Stupid question

[gf]

Hi,

I was talking to my brother and he says that a current of 1 ampere can easily kill someone.

I know that hammond transformers has a 6.3v at 2 A. Will it kill you if you touch by mistake the heater wire connected to the transformer.

Thanks

[Peter Snowberg]

It actually takes only 5 milliamps to kill. If you pass 5 milliamps through your chest cavity, your heart will stop beating and go into fibrilation.

Luckily it's almost impossible to get a real shock when you're working with low voltage circuits. The one time I have heard of a death from 12 volts was in the case of a person cleaning a huge electroplating tank.

In general, if the voltage is less than 50 volts, you have nothing to worry about. I've been shocked so much that I can tolerate 120V fairly easily.

It's generally the voltage you have to worry about.... not the amperage.

Take care,
-Peter

[Torchy]

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html

Ive been hit by 17kV at milliamps off a radar pack that hurt. I also saw a spanner welded across two 28 volt 120 amp bus-bars. I respect current more than voltage. BUT If the gear is switched on I always keep one hand in my pocket when taking readings. I do not want a current-path straight across my chest

[niftydog]

At 6.3V it's unlikely, but not impossible.  I recommend you do not experiment!

I like the "water pressure" analogy.  Think of volts as water pressure and current as water flow.  Now think of your hand as the resistance.

If you had a tiny voltage and a large current - imagine a pipe the size of the palm of your hand connected to a full glass of water - you can stop the flow easily with your hand over the pipe.  The same set up, with a tiny pipe (less current or flow) is just as easy to block.

Now, replace the glass of water with the Hoover Dam.  A huge "potential difference" or voltage / pressure is now present.

The large pipe (large current) will simply blow your hand off.  The small pipe (small current) will blast a tiny hole through your hand.  Either way, you're screwed.

So while it's the current that actually does the damage to your heart, it's the voltage that delivers it to your heart by overcoming the resistance of your skin.

Generally, I consider anything over 30V to be dangerous enough to be real careful!  Of course, that isn't to say that a lower voltage isn't as dangerous!

Having said that, I work with 25kV all day long and many of my colleagues have felt it's wrath.  While they are all still very much alive, they had a bloody sore arm for a week or two!  And if it wasn't for their reflex action and the tiny amount of current available, they may well be not with us!

Keep in mind this little urban myth;  An army trainee engineer was taught one day about "internal resistance".  Later, he decided to check his bodies "internal resistance" with a multimeter.  The meter was powered with a 9V battery.  Holding the probes against each thumb he read a huge resistance; the resistance of his skin.  Now, for the "internal resistance" he thought.  He pushed the probes through his skin and died suddenly of electrocution.

Why?  We are mostly made of salty water.  Salty water has very little resistance to current flow.  Salty water flows in our veins.  The current path was from one thumb, via his heart, to the other thumb.  Even the modest current available from a 9V battery was more than enough to stop his heart.

[Paul Marossy]

I was under the impression that the amount of current mattered more than the voltage. At least that what I have been taught. That's an eye-opener that a 9V battery could theoritically kill you.  :shock:

I watched some educational broadcast once where they had this water that was deinonized and they discovered that it wouldn't conduct current. They also did say that the biggest factor in water conducting electricity was the salt content in it. I thought that was very interesting.

[Ge_Whiz]

Ohm's Law. When a power source is quoted as "20V @ 2A" it means that it will ALWAYS try to supply 20V, at UP TO 2A maximum current, depending on the resistance. Yes, it is the current flow that kills you (by stopping your heart, usually, but possibly via other neurological damage) but this can only be supplied subject to the push of the supply (= volts) and the amount your body will permit to to flow.

5mA can constitute a current sufficient to stop the heart. However, even when the skin is wet, the resistance of the path through the body via the skin is rarely less than 10,000 ohms. From Ohm's law, V=IR means that in the worst case scenario, 50V COULD push 5mA through the body when the path resistance is 10 kohms. Hence the safety limit for unprotected power supplies in the UK is a conservative 40V.

Just remember, the current rating of a power supply is the MAXIMUM available. For a 20V supply to provide the FULL 2A, the load resistance must be no more than R=V/I=20/2=10 ohms. (Things are slightly different for AC.)

People who use TENS machines for pain relief are warned never to connect them to opposite arms. It is, however, a reasonably safe and interesting experiment to connect the electrodes between wrist and elbow and to crank it up gently to see how it causes the fist to bunch up. I don't recommend this, I'm just describing an experiment that I have done. If you decide to try it yourself, don't overdo it - the enforced muscle contractions can injure tissues.

6VAC applied to the temples (DON'T TRY THIS AT HOME) will cause visual strobing effects, facial muscle twitching, uncontrollable eyelid flickering, and eventually a burning sensation under the electrodes and a painful headache.

[gez]

6VAC applied to the temples (DON'T TRY THIS AT HOME) will cause visual strobing effects, facial muscle twitching, uncontrollable eyelid flickering, and eventually a burning sensation under the electrodes and a painful headache.

GW, is there a dark side to you that we know nothing about?!  

[Paul Marossy]

Wow.  :shock:
I have more respect for electricity now than ever! Believe me, I already have a healthy respect for it, but more so with this new information I have just read.

[Mark Hammer]

Get a basic physiology book and read up on how miniscule the electrical changes are that are involved in the functioning of muscle cells and neurons. (microamps and millivolts)  It doesn't take a whole heck of a lot to disturb the functioning of some critical bits of gooey stuff in your chest and head.  Indeed, it only takes a couple of milliamps, selectively applied to actually fry neural tissue beyond repair.  I know; I used to do it for a living.

Of course, what is fatally destructive at the level of a few cells is less so at the level of entire muscles, organs, and such.  Good thing for us, giving the number of mic stands, carpets, metal drinking fountains, and angora sweaters out there in the world!  While I will happily lick a 9v to see if it was fresh, I would *not* do the same for an 8-pack of penlights, let alone an 8-pack of C or D cells.  

Yes, current matters more than voltage.  Voltage is just how badly the current wants to flow from here to there.  Current is how much might flow if circumstances allowed.  Consider the analogy.  You are barricading yourself in w washroom and a voice at the door says "We really, really, REALLY need to get in there"  Would you be more or less reluctant  if it was 100 people and one less "really" vs two people and 3 "reallys"?

[Paul Marossy]

Good analogy, Mark.  
I would definitely listen more if a hundred people were banging on the door wanting to kill me, so to speak, than a few... (not that either are a good thing!)

[Alex C]

While I will happily lick a 9v to see if it was fresh, I would *not* do the same for an 8-pack of penlights, let alone an 8-pack of C or D cells.  

Yes sir, a few years ago I made a power supply for a hobby rocket (to which I had attached four engines   ) with 8 AA batteries.  I wired them all in series for a total of 12 volts.  To check my wiring, I decided to touch the two ends to my tongue (thinking "it's only 3 volts more than the 9v batteries I lick all the time).  Bad idea.  I also tried to install a mini computer fan in my paintball mask with a similar pack, and since I didn't have a SPST or any other type of toggle switch, I used a pot to turn the power on and off- also a bad idea.  It sparked when I started turning it, then it smoked like crazy.  This was how I learned about current.

Alex

[gf]

Thanks for you explanations !

I'll be more careful now, even when working with batteries.

[Ge_Whiz]

GW, is there a dark side to you that we know nothing about?!

[Laughs] Yes, Gez, there is. I have an embarrassing psychological condition that causes me to remember in detail the most inane trivia from thirty-plus years ago, whilst forgetting my best friend's name or what happened the day before yesterday. The quote about the effects of 6VAC applied to the scalp comes from a warning accompanying an article for a brainwave monitor published by Dave Bollen in "Practical Electronics" in 1972 (no, I'm not looking it up, nor have I seen the article for several years!).

Incidentally, Gez, haven't we met somewhere?  :wink:

Indeed, it only takes a couple of milliamps, selectively applied to actually fry neural tissue beyond repair. I know; I used to do it for a living.

Now, that IS dark!  :twisted:

[Ge_Whiz]

:idea:  :idea: :idea: DEFINITELY don't try this :idea:  :idea:  :idea:

Here is an interesting observation that even some professional electricians of my acquaintance don't understand.

Consider a wall switch for a room light. If the switch is off, its resistance is infinite and the voltage across it is the full mains supply voltage (220V in the UK). Touching both contacts simultaneously will cause a shock.

If, however, the light is switched ON, the resistance of the switch is a fraction of an ohm. Now the voltage is dropped across the bigger resistance of the light bulb, and there is very little voltage across the switch. Touching the switch contacts simultaneously will now NOT produce an electric shock. HENCE IT IS SAFE TO TOUCH BOTH CONTACTS WHEN THE LIGHT IS ON, BUT NOT WHEN THE LIGHT IS OFF - the exact reverse of what most people would think (which is, light on - electricity, light off - no electricity).

I have a friend who has taken bets before demonstrating this. His physics is entirely sound - but I wouldn't do it.

[niftydog]

HENCE IT IS SAFE TO TOUCH BOTH CONTACTS WHEN THE LIGHT IS ON, BUT NOT WHEN THE LIGHT IS OFF

PROVIDED YOU DON'T PRESENT THE PATH OF LEAST RESISTANCE TO THE CURRENT!

If you were, for arguements sake, really well grounded, had wet skin and the circuit you touched was very low current (high impedance) you could easily sustain a fatal shock trying this "trick".

[Ge_Whiz]

Reply:

1. Note emphasized warning.

2. Do not attempt this trick while standing in a metal bucket of salt water.

3. An electric light is NOT a high impedance circuit - that's the point.

4. Do not attempt this trick.

5. Don't do it, just understand it.

6. Never try it.

[Sic]

2. Do not attempt this trick while standing in a metal bucket of salt water.

LOL

[ryanscissorhands]

I have never had this confirmed before, and was hoping that someone might be able to help out on this. I have heard that if you are going to test something for an electrical shock (only in a must-do-it-now-or-die situation) that you should tocuh the object with the BACK of your hand because it will cause your muscles to pull away from the object. If you touch it with your palm it will cause you to GRASP it and you won't be able to let go.

Is this true? Anybody know?

[aaronkessman]

this is sort of related, cuz it has to do with shocks...

i was working on an amp, similar to a marshall 18W and stuck my hand where i couldnt see what it was touching and got a shock touching some exposed terminals from the OT. anyone know what voltage/current I was dealin with there? i never bothered to measure...

i heard this loud buzz in my head, but that was probably coming out of the speaker. my whole arm shook (possibly more of me too), but i was able to pull it away. it was a REALLY weird feeling. I ended up with a tiny burn on the finger i touched it with.

Aaron

[smoguzbenjamin]

You just got a nasty shock Aaron Glad you're OK.

Now with this argument about voltage or current being dangerous, here's another thing to consider: the electrical resistance of your skin. If you're dry, so to speak, you'd have a higher skin resistance. If you were greasy or wet or both [enter at least 20 different possibilities here], your skin resistance would be lower - hence more current would flow. Since 230v will 'push' more current through your body than 6v, I'd say that the voltage is the biggest thing to worry about.

the exact reverse of what most people would think (which is, light on - electricity, light off - no electricity).

well since you're closing the circuit when you touch them when the light is off, I'd say it's pretty logical that you get shocked... or something. Ah man I ain't making sense again