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When Do You Get a Shock?

09/30/2008 12:43 AM

This may be stupid question to ask.....i know touch a live phase of A.C....we gwt shock..how abt in D.C

I think most of u have seen a 15V/230V (or some high magnitude,rectified) DC Supply.It has Two terminals + and -.

Q1)Assume u r standing on a Rubber Sheet(Insualtors used in my Elec. Machines Lab),Do U Get a Shock by Touching a Single Terminal Or Both the Terminal of D.C Supply? or U Don't get Shock at all.
in this case.

Q2)Now When U are not standing on insulator(assume connected to Earth), Do U Get a Shock by Touching Either Single or Both Terminals of D.C Supply?

The main thing confuses me is whether we get shock only by touching one terminal(like A.C) or both in case of DC

....sry for my poor english or terminology in above statements....

awaiting for rep
thankx in advance

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#1

Re: When do u Get a shock

09/30/2008 2:49 PM

The very simple answer is that you need to have create a circuit (or path) to get the current to flow (which results in a shock).

Whether you get a shock depends on the level of voltage (needed to overcome the insulating resistance of skin on the human body and allow a suitably high enough current to flow).

Whether you get a shock from touching a single terminal when not standing on an insulator depends on the level of voltage (needed to overcome the insulating resistance of skin on the human body, insulation of your shoes, floor, etc). Also there would need to be a return path from ground back to the terminal (some electrical systems are isolated by such devices as transformers which can prevent this).

There are other cases but they are a little more difficult to understand without a better understanding of electrical theory.

Oh and it is possible to get an electrical shock by touching two terminals and creating a path using only a single finger, just as easy as it is to touch two terminals (one with each hand) creating a path between your two arms (and across your heart).

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#20
In reply to #1

Re: When do u Get a shock

10/01/2008 10:51 AM

GA. This is the basic truth. For me, this is an occupational hazard that I've experienced more times than I can count. This is a similar discussion I've heard on various job sites I've been on. I do my best to avoid having myself exposed to two different voltage potentials at the same time.

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#2

Re: When Do You Get a Shock?

09/30/2008 11:00 PM

An American researcher named Charles Dalziel performed experiments with both human and animal subjects to determine the effects of electric currents on the body. A table showing his research data is presented here:


Important Note: Dalziel's human test subjects were men and women in good health, with no known heart conditions or any other abnormalities that would have compromised their safety. In other words, these data points represent best-case scenarios, and do not necessarily reflect the risk to persons in poorer states of health.
Assuming a skin contact resistance of 600 W for a sweaty hand, 1000 W of resistance for foot-to-ground contact, 50 W internal body resistance, 70 W of resistance through the soil from the person's location to the earth ground point, and a male victim, calculate the amount of voltage necessary to achieve each of the listed shock conditions (threshold of perception, pain, etc.) for the following circuit:

Slight sensation at point(s) of contact: 0.69 volts ? Threshold of bodily perception: 1.9 volts ? Pain, with voluntary muscle control maintained: 15.5 volts ? Pain, with loss of voluntary muscle control: 27.5 volts ? Severe pain and difficulty breathing: 39.6 volts ? Possible heart fibrillation after three seconds: 172 volts

Notes:
Not only does this question introduce students to the various levels of shock current necessary to induce deleterious effects in the (healthy) human body, but it also serves as a good exercise for Ohm's Law, and for introducing (or reviewing) the concept of series resistances.
For the morbidly curious, Charles Dalziel's experimentation conducted at the University of California (Berkeley) began with a state grant to investigate the bodily effects of sub-lethal electric current. His testing method was as follows: healthy male and female volunteer subjects were asked to hold a copper wire in one hand and place their other hand on a round, brass plate. A voltage was then applied between the wire and the plate, causing electrons to flow through the subject's arms and chest. The current was stopped, then resumed at a higher level. The goal here was to see how much current the subject could tolerate and still keep their hand pressed against the brass plate. When this threshold was reached, laboratory assistants forcefully held the subject's hand in contact with the plate and the current was again increased. The subject was asked to release the wire they were holding, to see at what current level involuntary muscle contraction (tetanus) prevented them from doing so. For each subject the experiment was conducted using DC and also AC at various frequencies. Over two dozen human volunteers were tested, and later studies on heart fibrillation were conducted using animal subjects.
Given that Dalziel tested subjects for the effects of a hand-to-hand shock current path, his data does not precisely match the scenario I show in the schematic diagram (hand-to-foot). Therefore, the calculated voltages for various hand-to-foot shock conditions are approximate only.

On another Note AC is constantly changing from positive to negative while DC is constant, as long as you poke your finger in the ground or neutral pin you should not get a shock(well if you have no static electricity)

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#3
In reply to #2

Re: When Do You Get a Shock?

09/30/2008 11:10 PM

GA from me. But the units of resistance are wrong.

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#4
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Re: When Do You Get a Shock?

09/30/2008 11:52 PM

Thanks,

I did not notice that, it is not good to make an answer just before lunch break

W should be Ω(Ohm) off course.

Sorry.

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#7
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Re: When Do You Get a Shock?

10/01/2008 12:09 AM

And not even after lunch, as you may be sleepy then.

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#8
In reply to #4

Re: When Do You Get a Shock?

10/01/2008 12:29 AM

Hello Epke:

I like it. Your jokes are as bad as mine! Thanks, you made me chuckle

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#9
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Re: When Do You Get a Shock?

10/01/2008 12:34 AM

No, No My jokes could never be as bad as yours, i am not in your league

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#10
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Re: When Do You Get a Shock?

10/01/2008 1:42 AM

GA Epke.

Regards

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#11
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Re: When Do You Get a Shock?

10/01/2008 1:58 AM

Thanks, i wonder if i ever get a GA from you know who

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#15
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Re: When Do You Get a Shock?

10/01/2008 3:16 AM

You are not going to believe it but I was actually thinking the same.

Maybe if you burn your "Japanese" passport and denounce all things Japanese then you might be in with a chance. (Yes I know that you are not Japanese but certain people do not seem to realise this)

Oh, bye the way, I only gave you a GA so that you would not start fighting with me

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#16
In reply to #15

Re: When Do You Get a Shock?

10/01/2008 3:32 AM

Depends on which Country you are working in

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#21
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Re: When Do You Get a Shock?

10/01/2008 10:55 AM

GA, but resistance across the body tends to be higher, say 10kΩ.

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#23
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Re: When Do You Get a Shock?

10/01/2008 8:24 PM

Where do you base that on?

The most resistanse comes from the skin resistance, the inside of the body is 70% water with a lot of impurities, like salts (kalium chloride and Natrium Chloride) all very good for conducting.

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#28
In reply to #23

Re: When Do You Get a Shock?

10/02/2008 10:55 AM

Actual physical measurement and personal experience. Take a standard ohmmeter and hold a probe in each hand, and squeeze as tightly as possible for the best contact. While the readings vary, they are certainly much higher than 50Ω.

Perhaps this is just semantics as I am referring to the resistance across one's body completely, not just internally or "under the skin". I could not say how conductive blood is. However, the complete resistance you cited in your example is low, less than 2kΩ.

While working on receptacles, I've been shocked across my hand with a potential of 120VAC. If I used the values in your example, I should expect severe burns as a minimum, but this was not the case. This particular aspect of electricity I have more experience with than I care to admit.

It is not my intent to insult you. I thought your post was excellent, and gave it a GA. It's just the resistance numbers you cited in your example that gave me pause.

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#30
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Re: When Do You Get a Shock?

10/02/2008 8:21 PM

Well people's resistance is not a constant thing and is modified by a lot of things so there is a certain resistance swing depending what the condition of the person is.

I'm sorry that i gave the impression that i was insulted.

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#25
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Re: When Do You Get a Shock?

10/01/2008 9:59 PM

Thus chart clearly show that shock is due to current and not because of voltage.

Neon Lighting power supply transformers have high voltage like 25K or more, but current saturation to vary small value. Thus it give very small shock. Please correct me if I am wrong.

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#26
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Re: When Do You Get a Shock?

10/01/2008 10:11 PM

yes current kills, the voltage is only needed to conquer the skin resistance.

hypothetical speaking you could but a needle in one side of the body and a needle in the other side of the body piercing the skin and if the current flows through the hart. The amount of voltage would be far less.

Don't try this at home kids!

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#5

Re: When Do You Get a Shock?

10/01/2008 12:02 AM

Touching 1 terminal, and your "Ground Isolated" OR the terminals are "Ground Isolated" (Isolation transformer), then no shock

Think of a "Bird on a wire"

A Bird lands on a single wire, won't get zapped, but the cat climbing the pole to get that bird, crossing the wires will get zapped

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#6

Re: When Do You Get a Shock?

10/01/2008 12:03 AM

DON'T TRY IT. (EVER)

We see technicians working on live 333,000 volt AC lines and getting no shock, we get static shock from an insulated cable spool.

Whether you get a shock depends ABSOLUTELY on the surrounding conditions (including the return path options) and materials and there is no single answer to this broad question.

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#12

Re: When Do You Get a Shock?

10/01/2008 2:23 AM

This may be a silly question.

WHY ARE YOU TOUCHING LIVE TERMINALS? You have no business doing that, ESPECIALLY if you don't understand how electricity works! People die doing that!

Sorry, but nobody else addressed it, and it seemed obvious to me.

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#13
In reply to #12

Re: When Do You Get a Shock?

10/01/2008 2:51 AM

You get a shock when you take for granted what you have done so many times without a shock by repeating what worked before- but this time your mind has not recognised that something is not the same- this is how experienced electricians are electrocuted- familiarity breeds contempt in the hooman animal- that is why I triple check everything before applying power- & be very sure of correct wiring/plugs(maybe ok in your workshop, but in someone elses house?- check first!.)

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#14
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Re: When Do You Get a Shock?

10/01/2008 2:54 AM

I thought new assistants were there to test, if it is safe to work on the Machine/wires

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#17

Re: When Do You Get a Shock?

10/01/2008 4:38 AM

Hey thanks all for the reply........

i know this is silly thing to ask being an electrical eng. under grad.......but profs suck big time....all they teach is about theoretical things.......so iam not blaming them but they don;t impart any practical knowledge....hmm what to do.....i can;t comprise my curosity.the forum is the only help

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#18
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Re: When Do You Get a Shock?

10/01/2008 7:54 AM

You are correct on the profs thing - the only good profs I had in 6 years of college were those that had actually had an engineering job for at least 10 years before deciding to teach. That was about 2 profs. The best one had worked for Bell Labs for 20 years. The second best had worked for a power company for 25 years. The rest of them, all they knew was the theoretical bull dung.

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#19

Re: When Do You Get a Shock?

10/01/2008 9:48 AM

While I agree wholeheartedly w/ Epke's great answer regarding grounded AC circuits, your question asked about DC power supplies. If there is a "Common" terminal, that post is normally grounded/earthed, and Epke's sketch still applies if you touch the non-grounded side. If the terminals are "+" and "-" (as you describe), both sides of the power supply are floating (neither side intentionally connected to earth), and an additional factor must be considered.

Whether the system is grounded or not, in all cases of contact with a live conductor, current will flow through the subject's body, simply because there is no such thing as ideal insulation. The current leaves one terminal and travels through the subject's body, then earth, then the insulation between earth and the opposite polarity terminal to complete the circuit. It may be picoamperes or smaller, but there will always be flow.

For floating power supplies, the power supply's resistance to ground must be added to determine the total loop resistance and calculate current flow for any given output voltage. Epke's table can then be used to determine the effect on the body.

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#22
In reply to #19

Re: When Do You Get a Shock?

10/01/2008 8:21 PM

In the table DC is also mentioned

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#29
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Re: When Do You Get a Shock?

10/02/2008 12:51 PM

Epke, your answer and table are great. I was just expanding on your drawing of a grounded system so the poster could apply the table to non-grounded systems as well.

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#24

Re: When Do You Get a Shock?

10/01/2008 8:34 PM

i have made the mistake of messing with a car battery in the car while my cloths were wet with sweat. lean against the car with wet cloths,and you can get killed touching the positive terminal. also high output dc car coils, 50,000vdc,etc have warnings on the package telling you it can kill you.

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#27

Re: When Do You Get a Shock?

10/02/2008 9:30 AM

You seem like you might be going down the path of learning things the hard way. It's a nice path, I know it well.

VERY IMPORTANT NOTE

When grabbing electrical things that people with good common sense don't grab, especially the DC lines from batteries:

1) Remove wrist watch or any items (bracelet, whatever) that is on your wrist and could conduct electricity.

2) Remove any metal rings. Shorting a battery with a metal ring can burn your finger so badly that the doctors might have to cut it off. At a minimum, you will be amazed at how a small, simple mistake can cause great pain.

My wife put a ring on my finger and then fed me so much food that the ring will not come off. I put 3 layers of electrical tape over the ring before I go out to play.

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#31

Re: When Do You Get a Shock?

10/02/2008 10:35 PM

Dry human skin is good at keeping electricity outside the human body at voltages up to about 50 Volts. Whether it is AC or DC is unimportant. Wet skin offers much less protection. Once past the skin there is very little resistance to electricty. In fact the wet fluids (electrolytes) inside the human body conduct electricty very well. I have seen reports that currents as low as 40 milliamperes through the heart can disrupt the rythmic operation of the heart. One researcher was even more cautious saying only ten's of milliamperes through the heart can disrupt it. Perhaps 40 milliamperes is only tens of milliamperes. I suppose precice numbers are hard to come by because the consequences -- death --are severe.

The report from Dalziel cannot be used directly to calculate the breakdown voltage of human skin. There is not enough information presented in the table to work backward like Epke has done. Everything in Dalziel's table appears to be after skin breakdown has occured.

There is a standard you might want to investgate. It is called SELV which is an acronym for Safety Electrical Low Voltage. If my memory serves correctly, the upper limit for low voltage circuits is set at 42.5 Volts in this standard. Thus people carelessly touch "low voltage" circuits found in computers without difficulty. AC power is distributed in countries all over the world at voltages considered to be "high voltage" by this standard.

When touching a high voltage wire to see if it really is de-energiezed I always use these two safety precautions. I keep one hand in my pocket; and I always test it with the back of my finger. If you touch a live wire with one hand in your pocket then the voltage has to be high enough to break down your rubber soled shoes in order to harm you. If the wire is "hot" and you touch it with the back of your finger, the muscle will contract involentarially, pulling away from the live wire. If you touch it with the front of your finger and the muscle contracts involentary in such a way to make your finger grab onto the wire, then you die.

Ken Stewart

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#32
In reply to #31

Re: When Do You Get a Shock?

10/03/2008 12:08 AM

As we know, all body operations work on sort of piezo operations and small command electric signals.

It is wonder, how these signals pass to appropriate points, without getting shorted in body fluids.

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#33
In reply to #31

Re: When Do You Get a Shock?

10/03/2008 12:26 AM

GA!

When I taught physics and electronics we commonly tested for the minimum voltage students and I could feel (one wire in each hand, variable voltage source not visible to the test subject). It was years ago, and I no longer remember many details, but for most people they began to feel it at around 60V with dry skin. I'm an oily skinned person, and of course oils are insulators, so I needed more before I could feel it.

Another favorite activity was to make a big circle of all willing members of a class holding hands (they were all encouraged to participate, but never forced or required), then break the circle and put a hand-crank telephone ring generator across the break. someone would start cranking slowly, then speed up until someone let loose. The person that let loose would then be eliminated, and we start over. Today we would probably call it a survivor game. Of course as there get to be fewer people in the loop, each person gets a larger voltage. I, being used to it, was nearly always one of the last two. This was a two-pole slip-ring generator (alternator) with several big U magnets. I'm pretty sure you could feel the low-frequency pulses more than you would feel DC.

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#34
In reply to #31

Re: When Do You Get a Shock?

10/03/2008 5:39 PM

Do you actually touch a wire to see if it's de-energized? This is baffling to me. In well over 20 years as an electronics tech working on equipment ranging from radars and guided missile systems to simple manufacturing equipment, I have never, repeat NEVER touched a wire to see if it is hot. That is a gross violation of every electrical safety practice I've ever heard of. Hasn't anyone here ever heard of a multimeter? If you are workng on an electrical system, you probably have one at hand anyway, so why touch a wire to see if it has power? That's like sticking your finger in the pot to see if it's about to boil, really.

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#35
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Re: When Do You Get a Shock?

10/03/2008 6:25 PM

GA

Ditto! I've worked on similar stuff for nearly 60 years, and have been shocked many a time, from a tingle to almost life-threatening, but NEVER intentionally to see if a wire is hot. This is especially true now that we have Fluke's Volt-Alert (or similar devices from other companies).

Dick

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#38
In reply to #35

Re: When Do You Get a Shock?

10/06/2008 5:56 PM

Thanks, Dick, I appreciate that. I'm not saying I've not been shocked, Lord knows that I have. You know as well as I do, many of the alignments and troubleshooting that goes on is done on energized equipment, and sometimes shi... uh, stuff happens. But if I have to be touching wires for repairs, I always de-energize and verifiy with the meter. I'll also usually ground the circuit too, just to ensure there is no residual voltage waiting to spring. Always check with the meter! I think that if I have another kid, I'll name him Fluke!

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#39
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Re: When Do You Get a Shock?

10/06/2008 7:21 PM

"I'll also usually ground the circuit too, just to ensure there is no residual voltage waiting to spring"

I have to confess that's a great idea I hadn't thought of (other than for CRT anodes). Just a couple of weeks ago I was working on the electrical system of an old Horizontal boring mill that doesn't even have a master switch. It does have a master breaker, but its near the center of the panel, and power does go to several places (aux transformer inputs) before it goes through the breaker, so even with the breaker off, there are a number of hot points. I have requested a master switch be installed so I can lock it out.

BTW although I live in northern CA, this was at our plant in the Scripps Ranch area of SD - I spend a bit less than half my time in the SD area, roughly every other week.

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#40
In reply to #39

Re: When Do You Get a Shock?

10/08/2008 5:44 PM

Scripps Ranch, eh? I live just south of there. You need a full time tech? I'm tired of driving the 55 miles one way to Temecula every day!

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#36
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Re: When Do You Get a Shock?

10/04/2008 3:04 AM

You have to touch the wire in order to work with it. There is no getting around it. I de-energize the circuit and sometimes put a pad lock on the disconnect. Then I measure it with a voltmeter. Is it safe now? Was the voltmeter connected to the same ground as your feet are touching? Or your other hand? Or your leg that you are using to brace against something? I ALWAYS touch high voltage wires with the back of my finger before I wrap my fingers around it. Always -- even after the voltmeter test.

Ken Stewart

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#37
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Re: When Do You Get a Shock?

10/05/2008 11:42 PM

Some in the auto industry touch the HOT lead to the plugs to see if they are sparking enough, Higher energy shocks from the newer electronic stuff tho get you laughing ;o)

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#44
In reply to #34

Re: When Do You Get a Shock?

01/28/2009 4:44 AM

Hi, LCAC32!

Working around live electricity is a VERY nervous occupation in which one has to be alert all the time to avoid harmful contact. Even though I mainly work around North American household voltages, I don't like it; but because it's simpler for me in my work than shutting down an entire building or working in the dark, I do it. Naturally, I'm very careful; and after taking usual precautions such as using a circuit tester on the site and despite a bite or two from a concealed conductor over the years, I'm still here intact to tell the tale. And I don't test live circuits with my body.

But I have a confession to make (he said shamefacedly ). When I do cut the power, even if it's only at a local switch and I've tested the system to make sure that the shutoff switch is in absolute control of opening the circuit on which I'm working, I gingerly touch the conduit with the back of my finger just to be sure I can greedily hold onto it when I'm working with it.

While I've not yet been bitten by a hot line in these instances (except for the unseen conductors I mentioned), I have received a very mild bite from a couple of neutral lines, because the sneaky little s.o.b.'s can sometimes be carrying a small load due to other connections along the circuit or poor installation practices, but not enough to show up on the circuit tester even though I test all lines against ground and each other before beginning.

Yet I persist because time is money, and I know (I hope!) what I'm doing. And absolutely I carry a couple of mouth-sized flashlights with fresh batteries in my electrician's tools for when I'm working perforce in the dark due to either working on tricky connections within a panel itself (in a windowless basement at night, for example ,) or on an appliance with 'buried' wiring (e.g. accessed through a small opening and therefore not visible ) that poses a definite safety threat. Yes, there's a multimeter in the toolbox; but I don't haul it out every time I tackle a small job because I don't have three hands to use it each time I'm working on a job.

So, I don't stick my finger into the pot to see if it's about to boil. I stick it in to see if the stew is cool enough to serve, as it were. Formally, I know I shouldn't do it; but in practice, I find it soothes my nerves around working with an appliance or fixture that I have previously assured myself has been cut off from power, and gives me that little bit of information that I would otherwise have had to use the multimeter to discover on lines that are, ostensibly, not alive anyway.

So the gingerly 'touch' is a rock and a hard place for me, LCAC32. I think my biggest protection is my complete respect for the damage that energy form can impart if I screw up. My colleagues are of the same stripe as I, generally, in the way we treat electricity on the job.

Mark

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#45
In reply to #44

Re: When Do You Get a Shock?

01/30/2009 12:27 AM

Hey Mark!

All good points, I even agree with most of them. (Can't agree with them all. Not conducive to good discussion!) I often work with, or around live circuits myself. I just never check them with my skin. I always use a meter. ALWAYS. This is because in my former profession, the voltages being measured were often measured in the thousands, so we didn't take any chances. That habit has followed me into civilian life.

In the Navy you get it ground into you that safety procedures are written in blood, and they usually back that with bloody, gruesome training videos. After 25 years, it tends to leave an impression. I guess with me, I feel that if I'm being "too safe", I won't get hurt, as opposed to what could happen by being too lax. I've been bitten by a wide range of voltages, both AC and DC, (For the record, 400Hz feels better that 60Hz.) but it's all been incidental contact, re-enforcing the safety message.

I just don't ever want to be that guy on the safety bulletin that people read about and say "What a dumb-@$$!"

Enough of the soapbox, just be careful!

JP

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#46
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Re: When Do You Get a Shock?

01/30/2009 9:57 AM

Hi, LCAC32!

Agreed.

Mark

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#41

Re: When Do You Get a Shock?

10/15/2008 2:40 AM

If your hands are wet and or you have a cut on the hand that is exposed to electricity ac or dc the electrical circuit can readily travel down nerve pathways. My wife is a nurse and worked with patients that have been electrocuted and had almost undetectable tissue damage on surface of the skin and critical internal damage which the surgeons pointed out were nerve/arterial pathways. his leg and arm cooked on the inside and was visible through the outside of the skin.[days later]

on the horse/livestock electric fences the wire is meant to give enough of a jolt to "cause an involuntary muscle reaction" to be effective. at 7200 volts DC pulse in summer on dry bark mulch and dry running shoes you can hold the wire and experience it as slight jolt. with wet hands and wet boots it feels like a auto spark plug shock with wet hands and the pain is excruciating.[kids did it on purpose while I was climbing underneath]

even with DC I would [as stated above] tape rings or remove necklaces and avoid handling with minor cuts or sweaty hands. I was told of a finger blown off with a wedding ring touching a truck battery and a frame at the same time.12 volts! friend describing mechanics school.

correct if this wrong, Canada allowed 26 volts Ac leakage in appliances and USA allowed 40 volts. seems awfully high. Cannot remember were I found this out. I had a leak in a fridge many years ago and it felt like full shock with wet hands fridge/me/oven.[2 wire,no ground] when measured it was only about 1.8 volts. I had fresh cuts in my hand when detected. put a ground strap frig to stove anyways. It was this fault which prompted me to search out AC leakage tolerances. It may have been after a "destructive drop" test or a design test.

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#42
In reply to #41

Re: When Do You Get a Shock?

10/15/2008 6:31 PM

To test an electric fence,

Get a long bade of grass, and from the furthest end, slowly move towards the wire until you feel the tingle.

If your camping at night, don't piss near any fences, This goes for you girls too, grabbing something as you try to balance yourself can give almost the same reaction as a guy shooting the wrong direction.

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#43
In reply to #42

Re: When Do You Get a Shock?

10/16/2008 2:55 AM

Snaketails, "If your camping at night, don't piss near any fences" - probably the best piece of advice that was ever given out on CR4. Thanks

Kind Regards

Mr. W.A Snow

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