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Topic: Electrical Safety (Read 14101 times) previous topic - next topic

darwin44

I know the mantra, "it's the current that kills you, not the voltage," except it seems that's not the full story.  Presumably the voltage does matter, because of the high resistance of skin (maybe cause by inefficiency)?  Or is X amps at Y volts really the same as X amps at 10*Y volts, when applied to the outside surface of the skin?  Then there seems to be time factor.  For example, when you walk across carpet and then get a jolt from touching the doorknob, the voltage and current are pretty darn high (I think) but because the duration is so short it doesn't injure you.

So...it seems there are 3 variables: current, voltage, and time (duration).  Can anybody point me to a resource that might include all 3 of that might help me come up with some real-world numbers?  I'm interested in both perception (at what point a human can feel it) and safety (at what point it is injurious).  Can anybody point me to a good resource for this?

Some background: I'm going to be using an Arduino to control something like a photoflash charger (depending on what voltage, current, and duration I come up with) to send out bursts of electricity.  It's going to be used in a saltwater environment.  Having this info will help me determine the voltage, current, and duration of pulses, because it would be ideal if it didn't kill anybody if a careless wet person happens to grab onto the contacts.  For now I'll assume they don't have a pacemaker.  Bonus points if a person in the water next to it can't even feel it if it drops in the water.

seanz2003

#1
Feb 10, 2014, 05:06 am Last Edit: Feb 10, 2014, 05:15 am by seanz2003 Reason: 1
Depending on who you ask, 30 ma is considered lethal current. Last time I measured my resistance finger to finger it was in the 100k ohm range. So ohms law tells me the lethal voltage is pretty high. But there are a lot of other variables that factor in. The time is relatively short, however long it takes to cause fibrillation. I've felt shocks at twelve volts from a car battery capable of hundreds amps- not very painful. And I have been electrocuted from 240 mains voltage which is quite painful and I consider myself lucky I received no burns or lasting injuries. Static electricity is typically very high voltage and very low current.

cld_1

#2
Feb 10, 2014, 10:27 am Last Edit: Feb 10, 2014, 04:00 pm by cld_1 Reason: 1
http://en.wikipedia.org/wiki/Electric_shock

Electricity and water don't mix well -- and salt water conducts particularly well...

MarkT

Skin resistance drops dramatically as the voltage increases and individual cell walls
rupture (or whatever).  240V will mean amps can flow and you die.  24V on dry skin
means microamps flow and it might only tingle. 40V when submerged in salt water
might also kill, since the skin absorbs the salt water and the area of contact is much larger.

Your blood vessels conduct well passing the current straight to the heart, which is the
immediate danger.  Nerves also conduct well and cause muscle spasms that can throw
you across a room (DC is especially bad for muscle spasm).

One handy tip when working on circuitry where high voltages might be present and
you are stupid enough not to wear insulating gloves is to touch surfaces with the back of
a knuckle first - any muscle spasm makes you grip tightly, and you dont want to
involuntarily clamp onto the conductor that's electrocuting you.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Peter_I


I know the mantra, "it's the current that kills you, not the voltage," except it seems that's not the full story......

So...it seems there are 3 variables: current, voltage, and time (duration).  Can anybody point me to a resource that might include all 3 of that might help me come up with some real-world numbers?  ...


Mantras should be taken as the semi-religious half-truth they are.
They may have a core of reason, but they are repeated senselessly rather than understanding the background. And worst of all, they are not updated when new knowledge comes around.

And there is a difference between AC and DC.

Under European legislation, the limits for "reasonably harmless low voltage" (CAT I) have been set at 30 V AC and 60 V DC.
You can still get hurt from arching and red hot wires, but you are not likely to get "killed in your tracks".

When I go above that, I'm very careful.

"Nothing is foolproof to a sufficiently talented fool"

jackrae

Irrespective of what EU documentation may state, when you stick your fingers onto 60VDC you are definitely going to feel it (based on personal experience).  Manufacture of any electrical apparatus liable to cause "excitement" subjects you to potential litigation, not only by the afflicted but also by relevant statutory bodies.  The response of the human body to electric shock is not only affected by the drive voltage or resultant current, but also the physiology of the recipient, his/her state of health at the time, the manner of connection etc etc etc.

First rule of construction  -  prevent hazard by physical separation

Peter_I


Irrespective of what EU documentation may state, when you stick your fingers onto 60VDC you are definitely going to feel it (based on personal experience).  Manufacture of any electrical apparatus liable to cause "excitement" subjects you to potential litigation, not only by the afflicted but also by relevant statutory bodies.  The response of the human body to electric shock is not only affected by the drive voltage or resultant current, but also the physiology of the recipient, his/her state of health at the time, the manner of connection etc etc etc.

First rule of construction  -  prevent hazard by physical separation


I totally agree!

And not to mention the shock, discomfort and outright anger that will be the reaction, no matter if it is dangerous or not.
"Nothing is foolproof to a sufficiently talented fool"

mirith

Also frequency of conduction matters for AC.  I know that 50/60Hz main will hurt you differently than RF frequencies.  That being said, if dealing with water in general, I recommend just filling the entire contraption with either silicone or other potting compound (Epoxy also works well, just make sure its rated for the voltage.  If you are staying at relatively low voltages (24V or less) then most hardware store epoxy is plenty good enough).  This will serve to water-proof your circuit well, as long as you allow it to 100% surround your circuit.  Just remember that you need to have a closed loop for conduction, and as I understand it, salt water is more conductive than people.  This means that if you are submerged in salt water, then most of the current will flow through the water, and not through you.  Not unless you have an insulated grounding tether to earth or the negative terminal.

Also, my mantra has been Volts Hurts, Amps Kill.  Voltage alone can still kill you, but it requires being at extremely high voltages (At least 20kV, though this is a guess, I know that I've watched a coworker get shocked by about 15kV without lasting harm, though it hurt) and usually results in a major explosion where the concussion kills you.  However, that is assuming no current.
 
If you are sticking a typical Arduino circuit underwater (LEDs, etc) then most likely it won't be able to cause damage to a person.  If its connected to a mains power, then you need to be a bit more careful.

DVDdoug

Of course, don't trust you life or health (or anybody else's life or health) to what you read on a forum or the Internet! ;)

I'd probably just insulate the connections (so that they can't be touched), plus waterproof the thing.   Maybe you can design-in a fail-safe so if water gets in, it shuts down.    There's actually a good chance the capacitor will fail to charge-up with the circuit submerged in salt water.

Just remember that you need to have a closed loop for conduction, and as I understand it, salt water is more conductive than people.  This means that if you are submerged in salt water, then most of the current will flow through the water, and not through you.  Not unless you have an insulated grounding tether to earth or the negative terminal.


Also, you generally are not going to get killed or injured unless the current passes through your heart or your brain.   Typically, it's when voltage passes from one hand to the other, or from your hand through your feet to the ground. 

I've felt shocks at twelve volts from a car battery capable of hundreds amps- not very painful.
It probably wasn't 12V...  There are inductors in a car that can give you a high-voltage "kick" when you connect/disconnect a battery.  That's why you sometimes see a spark.   I've never felt anything from 12V....  Except you CAN test a 9V battery by touching the terminals to your tongue!  So, if you put 12V terminals in your mouth you can probably feel it! :D   


seanz2003

Quote
It probably wasn't 12V...
most definitely was. The car was off. My arm was slick with perspiration when I rested it across the battery terminals to reach down into the engine compartment to locate a dropped bolt. Stung mildly, (similar to licking a 9 volt).  Much different sensation then getting hit with the high voltage that causes the arcing in the spark plug which I have also experienced.

cjdelphi

If your skin resistance is low enough, sweaty etc... or immersed in a water solution of salt or eroded copper flakes, you can feel a tingle even more so with an open wound... just from 12v!

But (and please don't try it)

Stick 240v into bucket of water, spread the wires apart and then stick your hand in, you should be safe as the current will always flow where the least resistive path, place both hands in and the current will flow up your arm then through your chest (ciao, see you at your funeral)  down through your other arm and hand..

Youtube it...

There's an exception, you can use high frequency with 240ac called the skin effect which makes it safe ...


So current kills not voltage, static produces thousands of volts.

fungus


So...it seems there are 3 variables: current, voltage, and time (duration).  Can anybody point me to a resource that might include all 3 of that might help me come up with some real-world numbers?  I'm interested in both perception (at what point a human can feel it) and safety (at what point it is injurious).  Can anybody point me to a good resource for this?


There's way too many factors involved for this to have a nice simple answer. A 9V battery can kill you under ideal conditions but we don't take any precautions when we handle them.

Is it summer? Winter? Are you sweating? What's the floor made of? What shoes are you wearing? Which hand did you touch the wire with? How long did you touch it? Where did the current enter/leave the body? What path did it take?

etc. etc.

If no electricity reaches the heart then you only have to worry about burns.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Coding Badly

If no electricity reaches the heart then you only have to worry about burns.


Incorrect.  If it can cook a hot dog... http://www.evilmadscientist.com/2007/cooking-hot-dogs-via-electrocution/ ...it can cook internal organs.  And cooked internal organs generally have trouble functioning correctly.

That is exactly how a classmate died from a lightning strike.  The person felt sick but otherwise appeared to be OK.  They died a few hours later because of the damage done to the hidden bits.

sensai


http://en.wikipedia.org/wiki/Electric_shock

Electricity and water don't mix well -- and salt water conducts particularly well...


Just to put my two bits in,,, rain water is great, it doesn't conduct electricity. It is actually the minerals in water that do the conducting, not the water itself.

GoForSmoke

Rainwater will have a little dust in it, it's not pure.

60 cycle power is just at the right frequency to disrupt heart rhythm. 50 cycle is not supposed to. A hand that grabs an AC source will grip and you can't let go, that's when you generally need someone else to take quick action.

You can take a shock just in an arm (or hand or whatever) that burns a path through. If/when the ends heal but the inside doesn't what results is gangrene. Electrician's insurance has to cover that, or used to.

Know as much as you can about what's safe but don't take chances on what you don't know.
1) http://gammon.com.au/blink  <-- tasking Arduino 1-2-3
2) http://gammon.com.au/serial <-- techniques howto
3) http://gammon.com.au/interrupts
Your sketch can sense ongoing process events in time.
Your sketch can make events to control it over time.

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