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#50725 04/14/05 03:33 AM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and
Member
One way to look at this (getting shocked thing), is to relate the Wattage delivered to a Human's Body during a shock scenario.

If, for example, a Person receives a painful shock from an AC Power source, at a Voltage of 120 VAC, and the Current that flows through this Person is 6 ma (Milliamps), the resultant Wattage falls well below 1 Watt (0.72 Watts).

The same 6 ma at 240 VAC will result in a True Power draw of 1.44 Watts, and at 277 VAC, the draw is 1.662 Watts.

In this scenario, the Voltage, Resistance and Wattage change, but the Amperage remains the same value.

A drammatic rise in True Power drawn as the Voltage increases.
Looking at it the other way, there is a drammatic drop in True Power drawn as the Voltage decreases.

This would be results from something having a "User Changable" level of Resistance, which the Resistance may be increased, or decreased, in value in order to connect to a different Voltage level - and still draw the same 6 ma.
Definitely not going to be the results of a Fixed Resistance level, which is somewhat typical for People.

Scenario with a fixed Resistance value will result in large increases in Wattage being delivered, along with increased Amperage, as Voltage increases.

Same values from the AC power source - 6 ma flows when the Person gets connected to a 120 VAC power supply.
This equates to a Resistance of 20 K (20,000 Ohms), and the resultant True Power drawn from the supply is 0.72 Watts.

The same 20 K load (fixed Resistance) connected across a 240 VAC supply results in a 12 ma flow, and a corresponding 2.88 Watts drawn from the Supply.

As you can see, doubling the Voltage resulted in the Wattage drawn being 4 times larger.
Lowering the Voltage by ½ reduces the drawn Wattage to ¼ of that value.

In this scenario, the Voltage, Amperage and Wattage change - but the Resistance remains the same value.

Apply this to any Circuit, any Power Source, at any point on the Source, and get the same results.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#50726 04/14/05 05:28 AM
Joined: Mar 2005
Posts: 1,803
Member
Exactly put Scot. You can't tell what severity of shock has been received by a victim, because you won't know the atual resistance value. Some values in milliamps-
1ma- just about felt.
5-10ma- If victim is grasping the conductor -he can't let go!
20-50ma- possible fatality due to fibrilation of heart system and temporary paralysis of respiration.
60-800ma- probability(!) of death, burns.
Above that, muscle damage, internal and external burns, VERY life threatening. Or-

At 120v, dry skin 100kohm = 1.2ma=tingle
At 120v, wet skin 1000ohm = 120 ma=serious.

WHAT TO DO in an ELECTRIC SHOCK EVENT?-
* DONT TOUCH VICTIM until SURE he/she is disconnected from the source of the shock.
-seems obvious, but many victims are hapless 'rescuers'.
* Call 911 (999 UK) for urgent medical assistance.
* Apply first aid, possibly CPR (artificial respiration UK). You MUST know what you're doing- a wrong actio can make it worse, but prompt & proper action may save a life, because a common symptom is a temporary breathing stop. This isn't the place to discuss the finer details of CPR- Go on a First Aid course!!-Far better to say to a victim's family "Aw-it was nuthin!", than going out to buy a black tie....
Alan


Wood work but can't!
#50727 04/14/05 01:14 PM
Joined: Aug 2001
Posts: 7,520
P
Member
I've always been led to believe that it's the outer layers of skin which provide the greatest resistance. Once down to the flesh below, the resistance of the body can be very much lower, making shock from sub-cutaneous contact very much more dangerous.

Higher voltages also apparently have the effect of breaking down the insulation of the outer skin.

We had a discussion on this a couple of years ago. Some of you may remember the following link, which shows that under extreme circumstances (and obviously deliberately applied electrodes in this case), the resistance of the body can be very low:


http://www.dc.state.fl.us/oth/deathrow/drorder.html

(See the table of voltage/current/resistance values about one-third of the way down the page.)



[This message has been edited by pauluk (edited 04-14-2005).]

#50728 04/14/05 05:56 PM
Joined: Mar 2005
Posts: 1,803
Member
Oh dear, reading that has put me right off my tea!!

Alan


Wood work but can't!
#50729 04/14/05 06:58 PM
Joined: Dec 2004
Posts: 1,064
D
Member
Getelectric, another easy way to remmember this.
Static electricity runs in the thousands of volts. If you walk across the carpet and then touch something, and Zap, that's static electricity. But there are no amps to be pushed, very liitle current very high voltage.

Lightning runs in the millions of volts, lot more amps. Don't get Zapped by that.

Tazers, stun guns, same principle.
50,000V knock you down pretty quick, very little current.

#50730 04/17/05 05:09 AM
Joined: Apr 2005
Posts: 13
R
Member
FOR THE GUYS WHO DON'T KNOW, all those threads don't help.
FILL IN THE BLANK FUN;

Electricity follows the ____ of least resistance.

Electricity always returns to ______.

If I provide no ____ to ______, Electricity cannot flow through me.


"4 INCHES FROM YOUR CHEST, PYLE!!!, 4 INCHES!!"
#50731 04/17/05 05:10 AM
Joined: Apr 2005
Posts: 13
R
Member
Oh yeah, and don't become the neutral either.


"4 INCHES FROM YOUR CHEST, PYLE!!!, 4 INCHES!!"
#50732 04/17/05 07:00 AM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Quote
Electricity follows the ____ of least resistance.

I assume you are looking for the word 'path'?

That is true electricity follows the path of least resistance and at the same time all other available paths.

This is important to remember.

Quote
Electricity always returns to ______.

The correct answer here is 'the source' if you are looking for the word 'ground' or 'earth' that is not correct.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#50733 04/17/05 08:13 AM
Joined: Mar 2005
Posts: 1,803
Member
At the Ammo plants' Danger Areas and some of the Bomb Dumps that I worked in, we had to wear special conducting-footwear and pass a test by a machine at the entry points which showed we were conductive to ground. All except the Electrical Department, who all had a concession from the Director to wear special non-conducting boots. If the Safety Man caught you in the wrong boots on 'Clean-side' without a concession, (the outside world was Dirty-side!), you were on a v. serious fizzer; (wrong boots- couldn't have passed the test); and usually got 3 days suspension no pay. The very real danger from a static-spark putting a dozen men into low orbit outweighed the remote danger from electrocution- except for the guys who had to meddle in the iron-clads. Electric dets. were the worst- even the current your body uses in the nerves to control the muscles in your hand would set them off, so we used to pick them up with loooonnnnggg wooden tweezers behind a poly-carb screen!
Also, I often saw 'sparks' put the fuses from any circuit he isolated into his pocket before starting work, so no idiot could re-energise it while he was working, which I thought was a good idea.
Alan


Wood work but can't!
#50734 04/17/05 09:01 AM
Joined: Mar 2001
Posts: 2,056
R
Member
pauls point about the ouyer layer of skin is well taken.
I remember taking a course with Joe Tedesco about 15 years ago and we discussed an electrician being electrocuted by the filament of a broken temporary light.
The energized filament punctured the skin on the back of his neck and killed him.
To illustrate this, next time you have a minor cut, simply touch the probes of a DMM with your dry fingers then do the same with one finger and the wound. Compare the readings. You can also try comparing dry finger readings to that with a little bit of saliva on your fingers. Again, a dramatic decrease.
Doing the math will show that you are much more likely to get to the 70-100 mA threshold of defibrillation through an open wound or while damp.
Also remember that surface area plays a big role.
The resistance from finger tip to finger tip is much higher than that from a the palm of your hand wrapped around a pipe, or your forearm tightly pressed against steel work.

Scott's point regarding wattage seems to more so addresses the "cooking" of internal organs, than heart and muscle spasm type electrocution effects.




[This message has been edited by Redsy (edited 04-17-2005).]

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