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Dave Richards
09-15-2006, 4:13 PM
...on People.

Bob's "Stupid Electrical Question" post got me to thinking about electrical shocks and I thought I'd share the following. someone might find this interesting. By the way, I didn't think Bob's question was stupid at all.

Electrical shock involves electrical stimulation of tissueand its effects range from a tingling sensation to the violent reactions of muscle tetanus to ventricular fibrillation. Thus electrical shock is measured in terms of current intensity at specific frequencies.

Macroshock is defined as a high value current level (mA) which passes through the body by skin contact with a voltage source. There must be two points of body contact. If the current passes through the heart, it may cause ventricular fibrillation or death.

Microshock is defined as low-value current (μA) which passes through the body. Generally, intact skin has high enough impedance to prevent microshock level currents from entering the body. However a beak in the skin; a cut or wound or perhaps even a metal sliver could lower the impedance enough to allow small currents to pass. Wet skin also has lower impedance. In this case even micorshock curents can be lethal.

Larger currents are required to cause death from Macroshock because the skin is a relatively good insulator. These are the effects of various current intensities in macroshock. This is at 60 Hz, 1 second contact, arm-to-arm.

1 ma Threshold of perception.
5 mA Accepted as maximum harmless current intensity.
10-20 mA "Let go" current before sustained muscular contraction.
50 mA Pain. Possible fainting, exhaustion, mechanical injury; heart and respiratory functions continue.
100-300 mA Ventricular fibrillation will start but respiratory center remains intact. Death may occur.
6A Sustained myocardial contraction followed by normal heart rhythm. Temporary respiratory paralysis. Burns if current density is high.

Remember this is assuming 1 second of exposure. If you grab a hot conductor and the current intensity is high enough, a sustained muscular contraction might make it impossible to let go of the conductor resulting in much longer exposure.

In contrast to the above, micorshock currents of 10 μA to 100 μA can cause fibrillation of death.

The frequency of the current is also important when considering shock. frequencies between 50 and 60 Hz are particularly potent where as lower or higher frequencies are not as threatening. 1 mA at 60 Hz establishes the threshold of perception for most people and 100mA at 60 Hz may cause respiratory difficulty, ventricular fibrillation and/or death. However if the frequency is raised to 1 KHz, these current levels do not cause such sensations or life threatening phenomenon. High fequencies in the megahertz region, for example, will not cause shock at all. Electrosurgical units operate in this frequency range and do not induce electrical shocks although they can cut, burn or cauterize.

The most danger from shock is ventricular fibrillation. Death can occur within minutes if not treated with CPR or defibrillation. (BTW, contrary to popular belief a defibrillator doesn't restart a stopped heart. It cause a momentary myocardial contraction which when released should 'reset' the heart. It requires a current of 6A or more passing through the chest.)

Leakage Current is defined as the low value electrical current (μA) that inherently flows (leaks) from the energized portions of an appliance or instrument to the metal chassis. All electrically operated equipment has some leakage current. This current is not the result of a fault but is a natural consequence of electrical wiring and components.

Leakage current has two major parts: Capacitive and resistive. Capacitive leakage current results from distributive capacitance between two wires or a wire and a metal chassis/component case. For example the "hot" copper wire forms one plate, the insulation forms the dielectric and the metal chassis (ground) forms the other plate of a capacitor. This capacitor is actually distributed over the entire length of the power cord and the longer the cord the greater the capacitance. A capacitance of 2500pF at 60 Hz on a 120 V power system gives approximately 1MΏ of capacitive reactance and 120 μA of leakage current. Examples of components that cause capacitive leakage current are power transformers, power wires and motors.

Resistive leakage current arises from the resistance of the insulation surround the power wires, transformer primaries and motor windings. Modern thermoplastic dielectrics on power lines and cords are of such high resistance that the resultant leakage current is negligible compared to capacitive leakage current. Older equipment however might exhibit higher resistive leakage current due to less resistive or compromised insulation.

The classical remedy for excessive leakage current is the third or safety ground wire. The hot wire in US systems is black and is the ungrounded wire. The neutral ground wire is white and is the return wire connected to earth ground in the main power/fuse panel. The safety ground wire (which normally carries no current) is green and is the ground current return only under leakage and fault conditions. Actually, two purposes of safety grounding are to drain off leakage current and blow the fuse or trip the circuit breaker in the hot line in case of catastrophic fault (such as hot wire shorts to grounded metal case or overload).

As an example of the effect of safety ground consider a piece of electrical equipment connected to a power system in which the leakage current through a 1 Ώ ground resistance is assumed to be 100 μA. If a person of 500 Ώ resistance touches the metal case, 0.2 μA of leakage current flows through the person and 99.8 μA flows through the safety ground. Clearly the safety ground is a much lower resistance connected in parallel with the person. Hence most of the leakage current flows through the safety ground. If the safety ground connection should become broken or defeated (3 to 2 wire adapter or 2 wire extension cord for example) all the leakage current flows through the person.

You can do the math for 1 A or 10 A of current in the event of a nonexistant safety ground and direct contact with the hot.

It is worth periodically checking the integrity of the safety ground especially in quipment that might get plugged and unplugged frequently. It might also be worthwhile to add an additional parallel safety ground that is independent of the power cord.

Just be careful.

Note: most of the italicized text above comes from my biomed textbooks. It was written with a hospital environment in mind however I think the information is valid for us as woodworkers or as users of electrical equipment in general. The same considerations to maintaining proper grounding of equipment apply. Periodically I check the safety ground integrity of my tablesaw, jointer, bandsaw, etc. These machines get plugged in and unplugged and tend to get moved around my shop so I like to know that the safety ground would be effective in the event of a problem.

Checking the safety ground is a simple matter with a DMM. Just measure the resistance between the ground pin on the plug and the metal chassis of the equipment. Ideally you should see no more than 0.5 Ώ of resistance.

Sorry this is so long.

Bob Childress
09-15-2006, 4:52 PM
Dave,

That definitely clears it up. ;)

Since I was a history major, I spent my formative years contemplating the political impact of the Russian Revolution instead of learning stuff I could actually use. :) On the other hand, that's why I joined this forum, to benefit from the expertise of the members.

I appreciate your exposition and have now decided not only to turn off the current at the breaker, but to wear a foil hat to keep the "leaking current" from scrambling my brains. :D :D

Dave Richards
09-15-2006, 4:56 PM
Just keep your head out of the microwave oven while wearing that foil hat. It'll make sparks. DAMHIKT :D :D

Dennis Peacock
09-15-2006, 6:04 PM
Understood.....but when I was in the 10th grade, our shop teacher required each person to come by a table and stick your finger inside a light socket that was plugged up and turned on.....and be able to do it without getting shocked. Some got shocked and some of "us" did not get shocked. The key was to ONLY touch the center conductor in the bottom of the socket and NOT the side "while" touching the center conductor. :rolleyes: :eek:

I'm not saying, go home and try it....and don't blame it on me if you get smacked by a/c power for trying it. I'm NOT liable for your.......injury. :D

Larry James
09-15-2006, 6:55 PM
Quoting Dave R. - " ...The hot wire in US systems is black and is the ungrounded wire. The neutral ground wire is white as is the return wire connected to earth ground in the main power/fuse panel. ..."

Dave, you refer to the neutral ground wire. This description may (or may not be) be technically correct. The problem I have with this is the word ground. I think it is more correctly referred to as the grounded conductor. The black and white wires are the conductors that "complete" a circuit in 120 volt wiring. The neutral wire is connected to ground in the main power/fuse panel, but is not a ground wire and should never be substituted for a ground wire, and a ground wire should never be substituted for a conductor.

"... The safety ground wire (which normally carries no current) is green and is the ground current return only under leakage and fault conditions. ..."

As I recall, the ground wire may be a bare wire, green, or green with a yellow stripe.

For example, wire marked 12-2 with ground tells us there are two conductors size 12 gauge and a safety ground wire. Again, the important point is that the white grounded conductor is only grounded at the main power/fuse panel and shall not be used as a ground.


"It is worth periodically checking the integrity of the safety ground especially in equipment that might get plugged and unplugged frequently."

Good Advice!

As you said Dave, we should all - "Just be careful."

Larry

Dave Richards
09-15-2006, 9:15 PM
Larry, I agree that the white and green should never be substituted for each other. Perhaps the terminology could be changed--I wrote it as I got it--but the point is that the white neutral, ground, whatever you wish to call it is the side that is connected to earth in the panel. That side of the circuit should be at equal potential with the green safety ground.

Theoretically the white, neutral, ground doesn't have to go to the panel. It could go out the window to the nearest earth and if you think of it that way it would indeed be the ground.

The green is the safety ground. and is not intended to be the return path for the current except in cases of catastrophic fault. There should not be any confusion there.

You are also correct that the safety ground may be bare, green or green with a yellow stripe. Again I was writing what was in the text.

Ken Fitzgerald
09-15-2006, 10:06 PM
In my mid-20s, working on a test bench developing a new alignment procedure for the receiver of an X-band precision approach radar used for landing aircraft, I got the worst shock I've endured. I was holding the feedhorn for a spectrum analyzer in my left hand and mechanically adjusting a reflex klystron (2K25 IIRC) with my right while watching the display on the spectrum analyzer. My right thumb managed to make contact with the repleller voltage cap on the klystron. I took 1200 vdc from my right hand to my left hand. In that brief moment, I picked that 65 lb. chassis up with my right hand and waved it in the air like somebody shaking a blanket. I was ORDERED to go to the Naval sick bay at this Naval Air Station.

A couple of years later working on a navigational radar on a torpedo recovery ship in Charleston, SC harbor, I was again adjusting a klyston (another 2K25 IIRC). I had been up that mast several times wearing a harness but had never taken time to tie off. This time I did. Watching the face of a portable oscilloscope, I again placed my right thumb on the repeller. When everything settled down.....I was hanging by the rope on that harness from the mast some 20+ feet in the air.

In both instances I could not turn off the source because the alignments I was performing required the equipment to be energized.

Shocks are no fun.......they are life threatening........dangerous......

Safety is no accident. You live by it...........

Use the breakers.......the extra walk is good for your heart too!:)

Larry James
09-17-2006, 11:28 AM
Larry, I agree that the white and green should never be substituted for each other. Perhaps the terminology could be changed--I wrote it as I got it--but the point is that the white neutral, ground, whatever you wish to call it is the side that is connected to earth in the panel. That side of the circuit should be at equal potential with the green safety ground.

Theoretically the white, neutral, ground doesn't have to go to the panel. It could go out the window to the nearest earth and if you think of it that way it would indeed be the ground.

The green is the safety ground. and is not intended to be the return path for the current except in cases of catastrophic fault. There should not be any confusion there.

You are also correct that the safety ground may be bare, green or green with a yellow stripe. Again I was writing what was in the text.

Dave, I don't want to get in a debate over terminology, but I do want to be sure Creekers reading this thread are not misinformed by us. I no longer have a copy of the NEC, but referring to an electrical FAQ that has been on the web for many years:

FAQ-Subject: "grounding" versus "grounded" versus "neutral".

According to the terminology in the CEC and NEC, the
"grounding" conductor is for the safety ground, i.e., the green
or bare or green with a yellow stripe wire. The word "neutral"
is reserved for the white when you have a circuit with more than
one "hot" wire. Since the white wire is connected to neutral and
the grounding conductor inside the panel, the proper term is
"grounded conductor". However, the potential confusion between
"grounded conductor" and "grounding conductor" can lead to
potentially lethal mistakes - you should never use the bare wire
as a "grounded conductor" or white wire as the "grounding conductor",
even though they are connected together in the panel.

Electrical Wiring FAQ (Part 1 of 2) may be found at - http://www.faqs.org/faqs/electrical-wiring/part1/ This FAQ is not always up to date, so the NEC (National Electrical Code), or the CEC in Canada, should be checked - if you can interpet it.:rolleyes: The speaker at a code seminar I attended said - "Codes should be looked at as guides, not bibles."

Larry

Jason Roehl
09-18-2006, 9:04 AM
Larry, this is a perfect example of bureaucracy at its best. They can't see the forest for the trees. They're trying to reduce confusion and make things safer, but the terminology only increases confusion. "Grounded" vs. "grounding" conductors? I'm sorry, but that's retarded. Virtually anyone in the English-speaking world outside the electrical industry who has ever even picked up a wire knows the difference between a neutral and a ground, but I'd bet a survey of electricians would show that many of them even get the "proper" terminology wrong. Personally, I'll keep calling them the neutral and ground so that people know what I'm talking about.

Larry James
09-18-2006, 10:00 PM
Larry, this is a perfect example of bureaucracy at its best. They can't see the forest for the trees. They're trying to reduce confusion and make things safer, but the terminology only increases confusion. "Grounded" vs. "grounding" conductors? I'm sorry, but that's retarded. Virtually anyone in the English-speaking world outside the electrical industry who has ever even picked up a wire knows the difference between a neutral and a ground, but I'd bet a survey of electricians would show that many of them even get the "proper" terminology wrong. Personally, I'll keep calling them the neutral and ground so that people know what I'm talking about.

Jason, did you just call me "retarded" or were you referring to THE bureaucracy? :rolleyes: I agree, common usage is neutral and ground. What concerned me is someone reading the post may have believed that the two are interchangeable. Dave R., a knowledgeable guy, started the thread and he was making a point about safety - point well taken. Having made our point, be safe when working with electricty, let Dave, Larry and Jason ride safely off into the sunset.

:) Larry

Jim Dunn
09-18-2006, 10:47 PM
Well all that may be fine and dandy, but what about 3phase. Can you splain dis Lucy?

Chris Padilla
09-19-2006, 4:16 AM
Folks, "ground" is for carrots and potatoes and has nothing to do with electricity! ;)

We should all be retrained to think of "return path" since current always makes a loop. Now back to your neutral and ground.... :p

Russ Filtz
09-19-2006, 7:41 AM
What about the UK with "Earthing"!

All 120 current ends up in the ground via the ground rod anyway, hence the term! That's where it goes once the neutral and ground are bonded in the panel. (or to buried metal pipes in some systems that act like a ground rod)

Jason Roehl
09-19-2006, 8:56 AM
We should all be retrained to think of "return path" since current always makes a loop. Now back to your neutral and ground.... :p

"Return path" would only be for DC...half the time in A/C it's the "starting path"... :rolleyes: Maybe it should be called the "circuit path" or something like that.

BTW, Larry, I didn't call YOU "retarded". Not my style. I was calling the code terminology that.

Larry James
09-19-2006, 9:39 AM
Had he known the direction this thread would take, maybe Dave R. would have titled this thread: Physiological Effects of Electricity on Carrots and Potatoes in the Ground of 3-phase English Gardens.

What is it all about? Quoting Dave R, again:
..."It is worth periodically checking the integrity of the safety ground especially in quipment that might get plugged and unplugged frequently. ...

...Just be careful. ...

... Periodically I check the safety ground integrity of my tablesaw, jointer, bandsaw, etc. These machines get plugged in and unplugged and tend to get moved around my shop so I like to know that the safety ground would be effective in the event of a problem. ..."

BTW Jason, I knew that.:cool:

Larry, grounded in Michigan.

Mark Cothren
09-19-2006, 11:21 AM
Understood.....but when I was in the 10th grade, our shop teacher required each person to come by a table and stick your finger inside a light socket that was plugged up and turned on.....and be able to do it without getting shocked. Some got shocked and some of "us" did not get shocked. The key was to ONLY touch the center conductor in the bottom of the socket and NOT the side "while" touching the center conductor. :rolleyes: :eek:

I'm not saying, go home and try it....and don't blame it on me if you get smacked by a/c power for trying it. I'm NOT liable for your.......injury. :D


Only in Alabama.....:rolleyes: :eek: :D