The human body is subject to Ohm's Law the same way any other resistive material is. A voltage applied to it will cause current to flow. These currents can cause injury or death. Many safety engineers use the figure of 1,000 Ohms as the resistance for a typical human from hand to hand, and a current of about 10 milliamperes (still a "let go current") as being hazardous to human life. While these numbers vary widely, depending upon the individual and the conditions at the time of a shock, they are considered representative. In most cases of shock at low voltages, the current kills by disrupting the heartbeat and stopping breathing. Voltages as low as 40 Volts can be fatal.
The 120-Volt level commonly used in residences can clearly be lethal. With care, however, there is no reason for a hazardous situation. To understand why ground-to-earth resistance must be low, you need to apply Ohm's Law: E= I x R. (E is volts; I is the current in amperes; and R is the resistance in ohms). For example, assume a 4,000 Volt supply (2,300 Volts to "ground"') has a resistance of 13 ohms and assume an exposed wire in this system touches a motor frame that is connected to a grounding system that has a 10 ohm resistance to earth.
According to Ohm's Law, there will be a current of 100 amperes through the fault - from the motor frame to the earth. If a person touches the motor frame and is solidly grounded to earth, that person could be subjected to 1,000 Volts (10 ohms times 100 amperes) . . . more than enough to be fatal.
The live parts of an electrical system are expected to be energized and are recognized to be hazardous. However, other parts of the system are not ordinarily dangerous but could become energized under abnormal conditions and cause injury or death to persons touching them. Proper grounding of the metal parts of all equipment can greatly reduce this hazard. The purpose of properly grounding electrical equipment is to prevent any dangerous voltages from occurring between any exposed pieces of the metal structures or apparatus that a person could touch. The three most likely paths for current to flow through the body are these:
It is also possible for the head to contact an energized part, but this is less likely if the safety hat is non-metallic. These paths are sketched in the figure below.
