Why?...Get Shocked

Yes, you will. The current is what you feel. Although, your perception for AC current is better than DC.

This has been discussed before at:

http://cr4.globalspec.com/thread/3212/Which-is-More-Dangerous-AC-or-DC

Blimey guest, I can tell you've never worked on TV or oscilloscope power supplies have you?

EHT of 25kV dc can give you one heck of a belt

John.

Here's where I think this guy is coming from...

I pick up a 9V battery and place my fingers across the terminals and I get no shock. I even have a 12V car battery, I place my fingers across the terminals and I don't get a shock. Then, I place my finger across the leads of a 12V step-down transformer connected to AC and I can feel the shock! This is a tingle that is somewhat uncomfortable.

Why can I feel it when I use AC, but not DC of the same voltage?

Read my comment in Post # 5, which shows the reason for different perceptions.

I'm not sure Post #5 quite covers it. AC would likely result in greater peak current flow due to the additional current flow due to the capacitance reactance of the body as well as resistance. Also, a 12 VAC transformer has a peak voltage of about 17 volts and so is really a higher voltage. Also at these low voltages the effect on internal tissues is negligible.

I've felt the tingle as low as six volts on an stepdown transformer. Isn't there some thing about when you get shocked by DC, it immediately drops to ground - so you feel a jolt and them that's it?

Current (that's what you actually feel) from the initial capacitive effect will drop to zero.

I just measured the capacitance of my own body and it seems to be about 8nF which calculates out to about a 3260 ohms impedance at 60 Hz. At 6 volts (peak) that will create a peak current of 0.00184 amps! That is definitely adequate for you to feel.

The DC resistance I measure to be in the hundreds of thousands of ohms and so consequently the resultant current would be a couple of orders of magnitude less. I am surprised at the difference and how much effect the capacitance of the body has but it would seem to corroborate and explain everyone's observations.

Different from the DC resistance the capacitance does not seem to vary as much with skin surface moisture. Clearly the variation in DC resistance is swamped by the much lower AC impedance.

<Isn't there some thing about when you get shocked by DC, it immediately drops to ground - so you feel a jolt and them that's it?>

The voltage remains on the device in DC, it's just that the body can't detect it so easily. In the simplest of terms, as well as resistance the body also possesses capacitance. Capacitance is a block for DC, so no current flows. However, capacitance forms a path for additional current in AC over and above that which would flow through the resistance in DC. That's why 6VAC will give a tingle, whereas 6VDC or even 12VDC will not, usually. AC is simply more noticeable.

However, push the DC voltage up a bit, say 50V upwards, and the contact made by the skin begins to deteriorate until a greater current begins to flow at the contact point; the resistance drops. It gets into the nervous pathways and muscles and causes them to go into spasm. Further, the skin starts to sweat at the contact point and current continues to flow. With the spasm it becomes very difficult to release hold of the DC contact point, and so a burn usually results on DC.

With AC reversing polarity 100 or 120 times per second, the muscles have a better chance to function and it's easier to let go. Which is why AC is shocking, in the main.

Good explanation! Thanks!

If you're not getting shocked it's because you don't have the voltage high enough. Try again.

The reason you feel more from the AC voltage is the effect it has on your muscles, alternately contracting and expanding them at the AC frequency. With DC, there's just the initial clench, no "vibrating" feeling.

This is why if you grab a live AC wire and get shocked, you can let go, but if you grab a live DC wire and get shocked, you can't let go - your muscles contract and stay contracted.

The only electrocution death I have ever witnessed in 30+ years in the business was a DC drive technician who apparently grabbed a tool that was making contact with the 750VDC bus. We don't know how long he was locked on to it, it may have been as long as 3-5 minutes. When we happened to come buy we thought he was goofing around. We grabbed a 2x4 and knocked him off, but he was already dead when he hit the floor.

AC kills, DC kills. Ergo, electricity kills.

You said:-

This is why if you grab a live AC wire and get shocked, you can let go, but if you grab a live DC wire and get shocked, you can't let go - your muscles contract and stay contracted.

My experience with (far too) many shocks both AC & DC is actually the opposite of what all the good doctors (who have never had shocks from both as I have) think will happen.....

This sentence of yours is current (pun intended) thought, but still wrong at voltages up to 300 volts....over that it does not really matter!! Then both are highly dangerous....

I'd like to see experimental verification of your hypothesis.

Whether its AC or DC, your ability to control your muscles becomes small once the current is about a factor of 4 or 5 higher than you can feel (and that depends on how the current spreads through your body, so I shan't give a value).

However, there are still differences, as the different frequencies have different effects. Conventional wisdom has it that 50-Hz** is more likely to throw you across the room than bind you to the wires. Similarly, I was told that 60-Hz was more likely to make you hold on, and that DC effects depended rather critically on where the current flowed. So far, my experience is restricted to moderate shocks from 250-Volt 50-Hz mains and the odd stored charge on a capacitor - and that is where I hope to leave it - so I can't vouch for the accuracy or otherwise of these statements.

Summary: handle with care - electricity can be dangerous irrespective of frequency.

**That implies that 60-Hz is more dangerous for electrocution. However, it is better for optical strobing effects and requires smaller transformers and motors for a given operational power. I imagine that the reduced strobing will result in fewer accidents, as will the reduced weight of hand-held power tools, so 60-Hz may actually woork out safer overall.

"This is why if you grab a live AC wire and get shocked, you can let go, but if you grab a live DC wire and get shocked, you can't let go - your muscles contract and stay contracted"

I strongly beg to differ. I got a good bite once, from some good ole 120V AC, and there was no letting go. I was lucky that I was squatting on one knee close to a wall. I managed to push my self over, using my other hand, away from the wall. Even that arm was hard to move and it was not even part of the electrical path. Ever had a Charlie horse? It felt like every muscle in my body was tensed to the point of breaking. Definately strenthend my faith in God that I am here today to type this.

So trust me, AC will grab you just as well as DC.

I believe the statement on AC and DC to have originated in Europe, where we use 50-Hz. The "ability" to let go has nothing to do with "gaps" in the signal as implied in that post, and everything to do with how your muscles respond to different frequencies - and the grab or let-go response would not be in any way voluntary. So, even if this statement is true in Europe, it wouldn't apply in the US.

Your shock doubtless 60-Hz - which is reputedly even more grabworthy than DC. From the sound of it, you released yourself using muscles that were not under the control of the mains.

Testing in earlier years found the most dangerous AC frequency for the body was 16+²/₃ cycles per second. (We now call the cps Hertz)

That was the "ringing current" on the phone line, for the electromagnetic/polarised bells found in the phone, or in a big black box on the wall - the days when the phone really rang, not chirped, whistled, or played merry selectable tunes.

If you ever tried hanging on to 75 Volts at 16+²/₃ Hz, which was evidently not sufficient to kill, in the earlier days of rotary or step-by-step telephone exchanges, for any length of time, your muscles were sore for days.

As children, we used to do that as a sort of challenge, to see who could endure the pain the longest....

Hello Physicist, and other readers.

The original system of automatic telephone exchange equipment was a DC electric motor between a double row of equipment, plus the mechanical gears and shafts etc,

Invented by Strowger, who was an undertaker, because he was concerned that callers to his telephone were being diverted to the opposition undertaker by the Manual Telephone Exchange Operator, who was presumed to be taking under-the-table payments from that rival, for each diverted call.

This equipment was made by Western Electric of the US.

This automatic equipment was still in service in the 1960's in New Zealand:

Refer: http://www.dmine.com/phworld/switch/sxs.htm

Step by Step:

The later equipment in NZ was step-by-step which had dispensed with the DC Motor, gears, and shaft arrangement, all now being driven by electromechanical relays:

Uniselector - One for each number connected - whizzed round until it located a free "first selector switch" at which point it stopped, and you received "dial tone" from the motor-driven "Tone generator" of which there were two "Tone generators" in each exchange, and alternated weekly.

"I think the UK was over 20-Hz, but wouldn't be certain (of course, these would have been generated by mechanical line interrupters, so the harmonic content would have been ferocious as well"

The "Tone generator" was a collection of AC alternators, mounted on a common shaft with a DC Motor, which ran from the 50VDC lead-acid batteries which ran all the exchange equipment.

The "Tone generator" generated all tones and ringing current for that entire Exchange, also generating the 75 Volt - 16+²/₃ Ringing current, sent down the line to ring the actual electro-mechanical polarised bells in the called premises.

There were no harmonics, the ringing current and the tones were always pure sine waves at all times.

Stepping switch:

Row upon row of thousands of individual relays, sometimes up to 50 in each individual stepping switch "can" or enclosure.

Each individual relay could have up to 40 different contacts, with various contact configurations:

• NO = Normally open = Make
• NC = Normally Closed = Break
• Changeover = change from one to another
• BBM = Break before Make
• BBB = Break before Break
• MBM = Make before Make
• MBB = Make before Break
• And so on....all cleverly designed to interact to achieve the desired results.....

With all those hundreds of thousands on individual relay contacts being silver, which had the nasty habit of growing "whiskers" - refer the excellent paper and explanation here:

https://nepp.nasa.gov/whisker/reference/tech_papers/chudnovsky2002-paper-silver-corrosion-whiskers.pdf

Whiskers which were conducting, thus confusing the entire operation, and took much time and effort to locate the faulty contact/s.

Other contact metals were tried, but most had the whisker troubles....so it was a case of continual maintenance.

Maintenance was very costly in terms of labour, cleaning equipment, and very toxic solvents - Trichlorethylene, Carbon Tetrachloride, and Ethyl Alcohol (Rectified Spirits) giving the users violent headaches, and perhaps brain damage to be found out later.

NZ Step-by-step equipment normally was supplied under contract, by STC = Standard Telephone & Cable, of the UK - Refer:

https://en.wikipedia.org/wiki/Standard_Telephones_and_Cables

All that old equipment has long ago gone to the junkyard, except for small parts which ended up in MOTAT - Museum of Transport & Technology in Auckland, NZ.

Kind Regards....

Thanks for the additional info.

Dendrites - yuk. Were the exchanges humidity controlled? (I suppose Au or Pt were too expensive??)

No humidity control in those days.

Most cities had a Municipal Gasworks, coal-fired Steam trains were used, and coal-fired open fires everywhere.

So, the solution was remove the expensive linoleum, which had been made in the UK, then coat the floor with a special bitumastic base compound, which was around ¹/₂ inch thick, lay more linoleum over the top.

What was happening was that moisture had been drawn through the concrete by capillary action, and causing the higher humidity level in the telephone exchanges.

As workers went in and out, air contaminated with sulphur compounds plus the higher humidity level from the water via the concrete slab, gave impetus to the "whiskers" growing.

I do know that STC eventually failed, and no doubt that was assisted by claims against them as designers/manufacturers/suppliers, from many places the exchange rack equipment went to.

In those earlier days, there was no such thing in New Zealand as "dehumidifiers or Air Conditioning".

Once the Gasworks, Steam trains, and coal fires vanished, Silver "whisker" growth ceased to be a problem.

Many other metals may grow "whiskers" or dendrites,often associated with Tin electroplate being in contact, or nearby, under certain atmospheric conditions.

Refer:

http://nepp.nasa.gov/whisker/other_whisker/

If you look at the extensive links at the above website, you may be surprised how dangerous metal "whiskers" or dendrites really are, and how widespread the phenomenon.

This is an area worth exploring, to try and solve why the "whiskers"/dendrites form, and what may be done to prevent growth, as the results may be dangerous or fatal, even in quite common situations.

Happy Christmas.....Kind Regards....

The problems you describe were almost certainly quite localised, and you would have had to take some specific action regardless of manufacturer - unless, as I implied, you either had dehumidifiers (even at that time these were sometimes specified for locations where problems such as you describe were anticipated) or your technical people specified a heavy non-reactive metal such as Au or Pt - which were not too common on open relay contacts at the time (even there, problems can occur with intermetalics - but the Gold and Platinum are not so strongly effected by ionic solutions). Incidentally, I remember heavy black smogs in the UK from the late 1940s, but I don't believe UK exchanges suffered to anything like the same extent as you report.

BTW, the reasons for STC's failure were far more truly caused by financial mismanagement than being sued or customer dissatsifaction. STC** had been part of ITT until problems with the losses of Hartford Insurance (ITT didn't understand the risks they were taking when they bought Hartford as a supposed "cash cow") forced ITT to offload its overseas subsidiaries (there were other issues with purchasing of electronics by the military that made it sensible to have companies owned in the jurisdictions of operation, but in my opinion those were secondary).

Now, ITT had a very rigorous internal financial reporting systems, which pretty-much forced directors of subsidiaries to keep their focus on the bottom line. When that reporting requirement was removed, the directors of STC seem to have heaved a sigh of relief at the removal of this onerous burden - and apparently lost contact with at least some of the day-to-day detail altogether.
Be that as it may, they then bought a number of other companies for more than they were worth, and without a well-defined plan for what they would do with them*** (ICL being the most obvious - but there were others).
Also, in the absence of some of the detailed reporting, directors of some of the divisions were able to spend quite large sums of money on grandiose schemes that added but little to the company's output. It is rumoured that some of the costs of these schemes were inflated by materials and effort being diverted to the said directors' homes; of course, the direct cost of the diversion itself would have been relatively small, but the side-effect was that the facilities managers had no legal levers to allow them to control the cost of the original project.

... a company that has overpaid for some major acquisitions, without proper financial controls and with no firm sense of direction.
In due course, ICL was sold to Fujitsu, semiconductor technology to LSI Logic, and the majority of the original STC was taken over by Nortel. That's enough of this cautionary tale for one Christmas Eve, I think.

Enjoy your Christmas. (I certainly intend to take your advice on that)

Regards

Fyz

**SFIK, not intentionally short for S** the Customer - however it may have appeared in NZ.

***Consider "going short" on the stock whenever you hear the words "synergy" from the senior management of a company

You will kill yourself if you believe you will not be zapped by DC. Remember, it is not the voltage (E) that will kill you, but the current (I) that will do you in. As small as .1A is enough to kill a person. The resistance of dry skin when touching energized circuitry plays a role in what you feel as well, 120VDC is at a constant straightline 120V voltage, while AC has a peak-to-peak value, so 120VAC RMS is equivalent (120 x 1.41) x 2 = 340VAC peak-to-peak. So you can see why you feel AC more intensely.

A car battery as innocent as it looks, will kill you dead as a doornail if you are well grounded to the grounded (- charged) metal parts of a car, and get good enough contact to the positive (+ charged) circuitry or battery terminal. Again here we see that it is not the voltage, but the current passing through you. If you are sweaty when working anything, you have introduce water and sodium to your skin, which then creates a very low resistance path for current to flow through your body.

Disregarding the muscular contraction effect, the damage done to the human body by a DC shock is greater than with an AC shock.

The reason is that the DC shock mainly travels through the bone marrow, damaging it.

Thus a person after receiving a fatal DC shock, may be walking around, with a burn at each contact point, but with cooked bone marrow - which cannot be seen.

In a few short days, death results.

With AC the higher the frequency, the greater is the "skin effect", where the damage is not so great to deep structures in the body.

Here's an odd piece of info... When a person is hit by lightning and survives, the physicians monitor the patient very closely, because they have found that lightning can create smoldering fires within the nerves. If detected, the medical personnel rush to put them out.

'Fires' might be an incorrect description, as there is a lack of oxidant at these locations. Certainly biochemical systems react adversely to the effects of electrical current passing through them - there's the electrolysis and heating of their contents for starters.

An odd practice developed in this part of the world of late called "train surfing". It is (or was until recently) carried out by youths of low intelligence, usually male, as an act of bravado. They found their way onto the tops of moving electric trains and "surfed" between stations on the top of them. Of course, the chances of survival after coming too close to the overhead 25kVAC pick-up equipment are rather remote. Body fluids tend to turn to steam under the effects of the discharge current, and the resulting pressure is usually enough to rupture their container; the body quite literally explodes. It's a horrible event for any emergency service operative to attend. For that reason, modern rail vehicles are constructed with rather smooth body panels, so that climbing up them to do this becomes very difficult. The down-side is that rainwater pours down the roof into the entrance doorway when boarding or alighting.

The lowest DC I have felt a tingle from was a 48VDC battery charger while installing a charing system & batteries on a NCC Bouy in Fremantle (Australia)

I heard about (but can't cite any reference) someone killing themselves while tightening the terminals on a 12V car battery. They'd been swimming in the ocean and the saltwater had made their skin conductive. ffeJ

Put the 9vdc terminals of a small battery on you tongue, tastes sour

the more sour the larger the remaining charge.

Thats how I test 9 volt batteries. I can't do that with a car battery though.

I would be popular if I could.

James

Or you would be Gene Simons

Did you know, that in the UK about 10 people die each year by testing a 9 volt battery that way??

I don't remember whether it was due to electric shock or chemical or some other reason, it was a statistic that I remember as quite unusual!!

John.

I would guess that they probably got the battery lodged in their throat! I can hardly imagine the short current path through the tongue would be fatal.

Quality

not

quantity

I'll tell the story, about an incident, that could have killed me.

I was troubleshooting inside a large double door panel w/a center pillar between the doors. The actuator rod for the disconnect handle needed to be aligned so the rod would engage w/handle & allow the door to be closed.

Common practice was to reach behind the pillar & use a screwdriver to align the rod, while closing the door.

The problem was if you turned off the panel to reset the overloaded motor, the rest of the system would plug up.

A plc & some other interconnected systems were contained in this panel. The panel was 1 station of a network for 2 flour delivery systems, It would shut down production for 14lines, to turn off the power & lock out.

So I had reset the overload & was trying to close up the panel.

behind the pillar was the 480-120 transformer about shoulder height.

My arm shorted the 480 side to the pillar, I could smell my arm burning. I couldn't move it. I had to really concentrate to get my other arm to remove myself from the short. The arm was weak & useless for a few hrs afterwards.

scary stuff

I installed phenolic supports for the offending rods throught the plant [14 panels], over the next few weeks.

I was lucky cause the short was only going through my fore arm, if it would have been fatal if the power had gone across my heart. I was in a remote area & probably wouldn't have been found for 20-40 minutes.

remember the ABC's of safety

always be careful.

I was reaching into a control panel to make a voltage measurement when I was distracted. 277VAC through the finger to the deck plate I was standing on. My testicles were numb for two days.

Now I know how a eunuch feels

James

Ever work w'an old guy who'd lick a finger & swipe to check for voltage? This 1 old bird would even accuratly tell you how many volts

Ditto at 48VDC. It was the PSU in a small LF radio transmitter installation.

18VAC is quite enough too.

With AC you will feel current flow due to your body's capacitive coupling to ground every half cycle. With DC you get charged up capacitively only once when you touch it and as long as the DC leakage is negligible no further current will flow.

I used to like to draw an arc from the output of a television flyback transformer to my finger to show the other kids in electronics class that it wouldn't hurt you, he, he, he. The teacher asked me not to do that any more.

In the good old days of lurex (glittery metal threads) in neck ties, a TV technician was killed when his tie contacted the (EHT) Ultor terminal of a disconnected picture tube.

Ultor voltage for colour TV is approximately 40kV DC, the charge is on a greasy graphite layer inside the outer glass shell. Later design placed a very high resistor to automatically discharge the Ultor voltage, over a couple of minutes, so that the picture tube was safe to work on.

4 + 1/2 Volts DC can kill, if the shock occurs at the instant of heart/brain/heart impulse.

This can place the heart in a state of ventricular fibrillation, where the heart just pulses, without fully completing a proper muscular contraction.

In operating theatres at hospitals, where heart surgery is carried out, a "Faraday cage" metal screened and earthed room is required, so that even any induced voltages from external sources can not enter the room - cellphones are banned, of course.

Electrical equipment used on that operating room must be of the highest standards, because even a few micro-amps leakage can cause heart problems during the delicate surgery.

4.5V DC will is only dangerous if there is enough current available. It is not the voltage that kills. The voltage is the enabler of current flow, thus the higher the voltage the easier for current to flow.

Consider a static shock discharge when you touch something of different potential, this static charge can be millions of volts, but there is very little or near zero current available, if there was say 1/10th of an amp or so available, you would be killed by the massive arc and shock.

...Dear Guest..you are not aware of rudiments of electricity , it is absolutly wrong assumption that AC shocks and DC doesn`t just because you have experienced high voltage and amperage in AC and far lower Wattage in DC ment for electronics control, but actually DC is more dangereous at higher ratings and can kill instantly so be ware do not attempt to touch higher DC voltage carrying higher current even with precautionary gears and accidently .Always have experienced professional electrician when working with these invisible force if you don`t know......Thank you

And they say there are no stupid questions...

Everyone has to start somewhere.

No person can know everything about everything.

That's a good reason to try and help others, where possible.

Sparky,

I have no problem helping people who need help. However, much can be gleaned by doing a simple Google search BEFORE asking such a rudimentary question on an engineering forum.

My irritation got the best of me and I said "stupid". What I really detest is "lazy". I will not help someone who wants an answer spoon-fed to them. Part of what really helps us understand the principles behind questions we conceive is the effort it takes to answer them.

Mike

Since your probably sipping tea about now, answer me this Batman.

Do you think they (CR4) have someone on staff who may submit questions, just to get things moving?

James

i'M not batman

but I'll answer anyway w/an example

http://cr4.globalspec.com/blogentry/4009/Ray-Gun-Lawsuit?frmtrk=CR4digest

I was hiding out in Rancho Seco, a multi-megawatt nuclear power plant. When all of a sudden, I felt this terrible stinging sensation!!! Seems I had made the mistake of sitting directly on top of the atomic pile! That launched me about 300 feet across the plant, landing directly on the mains coming from the generator. My vision suddenly went blurry, and I started to make that blub-blub-blub sound from the cartoons!

I could just make out one of the workers saying, "Hey! What's that burning fuzz ball doing up there?! Thankfully, the blast from his fire extinguisher not only put me out, but knocked me free of the mains!

After that, we all had ice cream... Not that's what I call a sticky situation!

Did you get any super powers?

Maybe you're that fuzz I see flash the edge of my vision?

If you occasionally see dark things out of the corner of your vision, have someone give you the pen test. That's where someone holds a pen in front of you and moves it back and forth and up and down, while you track it with your eyes. It's probably a good bet that while you're doing this your eyes will jerk a bit. People that jerk their eyes while doing the pen test often see things out of the corner of their vision.

Oh, by the way, I saw this on Marcus Wellby... The patient died.

Wow. Try and explain that to the Veterans Administration. I lost some of my hearing during Gulf II. They pushed the papers forward quickly, thank you VA.

But could you imagine the look on their face when you say, Ray gun!

James

Actually the author not the content was the point!

Guess I'll have to share another shocking story.

My brother was working on the lawn mower & had removed the spark plug.

Hold this wire

zzt,zzt.zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt, zzt................................................................................

had removed the spark plug

I used to do that with my old beemer.

Fix the throttle on 2000 rpm and pull one cap off. Wearing very thick gloves would make it possible to see the drop on each cylinder and compare them to adjust the carbs equally.

Tzzzz, deng.....tzzzt, deng....tzzt deng that hurts. lets just switch it off and than put it back on shall we

Fix the throttle on 2000 rpm and pull one cap off. Wearing very thick gloves would make it possible to see the drop on each cylinder and compare them to adjust the carbs equally.

The drop of what?

Hey just trying to learn something.

I remember the question of which voltage garbs you and which voltage throws you has been debated for as long as I went to basic electricity and electronics school. The bottom line is if you grab something hot and your hand is wrapped around it you will not be able to control your muscles to let go. If your legs are bent when you contact either voltage and your muscles suddenly contract or straighten you may be launched across the room. Any electrician or technician that has to work around live voltages should know not to wrap their hands around something that might be hot, and only work with one hand in a panel in case shock would occur the path would not be directly from arm to arm through the heart.