Dangerous voltages

[BobT]

I couldn't find the thread where this was brought up so I thought I'd open a new one.

It has been suggested in at least one other thread that 12vdc or 24vdc is not going to be able to deliver a dangerous electric shock. I was concerned about this statement but wanted to find more reliable information before disputing it.

Here's some information from a very reliable source that you all might find interesting and enlightening. (The person this comes from is on the board of the NEC) Thought I'd pass it along.

Quote:

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The resistance of a human body varies from person to person and from moment to moment. It is lower when the person is tired or when the person has been sweating. But it is not unreasonable, for the purposes of discussions of electrical safety, to assign a value of 300 ohms. At some point in everyone's life, their body's resistance to electrical current can get that low.

An electrical current of 0.1 amps passing through a human body is sufficient to cause death. Multiply that value by a resistance of 300 ohms (using Ohm's Law, E = IR), and you get a result of 30 volts. Therefore, it is possible that a voltage as low as 30 volts can inflict a fatal shock.

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Always be aware that electricity is dangerous no matter what the voltage potential.

Bob

[fishermn]

This isnt out of the realm of possibility, but it is unlikely that you would be shocked by anything 24V or less... What is a bigger factor is called "Source impedance" or effectively how much the voltage of the source will drop when connected to a "load" or a short circuit. A system with a very low source impedance (such as 120 V household outlets) can drive a significant amount of current into a load without too much of a voltage drop. The human body being in that 500 - 1000 ohm resistance range can therfore easily and up carrying over 0.5 amps when exposed to a 120V low source impedance system.

Getting to the point of your question though, 12V or 24V DC systems are not likely to be dangerous as a shock hazard, however, you also have to realize that there is a tremendous amount of energy stored on a battery. The bigger hazards come from things like directly shorting the terminals of a battery when your wrench slips, or from the outgassing that occurs during charging.

In general, just dont make yourself a direct part of the circuit when you are working on any electrical device. Be conscious of which terminal you are woking on and that there is no direct connection through your body back to the source.

[icegrogg]

DANGER HIGH VOLTAGE!!!!

It takes 10amps to light a lightbulb.It takes ONLY A 1/2amp to KILL A PERSON!!! when in doubt disconect.

[JohnMickish]

It takes about 8 milliamps (.008) amps to put your heart into fibrilation (uncontroled irregular heart beats) under the right (or really, really wrong) circumstances. A properly working GFCI receptacle is set to trip at .004-

.006 amps. Voltage is irrelevant at this point. Where voltage comes into play is in the amount of tissue damage, especially at the exit point.

[eurolarva]

Human resitance is anywhere from 1500 to 1.5Meg ohms. Not 300 ohm. If you were to hook an ohm meter between two blood veins you might get 300 ohms however skin is an insulator. If you have an ohm meter wet both finger tips and test it yourself. Multiple places and sources have attempted to do personal harm with a battery and a human body and no matter how hard they tried they can not lower the human resistance low enough to cause damage.

This was cut and pasted from a web site

A car battery provides 12 volts DC, pretty low voltage. However, the current it can provide is about 240 amps. The car circuitry (starter motor wiring, inside of battery, connecting wires) has a resistance of about 0.05 Ohms (12 v / 240 amps = 0.05 ohms). The power delivered to the starter motor is 2880 watts ( 12v X 240 amps = 2880 watts). If you grab both terminals of a car battery, the resistance of your skin (2000 ohms or more) will drop the circuit current to 6 milliamps (I = 12/2000 = 0.006 amps), not enough to feel or injure you. Take that same battery and use sharp probes to stick into the chest under the skin to a lower circuit resistance (500 ohms) and now you can be harmed (24 milliamps

[BobT]

I don't know where some of this information is coming from that suggest electricity is not something to have respect for but without a doubt, it is! This is precisely why I opened this thread. There are too many out there that don't believe it can hurt you and when one begins to take it for granted, that is when he/she is open for disaster. It's not worth taking the chance.

I'll use a scenario from another person. Suppose once every day of your life you take a revolver and place one cartridge in the wheel, give it a spin, aim it at you temple, and pull the trigger and every day the hammer falls on an empty chamber. Does this prove that this practice is safe and without its risks? I should say not! Likewise, so goes the idea that electricity is not dangerous. Every person's body resistance is different and varies considerably at any given time. 12vdc under the right circumstances can be dangerous to just about anyone. Don't take it for granted.

Bob

[musky hunter]

This topic has legs. While small currents can and do kill, when was the last time anyone heard of a car battery electrocuting someone? It just doesn't happen. But a dead short can heat up a wire, wrench, etc. to red hot just like that, and make sparks fly too. How many kids played with an electric train set? Again, voltage under 24 volts, they don't even feel it if they do straddle the track. Welders are around low voltage (20-30 volt or so) high amp current sources day in and out. While they can get a shock it's rare. But when you think of the thousands of sweaty welders working around these current sources and contorting their bodies into a multitude of positions to perform the work, there must be someone touching the electrode every day, but they are not dropping dead left and right from these low voltage sources. I conclude that anything 24 volt or less presents very little shock hazard. I'm an engineer and well aware of Ohm's law. When working with boat batteries, one would be better advised to wear goggles to protect agains acid burns after a battery explosion caused by a buildup of hydrogen gas rather than to worry about an electric shock. My attitude is completely different when working on home wireing, which can shock and kill. Somewhere between 24 volds and 115 volts lies the threshold, but it's above 24 volts. Just my humble $0.02.

[Northlander]

Bobt I wouldnt compare a 12 or 24 volt system to a game of Russian Rulet. ( spelling?) Just not a fair comparison in my eyes.

I agree any time you are around electricity you need to be aware and take safety precautions. But the odds of getting hurt from a 12 or 24 volt system are low in my eyes. Im not saying to be unsafe when working with them but I cant see any comparison with putting a gun to your head.

[troutmaster]

FISHERMN Is right, i couldn't have said it better myself!

[JollyT]

The risk of cardiac arrest is very low with 12v. Even more so because it is DC Electric chairs are AC. But, you can injure yourself severely with 12v DC. Short out the positive terminal to ground and look out!! Lots of heat, maybe even red hot metal. Want to be wearing your wedding ring when it's red hot? (One reason mechanics don't wear jewelry.)

If this is a short to a hood or fender, while the metal heats rapidly, the battery will also heat up due to the chemical reaction that releases the electrons. Maybe even to the point of boiling. Probably will outgas hydrogen too. Sparks, explosion and boiling sulfuric acid flying through the air. I knew a guy who lost his sight this way.

Even if it won't stop your heart you should still respect it!

[Jeremy airjer W]

12 or 24 volts is not going to kill you! If there was a possibility then there would be auto and marine techs dropping dead all the time. This obviously is not the case. However as mentioned before, the possibility of a battery exploding on you is very real. I have personally witnessed two explode, and have had one explode while I was hooking up a battery charger. All three where violent, shooting pieces of plastic 4 and 5 bays away. So a battery shutting down your heart, I would bet you have a better chance of being bit by a shark while swimming in millacs!! A battery exploding on you, absolutely can happen!!! BE CAREFULL!!!!!!!

[Fishook]

I don't mean to get off the subject here but am really interested in this "battery exploding" situation. What are we talking about here (anytime or only when charging a battery?), what causes it, and what can you do to prevent it? I really do not want to experience this scenario. Thanks.

[Jeremy airjer W]

The possibility exists anytime there is a spark near the battery. A battery gives off hydrogen gas which is an explosive gas. A small spark from a charger, jumper cables, or hooking up the battery terminals is all it takes. Granted this is not a common thing to happen but the possibility is very real!!! I alway keep the thought in the back of my head whenever I'm dealing with a battery. I always where my safety glasses and sometimes turn my head when hooking one up.

[stmichael]

This is some excellent information thanks guys for posting.

[BobT]

The potential for explosion is in fact very real! Hydrogen gas is probably the most explosive gas we know of. Every time you connect jumper cables to a battery you are at risk. Because the dead battery is a potential load on the live battery, there is likely going to be an arc when you connect the jumper cables. You've probably noticed this.

To protect yourself and reduce the potential for explosion you should hook up the cables following this procedure.

1. Connect the red cable clamp to the live battery's (+) terminal.

2. Connect the other red cable clamp to the dead battery's (+) terminal.

3. Connect the black cable clamp to the dead battery's (-) terminal.

4. Connect the other black cable clamp to the car frame of the live battery away from the live battery as much as possible.

Any spark that is likely to occur will happen when you make that last connection. By clamping it to the car frame away from the battery, you will reduce the possibility of igniting the hydrogen.

Bob

[Fishook]

Should I remove/loosen the cell caps on my deep cycle battery when I charge it or should they remain on tight like a maint. free? I am not a mechanical person and I am still unclear as to where/how the hydrogen gas comes into the picture. It would seem by loosening the caps the gas could escape?? Thanks to all for your informative responses to my earlier questions.

[BobT]

I believe most batteries have ventilation designed into the caps.

The hydrogen gas is the byproduct of the chemical reactions taking place inside the battery. Like the chemical reaction taking place when burning gasoline, in which you get a few byproducts such as heat, carbon dioxide gas, carbon monoxide gas, water vapor, etc.

Bob