."

Not quite accurate I am afraid; radio frequency burns can be devastatingly penetrative and tissue destructive. We use RF in surgery in the form of "electrosurgery" or "diathermy" machines in many surgical procedures. These devices generate RF, usually in the range of a few MHz to tens of MHz in some embodiments.
A circuit is usually created by placing a large-area wet earthing plate against the patient's skin, often on the back or the thigh, in preparation for surgery.
The working tip of the machine, the "antenna" varies according to function; as does the waveform that is generated. The machines are most often used for closing small bleeding blood vessels during surgery by tissue coagulation. Another tip is simply a very fine wire that is used as an electronic scalpel. It is moved quickly and smoothly through the tissue and is very effective at clean cutting whilst closing capillary blood vessels as it passes. However if it is not moved quickly and smoothly it can cause deep tissue damage. I have a photograph for teaching that I took of sequestrum (dead bone) shedding through skin where an electrosurgery unit was used by a surgeon who did not understand how the machines work and who kept the electrode in static contact instead of moving it progressively. I have also seen a radio transmission technician who came into casualty (the "emergency room" for our USA viewers) with a mildy-browned area of skin on his finger. He was in no pain but had been told to come to casualty after he suffered a direct hit from the RF antenna output of a large commercial transmitter. The RF damage to the underlying bone was so severe that he lost the finger. He found it very difficult to reconcile the apparently minor surface damage with the actual severity underlying it.

I did some clinical research into the technology of clinical electrosurgery many years ago. If you think that impedance can be tricky to measure in electronic circuits you should try and determine a figure for impedance in humans! That, of course, is the reason for many of the discrepancies of views in this thread. There is a huge difference between wet skin and dry skin; between a human in nylon clothing in a dry atmosphere and a sweaty human in a high humidity environment. And between someone who has skin directly over muscle and someone who has several inches of fat! Cool feet in rubber-soled trainers will be radically different in earthing potential to sweaty feet in leather-soled lounge shoes. The danger of any electrical potential, whatever the voltage or frequency, depends upon the complete circuit and with humans this is mostly unmeasurable outside the laboratory. Remember that a cardiac pacemaker has such a tiny output that it can run for a decade on something little bigger than a coin cell but it is, of course, in direct contact with the heart. All electrical effects that can cause a LOCAL voltage in the heart at an equivalent level are hazardous; it just depends upon the circuit and that is where the variables exist so markedly. Overall, ALL electricity is potentially dangerous; it simply depends upon the circuit through the human. So it is not a question of what voltage is safe or what AC frequency is safe; it is a question of what circuit through the human is safe. And that is probably the proper answer to the question posed in the original posting in this thread.

Apart from cardiac arrest and direct tissue damage, remember that our nerves are a specialised form of electrical conduit and in some circumstances temporary or permanent nerve conduction damage can occur; but I am not going to elaborate further on this more complex form of damage.

There are many "amusing" anecdotes in various professions that pay regard to the dangers of RF. From the old BBC stories of technicians climbing up unisolated transmission masts and impairing their procreative potential through to hospital doctors switching on diathermy machines and sparking flames from alcohol-based skin preparations. But seriously, do not underestimate the power of high frequency electricity to do deep damage to human tissue.

 

[Paix]

Yes, the thread got very interesting - RF does often flow over the skin and then suddenly enters, at which point as has been mentioned by Jedy' the burns that you receive are full thickness burns, probably all the way to the bone and maybe out elsewhere. Like the cited guy with the burned finger who couldn't comprehend the connection between the cause and the result, the first thing that gets burned is the nerves and so an RF burn is often relatively painless, bellying the serious damage that has been done.

Even with just skin damage, that's a lot of layers of skin to shed before the damage is anywhere close to healing and the chances of secondary infections remains for a protracted period of time as the original wound tries to heal.

You don't plan on waggling 240V among your 5V or 3.3V logic because of a healthy regard for the damage it will do to the logic, so why chance screwing up your own mV logic system. Don't try to sign up for any Darwin awards that might be on offer under the hood . . .

 

[Dippy]

Yes, RF and HF electrical voltages can be a right bummer. I accidentally put my elbow on a messy battery inverter I was making. For several seconds I couldn't understand where the smoky bacon smoke was coming from. I had a sore elbow for days.


And sometimes even the light-weight stuff can catch you out.
Just last week, when I was merrily waving wires around and splashing power on to a 24V contactor coil, my finger tips 'detected' the reactive component!
Some people never learn , or remember their schoolboy Physics . It certainly woke me up


So, unlike me, I suggest you investigate, calculate and err on the side of caution.
A little time nerding the net will give some very qualified information.
Being careful is better than being burnt or being deaded.

 

[John West]

Something for us all to remind ourselves of, and something that is often an entirely new thought for electronics newbies, is the power of capacitors when it comes to danger from electrical shock, possibility of explosions, and high instantaneous discharge currents creating heat and sparks through low resistances.

Capacitors, specifically electrolytic capacitors, but really any capacitor, may be thought of as small (and sometimes not so small) batteries. They can hold a significant charge for minutes, hours, days, and even weeks, and catch one completely by surprise simply because they "weren't plugged in." And as Beaniebots mentioned, there are low power, low voltage sources (as low as a single AAA cell,) that can charge a capacitor up to what may be lethal levels.

Items like the flashes on inexpensive cameras can charge internal capacitors to dangerous levels, and may not discharge them for extended periods. An interesting example of a capacitor that didn't appear at first to even be a capacitor, and one attached to a charging source that didn't even appear to be a charging source, was my longwave radio antenna and its coaxial cable.

I'd built the antenna as a Hertzian design, (2 quarter wave sections isolated from earth.) While it was only 40 feet tall, I'd used over two miles of wire in winding it, and had attached about 60 feet of coaxial cable to it. The day it knocked my socks off was a very dry day with a light breeze blowing. The antenna had picked up enough of a charge from the air passing through the wires that it had charged up the coaxial cable (an outer shield isolated from an inner conductor by a dielectric foam plastic, i.e. a capacitor,) to a voltage I don't even want to think about.

The cable had not been connected to my longwave transmitter for several days, so I picked it up by the connector without a second thought. That was when it knocked me across the room and punched a small hole in my fingertip. The moral of the story is that almost anything may be a source of electrical energy, and almost anything may be a capacitor, and the combination can bite you severely if you don't have your wits about you.

BTW, if I'd known more at the time, I would have added a 1 M Ohm resistor across my antenna terminals to provide a path for static charge to trickle away and avoid that unfortunate development.

Just a further concern for those of us engaged in this shocking hobby.