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Electric Shock

The primary variable for determining the severity of electric shock is the electric current which passes through the body. This current is of course dependent upon the voltage and the resistance of the path it follows through the body. An approximate general framework for shock effects is as follows:

Electric Current
(1 second contact)

Physiological Effect

1 mA

Threshold of feeling, tingling sensation.

10-20 mA

"Can't let go!" current - onset of sustained
muscular contraction.

100-300 mA

Ventricular fibrillation, fatal if continued.

One instructive example of the nature of voltage is the fact that a bird can sit on a high-voltage wire without harm, since both of its feet are at the same voltage. You can also see that the bird is not "grounded" -- you will not be shocked by touching a high voltage if there is no path for the current to reach the Earth or a different voltage point. Typically if you touch a 120 volt circuit with one hand, you can escape serious shock if you have insulating shoes which prevent a low-resistance path to ground. This fact has led to the common "hand-in-the-pocket" practice for engineers and electrical workers. If you keep one hand in your pocket when touching a circuit which might provide a shock, you are less likely to have the kind of path to ground which will result in a serious shock.

More detailed table Conditions for dangerous shock

Index

Voltage concepts

Practical circuit concepts

Reference
Nave & Nave
Ch 14

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Current Involved in Electric Shock

The electric current in amperes is the most important physiological varible which determines the severity of an electric shock. However, this current is in turn determined by the driving voltage and the resistance of the path which the current follows through the body. One difficulty in establishing the conditions for electrical safety is that a voltage which produces only a mild tingling sensation under one circumstance can be a lethal shock hazard under other conditions.

Will the 120 volt common household voltage produce a dangerous shock? It depends!

If your body resistance is 100,000 ohms, then the current which would flow would be:

But if you have just played a couple of sets of tennis, are sweaty and barefoot, then your resistance to ground might be as low as 1000 ohms. Then the current would be:

The severity of shock from a given source will depend upon its path through your body.

Index

Voltage concepts

Practical circuit concepts
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Shock Physiological Effects

Electric Current
(1 second contact)

Physiological Effect

Voltage required to produce the current with assumed body resistance:

100,000 ohms ....1,000 ohms

1 mA

Threshold of feeling, tingling sensation.

100 V

1 V

5 mA

Accepted as maximum harmless current

500 V

5 V

10-20 mA

Beginning of sustained muscular contraction ("Can't let go" current.)

1000 V

10 V

100-300 mA

Ventricular fibrillation, fatal if continued. Respiratory function continues.

10000 V

100 V

6 A

Sustained ventricular contraction followed by normal heart rythmn. (defibrillation). Temporary respiratory paralysis and possibly burns.

600000 V

6000 V

Electric shock
Index

Voltage concepts

Practical circuit concepts

Reference
Nave & Nave
Ch 14, Table 14-1.
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