drsparky

Voltage will not kill, current will not kill. Completing a proper circuit kills. A 480 amp car battery at 12 volts is harmless to the touch. A 12000 volt static shock from the carpet at 1 micro amp is just annoying. Of course in the right conditions (with proper resistance) together they kill.

retired 7373

THE HIGHER THE VOLTAGE THE LOWER THE CURRENT IN AN A/C CIRCUIT- CHECK THE FORMULA.

JohnJ0906

Really?

I=E/R

Seems to say that the higher the voltage, the higher the current. (At a fixed resistance)

If a person is shocked by 120v, then 277v, which will cause more current to flow through the body?

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

retired 7373

John,
I dont have the formulas at hand any more. Check the formula for a/c power.
10 amps at 120 volts will be 5 amps at 220 volts a/c. The higher the driving force the lower the amps. That is why power lines are so small since the voltage is so high.

JohnJ0906

What does this have to do with current through a human body? (See the title of the thread)

The resistance of the body is the constant. If the voltage is higher, the current is higher. Basic Ohm's law. Power has nothing to do with it.

__________________
John from Baltimore
"One day at a Time"
All responses based on the '08 NEC
It's not my fault, it's not my problem, I'm not your solution.

YOUNGY

This has been my first cection at tafe

drsparky

I have a lower body resistance than a lot of people that I have worked with over the years so I am more susceptible to getting lit-up than most. My resistance is about 200,000 ohms thumb to thumb.

I=E/R

So at 120V I would receive about .0006 Amps
277V I would receive about .0013 Amps

If your body was closer to 1 Meg Ohms thumb to thumb (more common).
120V .00012 Amps
277V .00027 Amps

Of course as your skin burns off and your resistance drops until your blood and fluids boil and dry up. Also remember that in a health care setting that an Intervenes line (IV) bypasses the natural resistance and basically puts a conductor strait into the nervous system (IV fluids are salt solutions that are very conductive) that is why they are so big on grounding any stray voltages.

Chuck

Zog

There are several factors that determine the severity of an electric shock

1. Amount of Current
The amount of current that passes through the body during an electrical shock is a large factor when determining the amount of tissue damage that will occur. The table listed below shows some values and their effects.

CURRENT

60HZ

None
Imperceptible
Perception threshold

Sensation of shock
Not painful, muscle control maintained

Ground Fault Circuit Interrupter Operates
Paralysis Threshold of Arms
Cannot release hand grip, victim may be thrown clear (may progress to higher current and be fatal)
Respiratory Paralysis
Breathing Stoppage (frequently fatal)
Fibrillation Threshold (0.5%)
Heart action discoordinated (probably fatal)
Fibrillation Threshold (99.5%)
Tissue Burning
Non fatal unless vital organs are burned


2.Current Path

A vital aspect of electrical shock is the pathway through the body that the current will take. The current path will determine which tissue and organs will be damaged or destroyed. However it is important to remember that current takes all paths, the damage to the human body is not simply between the entry and exit points.Often a shock victim is spared because the current path does not pass through a vital organ. For example, a foot-to-foot path is less likely to result in fibrillation than a hand-to-hand path because there is no current flow through the heart.

3. Duration
The amount of time that the current passes through the body plays a significant factor in the outcome. We discussed earlier that a 100mA shock is the value that OSHA uses as a fatal shock level, but as the duration increases the amount of current that can be fatal decreases exponentially. One-way to avoid getting stuck on a circuit is to use the back of your hand. Since AC causes the muscles in your forearm to contract, you may grasp onto a conductor upon getting shocked. By using the back of your hand, the contracting muscles may cause you to pull away. Studies have shown that at voltage levels above 2300V, an arc is drawn to the hand, causing you to retract before grasping on to the conductor or circuit part.

4.Body resistance
As discussed, the level of current is the factor that will determine the extent of the injury from an electric shock. But the resistance of the human body is the variable that we can control to limit the current level. Ohms law applies to human shock, the amount of current that flows through the body will be determined by the voltage the person contacts divided by the body resistance. The resistance of the human body can vary dramatically depending on several factors. Essentially it is the skin, along with such factors as area of contact, tightness of contact, dryness or wetness of the skin, and cuts, abrasions, or blisters that introduce the variables.Excluding the skin, human body resistance is about 250 Ohms per arm or leg, and 100-500 Ohms for the torso. The more muscular the person, the lower the resistance. Unless the skin is punctured, the skin will provide additional resistance. The worst-case scenario is considered to be 500 Ohms hand-to-hand. Some typical values of skin resistance are listed below.


Condition of contact
Finger touch
Hand holding wire
Finger-thumb grasp
Hand holding pliers
Hand around pipe (or drill)
Palm touch
Two hands around pipe
Hand immersed
Foot immersed


Using these typical values, a person can estimate their approximate body resistance. Let’s say a person grabs a wire in a 480/277V panel that they assumed was deenergized while touching the panel door with the other hand. The worker would have about:

250 ohms for the arm +
250 Ohms for the other arm +
32,500 Ohms for the skin
A total resistance of about 33,000 Ohms

277V/33,000 Ohms = 8.4 mA (Mild shock

Now lets look at the same scenario, but this time with wet or sweaty skin.

250 ohms for the arm +
250 Ohms for the other arm +
4500 Ohms for the skin
A total resistance of about 5000 Ohms

277V/5000 Ohms = 55.4 mA (Respiratory Paralysis, possible fibrillation, may be fatal)

At voltage levels above 600V, the resistance of the skin ceases to exist; it is simply punctured by the high voltage. For higher voltages, only the internal body resistance impedes current flow. At levels about 2400V and higher, burning becomes the major effect, lower voltages fibrillation and asphyxiation.

How well you are grounded is also a factor. Wearing rubber soled shoes or gloves along with the material between you and ground will add to your total resistance.



Voltage
For any given voltage the current passing through a body will be equal to volts divided by body resistance, however, higher voltages affect the path due to the “Skin effect”. High voltages (>20,000V) cause current to flow primarily on the outside of a conductor therefore a shock victim from these higher voltages will see more external burns to the skin and less of an effect on the internal organs.

Frequency
Studies have shown that the frequency best suited for traveling through the human body is 50-100 hertz. Higher frequencies (>200Hz) also have a “Skin effect” similar to that discussed above.

AC vs. DC
AC and DC also affect the body in different ways. AC tends to cause a muscular contraction that can cause the victim to grab hold of a conductor while DC tends to cause a convulsion effect causing the victim to jerk away. The minimum “let-go” current for AC is about 15mA while the DC “let-go” current is significantly much higher at 75mA.

Faultfinder1

If you're getting shocked you're doing something that is unsafe - voltage/amps/ohms/etc don't matter! If (hypothetically) I contacted a cable and received a "tingle" there's no saying that the same contact would not kill you. A 9V battery applied to your heart will kill you, so there is no way to actually tell how much voltage/amperage, etc will hurt you or kill you under any specific circumstances.
The bottom line here is safety - if you started your job with the right safety attitude and didn't allow yourself to become distracted you won't get dead!
Sorry if I sound pissy here, but too many electricians get killed every year simply because they refuse to learn and follow the simple safety rules. You seldom, if ever HAVE to work anything hot - so why are people taking these risks at all?
I want to go home to my family at the end of each and every day, and I hate hospitals - so I take the time to do things the safest way, not the easiest way.

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