tutorial on electric shock

[LCHIEN]

I'm always amazed at statements made on electrical power.

Mentally take this quiz:
Are you more likely to get electrocuted by
A. 120V circuit with a 20A breaker
B. 120V circuit with a 10A breaker
C. 120V circuit with a 1A breaker?
D. Equally likely with all the above

remember your answer.

People are injured by electrical power all the time. I categorize the injuries a being one of the following:
1 - shock leading to involuntary nevous pullback and resulting injuries from hitting objects and falling
2 - closely related to 1, but shock leading to muscle spasm (due to electrical currents) resulting in injuries from hitting objects or involuntarily clutching or spasms
3. electrocution deaths due to electrical currents in the heart and other vital organs disrupting the heartbeat.

Obviously 1 & 2 are of concern but number 3 is probably uppermost on most peoples minds.

It takes very little current to disrupt the heart. Various studies put the value at about 1 to 1.5 milliamps, that's .001 to .0015 amps. DC or AC, makes little difference.

However, the current has to go through the heart muscle. Thnk of current as a stream of water molecules, when splashed on a slightly sloping surface you will get rivulets of multiple paths of water trying to get from the high side to the low side.

If a person touches the 120V hot side with his left hand, and the 120V neutral or ground side with his right hand, he has completed a circuit with the path going through one arm into the trunk of the body and into the other arm.

Electrically speaking the body can be simplified to be each arm or leg is 1000 ohms and the trunk is like a junction point where they are all connected. This is pretty much true for everyone.

Added to the resistance of the limbs, is the contact resistance of your skin where you touch the exposed voltage. The resistance varies greatly from some people with very dry skin at about 10,000 ohms to very few ohms for people with sweaty skin, or perhaps wet with water, and the contact area size. (more contact area = lower resistance)

Assuming our hypothetical victim had sweaty, wet hands, his completed circuit now looks like 120V across 1000 ohms left arm, through the trunk and 1000 ohms right arm, then to ground/neutral. the current will be 120V/(1000+1000) or around .0006 amps, very close to the typical .001. Because the current may flow around the heart instead of through the heart he may be spared, or if he is on the low side of typical, he may die.

Assuming our victim was standing in a puddle of saltwater with both feet and had grabbed the electrical contact with both hands, he would double the current in his body, due to paralleling the limb loads top and bottom. This would be more likely to be lethal.

OTOH, assuming he put his left thimb on the hot contact and his left pinkie on the neutral contact, he would receive a nasty shock but would very unlikely to get electrocuted becaue the current would travel though his hand but not his heart.

So, back to the original question, the correct answer is D. None of the circuit breakers would protect you from electrocution, all of them will trip at 1000's of times the current level that would possibly kill you. Circuit breakers protect in two ways: one, they prevent fires, their primary puspose. they keep wiring and to a secondary point, appliances from overheating due to excessive current. They also will open the circuit upon a short. Most equipment with metal cases is grounded so that a loose hot wire will contact the case and short and open the breaker before you have a chance to contact the exposed wire.

GFCIs can help with electrocution injuries, they trip at current in the sub-1 milliamp area, below the threshold of heart disruption. They only work in limited ways though... the sens the difference in current between the hot and neutral. If not exactly equal within the milliamp tolerance then the the assumption is that the current is sneaking off through a ground fault, one of which may be you standing in a puddle of water while holding that hot contact in your left (or right) hand. So it will trip quickly and spare you. It does however require the alternative ground path and won't protect you if you get across the hot and neutral.

I might add that most of the electrical contact injuries are from causes 1 & 2, not from electrocution. Most people get hurt jerking their hand back and getting seriously cut on hard and or sharp objects and from falling off ladders and breaking bones. Fortunately. shock injuries from 1&2 are seldom fatal.

[cabinetman]

Good info, but very shocking. I've been zapped by 110V, 220V, but the worst blast was from a lawnmower (running of course). I was a kid, and I may have been barefoot...can't remember...it was in the last century. Anyway, it was the kind that had a grounding strip of metal that you pressed against the spark plug to stop the motor. Well, it knocked me right off my feet, not just a buzzing of the upper body.
.

[Stytooner]

Thanks Loring. I did choose the correct answer. Lethal current is much less than 1 amp. I'm not sure how much voltage a human can typically withstand without the presence of current or very little anyway. I have been shocked several times by the coil on an engine. Been zapped a couple times by 16,000 VDC by my powder coating gun. Numerous times on house voltage. You can certainly tell the difference when one hand makes the contact or both hands. Big difference in the feel.
I was helping an electrician in Germany and he held these two naked wires. He said "Here, hold these like this." I did. One lead in each hand. I didn't hold them long though. They were live with 220 VAC @ 50 cycle.
He told me the reason that he made me hold them is so I know what it feels like to get shocked. That way I would be less likely to fall off a ladder and break my neck.

Well, then I told him that I had been a generator mechanic in the Army and I had already went through all that kind of training will hands on learning. It was done to us in class. I told him thanks anyway and he could hold his own wires in the future.

I try to be as careful as possible and still get bit, so even extra precautions every time is really worth the effort. If you have heart arithmia problems or pacemakers and things, you should not even fool with electrical stuff IMO. Get someone else to even change a light bulb if possible.

Electricity can indeed travel through the air. On my PC gun, I didn't actually touch the probes to get shocked. I just got within an inch or so with my other hand once and the second time was with a rubber tipped air gun. Bit me through the gun.

[Tom Slick]

Thanks for the reminder Loring.

A couple of stories;

My uncle was killed as an indirect consequence of being badly shocked. He had shut off the correct breaker but cut into a large 208V feed that was the wrong circuit. He suffered all the effects of electrocution short of being killed but a few days later "felt fine". Two weeks later he was on a motorcycle ride and seemed to have lost some of his "sense" for riding, he was known for being an outstanding rider. He proceeded into a two-way intersection without looking and was run over by a truck.

An electrician I know was paralyzed as an indirect consequence of someone else being badly shocked. He was working on a house that had a large heavy door that swung upward to expose the panel. that door was propped open by a 2x4. He was out at his truck when the homeowner started poking around in the panel. The homeowner touched something in the panel and was getting shocked bad enough that he couldn't let go. The electrician got a running start and was going to knock the homeowner away from the box. In the process the 2x4 prop was knocked away and the door fell directly onto the electrician's head. that caused spinal damage and he was immediately paralyzed. it took him 2 years before he could walk normally again, he still can't sit for more then 20 minutes or lift anything.

The worst I've done is shorted my pinky finger across two legs of 480V 50A supply, that's 277v across my "pinky circuit". That happened because I shut off a disconnect on a control box and assumed a circuit was safe. it wasn't, I was on the supply side. Luckily, it did nothing more then scare the @#$% out of me and give me a reminder in electrical safety. After that I invested in some electrical safety devices including non-contact sensors for all maintenance employees and revisited the company's non-existent lock out/tag out procedures.
that was all so that I could pull some comm wire into the enclosure to wire up some PLCs.

moral of the stories,
Doing the "mechanics" of electrical work is easy. in home/farm wiring the engineering is easy because it is all "rule of thumb" and looking up sizing/ratings on charts. it is the SAFETY aspects of doing the work that will KILL you. It even happens to professional electricians. Much the same way that running a table say isn't inherently dangerous, but if you don't have good safety practices you are going to be injured or killed running the saw.

[Mr__Bill]

While from an electrical/medical stand point answer D is correct I think that C from a quantitative point of view is more accurate. Let me explain. I think that more people are likely to be sloppy when handling the low power circuit than the high power one. Thus, if I am more likely to mishandle a 1 amp circuit than a 20 amp circuit I therefor am more likely to be electrocuted by the 1 amp than any of the others.

Option two:
There are more 15 amp circuits in a home than 20 amp and almost no 1 amp circuits. Thus I am more likely to screw around with a 15 amp one than the others and thus a 15 amp circuit is more likely to kill me.

Hey, lick your fingers and see it that wire is hot!

[Uncle Cracker]

I have been hit by lightning and blown across the room. Thank God I did not have my calculator with me, or I might have done the math and realized that I should have died...

[MilDoc]

Only thing not taken into account is frequency. The internal electrical frequency of the heart is in the range of 60 Hz which is of course power line frequency. At 60 Hz and far less than 100 milliamps the heart fibrillates just as it does after a major heart attack and you die quickly.

Hospital electro-surgical equipment operating properly often runs above 100,000 Hz and current passes through the body with no effect on the heart or breathing.

The "can't let go" current is about 10 mA for adults and 5mA for small children. Prolonged contact due to muscle spasms ("can't let go") running across the chest does not have to effect your heart at all. It will cause the diaphragm to contract completely, causing suffocation.

[Bill in Buena Park]

Good stuff to remember

Thanks for the post.

I have been shocked quite a bit - by home current when working my own electrical, and automotive coil current - and its always a hair-raising experience. I've never sustained any real injury from any (darn lucky, if you ask me), but accidentally contacting that wire that goes from the coil to the distributor has hurt more than any other shock I've had - feels like you got whacked with a fan blade, and is sore quite a while after.

I hate the sneaky ones that have gotten me by surprise. When I was a kid and tagged around with my dad in the yard or garage, he had rigged an ungrounded extension cord/box thing with an old metal outlet box, but failed to protect the wires from the metal where he punched out the slug... which over time abraided through to contact the hot side in the cord. I found this out by carrying the box, in my bare hand, while it was plugged in, over a dirt area while barefoot - that building tingle in my hand baffled me until I recognized it and tossed that box while jumping about 2 feet off the ground.

Once, when working a live circuit, I contacted my linesmans pliers, in which I had gripped a hot line, to the side of the ground box. There was a POP, a bright flash with some radiant heat (very close to my face) before the breaker kicked - left the pliers welded to the box. The pop deafened me momentarily. Aside from knowing the pop must result from the light and heat being created at shorting the circuit, I was never clear on the physics of what causes the pop. Anyone know?

[Stytooner]

I think that pop is the same as thunder. Now if you know what causes thunder, we are a step closer. Perhaps light or heat breaking the sound barrier by traveling as fast as the speed of light?