Over the years I have come across a lot of electricians that do not understand the relationship between V, A, & Resistence. I would say you can touch a recepticle in a home or go out and touch one side of the main feeder lug in the panel and it would be the same (standing on the same surface of course) because even though the main lug is capable of putting out more amperage, you are not going to draw it, as long as the resistence in your body is the same for both. Basically.. the only way to draw more current is to either raise the voltage or lower the resistence. Simple concept but many even experiened electricians do not understand this.
Has anyone come across this?
:-)

Have to re-remind myself all the time.

Yeah, when I have to climb up on a roof and cut the POCO service lateral for a srvice change or damage repair, I have to remind myself too. I understand it perfectly, but it doesn't make my spine any stiffer.

That's a good question...a #14 will drop you just as quick as a 500KCM...but I never sweat an outlet, only the service lateral...maybe it's because it's not something I do sitting on a spackle bucket

You're probably thinking of the hazards of an arc blast, which a 15 amp circuit won't create much of. Compared to incoming power from the street.

Years ago.... Closing a can on 20' ext ladder, and pushed through the tape on a kearny. Went off like a grenade! Knock me off the ladder, very luckily un-hurt. Had to fix the damage the next day, I shook so hard it took a few hours to get the nerve to climb the ladder. I wonder what the AIC rating of a can cover is?

Yes, I find people who don't understand the relationship all the time, and assume that if you touch a 240V 30A circuit you're going to get 30 amps going through you.

It must be over a year ago now, but ECN regulars might remember that we discussed a thread on another forum where somebody was arguing just that point. He was saying that even on 12 volts you could get a massive shock if the current in the circuit was high enough. No amount of explanation would convince him otherwise.

aldav53, it is called ohm's law, day 1 theory after algebra class. However the is a heck of difference between the terminals of a service transformer, and a branch circuit with 50-feet of #12 AWG wire after the main panel.

To learn this difference simply short out the terminal from L-N or L-L on a service transformer, then repeat at a receptacle (use an apprentice to make the connections). Measure the fire-ball produced from both and see if there is any measurable difference. If so, why?

dereckbc,
True, thats because the #12 circuit trips the circuit when it reaches 20 amps. (remember thats basically 0 resistence).
But you still won't get shocked any different by touching either one. (providing "your" resistence is the same).

pauluk,
Good example on the 12 volts. A car battery is 12 volts and capable of hundreds of amps, but the voltage is too to draw it. Unless you lay a metal bar across it (0 resistence).

I,m not so sure I understand myself. so you are saying that if i grabbed with one hand a 400a circuit and grounded myself out that it is the same as if it were 15a being that both voltages were 120.

pauluk,
Good example on the 12 volts. A car battery is 12 volts and capable of hundreds of amps, but the voltage is too low to draw it. Unless you lay a metal bar across it (0 resistence).

added the "low"

GETELECTRIC,
Yes, thats exactly what I'm saying. If your resistence was the same when touching either one, you will draw the same amount of current on either one. Amps are not just there, they are "drawn" depending on the resistence and voltage. As the resistence goes down, or voltage up, you will draw more current. Of course the 15 amp circuit will reach tripping much sooner than the 400 amp, because it will trip at 15 amps, and the 400 amp will trip at whatever it is fused at.

And, irrespective of the volts, if 500ma passes through you, somebody else will be doing the math, because you will be ....
.......dead!

So what kills voltage or current,

Volts gives you the jolts, but Amps puts out your lamps!!

Alan

so what is your answer

Your body's resistance, which can vary widely, is around 10Kohms. Much lower if you're in the bathtub taking a bath and touch a wire with wet hands. GFI's are set to trip at about 7ma of imbalance. 7ma of course will not trip a regular 15 amp circuit breaker or a 400 amp breaker. A major variable if you live or not is the path the current takes thru your body. One hand touching the 120V and the other hand touching a ground will cause the current to pass thru the area of your body where your heart is. That ain't good, might goof up the heart beat.

So the answer is: the fault current is the killer, but because of Ohm's law what voltage source you happen to touch while part of you is also touching another voltage source or ground will determine what fault current you'll pass thru you. A 12V car battery won't create enough current to harm you (but do be careful of metallic rings on fingers don't create a dead short from the + terminal to the car's frame and get very hot and burn your finger off).

Higher voltages will create higher fault currents thru your body given the same resistance. Also a poor enough contact that would let you survive a contact of 120V to ground can kill you at 277V.

If you touch 120V or even 277V and nothing else (because you are standing on insulation) you are an open circuit and no current flows. That's the reason for the ham radio safety rule of placing the other hand in your pocket while poking around high voltage in a radio transmitter. Not that you purposely touch high voltage, but if you slip with the voltmeter probe this precaution offers redundancy to prevent shock.

[This message has been edited by wa2ise (edited 04-13-2005).]

'So what kills voltage or current,"

Neither.

It's a combination of factors:
- length of time "hung up"
- amps passed through the body
- path current takes through body

Here's a 13 minute long video: http://www.electrician.com/safety/electricalsafety_01.htm

*****WARNING****
The video contains graphic images of the effect of electric on the body...not for the faint of heart ~ if you can't bear to watch the video, you have NO BUSINESS being IN BUSINESS.

"True, thats because the #12 circuit trips the circuit when it reaches 20 amps."

I have seem 20A breakers hold 20 -30 amps for a long long time.

Also during direct or partial shorts this not exactly true either..... a short, acts slightly different than an over-load, and has more to do with the AIC rating of the breaker/fuse and size of transformer, and distance to it via the conductors than simply "20A". The main reason you'll get that fireball/arch-flash effect on a service short, as opposed to a feeder or branch circuit is that circuit breaker or fuse has an AIC rating determined by many varying factors, and it should clear below those ratings safely. Exceed the ratings, as many people dont even think about them during most installs, and you could have a breaker or fuse that fails to clear a fault in time, explodes in the proccess, or worse not at all.

Most residential ditribution systems the idea is to limit to be 10,000 AIC or below, and likewise some comercial, but some 22,000, 30,000, and upwards for some types of industrial work.

Theres kind of an explaination here: http://www.currenttechnology.com/pdf/Current_Terminology.pdf
http://www.mikeholt.com/mojonewsarchive/EES-HTML/HTML/ElectricalCircuitBreakers~20030621.htm

And, I'm sure some of the other guys will have sometime else to add.

Its really the current thats gets you. But you have to have either enough voltage to push it, or a low enough resistence to allow the current to flow. All 3 relate to each other.
Again, a car battery is a good example. Go out to your car, touch the pos & neg terminals with your hands and you will not get shocked, yet a car battery is capable of putting out hundreds of amps. So why don't you get shocked? Because the voltage is to low. But on the other hand, if you lay a metal bar across the 2 terminals (don't do it), you will get high current flow through the metal bar (arking, heat, etc) because the resistence is almost zero in the metal bar. If you could gradually lower your resistence while touching the post, or gradually raise the battery voltage, you would eventually start feeling a tingle increase more and more.

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

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

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).]

Oh dear, reading that has put me right off my tea!!

Alan

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.

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.

Oh yeah, and don't become the neutral either.

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.


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

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

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).]

Call this urban legend rather than confirmed fact...but http://www.darwinawards.com/darwin/darwin1999-50.html

-Jon

the real problem then is the amount that I sweat around a main circuit, so why are the always in the hottest part of the building???

but when it returns to source,i.e. panel, it uses the grounded conductor to return to earth,...right?

The source would never be a panel, the source would be a utility transformer, a generator, photo voltaic system etc.

If we are talking about a grounded system part of the current may flow through the earth (in and out) to get back to where the source is bonded.

now you're confusing them again. Without formal training, one can only ponder the meanings and visual aspects of these different terms. I was only trying to make it simple, we can go on for days with the theories and principles of electron movement, but is that gonna help a guy on top of a ladder holding on to an I-beam digging around in a 4"sq.?

I didn't want to argue about it, so if you say a duck dips snuff, a duck dips snuff.

We never quite hit the physics of it all...

Anyone care to discuss valence rings, and conducting materials? 1-3 ;D Think of the numbers 5, 7..... and 8. I'm going to venture to think that a lot of the body is made of items with 4 in the valence.......

If my memory serves me, any atom with more than 8 is very stable making it less likely to be a conductor. Any atom with less than 4 valence electrons is too easily...

Man I don't have my Delmar's textbook with me and I can't remember all the specifics. It's been years since I learned this(1st yr. apprenticeship) and I've been piling up on everything else since, so I'd hate to post something less than perfect.

I'd bet you a dollar to a doughnut that iwire would be there to correct me though!

Your memory serves you well....

1-3 are conductive.
5-8 are non-conductive.
And materials with 4 are semi-conductive when doped with impurities, can go either way with a potential.

I'll bet that much of nervous, and other tissues stand in the realm of 4??????

Life as a giant SCR......

Cool, some Electrophysics stuff!

It has been a few Eons for me, so please bare with my Short-Term memory loss problems!

Well, first off - one statements has to be made:
"Virtually ALL Substances, in Solid or Liquid State, posses the property of Electrical Conductivity - in some degree"

Some are very efficient Conductors (meaning the greater the density of current for a given potential difference, the more efficient the conductor is said to be),
while others are very efficient Insulators (Non-Conductors, or higher the density of current at a given potential difference, the less efficient they are at conducting).
Some are right between these extremes - and are said to be "Semiconductors".

Basic Theory describes that Atomic Structures with at least 1 *"Free"* Electron in the outermost shell, will act as Conductors (the free electron is bounced between Atoms, etc...).
How much Energy is required to achieve this is another issue

Typical Efficient Conductors are Metallic - and include substances such as Silver, Copper, Aluminum, Iron, Nickel, Cobalt, Zinc, Gold, Calcium, etc.
Some substances are "Semiconducting", such as Silicon, Germanium, Sodium, Chlorine, Arsenic, etc., while other substances are efficient Insulators - such as Hydrogen and Oxygen.

Elements which are thought of as "Unstable" (Uranium, for example), have many free Electrons in many shells - and are very large Elements too.
They lose "Free" Electrons in natural states (without any intentional influences).

The Curies (sp???) discovered the Element "Radium", due to how the "Free" Electrons caused an effect in the Fog, when they flew away.
The effect looked like tiny Comets in the light Fog.

Back to Electricity!

As mentioned, we use Efficient Conductors for doing the job of carrying Electrical Energy (the complete package) from Generating Source, to desired Load Device.
Energy is Transported along the Efficient Conductors by taking advantage of "Free", or "Unbound" Majority Charge Carriers.
This would include an Electron going forward, falling into a "Hole", which goes backward (just including directions for simplicity).

Any opposition to the free transference of this results in a given level of "Work" being done.
This covers Voltage Drop, Losses, and actual Power Transduction at the intended Load Device.

To eliminate major loss of Smoke , Insulators are used to keep things in check.
These are efficient "Non-Conductors", but are still Conductive to some extent.
They allow a very small level of flow to take place, and act more like Dielectric substances than Insulators (i.e. Capacitive Coupling, Line Charging, etc.).

Semiconductive substances conduct efficiently in one certain direction, and poorly in the opposite direction (flow directions).
They may also work better with certain Majority Charge Carriers - like the way Salt does (Na Cl).
One Element conducts Negative Charges easily, the other conducts Positive Charges easily.
The two Elements bound together become a complete charge carrier.

I've gotta take off now, let me know if we should carry on with this discussion!

Scott35

Where I worked we detected an element we tentatively called Administronium. The heaviest element yet discovered, with 1 neutron, 12 assistant neutrons, 75 deputy neutrons and 111 assistant-deputy neutrons, giving an atomic mass of 312. These are all held together by a force called morons, surrounded by vast numbers of peons. Administronium, having no electrons, is absolutely inert, but it can be detected because it impedes any reaction which contacts it. A tiny quantity causes a reaction normally taking a few seconds to drag on for days or weeks. It has a half-life of about 2.5 years, but it doesn't decay- No! - instead it reorganises a portion of the assistant and deputy neutrons to change places. In fact its mass always increases over time, as morons join assistant-deputy neutrons to form isodopes. Some scientists think that this characteristic of 'moron promotion' means that Administonium forms spontaniously whenever enough morons gather together, the so called "Critical Morass".
Is it a good conductor of electricity? - I dunno- but I'd love to find out!!!!!
Alan

Joking aside, these theories of why 'conductors' conduct, ie valence, electron clouds etc. don't explain to me why:
I've seen electron 'flow' quoted at approx. 1/32" per second in a conductor- how then do 'signals' travel in a wire at near light speed? If conductivity is a function of electrons at the outer (shell) orbit of atoms, why don't all metals have identical conductivities? Why is a larger % of current supposedly carried at the surface of a conductor?
quote "semiconductors have a valence of 4"- So does carbon, a good conductor.
Duh! I await enlightenment!
Alan

This "electricity always goes to ground" myth is one which seems to be widely held.

In a grounded system, the earth can provide a convenient path back to the source (either intentionally or by accident), but the current is still returning to source, not to ground. The ground just happens to be a suitable path back to the source.

If you don't believe this, take a battery and bulb and connect them up so the bulb lights (no other external connections, so this is an ungrounded system at this point). Now, using an ammeter in series, connect one side of the battery to ground. You will see a zero reading, because only one point on the system is grounded.

(If you drove another ground rod and connected it the other side of the battery, then you would register a current on the meter.)




[This message has been edited by pauluk (edited 04-20-2005).]

ROTFL!

Ditto to Paul!!

LOL!

Double Ditto on the LOL for the Element "Administratium"!!!
ROTF, LMAO

Alan, may I print that out and use it at the next Staff Meeting?

It looks totally serious, until the second paragraph is read - then it kicked in for me!

Scott35

Show away, mes chers amis, anything on the Net is in the public domain! However, if you DO show it to any Admin people, please try and emphasise several times that it is, in fact, a joke, and that you won't actually be using any Company elecricity to accomplish the final line, (unless very severely provoked).
PS. More humour/humor in the same vein may be found in Scott Adams' 'Dilbert' books-the author used to 'work' (in its loosest meaning) for Pacific Bell you know.........
Alan

Just because it is a semi-canductor, doesn't mean it's not a conductor....

In a breif search for something enlightening, I found this: (Scroll down the page. I'm sure you'll find something of your brand of humor in it...) http://hypertextbook.com/facts/2004/AfricaBelgrave.shtml