Electrical safety

[dominiz]

1. The electrical properties of the body that changes when it is wet and how it affect electrical safety. 2. Way electrical shock can be minimise.
3. What is consider as a fatal current.

 

[jpanhalt]

This type of question comes up from time to time. The simple answer is that sharply defined limits for voltages and currents that will cause death versus those that will not cause death do not exist. There are, of course, general guidelines. In addition, electrical shock can cause burns, fractures, and other body damage without ultimately resulting in death.

Unfortunately, some very interesting articles on the subject are not freely available on the Internet, but may be available to you at school.

Here is one source: https://www.journals.elsevier.com/forensic-science-international/

If you search on "electrocution," you will get several articles. I found the second article by Z. Peng and C. Shikui particularly interesting. That article reports seven case studies of electrocution in welders. The lowest open circuit voltage (OCV) that resulted in death was 47V DC, and welding voltage was only 24 to 30V.

This search tool may also help you: https://www.ncbi.nlm.nih.gov/pubmed/

Unfortunately, that tool seems to be limited in how far back it searches. In 2008, it found the following book by Winburn; today, it did not. Here is a link to that book found on Google: https://books.google.com/books/about/Practical_Electrical_Safety.html?id=OjboRgTkKq8C

Several sample pages are provided for your review. I highly recommend it, as it presents factual information on such things as "let go" currents and voltages in men and women. Reported values for AC limits are considerably less than DC:

Let-go current (AC) = 15.9 mA (men) and 10.5 mA (women)
Let-go current (DC) = 76.1 mA (men) and 50.7 mA (women)

The corresponding values for voltage are reported as:

Let-go voltage (AC, 60 Hz) = 21V
Let-go voltage (DC) = 104V

Remember, skin resistance, e.g., dry vs. sweaty, makes a significant difference.

Finally, there is a series of articles by Raymond M. Fish on treatment and types of injuries one sees from electrical shock. It is often referenced in such discussions. See: Raymond M. Fish, J. Emergency Medicine, 17(6):977-983 (1999); loc.cit. 18(1):27-34(2000); and loc.cit. 18(2):181-187(2000).

Regards,

John

 

[MrAl]

Hi,

We are always looking for volunteers to participate in experiments where we send a current through your body at different places and see if you die or not. Anyone care to sign up?

Seriously though most of the deaths come about from current through the heart. It only takes maybe 5ma or so. There have been some interesting cases where people have been struck by lightning and still live because there was something bypassing their heart that conducted all the current. One case involved a guy and a golf club made of metal, and the club handle conducted all the current so it did not go through his heart. Lucky guy

 

[tvtech]

I read somewhere a long time ago that 60mA flowing through a persons body will kill that person.

For the life of me, I canno't remember where though

Regards,
tvtech

 

[DerStrom8]

I always learned that 30mA through the heart will almost definitely cause fibrillation, though it could take less if the conditions were right (or wrong, rather). There is no correct answer for #3.

You won't get much luck by just posting the questions here without giving your own thoughts about each one. We don't do your homework for you--you need to try it on your own first, and tell us what you've got. We can then pick up from there.

Just a suggestion from my point of view.

Matt

 

[tvtech]

Hi,

We are always looking for volunteers to participate in experiments where we send a current through your body at different places and see if you die or not. Anyone care to sign up?

Seriously though most of the deaths come about from current through the heart. It only takes maybe 5ma or so. There have been some interesting cases where people have been struck by lightning and still live because there was something bypassing their heart that conducted all the current. One case involved a guy and a golf club made of metal, and the club handle conducted all the current so it did not go through his heart. Lucky guy

Hi MrAl

More or less like the "one hand in pocket" rule I adhere too. Especially now that I am older Not fun to get zapped.

Regards,
tvtech

 

[DerStrom8]

Hi MrAl

More or less like the "one hand in pocket" rule I adhere too. Especially now that I am older Not fun to get zapped.

Regards,
tvtech

When working with mains circuits the 1-hand-in-pocket rule doesn't keep you from getting shocked, it just keeps you from dying (prevents current from flowing through your heart). I've gotten to the point where I barely feel mains shocks. I only feel them if I use both hands.

 

[tvtech]

Hi Guys

I posted a while back about a CRT biting me repeatedly in the W/S. I posted my crappy experience for you guys to see...

Latest in...all the sets are now doing the same thing from the same LOPTX/TV Manufacturer.

The EHT lead develops a place where it arcs through the lead insulation. Cannot smell Ozone. Cannot hear arcing. Something tells me all is not good.

Bring a hand or nose anywhere near the place....27KV likes you and uses you to get to Ground.

Extra weary now. Good sets and the leak is easy to seal.....but you must find it first. Don't let it find you.

Regards,
tvtech

 

[jpanhalt]

When working with mains circuits the 1-hand-in-pocket rule doesn't keep you from getting shocked, it just keeps you from dying (prevents current from flowing through your heart). I've gotten to the point where I barely feel mains shocks. I only feel them if I use both hands.

I don't understand how current from a shock when one contact is a single hand -- the other being another body part -- doesn't go through the heart, but when both contacts are hands, it does.

Can you explain?

John

 

[DerStrom8]

I don't understand how current from a shock when one contact is a single hand -- the other being another body part -- doesn't go through the heart, but when both contacts are hands, it does.

Can you explain?

John

Hi John. It's a very generalized explanation that doesn't always hold true, but it's the idea behind the one-hand rule. If you touch a live wire with one hand, it will generally flow through your arm, down your side, and through your legs. A majority of the current is said to miss the heart, due to where you're grounded. If you touch a live circuit with one hand and a grounded part with the other hand, the shortest route is through one arm, across the chest, and out the other, carrying current directly across the chest.

Again, this is very general and isn't always true in every case, but it's the theory that the one-hand rule is based on.

Matt

 

[jpanhalt]

If you touch a live wire with one hand, it will generally flow through your arm, down your side, and through your legs. A majority of the current is said to miss the heart, due to where you're grounded.

That is what I don't understand. There is a lot of lore about electrical safety that has evolved over time. I wonder if the one-hand rule was initially and simply to reduce the chance of shocks, e.g., with one hand, you are less likely to touch two different conductors at the same time than if you had both hands touching things. Maybe it has evolved to include an explanation that is not entirely accurate, but which is certainly more scary.

The heart is relatively low in the chest. Point-to-point conduction from one hand/arm/shoulder to the other hand/arm/shoulder misses the heart entirely. If one argues that the path of least resistance is through the major vessels, you still miss the heart. However, if you are sitting or standing, the major vessel hypothesis would put the current directly through the heart.

Likewise, if you are talking about some sort of current flow dependent on the bulk of the tissue, the legs, belly, or seat would be more risky points for the second contact than the other hand. Remember, the distance between our finger tips is similar to our height, and the arms have much less bulk than the legs or torso. Perhaps, we could extend the lore to suggest that people should always stand on the ipsilateral leg to the hand they are using.

Are you aware of any data on it?

John

 

[DerStrom8]

That is what I don't understand. There is a lot of lore about electrical safety that has evolved over time. I wonder if the one-hand rule was initially and simply to reduce the chance of shocks, e.g., with one hand, you are less likely to touch two different conductors at the same time than if you had both hands touching things. Maybe it has evolved to include an explanation that is not entirely accurate, but which is certainly more scary.

The heart is relatively low in the chest. Point-to-point conduction from one hand/arm/shoulder to the other hand/arm/shoulder misses the heart entirely. If one argues that the path of least resistance is through the major vessels, you still miss the heart. However, if you are sitting or standing, the major vessel hypothesis would put the current directly through the heart.

Likewise, if you are talking about some sort of current flow dependent on the bulk of the tissue, the legs, belly, or seat would be more risky points for the second contact than the other hand. Remember, the distance between our finger tips is similar to our height, and the arms have much less bulk than the legs or torso. Perhaps, we could extend the lore to suggest that people should always stand on the ipsilateral leg to the hand they are using.

Are you aware of any data on it?

John

You mean medical journals? No, I can't think of any off the top of my head, but ask any professional electrician. It's also all over the web, in electronics handbooks, I even think it might be in the safety section of the National Electrical Code (NEC) handbook that every electrician is required to know. It's a commonly accepted rule, however general.

 

[jpanhalt]

I am not questioning whether the one-hand rule exists and is followed. I am talking about its purpose and questioning the reasoning behind it.

The location of the heart is a simple matter of human anatomy in most people.

John

 

[DerStrom8]

I am not questioning whether the one-hand rule exists and is followed. I am talking about its purpose and questioning the reasoning behind it.

Yes, I understand that, and it is why I said that it's just a general rule, and may or may not be based in reality. Obviously it's mostly theoretical.

The location of the heart is a simple matter of human anatomy in most people.

Of course, but current flowing across your chest (arm to arm) still gets closer to the heart than it would if it goes down your side (again, in theory). It all has to do with probability--you're more likely to get a dangerous shock when you hold onto the circuit with both hands than with one hand.

Also, it does not have to be tied directly to your heart. There are several important organs in the chest-area that can be damaged if enough current flows near them. Just thought I'd throw that in there.

 

[tvtech]

Hi Guys

One of the first things I was taught when I did my TV repairers course in 1990 was the "one hand in pocket rule". It has served me well thus far. And I speak strictly from a TV techs experience where EHT @ up to 27KV on CRT sets can seriously hurt people. Not necessarily the person being shocked....but those around them where the guy being shocked starts trying to get rid of the thing shocking him and literally smashes the TV, tube and anything else around him.....

He was holding the Chassis with both hands when the EHT bit him. There is a post here somewhere where I commented a while back that he was going to get zapped sooner or later....I could see it coming.

Anyway, I have a healthy respect for EHT. Been zapped before, but not that much more damage than a hole in my very sore zapped finger

Regards,
tvtech

Anyway, here is Bill: **broken link removed**

 

[jpanhalt]

Of course, but current flowing across your chest (arm to arm) still gets closer to the heart than it would if it goes down your side (again, in theory). It all has to do with probability--you're more likely to get a dangerous shock when you hold onto the circuit with both hands than with one hand.

Wrong. Anatomically, arm to arm stays further from the heart than arm to either leg, butt, back, or stomach. That was the point I raised at the outset. Look at a chest X-ray.

Anatomy aside, it was this comment of yours that got my interest:

When working with mains circuits the 1-hand-in-pocket rule doesn't keep you from getting shocked, it just keeps you from dying (prevents current from flowing through your heart). I've gotten to the point where I barely feel mains shocks. I only feel them if I use both hands.

Realizing the potentially dangerous implications someone might draw from that sentence, I simply asked for the authority on which you based such a statement.

As best I could find, the one-hand rule arose from the increased likelihood that two hands inside an electrical enclosure (for example) are more likely to touch opposing polarities than a single hand. Ironically, this OSHA statement on electrical safety (**broken link removed**) doesn't even mention the one-hand rule.

John

 

[DerStrom8]

Wrong. Anatomically, arm to arm stays further from the heart than arm to either leg, butt, back, or stomach. That was the point I raised at the outset. Look at a chest X-ray.

Anatomy aside, it was this comment of yours that got my interest:

Realizing the potentially dangerous implications someone might draw from that sentence, I simply asked for the authority on which you based such a statement.

As best I could find, the one-hand rule arose from the increased likelihood that two hands inside an electrical enclosure (for example) are more likely to touch opposing polarities than a single hand. Ironically, this OSHA statement on electrical safety (**broken link removed**) doesn't even mention the one-hand rule.

John

Interesting. You may be absolutely right, but you also have to consider more than just distance. The human body is not uniform--some parts of it have more blood vessels, nerves, organs, etc than others. I guess there are really too many variables to take into account, at from my perspective. I really don't know anything about anatomy

Thanks for the comments though. I'm just going from what I've learned over the years, and what I was taught in an electrician's safety course I took some years ago. It seems things have been expanded to have several different meanings, whether accurate or not.

I would be interested to know what a medical expert in the field of electric shock would say about this....

Matt

 

[tvtech]

Hi Guys

It works. That is what I know

Why challenge it now only ????

Common

Regards,
tvtech

 

[tvtech]

Hi Guys

Being or getting shocked is part of life when you work with dangerous Voltages daily. No matter how careful you are, a zap can happen unexpectedly...

That is hard to prevent.

The dying part is preventable though. And that is what the "one hand in pocket" rule does if you follow it. You still get zapped, but you live to tell the tale afterwards. Even after multiple bad zaps, I have yet to break anything around me.

23 Years of dealing with the unexpected. Successfully. And "Im still standing"

And my heart is 100% still fine

Regards,
tvtech