Why don't I get shocked by batteries?

[Rick]

Supernate read again what I wrote

FranticMad, there are a few things you need to learn about how this electrical stuff works. CRafter_Man is right, current capability doesn’t matter.

Let’s say that my car’s starter motor draws 120A, and I change out the battery for one that has twice the amperage capaicity. What will happen? Will my engine crank twice as fast? No. The reason is ohms law. It takes one volt to push one amp of current through 1 ohm of resistance. V=AR (For the engineers in the group I’m not using E and I cause I don’t feel like doing the expalantion) So in my starter circuit 12V/0.1ohms = 120Amp draw to the starter. It does not matter if the battery is capable of 200A or 500A as long as the resistance doesn’t change the current flow won’t go up. **Votlage and resistance determine current flow. **

(added bolding)

The topic under discussion was car batteries. Putting a larger battery (more amps, same voltage) in a car won’t cause current flow to go up. Just the same as changing a circuit breaker in your house from 15A to 30A won’t make you computer run faster. (although this is a bad idea for other reasons)

[Crafter_Man]

*Originally posted by Supernate *
**I may be taking what you are saying the wrong way… I hope you are saying that the lower the resistance, the more current will flow through the body so it is the resistance that matters, not the current.

I think any EE will tell you that current is the ONLY thing that matters as far as safety with electricity is concerned.**

Well I’m an EE, and I wrote it…

Some might think that a shock of 10,000V would be more deadly than a shock of 100V, which simply isn’t true.

You are wrong.

Common sense should tell you that, all else being equal, 10,000 V is much more deadly than 100 V.

The biggest problem I’m seeing in this thread is that no one is modeling the situation properly. You can’t analyze something without a model.

A first-order model is actually quite simple. It’s a voltage source in series with two resistors. One of the resistors represents the source impedance of the voltage source. The other resistor represents your body resistance, which is dependent on the actual path through your body.

If you know: a) the voltage of the voltage source, b) the source impedance, and c) your body resistance, then you can calculate current. If the current is below 10 mA it is generally safe.

Now to address your statement… a 10,000 V source with a source impedance of 1000 ohms would be quite dangerous. On the other hand, a 10,000 V source with a source impedance over 1,000,000 ohms would be fairly safe. I’m neglecting other factors (such as energy storage in capacitors), but you get the drift.

[Patty_O_Furniture]

Current capability doesn’t matter.

I may be taking what you are saying the wrong way…

You did misunderstand. Somebody said that car batteries can deliver 100 amps, so watch out! Crafter Man replied that a source’s current delivering capabilities are not relevant, meaning it doesn’t matter if a source can deliver 100 amps or 1000 amps. If the load only draws a few µA, it’s all academic.

I’m not sure where the 300-600 ohms for skin resistance came from in the earlier post. I grab a meter lead in each hand and get 200K-300K ohms. If my fingers are damp from perspiration or I pinch the pointed ends of the probes into my skin, I can get it down to a few tens of K ohms. From a 12V source (car battery or smoke alarm battery, it doesn’t matter), that’s good for a few hundred µA at best.

[Crafter_Man]

*Originally posted by Attrayant *
**I’m not sure where the 300-600 ohms for skin resistance came from in the earlier post. I grab a meter lead in each hand and get 200K-300K ohms. If my fingers are damp from perspiration or I pinch the pointed ends of the probes into my skin, I can get it down to a few tens of K ohms. From a 12V source (car battery or smoke alarm battery, it doesn’t matter), that’s good for a few hundred µA at best. **

It’s impossible to predict body resistance with any certainty; the values are all over the map. About all we know is that skin resistance can be quite high, while your body’s internal resistance can be fairly low. To complicate things further, the I-V relationship is probably non-linear (i.e. the resistance is a function of current). And because we’re dealing with a medium saturated with a salty and acidic liquids, the AC resistance will be different than the DC resistance.

Suffice to say, it’s impossible to pinpoint body resistance due to the sheer number of unknowns.