I know dumb question but I have to ask, It is my understanding that 3 ea 12V 100ah batteries in parallel will not shock you because the voltage is only 12V and that is not enough voltage to push the electrons through your body. Is this correct. in other words the 12V voltage going into the inverter will not shock you but the 120V voltage coming out of the inverter will, Correct? Help me to understand this, please.
Voltage
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The danger to a human from electricity is the amount of current passing through your body. Your body acts a resistor (whose characteristics widely depend on if you are wet or not, breaks in your skin, insulation of your shoes, etc), which when applied to Ohm's law you can see that higher voltage will result in more current (or as you put it, more electrons being pushed through your body). It only takes on the order of tens of mA to cause injury, so even 12 Volts can be dangerous in the worst case conditions. Having said that, NFPA 70E considers voltages less than 50 V to be generally permissible to work on live. There are differences in how AC or DC effects a person. Your skin acts as a capacitor, which has decreasing impedance with increasing frequency, therefore AC works against you in that case. However, since AC crosses the zero voltage line 60 times a second, you are more able to 'let go' in the case of shock.
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Just to correct something else you said though. 3 x 12V batteries in series will be 36V. It would only remain 12V if they were in parallel.
Speaking from experience, I have been badly burned by touching the positive post of a 12V car battery while leaning on the frame (and sweaty). It may not have killed me, but for sure it was not damage free. I still bear the scar. Also, electric fences in remote areas are often run off of 12V car batteries. Yes, they do that at low voltage so that they are not LETHAL, but it is still giving out a shock.
Speaking from experience, I have been badly burned by touching the positive post of a 12V car battery while leaning on the frame (and sweaty). It may not have killed me, but for sure it was not damage free. I still bear the scar. Also, electric fences in remote areas are often run off of 12V car batteries. Yes, they do that at low voltage so that they are not LETHAL, but it is still giving out a shock.
BillyMac59
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Current is the danger, not voltage. yes, voltage "pushes" the current through you. Think of this analogy: If your being stabbed by a long, thick sword, does it really matter how fast the thrust is?
drcampbell
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Um, no.
The zero crossings occur much, much faster (120 Hz) than a person's neuro-muscular system can respond. It takes a large fraction of a second to let go, at least 0.2-0.5 seconds, by which time the zero crossing will be long gone and the current will have resumed.
And 50-60 Hz is approximately the worst-case frequency for electrical contact injury -- the most biological response for a given current. Either DC or 400 Hz is about half as effective as 50-60 Hz, and increasing frequencies beyond 400 Hz are diminishingly effective.
My mistake, thank you for the correction.
Barbqranch
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Electric fences are typically 5000 volts or more, the fence charger is run off low voltage (12 or 120 or ?) which pulses the primary on a transformer. High voltage low current pulses every 5 seconds or so.
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Huh, weird that I read it as series…
Was he not right in his OP?
Brain fart I guess, or it was edited after I posted that.
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Makes sense actually, my bad for ASSuming. I installed several “Bull Dozer” brand fence chargers at a ranch I worked at as a teenager (long before I was an electrician) that were powered by car batteries, but I never opened one up to look inside.
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Love than analogy, I’m going to plagiarize it.
The one I have been using is that if you are electrocuted, the voltage has no effect on the amount of death you experience. But that seems to confuse some people. I like the sword idea better.
We got regular electrical safety training from Cooper Industries. Electrocution was a danger when it was at about 70V or above. AC or DC. But we had one electrician could use a thumb to check a 250V socket. Andrew "Digits" Leck.. He was also a good cricket player,
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drcampbell
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Electric fence chargers use a pulse-mode power supply which charges up an inductor from the battery and discharges it into the fence. The inductor stores a specific and limited amount of energy each time it's charged. When that energy is discharged into the fence, the voltage, current and duration are determined by the impedance of the fence and whatever's in contact with it.
If the soil is damp and your hands are wet & salty, the energy will be dissipated quickly, with a lot (relatively) of current and little voltage rise. If it's dry, the voltage will rise higher, the current will be less and it will take longer. And if there's nothing touching the fence at all, the voltage will be quite high and stay there for a long (again, relatively) time while the inductor's energy is dissipated into the air.
Because the energy is limited, the voltage or current (bot not both) can be quite high without risk of electrocution.
It's not just the current that kills, it's the current and the duration. I2t ... perhaps you've seen that somewhere else?
If the soil is damp and your hands are wet & salty, the energy will be dissipated quickly, with a lot (relatively) of current and little voltage rise. If it's dry, the voltage will rise higher, the current will be less and it will take longer. And if there's nothing touching the fence at all, the voltage will be quite high and stay there for a long (again, relatively) time while the inductor's energy is dissipated into the air.
Because the energy is limited, the voltage or current (bot not both) can be quite high without risk of electrocution.
It's not just the current that kills, it's the current and the duration. I2t ... perhaps you've seen that somewhere else?
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DC cannot cause fibrillation. It can however break bones from causing the muscle to contract or with enough current cause burning and organ damage.
That being said the normal DC cutoff is over 100 V. Only idiots on the 70E Committee are unaware of the hazards and easily confused and write dumb things.
That being said the normal DC cutoff is over 100 V. Only idiots on the 70E Committee are unaware of the hazards and easily confused and write dumb things.
kwired
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Some ranchers have up to 15 kV fence chargers that are intended to energize many miles of fence. Current and/or duration however is pretty low. They still can knock you on your behind though.
I was working near one once, messing with something on the ground near the fence and happened to hit the fence with top of my head. someone was there to witness said I was unconscious for a little bit. All I remember was I was doing one thing and next thing I remember was picking myself up from the ground.
GeorgeB
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Independent of your analogy, I prefer to think that POWER, the product of current and voltage, is the danger. Dry skin has high resistance and it is difficult to get sufficient current at 12V. But even 1.5V on exposed cuts is sensed. The Van de Graaff generator is very high voltage at substantially no current and does "no" damage.
No. It varies on voltage (and higher than discussed here) but minimum resistance through the skin is 1000 ohms. Internal resistance is much less. Current is the issue. At a about 100 mA with current through the heart AC will interfere with normal firing of the muscles and trigger fibrillation. At about half that it starts interfering with breathing but it’s not fatal. At around 1 A there is enough current to cause organ damage (burning) AC or DC. Well before than DC causes muscles to contract severely and sudden removal causes violent movements. There are also differences in body weight and males vs females.
The effects are constant with current, not voltage. We use voltages like a maximum safe voltage of 50 VAC because it’s convenient to think in those terms and not try to analyze every scenario involving wet or dry skin. So speculating about power has been disproven. There are 4 technical papers by Charles Dalzeil back in the 1950s that are public domain giving full details on the original experiments.
kwired
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I know of an incident where a mason was washing finished bricks with acids and was woke up pretty good by 16 volt doorbell circuit, even was wearing rubber gloves to protect him from acids. The right conditions you still subject to shock from some low voltages that you normally can touch without feeling a thing.
drcampbell
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Maybe his rubber gloves aren't insulators. Some polymers (plastics & rubbers) are conductive. Some casually, some deliberately -- automotive tires, for example, are deliberately conductive to dissipate any static-electricity charge that might be on the body when you pull into a filling station.
Seven-Delta-FortyOne
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After all of that, yes, 3 12v batteries in parallel is still 12v.
You would have to work very hard to feel anything from 12v. Not really an issue in normal circumstances.
Yes, DC can injure or kill you. The threshold is generally considered to be a bit higher than with AC.
Yes, DC can cause dysrhythmias in the heart. That’s how it kills you, along with burns and other organ damage.
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