Electrical Safety

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.

darwin44:
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.

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

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

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.

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.

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! :wink:

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.

mirith:
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.

seanz2003:
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! :smiley:

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.

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.

darwin44:
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.

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

Incorrect. If it can cook a hot dog... Cooking hot dogs via electrocution | Evil Mad Scientist Laboratories ...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.

cld_1:
Electrical injury - Wikipedia

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.

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.

Being cooked is a type of burn, isn't it...?

One could argue that cooked is merely an instrument in which heat is produced to be able to cook with?

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.

InPhase277:
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.

fungus:
Being cooked is a type of burn, isn't it...?

So it is. My apologies.

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.

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.

Voltage without current, otherwise known as static electricity....

GoForSmoke:
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.