Human resistance...

Not a torture question, but close.

There were talks a while ago from the auto manufacturers, because of the new electrical demands, to shift away from the 12V standards up to 48V.

Most experienced auto mechanics aren’t too worried about getting shocked by touching both poles of a 12V battery. Our electrical resistance is too high to pass any meaningful current, you can’t really get hurt.

At 48V though, I don’t know… At what voltage does the human body start letting some decent current getting through? Would 48V be dangerous to some unknowing (yet unexperienced…) mechanic?

48v is considered potentially dangerous, especially with wet or sweaty hands. I think it’s pretty unlikely to cause serious harm, but under the right circumstances, it could cause electrocution.

You have to define harmful. The highest resistance in the body is through the skin (makes sense). If you’re wet, you have to worry about inducing arrhythmia but if you’re dry, you have to worry about burns. I have no idea about the values though.

This is exactly right. I just want to add that 42 volts is generally considered the threshold for ‘finger-safe’ voltages in industry. In a factory for instance, any terminals that are at 42V or lower are allowed to be exposed; higher than that they are to be guarded and accessible only to trained personnel. This is in Canada, I think the U.S. regulations are pretty similar. As beowulff says the risk is small but not zero. There has been at least one fatality recorded in industry at 42 Vdc.

ETA: Short link on the topic which could explain things better.

Interesting. I may be wrong but the ‘industry’ current, being supplied by transformers, must be somewhat less deadly than a battery’s peak amperage.

One would think that the peak amperage of a 48V battery would be a 1/4 of a std battery, still a 150-200 amp shock is something you’d remember (or not for long…).

Remember that it only takes a tenth of an amp to cause death. Transformers can be sized to limit current but usually, where I am anyway, it’s done through fusing. Transformers can be sized to easily feed thousands of amps if needed. Current kills you but voltage is what enables it to flow. A 48V battery might be capable of delivering 100s of amps but that’s with a low resistance across the terminals. A healthy person with normal skin resistance won’t have that current pass through them. The problem is if someone comes along and is sweaty or has cuts on their hands and they grab the terminals because they’ve done it a hundred times. If the conditions are right it could kill them.

Not even sure if it’s possible to receive a 150-200 amp shock. Well, I supposed it’s possible, but it would take one heck of a high voltage if it’s through dry skin. Not only that, but the power source would have to be capable of supplying 150-200 amps at that voltage.

To elaborate - a 200A shock is something you wouldn’t remember, because you would explode! If we assume that the human body has 1,000Ω of resistance (hand-hand), then 200A of current would mean that 200^2 * 1000 = 40,000,000 Watts of power was being dissipated. This is the amount of power being generated by a small power plant - enough to run 32,000 homes.

All I can say is BANG!

When I was young we had a guy on our block Ed, and he used to be able to fix our electronic things, and he’d get shocked all the time. I’d say “How can you stand that.” He’d say “You get used to it.”

I wonder if you can build up some sort of tolerance to that sort of thing? Of course he was weird and usually on drugs but then again, no matter what electronic thing from lamps to TV (this was the 70s) he was able to fix it.

I don’t know if they build up a tolerance or they’re born with or what but I know guys who don’t think twice about grabbing hold of a 120 or 208V ‘hot’. I should put a meter on one of them and see what resistance I measure; I sure as hell can’t do it.

BTW, I got quite a few lifts over the years and never “got used to it”.

If they’re not grounded and they’re only touching the hot line, they don’t feel a thing; there’s next to no current flow other than the capacitive coupling between them and the ground. I’ve touched live 600-volt lines (3-phase, 346 V line-ground) here at work and felt nary a tingle.

I know what you’re saying but I also know exactly the type Marxxx is referring to. There’s no working with one hand behind the back for them. They’ll have a hot and be grabbing the frame of the cabinet with the other hand bare. I don’t know if they have thicker or tougher skin or what it is. Obviously they’re not the safest guys to work around since they tend to forget not everyone can do it. Now that high voltage gloves are required for all live work I don’t notice it as much as I used to.

Shit, my connection failed before I could edit the last post. I meant to put they accidentally ground themselves to the frame once in a while while holding the hot not continuously. Mostly I used to see it when snipping loose strands from an end or redoing a termination.

Well, that was my point. A 12V battery can deliver an 800amp peak surge to a starter but be perfectly safe to us by handling the two poles…

The discussion brought me to a point on the original subject, if car maufacturers were to adopt 48V, could the frame still be used as ground? Or just cover the car with safety warnings stickers…

Sure- the GROUND is used as “ground” for transmission lines carrying thousands of volts- no problems there!

Sure. Why not? If (for example) a car manufacturer were to adopt 100V, they could connect the battery’s negative terminal to the car’s chassis and it would be safe. Heck, they could otherwise connect the battery’s *positive *terminal to the car’s chassis, and it would still be safe.