Why Aren't Laptop Computers Grounded?

[technoextreme]

A 9V battery has 500-800mAh (that means 500-800mA over an hour)
A AAA battery has 1150mAh (that means 1150mA over an hour)
A AA battery has 2850mAh (that means 2850mA over an hour)

All very low current. I would expect a hearing aid battery to be somewhere lower than the 9V...of course it can't kill you, the amps aren't there.

Muahahah...... Thats enough current to kill.

 

[Akhenaten]

No. It doesn't. That's how DC works. There is no frequency (well, ok, 0Hz...if you want to be pedantic). AC has frequency, DC does not.

At what point between 0 Hz and 1 Hz does DC become AC?

 

[technoextreme]

At what point between 0 Hz and 1 Hz does DC become AC?

He's right though. I was being pedantic. For some inexplicable reason I was thinking of the Fourier Transform (Frequency Domain) where the DC signal is at 0 Hertz.

 

[jsfisher]

Given that skin resistance is usually in the thousands of ohms

More like hundreds of thousands to millions of ohms.

...your 19V powersupply will have a hard time killing you - UNLESS you do something spectacularly stupid so that the current flows through your blood and goes through your heart.

Current from 100 to 200mA is the generally accepted range for fatal electrical shocks. Even with very wet skin, skin resistance drops to only about 1,000 ohms, so applying Ohm's Law, worst case you'd experience only about 19mA, well below the 100mA fatal mark.

The one problem that can arise, though, is that you are getting into the "can not let go" range so other factors may come into play (difficultly breathing, injury from falls, etc.).

It is a little far fetched, but not completely out of the questions.

 

[Akhenaten]

He's right though. I was being pedantic. For some inexplicable reason I was thinking of the Fourier Transform (Frequency Domain) where the DC signal is at 0 Hertz.

Cool. My next question is:

If an AA battery has a capacity of 2850 mAh, and will produce 2850 mA for an hour, why won't it kill you with 2.8 Amps over 3 seconds or so, kinda sneaky like?


ETA: Rhetorical only. Please don't waste too much time answering it, if any at all.

 

[Akhenaten]

<yup>

The one problem that can arise, though, is that you are getting into the "can not let go" range so other factors may come into play (difficultly breathing, injury from falls, etc.).

It is a little far fetched, but not completely out of the questions.

See. Mr Westinghouse was right all along.

 

[technoextreme]

Current from 100 to 200mA is the generally accepted range for fatal electrical shocks.

There is two accepted ranges. The range that you are citing is for macroshock. Macroshock is what most people traditional associate with electorcution. You grab a wire and you sizzle. The other form of electroctuion is microshock. That form of electrocution is commonly cited in the microamp range. It very specialized and I doubt most people who undergo electrical safety training know of it unless you work in a hospital. Mircoshock is when you have current directly across the heart. The only time when this is a problem is when you have catheters and other doodads providing a path of current directly to the heart. This difference is quite important and very exploitable in applications like pacemakers.

ETA: Rhetorical only. Please don't waste too much time answering it, if any at all.

Technically I should know the answer to that question.

See. Mr Westinghouse was right all along.

No. Edison was right. 60 Hz is among the most dangerous currents in terms of not beign able to let go and inducing cardiac fibrrillation. Last I checked DC has none of those problems.

 

[jsfisher]

See. Mr Westinghouse was right all along.

Indeed he was. And I take special caution every time I get anywhere near that -48VDC power supply we have in our computer room just because of Mr. Westinghouse.

 

[Akhenaten]

Re: High current/short time discharge for a battery:

Technically I should know the answer to that question.

Apart from the Ohm's law bit of the thing, a dry battery simply can't carry out its internal chemical reaction fast enough to produce that sort of current.

No. Edison was right. 60 Hz is among the most dangerous currents in terms of not beign able to let go and inducing cardiac fibrrillation. Last I checked DC has none of those problems.

I know the fibrillation is a worry, but I always thought the grabbies was a DC only problem. Live and learn, I guess, and that's why we use the back of our hand, eh?

Indeed he was. And I take special caution every time I get anywhere near that -48VDC power supply we have in our computer room just because of Mr. Westinghouse.

I've had some of my worst beltings off DC. TVs are my nemesis and regularly throw me about the room.

 

[technoextreme]

I know the fibrillation is a problemo, but I always thought the grabbies was a DC only problem.

I thought the grabbies were something different from let go current. This is actually a rather quite interesting subject. A lot of the information involving electrocution comes from the man who invented the GFCI.

 

[Akhenaten]

I thought the grabbies were something different from let go current. This is actually a rather quite interesting subject. A lot of the information involving electrocution comes from the man who invented the GFCI.

That makes sense.

I understand that the speed with which a GFCI trips is vital in preventing a potential shock from continuing long enough to do harm.

In other words, lethality is a function of current across the heart and time.

I'm guessing a bit.

 

[jsfisher]

It was my understanding that tetanus (locked muscles, can't let go, grabbies) was more of a problem with DC than with AC, but was somewhat compensated for by the tendency of DC to throw the victim clear of the conductor.

Other than that, though, volt for volt, AC was 3 or 4 times worse for ones continued existence than DC.

 

[BobTheDonkey]

I thought the grabbies were something different from let go current. This is actually a rather quite interesting subject. A lot of the information involving electrocution comes from the man who invented the GFCI.

DC causes muscles to contract, and since it doesn't fluctuate, keeps the muscles contracted (grabbies).

AC's fluctuation is what may save you from the grabbies...but b/c of the frequency is more likely to interfere with normal heart fibrillation which is why AC requires few(er) amps to be deadly...

 

[Akhenaten]

Thanks guys. I've been a sparky all my life, but those last few posts have clarified stuff that I seem only to have ever half understood.

In any case, I take the same precautions with both flavours, and regard them both as potentially bitey. Kind of like an 'all guns are loaded' thing.

 

[GlennB]

Great thread.

Started with a minor innocent static shock and ended with violent disagreement about various means of electrical death.

This is what JREF is all about! Keep it up

 

[MikeMangum]

Perhaps. But, I also have a couple of desktop machines, all with grounded plugs, and they don't give me a spark.

...

But, I think it does build up static in some of the places I put it.

I've worked in labs with literally thousands of computers, all of them either desktop machines or rackmount machines, and I frequently would get shocked touching them, many times a day. I got to the point where I instinctively touched a machine with the back of my hand first (hurts less) to discharge any static that I had built up. Which brings me to the second point: your laptop is not building up static, you are.

 

[Akhenaten]

Great thread.

Started with a minor innocent static shock and ended with violent disagreement about various means of electrical death.

This is what JREF is all about! Keep it up

It's when you see all the sparkies starting to agree on the best methods that you really need to start worrying.

Also, if you really want to see the sparks fly (heh), just mention Uncle Nikola.

It's the electrical equivalent of Godwinning a thread.

 

[Akhenaten]

I've worked in labs with literally thousands of computers, all of them either desktop machines or rackmount machines, and I frequently would get shocked touching them, many times a day. I got to the point where I instinctively touched a machine with the back of my hand first (hurts less) to discharge any static that I had built up. Which brings me to the second point: your laptop is not building up static, you are.

It's quite logical really.

Things that move around build up static, but things that are static, don't.

Umm . . . hang on. Think music please . . .

 

[BobTheDonkey]

Thanks guys. I've been a sparky all my life, but those last few posts have clarified stuff that I seem only to have ever half understood.

In any case, I take the same precautions with both flavours, and regard them both as potentially bitey. Kind of like an 'all guns are loaded' thing.

Agreed.

And I'll admit that my previous comment that started this side-discussion was a bit excessive. What we do is we look at it like this:

We have a current (8A), we know that the voltage is 19V...putting those into ohms law (to solve for the resistance we'd get those values at: R = V/I ) we see R = 19/8. If your skin was ever to get to around 2ohms, you'd see the full dose of 9A.

Solving for the current flow of 19V at 1000ohms (wet skin), we use I = V/R > I = 19/1000 > I = 19mA. Not near enough for a lethal dose, so on that point I concede. My power supply would not be able to kill a human (unless you used it to bludgeon someone with or the voltage regulator goes bad).

However, I maintain that voltage isn't the killer. It's the current flow (amps) through the heart causing fibrillation (heart attack).

 

[tesscaline]

It was my understanding that tetanus (locked muscles, can't let go, grabbies) was more of a problem with DC than with AC, but was somewhat compensated for by the tendency of DC to throw the victim clear of the conductor.
.

You mean tetany, not tetanus.