Contridiction in Safety Lesson

In the body of your "Physiological effects of electricity" lesson, it states:
"Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source. [MMOM]"

ie AC current freezes it's "victim"s hand to the conductor.

But in the lesson Review at the bottom, it says that DC is more likly to freeze its victim.

"Direct current (DC) is more likely to cause muscle tetanus than alternating current (AC), making DC more likely to "freeze" a victim in a shock scenario. However, AC is more likely to cause a victim's heart to fibrillate, which is a more dangerous condition for the victim after the shocking current has been halted."

Not sure which type of current does in fact "freeze" a person. I always thought it was DC that did it, but I'm not sure now. Can someone let me know?

PS, I love this site. I forecast myself spending time here learning alot, maybe even teaching a little.

Welcome to AAC!

I claim no expertise in this, but my understanding is DC does tend to make muscles clench, while AC makes them spasm. The clenching means you tend to "grab" the wire, while AC may throw you off.

Both can be deadly, so it doesn't matter a whole lot. The subject keeps coming up in the Electronic Chat forum.

Feedback and Suggestions Forum for providing feedback and suggestions about All About Circuits, including corrections to the e-book. This forum is not for getting help with technical questions.

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I would have moved this, but I get the feeling you have a suggestion, or at least something more than a technical discussion. If I am wrong I will move it at a later date. One of my jobs as moderator it to keep things tidy.

You might introduce yourself in this thread...

Who are you?

Let us know a bit of your background.

Me, I write a bit (I have several articles in the AAC book), and am one of the more verbose folks around here.

Once you have 10 posts or so extra privileges click in, hit and run homework types forced rule changes when they deleted their questions and problems, afraid their teachers would see and denying their competition help. This concept is anathema to what this site is about. If you have not done so I strongly recommend you read our Terms of Service.

We have local albums for storage of schematics, as well as allowing attachments to the posts. Png or GIF files are preferred, JPG files tend to blur out schematics.

If you have any questions please feel free to ask.

In my experience, I have been shocked with all sorts of electricity. 700+V DC (high power), 1200VDC (low power) 480VAC, 240VAC, 120VAC - they all caused me to jerk my hand out of the death zone before I even knew what was happening. I've never experienced the "death grip", but I know people who have. Old electricians would probably tell you that 120V is more dangerous than 220V because 120V will give you the "death grip" but 220 will make you draw your hand back. I can't confirm that, but lots of people say it.

Yes, so far I have not had the "death grip" either, which may explain why I'm not dead yet. We have to be really respectful of the mains here in the UK though, as our house current is mainly 230V 50Hz AC - or maybe even 440V if you have any really big kit installed.

While ideally one should never approach a body part to anything that might be electrically live, it may be worth noting that the arm and hand muscles responsible for gripping and pulling towards the body are generally stronger than those for relaxing. Thus when approaching any conductor, bringing up the back of the hand may be marginally less dangerous than a palm-first approach. (Note my cautious wording, less dangerous ≠ safe). Laying your palm on to a live wire may be extra dangerous, as the dominant gripping muscles may lock on. The other way around, jerking away may be more likely.

There is always the possibility though that jerking away* may take you into contact with something else nasty though, like another live wire or a sharp object, or knock you off a ladder / scaffolding etc, so having survived the shock you might still break your neck!

Edit: *Don't assume from this that UK electrical engineers and electricians spend much of their time jerking away. We are always careful about what we do, and naturally we don't understand any coarse slang that may be used in distant lands.

Adjuster said:

Yes, so far I have not had the "death grip" either, which may explain why I'm not dead yet.

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Haha good point

While ideally one should never approach a body part to anything that might be electrically live, it may be worth noting that the arm and hand muscles responsible for gripping and pulling towards the body are generally stronger than those for relaxing. Thus when approaching any conductor, bringing up the back of the hand may be marginally less dangerous than a palm-first approach. (Note my cautious wording, less dangerous ≠ safe). Laying your palm on to a live wire may be extra dangerous, as the dominant gripping muscles may lock on. The other way around, jerking away may be more likely.

There is always the possibility though that jerking away* may take you into contact with something else nasty though, like another live wire or a sharp object, or knock you off a ladder / scaffolding etc, so having survived the shock you might still break your neck!

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This is a pretty concise but a thorough explanation and sound advice.
I have emboldened the underlying physiological reason.

Bill,
Thanks for your reply. I just recently found this site, and am reading the e-book to learn more. I saw the contradiction between the lesson and it's summary and hoped to give the author a heads up. I make the same type of mistake plenty in this cut and paste world. I posted on this particular thread according to the instruction to point out a mistake in the e-book. I was not trying to start a debate among the members. As for me personally, I avoid touching any live current, or voltage potential and I am sure all sane technicians do.

I will take a stab at it. There is an effect that takes place when current is AC.

First with DC - at certain level, the person will be unable to let go of the wire. Usually 10 mA is accepted as the threshold. At this point it is said that the muscles go into involuntary contraction and since all muscles spasm, you lock.

with AC something called skin effect occurs - at higher frequencies is pushed to flow along the surface which is why electrosurgical units are operated at very high frequencies - muscles do not go into contraction. But at lower frequency range, AC may actually be more dangerous than DC as skin effect is not very relevant.

check this out on current and safety: http://www.unc.edu/~finley/BME422/Webster/c14.pdf

When i did my training as a powerlinesman using 240v/440v AC we were taught to to use the back of the hand if unsure if you were insulated enough to work on live mains. Yes we had to work on live mains with bare hands with you body floating at 240V ac, on wooden ladders. I have seen a lines man hooked to 240V ac & unable to let go. We were able to remove him & he survived. In the 7yrs i was a lines man he was the only incedent of this nature at our depot. we were also taught to only use one hand when testing things.

@Philr07 - I suspect that Bill may have made the same mistake I did. I didn't realize that you were talking about the allaboucircuits.com e-book, I thought you were talking about some phsyical textbook you had in hand. I don't know why I thought that.

No, I understood. The problem is it is all true, it depends entirely on too many variables that are easily spelled out in a simple paragraph or two. That wasn't really its function though. It was trying to communicate the dangers in a very concise paragraph.

I have written parts of the book, pretty small pieces. I didn't write that one. I'm still not convinced it is incorrect though, so I'm sitting on the sidelines watching.

@Stantor: Yeah, I think you have it. Two moderators totally missed the meaning of my message. Maybe I need to work on more concise writing skills. I tried to private message "Studiot", (an e-book author), to explain.
I was unable to contact him to give him the heads up that his e-book needs to be edited on this. Apparently, I don't have that privilege on this forum.
I understand why, as it is very large. Thanks for your understanding.

@Bill,
Yes, one of the statements in the article is correct. The other is incorrect. It cannot be BOTH ways.
Please read the text of the article.

Hello, Phil.

I haven't received any PM so I don't know what went wrong.

We are always happy to discuss technical issues and you are welcome here by the way, it is a very friendly and caring forum.

I looked at the page you refer to and I note the following:

This is an extract from the body of the text

How AC affects the body depends largely on frequency. Low-frequency (50- to 60-Hz) AC is used in US (60 Hz) and European (50 Hz) households; it can be more dangerous than high-frequency AC and is 3 to 5 times more dangerous than DC of the same voltage and amperage. Low-frequency AC produces extended muscle contraction (tetany), which may freeze the hand to the current's source, prolonging exposure. DC is most likely to cause a single convulsive contraction, which often forces the victim away from the current's source.

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This is an extract from the summary at the end

Direct current (DC) is more likely to cause muscle tetanus than alternating current (AC), making DC more likely to "freeze" a victim in a shock scenario. However, AC is more likely to cause a victim's heart to fibrillate, which is a more dangerous condition for the victim after the shocking current has been halted.

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I see no contradiction here so please amplify your original observation.

The difference in the above statement is extended contraction vs single spasm (AC vs DC). I, too, admit it needs attention while reading, but it's not wrong, per se.

I agree. The statements are perfectly fine. They have to be read carefully though.