Amperage and voltage... what's dangerous?

[fixorater]

I purchased a beam scanner from a surplus store recently. It requires 24v DC power. Since I work at a computer repair place I looked around for some sort of power adapter that puts out 24v. I found a power adapter that puts out 24v at 1.875 amps- to use it I'll have to splice the end of the cable and there is a potential for shock.

What I'm unclear on is what constitutes a dangerous level of amperage... The impression I have is that this shouldn't be dangerous since its such a low voltage- but I've been told that amperage is generally what kills you.

So should I worry, would a shock just be a little jolt or something real bad?

 

[robk]

24V isn't going to hurt you, regardless of the amperage. Because of the resistance of your skin (dry), it would require somewhere from 90 V and up to even feel it. Even at line voltage, 117VAC, it would take more than a casual brushing against a conductor to do any damage - maybe grabbing a ground like a water pipe and cutting into a hot wire with uninsulated cutters. Don't get so worried about it. It takes a lot more voltage than 24V to overcome your skin resistance, then the amperage comes into play.
Rob

 

[fixorater]

Thx for the quick response. Thats what I thought but wanted to confirm before my monkeying around forces the girlfriend call 911.

 

[rdh226]

Agree -- 24V is quite safe. You can probably get a decent spark with 2A available,
but virtually no danger to wetware forms like us, externally at least (skin being a
fairly poor conductor).

Disagree, 90V is dangerous. Generally speaking, DC power up to about 44-48V
is considered "safe", over that starts getting "dangerous" (that's where various
gummint bodies start requiring warning labels and the like warning idiots to keep
out). If you grab a 90V line, YOU WILL KNOW IT, and it will be unpleasant.

-RDH

 

[Ragnarok]

Do your splicing and other work first, THEN plug it in. No need to work on it while it's live.

 

[fixorater]

Yeah figured I would be working on it then plug it it- but didn't wanna do a crappy job splicing it and have the thing come apart while live and fry the cat.

 

[mikeage]

Treat it as if it's dangerous.

Don't get me wrong... it's not.

If, however, you need to ask the question, then you don't yet have the experience necessary to decide these things (it's ok... takes time). Use the chance now to develop good habits.

I've worked on everything from 1.5V to 15kV, both live and disconnected. Why not practice doing things the right way, every time?

It won't take you all that long to apply principals of basic common sense (only one hand in the circuit at a time, never leave exposed terminals where they can be touched, etc), and the habits you'll develop will come in handy if you decide to work on something dangerous (house wiring, etc.)

But don't sweat it. You'd be hard pressed to hurt yourself with 24V (the biggest danger is a burn from a spark, not a shock).

 

[robk]

Agree -- 24V is quite safe. You can probably get a decent spark with 2A available,
but virtually no danger to wetware forms like us, externally at least (skin being a
fairly poor conductor).

Disagree, 90V is dangerous. Generally speaking, DC power up to about 44-48V
is considered "safe", over that starts getting "dangerous" (that's where various
gummint bodies start requiring warning labels and the like warning idiots to keep
out). If you grab a 90V line, YOU WILL KNOW IT, and it will be unpleasant.

-RDH

Bah, that's for sissies. I know electricians who test for live wires by touching them! Not to say you should do it, but the skin is not that conductive to low voltages.
Rob

PS- I've gotten shocks that literally threw me across the room. When I was a kid and building ham radio transmitters, I touched the plate cap on a 807 tube (about 450VDC) and I saw stars. But I'm still here. The rule that has always saved me was to keep one hand in your pocket when dealing with dangerous voltages - no direct path from one hand to the other through your heart!
Rob

 

[gadget_lover]

Please, please be careful with advice about voltage. 110 volts IS dangerous even if you are not grounded by grabbing anything external. The second wire in a power cord IS the equivalent path back to ground.

Lower voltages can be dangerous, especially if you do something dumb like stripping wire with your teeth. Stop laughing. It's been done.

It's the amps that kill. 20ma can be fatal. 60ma can be quickly fatal.

Dry skin has a fairly high resistance (1 - 2 million ohms) but sweat, blood, spit can drop it to the thousands. At that point 60 ma is not out of the question with 110 volts.

So just use caution, and always turn things off and unplug before you start working.


Daniel

 

[Ashton]

This, to me, begs the question why is high-frequency pwoer safe then? toys like van-d-graph gennerators and Tesla Coils emit electicity that, IIRC literally passes over your skin without ever entering your body.

 

[carbine15]

yes and stun guns and TASERs use hundreds of thousands of volts to stun and not kill... High voltage is fun until you get more amps behind it; then it becomes deadly.

 

[mikeage]

The rule that has always saved me was to keep one hand in your pocket when dealing with dangerous voltages - no direct path from one hand to the other through your heart!

Good advice -- but not quite enough to allow one to touch a live wire. If one's wearing thin shoes on a concrete (or metal!) floor, there's a pretty good path to ground...

That said, many accidents happen from too many hands in too small an area. Keeping one hand out of the way (behind one's back, pocket, etc) is an excellent safety precausion.

 

[SenKat]

Like Rob - I have learned the hard way - have a few minor scars to prove my battle success as well as knowing I have learned my lesson ! I am cautious (mostly) and will undoubtably be zapped again and again, working on live power outlets at 115v, I have blown a few circuit breakers in my time, especially in my old house - that house was 50+ years old, and the circuits were nilly-willy all over the place as far as the breaker box was concerned. Thankfully the new owner is completely reworking the entire electrical system - as NOTHING was grounded - with the exception of two rooms in the entire house - that I personally banged the 8 foot grounding rod outside, into the ground, ran the wire to the socket.

Always just be careful, and always have someone around if you are working on live circuits - OR DEAD. Always assume it is live - even after you have certified the circuit is off - it'll save you quite a few tingles in the long run !

 

[The_LED_Museum]

It takes at least 48 volts before you can receive an electric shock on reasonably dry skin that would be considered the least bit hazardous.
24 volts is safe to work with, regardless of the available current (amperage).

 

[Enigmafied]

Amperage is the dangerous one.

Cattleprods put out ~50,000 volts. Causes pain on the skin, to get the animal moving. Very safe because the electricity moves between the 2 prods.

Stunguns are basically the same thing as a cattle prod, except the voltage is ~20,000 to 150,000 volts, and it puts out more amperage. Relatively safe, as the electricity also only moves between the 2 prods.

Tasers have the same power as a stungun, except the prods are not in a fixed position, making them dangerous, as the route the electricity will take depends on where the "hooks" land. If that path is across the heart, and there is enough mA, well, thats why we have people who have died from being shot with a taser.

The problem is the mA required to stop a person, is also enough to stop their heart.

Here is a description of what different amperages will do:

Less than 0.5 mA No Sensation
0.5 to 2 mA Threshold of perception
2 to 10 mA Muscular Contraction
5 to 25 mA Painful shock, inability to let go
over 25 mA Violent muscular contractions
50 to 200 mA Ventricular fibrillation
over 100 mA Paralysis of breathing

Now, with dry skin having a resistance of 500,000 ohms or so, which is not uncommon, at 208volts only about 0.4mA is expected to pass, which according to the above chart, should not give any sensation.

However with wet/dirty/sweaty/bloody hands, the skin's resistance could drop to 500ohms, which would let about 400mA to pass... and thats not good for anyone.

 

[Stillphoto]

"5 to 25 mA Painful shock, inability to let go"

On film sets, we usually keep a "grip stick" nearby..also known as a 2x4 or a wooden broom stick. If one of the grips begins to get bbq'd you grab the stick and whack him/her out of contact with the circuit. That, or you can jackie chan them, but you must be flying in the air and kick with both legs...Significantly harder lol.

 

[matrixshaman]

The frequency of the voltage is a BIG factor also in how dangerous it is. 60 cycle AC (standard U.S. house voltage frequency) is amoung the most deadly. And just remember voltage is the amount of force and current is the amount of volume so it takes some of both. Be safe - if you have ever had a shock you'll respect it. I think my first experience was from an unmarked electric fence. Not fun.

 

[SenKat]

An unmarked electric fence ? That is just wrong....OW.

 

[Hemlock Mike]

Robk --

You must be an old ham too - to remember the 807, 811 and 813 tubes !!
That was an old expression I got too - "keep one hand in your back pocket" !!
I too went flying but it was a 6146 tube cap !!

Those stars are so bright

Mike

 

[David_Web]

This, to me, begs the question why is high-frequency pwoer safe then? toys like van-d-graph gennerators and Tesla Coils emit electicity that, IIRC literally passes over your skin without ever entering your body.

In this case it's not the voltage that is the problem but voltage-drop. It is no problem running 1000 volts through a led, just make sure you do something with the remaining 997 volts.
That is why a cable will survive but not you.
We are also talking about very low amperage here so the total energy will not get very high.
For high voltage it's actually better to be more conductive as the voltage-drop is a lot less than it would be if you where less conductive. Within reason that is.
But that require that there is something else to conduct the rest away, but there usually is when it comes to these "toys", mainly the ground (and sometimes air).

I am in no way certified or otherwise educated in high voltage so use these words with caution! (use high voltage with caution as well!!!)