Electrocution of the Human Body

Physiological effects of current density on the human body are shown in the table below. Contrary to popular belief, it is the current - not the voltage - level which is responsible for effects. According to Ohm's Law, of course, a certain voltage is required to cause the necessary currents to flow. Values show vary depending on the body.

Personally, I have been hit with 480 V while working on a 3-phase industrial motor connection. It didn't feel good.

See article on shock level versus pain level from Discover magazine.

The following table of quantity of electrical current and its effect on men and women is from work done by the inventor of the Ground Fault Interrupter circuit, Charles Dalziel.

Table retrieved from the "Deleterious Effects of Electric Shock," by Professor Charles Dalziel, 1961.

Above is the original table for this page.

Regarding how to properly handle the ground (aka earth ground) and neutral wire connections.

Per the National Electric Code, the neutral wire is bonded (physically connected) to the ground at only one point, which is where the service originates (referred to as "service equipment"). IAs with nearly all residential installations, the neutral and ground are bonded at the main circuit breaker panel; i.e., service equipment, aka service entrance panel. Without going into gory detail, the purpose is to ensure that the current being supplied by the two phases (120 V each) has a return path only through the system neutral.. If some other point in the system (e.g., a remote secondary breaker panel or in your case an inverter) has the neutral bonded (connected) to a separate ground (connected to a different ground rod), then some of the current that would normally return to the source via the neutral wire will instead go to ground. The division ratio of how much current goes to the alternate ground and how much goes through the system neutral depends on their relative resistances. Two problems are caused by that scenario. One is that there is an unbalanced condition at the service entrance point. The other is that the alternate ground wire will have current running through it which, since the wire going from the panel to the ground rod is usually uninsulated, an electrical shock hazard is present. Simply put, there must only be a single point in an electrical distribution system where the neutral wire and the earth ground are physically connected, and that is where the electrical service originates (in your circuit breaker panel). The separate ground wire in Romex cable is present as a "safety ground" whose purpose is to provide an alternate path for return current if the neutral is inadvertently opened, in order to prevent an electrical shock (a scheme which only works under certain conditions). Regarding the unbalanced condition, that is the principle on which GFI circuit breakers and receptacles work. A circuit inside the GFI device measures the difference between the current in the "hot" wire and the current in the neutral wire, and if there is a difference of just a couple thousandths of an amp, it shuts off. A tenth of an amp of AC current passing through the heart (e.g., from one hand to the other) is enough to cause defibrillation and kill you. - Kirt Blattenberger

Posted April 26, 2004