Impedance of person and fault current

[cppoly]

Mike Holt uses the impedance of a person as 500 ohms, and another source uses 1000 ohms.

That being said, I'm assuming this is only for a person and not with a person wearing shoes. Wouldn't this impedance be a lot higher, maybe mega ohms for rubber soles?

For instance, I've see fault calculations for a fault running through a person at 120V/1000ohm = 120 mA, which is enough to kill a person.

But if this calc were to include the shoe impedance, wouldn't it be a lot lower fault current running through the body?

 

[xformer]

Mike Holt uses the impedance of a person as 500 ohms, and another source uses 1000 ohms.

That being said, I'm assuming this is only for a person and not with a person wearing shoes. Wouldn't this impedance be a lot higher, maybe mega ohms for rubber soles?

For instance, I've see fault calculations for a fault running through a person at 120V/1000ohm = 120 mA, which is enough to kill a person.

But if this calc were to include the shoe impedance, wouldn't it be a lot lower fault current running through the body?

What about a hot in one hand and a ground or neutral in the other hand connection?

 

[cppoly]

Yes that would use only the impedance of the body, but I've never seen examples to include the shoe impedance. Does this mean that the fault current is so small, someone will not be hurt?

 

[xformer]

Yes that would use only the impedance of the body, but I've never seen examples to include the shoe impedance. Does this mean that the fault current is so small, someone will not be hurt?

Generally speaking, if a person energizes themselves and there is no outlet path for the electricity to flow then the impedance would be close to infinity. But, It doesn't take much current to kill someone. Most people think that that a severe shock (one that causes death) will leave a person with a large burn but such cannot be further from the truth. All that has to happen is for the electricity to stop the heart from beating, or throw it off rhythm. That is why the GFCI is set at the 5 mA range. Hope this helps

 

[zog]

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:


Condition of contact Resistance (Ohms)
Dry Wet
Finger touch 40 k-1 M 4-15 k
Hand holding wire 15-50 k 3-6 k
Finger-thumb grasp 10-30 k 2-5 k
Hand holding pliers 5-10 k 1-3 k
Hand around pipe (or drill) 1-3 k 0.5-1.5 k
Palm touch 3-8 k 1-2 k
Two hands around pipe 0.5-1.5 k 250-750
Hand immersed -------- 200-500
Foot immersed -------- 100-300

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. Some typical values for different materials:

Materials Resistance (Ohms)
Rubber gloves or soles More than 20 M
Dry concrete above grade 1-5 M
Dry concrete on grade 0.2-1 M
Leather sole, dry 100-500 k
Leather sole, damp 5-20k
Wet concrete on grade 1-5 k

(Resistance values for equal areas (130 cm2) of various materials)

 

[kwired]

Hands or feet with callouses will likely have higher resistance also, but if there are any cuts or abrasions on them that will change.

 

[templdl]

Hands or feet with callouses will likely have higher resistance also, but if there are any cuts or abrasions on them that will change.

That's the point. There are so many variables that van determine the magmetude of electric shock. It's basically ohms law. If you are exposed to 120v at a given resistance a give amount of electric current will flow. Reduce that resistance and the current will increase. Increase the voltage and the current will increase.
Them there is the path at which the current take through the body which often determines what affect it has on your heart. The current flow can cause a person to fall of of a ladder causing serious injury or even death from the fall.

 

[cowboyjwc]

Man, woman, child, makes a difference also.

 

[ggunn]

Generally speaking, if a person energizes themselves and there is no outlet path for the electricity to flow then the impedance would be close to infinity.

Have you ever seen the videos of how those guys work on high tension transmission lines from helicopters? First they connect the hovering helicopter to the line to equalize the voltage, and then the guy steps across and the chopper flies away and leaves him up there hundreds of feet off the ground with his body energized at however many thousands of volts the line is at. It's not for me; whatever those guys get paid, it ain't enough.

 

[GoldDigger]

Have you ever seen the videos of how those guys work on high tension transmission lines from helicopters? First they connect the hovering helicopter to the line to equalize the voltage, and then the guy steps across and the chopper flies away and leaves him up there hundreds of feet off the ground with his body energized at however many thousands of volts the line is at. It's not for me; whatever those guys get paid, it ain't enough.

I would not be particularly concerned about the HT, but the height alone and the helicopter that close to the wires would both cause me to change my pants.

 

[hurk27]

Also keep in mind that those workers are also in a Faraday cage sort of as the clothing they have on is very conductive as in high carbon or metal fibers, at the working voltage of some of those lines even the air can cause enough current to flow to be a problem, but even if the lines are de-energized, the static build up from the rotor blades can generate enough static current to kill a person, so energized or not they have to take the same precautions conductive suit and all.

 

[pfalcon]

Have you ever seen the videos of how those guys work on high tension transmission lines from helicopters? First they connect the hovering helicopter to the line to equalize the voltage, and then the guy steps across and the chopper flies away and leaves him up there hundreds of feet off the ground with his body energized at however many thousands of volts the line is at. It's not for me; whatever those guys get paid, it ain't enough.

http://www.mikeholt.com/HighPowerJobVideo.php

Because you mention it.

 

[pfalcon]

That's the point. There are so many variables that van determine the magmetude of electric shock. It's basically ohms law. If you are exposed to 120v at a given resistance a give amount of electric current will flow. Reduce that resistance and the current will increase. Increase the voltage and the current will increase.
Them there is the path at which the current take through the body which often determines what affect it has on your heart. The current flow can cause a person to fall of of a ladder causing serious injury or even death from the fall.

Current, Voltage, and Frequency all play in electric shock with contribution of each component relative to the overall Power delivered.

At very low power levels the body resists the shock.
Raise power a little and Frequency begins changing the rhythm of the organs, most notably the heart and brain.
Raise power more and Voltage can cause muscle contractions. AC resembles spasms.
Raise power more and Current vs Resistance develops heat in tissues.

At all levels all the above is actually taking place. Just like electricity attempts to take every available path in proportion to its resistance. Some problems are just have "less noticeable" effects at lower power levels.