Arduino Forum

Using Arduino => General Electronics => Topic started by: darwin44 on Feb 10, 2014, 04:30 am

Title: Electrical Safety
Post by: darwin44 on Feb 10, 2014, 04:30 am
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.
Title: Re: Electrical Safety
Post by: seanz2003 on Feb 10, 2014, 05:06 am
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.
Title: Re: Electrical Safety
Post by: cld_1 on Feb 10, 2014, 10:27 am
http://en.wikipedia.org/wiki/Electric_shock

Electricity and water don't mix well -- and salt water conducts particularly well...
Title: Re: Electrical Safety
Post by: MarkT on Feb 10, 2014, 01:03 pm
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.
Title: Re: Electrical Safety
Post by: Peter_I on Feb 10, 2014, 01:59 pm

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.

Title: Re: Electrical Safety
Post by: jackrae on Feb 10, 2014, 02:39 pm
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
Title: Re: Electrical Safety
Post by: Peter_I on Feb 10, 2014, 03:17 pm

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.
Title: Re: Electrical Safety
Post by: mirith on Feb 10, 2014, 04:39 pm
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.
Title: Re: Electrical Safety
Post by: DVDdoug on Feb 10, 2014, 09:35 pm
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   

Title: Re: Electrical Safety
Post by: seanz2003 on Feb 11, 2014, 06:23 am
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.
Title: Re: Electrical Safety
Post by: cjdelphi on Feb 11, 2014, 08:09 am
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.
Title: Re: Electrical Safety
Post by: fungus on Feb 11, 2014, 10:29 am

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.
Title: Re: Electrical Safety
Post by: Coding Badly on Feb 11, 2014, 10:41 am
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.
Title: Re: Electrical Safety
Post by: sensai on Feb 11, 2014, 11:47 am

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.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 11, 2014, 01:10 pm
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.
Title: Re: Electrical Safety
Post by: fungus on Feb 11, 2014, 02:35 pm

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.


Being cooked is a type of burn, isn't it...?
Title: Re: Electrical Safety
Post by: cjdelphi on Feb 11, 2014, 04:02 pm
One could argue that cooked is merely an instrument in which heat is produced to be able to cook with?
Title: Re: Electrical Safety
Post by: InPhase277 on Feb 11, 2014, 09:58 pm
Hello! First post, so please forgive any infractions of forum etiquette!

I would say that for any project that involves the potential for lethal current to flow in a body of water requires not merely trying to limit the current, but to detect the flow and disconnect power automatically. Americans and Canadians call it a GFCI, Europeans and Australians call it an RCD. Whatever you call it, it is absolutely, positively REQUIRED on the high-voltage mains supplying the equipment.

Of course, various levels of power supply isolation may cause the stray current to go unnoticed by the mains device, but GFCIs are available in many voltages, and probably wouldn't be too hard to construct from available components. The basic idea is to measure the current leaving one wire of a power supply and compare it to the current returning to it on the other. If they are equal, no stray current is flowing. If the current leaving one wire is NOT equal to the current returning, there must be another path outside the normal intended circuit. One or two hall effect sensors could be setup to monitor this pretty easily.
Title: Re: Electrical Safety
Post by: Peter_I on Feb 11, 2014, 10:08 pm

Hello! First post, so please forgive any infractions of forum etiquette!
....


Welcome!

When you have something relevant to say, it can never be a breach of forum etiquette.


I like RCD's.
They have saved me a couple of times.
Title: Re: Electrical Safety
Post by: Coding Badly on Feb 11, 2014, 10:45 pm
Being cooked is a type of burn, isn't it...?


So it is.  My apologies.
Title: Re: Electrical Safety
Post by: polymorph on Feb 11, 2014, 11:37 pm
60Hz disrupts the heart and 50Hz doesn't? I have a hard time believing that slightly lower frequency makes such a large difference.

I have heard that AC, 50 or 60Hz, is more likely to cause fibrillation and therefore death than DC, because DC is more likely to just cause the heart to lock up until after the shock ends. Subject to damage from heat, of course.

Quote
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.


This is a dangerous thing to say on any forum with newbies. As has been pointed out, 12V can shock you. If you were to pierce the skin in two places, say both hands or a hand and opposite foot, 12V could electrocute you (electrocute = kill, shock is if you survive).

Drop a wrench across the 240Vac line coming into your house, you'll see an arc flash with a concussion that can kill you and set nearby flammables ablaze. No way, shape, or form does it require "at least 20kV".

Your coworker was hit with, let me guess, a neon sign transformer? Those are current limited. As low as 10mA. FYI, there is no such thing as voltage with no current.

Look up arc flash on Youtube. In one case, there is a story of a worker who thought he was measuring 440Vac and put the wrong meter across a 2kV line. The resulting arc flash jumped from the meter to him and then across the lines, set his clothes on fire heat to toe, blew him back into the wall, and eventually killed him a day or two later.

Once an arc forms, the resistance is relatively low. Your flesh, once you get beyond the skin, has only a hundred ohms or so of resistance, even from one arm to another. I did some testing with a current limited high voltage power supply that I designed and built. And that is at very low currents. At higher currents, as someone pointed out, cell walls rupture and the resistance goes down.

And even with relatively casual contact, 60V can do you in. Get a hand locked in place by the current, and you get to hang on the wire and slowly cook to death. Or muscular reaction throws you into something, causing secondary injuries that may be fatal.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 12, 2014, 02:11 am
Voltage without current, otherwise known as static electricity....
Title: Re: Electrical Safety
Post by: sensai on Feb 12, 2014, 02:22 am

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


Correct but parts per million makes the dust speck in each drop of rain non applicable.
I actually tested this as a kid in grade 5/6.
perhaps with all the chemtrails these days it might produce a different effect. Maybe I will try it again,, if it would rain and not snow for a day or two.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 12, 2014, 03:12 am
Melt the snow, but sweep the top layer off first before you collect any.
I am sure that once power is applied that the small impurities will find an electrode quickly enough, maybe stick?
Pure water has a dielectric constant of 81 where air = 1.

Chemtrails? That's what clueless people who see contrails call them. I've seen one video where a California woman is showing the rainbow here lawn sprayer in making on a bright, sunny day. But to her it's not a rainbow, it's chemicals in the water supply just like the chemtrails chemicals! It's a huge plot to kill us all, not natural in the slightest! What a dizzy bitz!

About 30 years ago if not longer there was enough acid in acid rain to slowly kill trees. Today the rate of burning is higher, because when you're running out of something the thing to do is use it up faster and faster.
Title: Re: Electrical Safety
Post by: polymorph on Feb 12, 2014, 06:58 am
Quote
Voltage without current, otherwise known as static electricity....


Voltage requires charge displacement.

Why do you think you go "ouch!"

So-called "Static" electricity is just a build-up of voltage through charge separation. Lightning is "static" electricity. Within the realm of everyday life, I had a car that would just about knock me on my behind on dry days as I was getting out.
Title: Re: Electrical Safety
Post by: InPhase277 on Feb 12, 2014, 07:46 am

Within the realm of everyday life, I had a car that would just about knock me on my behind on dry days as I was getting out.


I learned to keep my hand on some part of the metal as I slide out. This allows a small current to flow over the time it takes to get out of the car instead of one enormous current when you shut the door. Insulation-blowing machines can also create giant 6" sparks from their hoses on dry days. It is quite surprising when you walk too close to one to say the least.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 12, 2014, 09:33 am
I got into static back in school over 40 years ago though I never built a generator.
Voltage is electrical potential.

If you work in a place with compressed air to run machines with, it's got a little oil in it, then take a plastic bowl and give it a good spraying. Then find a victim, smile, point into the bowl and say "juice" but don't say touch or not, let them stick a finger in to show you there's no juice in that bowl.  ]:D

Title: Re: Electrical Safety
Post by: Peter_I on Feb 12, 2014, 01:11 pm

I learned to keep my hand on some part of the metal as I slide out. This allows a small current to flow over the time it takes to get out of the car instead of one enormous current when you shut the door. Insulation-blowing machines can also create giant 6" sparks from their hoses on dry days. It is quite surprising when you walk too close to one to say the least.


I do exactly the same when the weather is dry, otherwise I can draw a spark that makes my arm buzz to the elbow!
:smiley-eek:

I actually think, that I and the car work as an electrophorus (http://en.wikipedia.org/wiki/Electrophorus), unless the electrons are allowed to flow back slowly.

And in dry weather, I touch the door handles at work with my keys or a penknife before opening the doors at work. I can draw a couple of cm long sparks on a "good" day.
Title: Re: Electrical Safety
Post by: cjdelphi on Feb 12, 2014, 01:39 pm
how about attaching a small neon bulb to the car, to absorb any static build up?
Title: Re: Electrical Safety
Post by: polymorph on Feb 12, 2014, 02:10 pm
How would you attach the bulb?
Title: Re: Electrical Safety
Post by: Peter_I on Feb 12, 2014, 02:40 pm

how about attaching a small neon bulb to the car, to absorb any static build up?


I think the problem is, that the charge build up is between the driver and the seat.

My hineys are busy rubbing electrons off the seat while driving, and when I get out, the charges are separated.
That also explains, why the effect of the "lightning rod" some people attach to their cars, is quite limited.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 12, 2014, 06:28 pm
Wearing plastic fiber clothes don't help.
I spent time at the local VA where the carpets were like some kind of Van Der Graff belt. Get up to a door and the habit is to grab the handle and get zapped. I took to touching the painted part of the metal door and while no zap, I could feel the effect of the charge bleed (human, I feel it after it's happened as if it still is). Some time in the past the DoD bought a bazillion gallons of oil based paint and it makes a good resistor.

I have a scare when there's casual static because I am a computer user.
Title: Re: Electrical Safety
Post by: fungus on Feb 12, 2014, 08:20 pm
https://www.youtube.com/watch?v=rMIdOThUYfo

Title: Re: Electrical Safety
Post by: Peter_I on Feb 12, 2014, 08:20 pm
But there is some fun to it!
Sneak up to a friend, and touch a finger to his ear.
]:D




When my kids were small, they had a plastic chute for playing. Sometimes they played "Pikachu", meaning that one of them would slide halfway down and stop, then touch a finger to the nose of the other, standing next to the slide, and draw an audible spark.
They considered it great fun.
Title: Re: Electrical Safety
Post by: Peter_I on Feb 12, 2014, 08:23 pm

https://www.youtube.com/watch?v=rMIdOThUYfo


My goodness!
:smiley-eek:

what was that?
Had he thrown some kind of conductive string over the wires?
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 12, 2014, 11:35 pm

https://www.youtube.com/watch?v=rMIdOThUYfo




Then later to complain about poor electrical service. It is way.
Title: Re: Electrical Safety
Post by: cjdelphi on Feb 13, 2014, 01:21 am
After thinking about it, the neon bulbs would have to be connected inside door frames so it sits between you and the car ... not as straightforward as I thought at 3am
Title: Re: Electrical Safety
Post by: polymorph on Feb 13, 2014, 02:00 am
Actually, while you are in the car, you have no charge (assuming you are touching the key in the ignition or some other metal part.

The car picks up charge as it moves through the air, and probably some interaction with the tires.

When you step out, you discharge the car. It isn't actually you that has the charge built-up. So what you need to help keep the car discharged is a light wire or chain hanging down.
Title: Re: Electrical Safety
Post by: GoForSmoke on Feb 13, 2014, 02:34 am
They used to be called ground straps.
Title: Re: Electrical Safety
Post by: Peter_I on Feb 13, 2014, 08:58 am

Actually, while you are in the car, you have no charge (assuming you are touching the key in the ignition or some other metal part.

The car picks up charge as it moves through the air, and probably some interaction with the tires.

When you step out, you discharge the car. It isn't actually you that has the charge built-up. So what you need to help keep the car discharged is a light wire or chain hanging down.


There are two theories, and you represent one of them here:
"The charge buildup is between the car, and the rest of the world. You get zapped, because you form the path to discharge the car"

The other one is:"The charge build up is between you and the car. When you get out, you separate the charges, increasing the voltage. The discharge is between you and the car"

I believe in the second explanation.
(And I have had ground straps on cars. In my case, they didn't work at all)
Title: Re: Electrical Safety
Post by: polymorph on Feb 13, 2014, 04:02 pm
Peter_I, I think both are correct. It just depends on the vehicle and the conditions.

I've had cars where I'm sure it is the vehicle building the charge. I can touch metal inside, nothing happens. I slide around without getting out, I can touch metal and nothing happens. But if I get out with a hand on metal I can feel a big Snap through my shoe to the ground. And if I get out without touching metal, I don't get zapped until I touch the car.

Other vehicles, I think it is me sliding across the seat, because I won't get shocked if I touch the keys while still inside, but if I start sliding around and -then- touch metal, I get a spark while still inside. If I get out without touching metal, sometimes I can detect a spark through my foot to the ground, and another spark when I touch the car. That would seem to indicate I have the charge.
Title: Re: Electrical Safety
Post by: cjdelphi on Feb 13, 2014, 10:17 pm
Between they key/ignition a neon bulb should sit the last thing to do after stepping out is get the key which discharges static build up then?
Title: Re: Electrical Safety
Post by: polymorph on Feb 13, 2014, 10:50 pm
I don't think the neon bulb is necessary, except as an indicator that there was a discharge. The current is low enough that you only feel the spark because it is very concentrated where it hits you. So in that sense, holding the neon bulb is as good as holding a key, coin, or other metal object.

There is always that "wireless" antistatic wrist strap... smirk.