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Old 19th January 2010, 12:50 PM   #41
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Originally Posted by BobTheDonkey View Post
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.
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Old 19th January 2010, 01:12 PM   #42
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Originally Posted by BobTheDonkey View Post
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?
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Old 19th January 2010, 01:16 PM   #43
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Originally Posted by Akhenaten View Post
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.
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Old 19th January 2010, 01:20 PM   #44
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Originally Posted by MortFurd View Post
Given that skin resistance is usually in the thousands of ohms
More like hundreds of thousands to millions of ohms.

Quote:
...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.
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Old 19th January 2010, 01:24 PM   #45
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Originally Posted by technoextreme View Post
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.
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Old 19th January 2010, 01:31 PM   #46
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Originally Posted by jsfisher View Post

<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.
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Old 19th January 2010, 01:33 PM   #47
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Originally Posted by jsfisher View Post
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.
Originally Posted by Akhenaten View Post
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.
Quote:
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.
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Old 19th January 2010, 01:33 PM   #48
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Originally Posted by Akhenaten View Post
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.
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Old 19th January 2010, 01:46 PM   #49
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Originally Posted by technoextreme View Post

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.


Originally Posted by technoextreme View Post
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?


Originally Posted by jsfisher View Post
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.
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Old 19th January 2010, 01:59 PM   #50
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Originally Posted by Akhenaten View Post
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.
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Old 19th January 2010, 02:07 PM   #51
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Originally Posted by technoextreme View Post
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.
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Old 19th January 2010, 02:11 PM   #52
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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.
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Old 19th January 2010, 02:34 PM   #53
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Originally Posted by technoextreme View Post
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...
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Old 19th January 2010, 02:44 PM   #54
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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.
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Old 19th January 2010, 02:53 PM   #55
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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
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Old 19th January 2010, 03:04 PM   #56
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Originally Posted by Wowbagger View Post
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.
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Old 19th January 2010, 03:07 PM   #57
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Originally Posted by GlennB View Post
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.
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Old 19th January 2010, 03:12 PM   #58
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Originally Posted by MikeMangum View Post
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 . . .
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Old 19th January 2010, 04:33 PM   #59
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Originally Posted by Akhenaten View Post
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).
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Old 19th January 2010, 05:06 PM   #60
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Originally Posted by jsfisher View Post
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.
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Old 19th January 2010, 05:27 PM   #61
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Originally Posted by SkeptiChick View Post
You mean tetany, not tetanus.
I'm not so sure. It seems like most of the online references for either tetany or tetanus take a solid medical perspective (where the former is calcium related and the latter, bacterial) for muscle contraction conditions. Wikipedia seems a bit broader than most for tetany by admitting "or other conditions". The "other conditions" certainly could be electrical shock.

The couple of dictionaries I tried tended towards the opposite. Tetany was solidly a medical condition related to abnormal calcium metabolism while tetanus could be more than just "lock jaw."

Bottom line: I'm confused and will admit it.
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Old 25th January 2010, 01:02 PM   #62
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Originally Posted by Wowbagger View Post

Which responses seemed particularly ignorant, to you?
Too many to choose from. I'll let you select them given that A/ anyone should know a laptop is not only not grounded, but is explicitly isolated from the power ground B/ a power ground isn't needed for a static discharge.

Quote:
Repeating what others had stated, in different words.... is addressing verbosity.... how exactly?
Munford's explanation wasn't up before I started. The others preceding are rubbish.
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Old 25th January 2010, 01:13 PM   #63
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Originally Posted by stevea View Post
Too many to choose from. I'll let you select them given that A/ anyone should know a laptop is not only not grounded, but is explicitly isolated from the power ground B/ a power ground isn't needed for a static discharge.


Why should 'anyone' know this?

Do you include people who have never seen a computer of any description? 6-month-old babies?


Originally Posted by stevea View Post
Munford's explanation wasn't up before I started. The others preceding are rubbish.


The lack of specific examples makes it seem that you might not have any. If you do, then at this stage, you'd better make them really, really good ones.
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Old 25th January 2010, 01:35 PM   #64
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Originally Posted by BobTheDonkey View Post
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).
Freaking hell. I go into detail about how that statistic is wrong and you still botch it up. Cut skin has a resistance of zero ohm's!!!!!!!!!! And its also a stupid metric. Given that my grandpa's pacemaker battery is capable of stopping my grandpa's heart and remains operational for about two years and is only the size of a hearing aide battery something is wrong with your logic. My grandpa would be dead if it was solely current that kills.
Quote:
Why should 'anyone' know this?

Do you include people who have never seen a computer of any description? 6-month-old babies?
Most newer power supplies for any electronics are two prong which lock stop and cocks a shotgun at the argument. Also, anyone who has done basic electrical work would know that the third prong doesn't do what the op or many people for that matter think it does.
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Old 25th January 2010, 01:51 PM   #65
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Originally Posted by technoextreme View Post
Freaking hell. I go into detail about how that statistic is wrong and you still botch it up. Cut skin has a resistance of zero ohm's!!!!!!!!!!


Oh rly?


Originally Posted by technoextreme View Post
And its also a stupid metric. Given that my grandpa's pacemaker battery is capable of stopping my grandpa's heart and remains operational for about two years and is only the size of a hearing aide battery something is wrong with your logic. My grandpa would be dead if it was solely current that kills.


I rather think the consensus is that you can't have current without voltage and saying that one or the other will kill you is a nonsense statement.


Originally Posted by technoextreme View Post
Most newer power supplies for any electronics are two prong which lock stop and cocks a shotgun at the argument. Also, anyone who has done basic electrical work would know that the third prong doesn't do what the op or many people for that matter think it does.


That's why people come here and ask questions.

Works well, doesn't it?
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Old 25th January 2010, 02:13 PM   #66
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Originally Posted by jsfisher View Post
I'm not so sure. It seems like most of the online references for either tetany or tetanus take a solid medical perspective (where the former is calcium related and the latter, bacterial) for muscle contraction conditions. Wikipedia seems a bit broader than most for tetany by admitting "or other conditions". The "other conditions" certainly could be electrical shock.

The couple of dictionaries I tried tended towards the opposite. Tetany was solidly a medical condition related to abnormal calcium metabolism while tetanus could be more than just "lock jaw."

Bottom line: I'm confused and will admit it.
Personally, I go with the CDC. I linked Wiki because it is generally more understandable, and in this case I knew it to be pretty correct (even if it didn't go into great detail).

Tetanus is the name of a disease which is caused by bacteria (C. tetani, aka "tetanus bacillus"), for which most people in developed countries are vaccinated against regularly. http://wwwnc.cdc.gov/travel/yellowbo...2/tetanus.aspx

Tetany is a medical sign in which the patient's muscles contract involuntarily. The most common cause is a calcium imbalance/lack. There are other causes, but those are either much more rare, or only occur in non-humans (cows, for example, have their own version of tetany). I'd link a definition from the CDC, except the CDC assumes readers already know the definition, so don't provide one. If you search the CDC site for it though, you'll get a big long list of results where it is a symptom of a larger disease/condition. You cannot vaccinate against tetany.

Neither one is technically applicable to the situation, as you're talking about a muscle spasm caused by an electric shock and not a disease or a symptom of a disease. But tetany is much closer to describing the results of an electric shock than tetanus is.
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Old 25th January 2010, 03:46 PM   #67
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Originally Posted by BobTheDonkey View Post
how many amps is that hearing aide battery?

Here's a few examples of your pleasure:

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.
Milliampere hours are not a measurement of how much current a battery can deliver, they’re a measurement of how much energy it stores (which is not relevant to this discussion). For example, in theory, a perfect 2000 mAh battery could deliver 8000 mA for fifteen minutes (or, say, 1000mA for two hours) before being drained.

The current a real battery can deliver is limited by other factors, which in circuit theory is represented by “internal resistance”.

For the sake of experiment a few years ago, I momentarily shorted a 2500 mAh AA cell through my multimeter. It read about 1800 mA.
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Old 25th January 2010, 07:13 PM   #68
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http://www.allaboutcircuits.com/vol_1/chpt_3/4.html
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Old 25th January 2010, 08:18 PM   #69
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Originally Posted by Akhenaten View Post

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

It's the electrical equivalent of Godwinning a thread.
No, it just makes the thread go off in a different arc....

V.
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Old 25th January 2010, 08:28 PM   #70
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Originally Posted by Verde View Post
No, it just makes the thread go off in a different arc....

V.
I re-fuse to go there.
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Old 25th January 2010, 09:23 PM   #71
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Originally Posted by jsfisher View Post
I re-fuse to go there.


No potential at all?
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Old 25th January 2010, 09:25 PM   #72
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Originally Posted by Akhenaten View Post
No potential at all?
Is that any way to conduct a polite conversation? You are just trying to switch the subject. Go ohm!
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Old 25th January 2010, 10:13 PM   #73
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I just measured my laptop power supply (which does have a "earth" pin), and the output voltage 0V is actually connected to the mains earth. That means the laptop's USB port etc are also connected to mains earth, just as for desktop computers.

This is better than my previous laptop, which didn't have an earth pin, and which would sometimes give me a very small "tingle" when I touched the output connector when standing on tiles. This was due to capacitive coupling in the power supply injecting a small 50Hz voltage on the output connector (ie, it was NOT due to static electricity).
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Old 25th January 2010, 10:41 PM   #74
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Originally Posted by MichaelN View Post
I just measured my laptop power supply (which does have a "earth" pin), and the output voltage 0V is actually connected to the mains earth. That means the laptop's USB port etc are also connected to mains earth, just as for desktop computers.

This is better than my previous laptop, which didn't have an earth pin, and which would sometimes give me a very small "tingle" when I touched the output connector when standing on tiles. This was due to capacitive coupling in the power supply injecting a small 50Hz voltage on the output connector (ie, it was NOT due to static electricity).


Yup, that sounds about right. I've got a few bricks and other power supplies that are earthed through the mains plug. Especially older transformer types.

You've reminded me of something else too.

I wonder how many non-sparky people realise that Earth isn't as 'low' as a voltage can get, and that in fact a lot of equipment has a negative rail that's 'below' Earth.
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Old 25th January 2010, 10:42 PM   #75
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Originally Posted by jsfisher View Post
Is that any way to conduct a polite conversation? You are just trying to switch the subject. Go ohm!


Can I just stay farad day?
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Old 26th January 2010, 07:09 AM   #76
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Originally Posted by Akhenaten View Post
Can I just stay farad day?
As O'Henry might say, only with great reluctance. Now my head hertz.
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Old 26th January 2010, 08:04 AM   #77
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I think we might be risking modulation here.

In seriousness, I can't think of anything else to add that might assist the OP, but I'm happy to clarify anything I might have said. A bit of general electrical knowledge is a good thing for everyone to have, I reckon. 'tis dangerous stuff, as much as I love it.
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Old 26th January 2010, 08:34 AM   #78
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Originally Posted by Akhenaten View Post
That's why people come here and ask questions.

Works well, doesn't it?
Not when BobTheDonkey keeps on giving out information that can get people killed.
Quote:
I wonder how many non-sparky people realise that Earth isn't as 'low' as a voltage can get, and that in fact a lot of equipment has a negative rail that's 'below' Earth.
Thats because most people don't realize that ground is what ever you define it to be. I can define a ground point to have a potential of 1000V in relation to earth ground. It doesn't matter at that point its 0V.
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Old 26th January 2010, 09:26 AM   #79
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Originally Posted by Verde View Post
No, it just makes the thread go off in a different arc....

V.
Originally Posted by jsfisher View Post
I re-fuse to go there.
Originally Posted by Akhenaten View Post
Can I just stay farad day?
Originally Posted by jsfisher View Post
As O'Henry might say, only with great reluctance. Now my head hertz.
I sense zero resistence to all this punnery .

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Old 26th January 2010, 09:39 AM   #80
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It seems that JREF has cornered the market on electricians-who-wish-they-were-comedians.

I wonder watt that's all about.
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