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,
|Slight sensation on hand
|Perception threshold, median
|Shock - not painful and muscular control not lost
|Painful shock - muscular control lost by ½%
|Painful shock - let-go threshold, median
|Painful and severe shock - breathing difficult, muscular
control lost by 99½%
|Possible ventricular fibrillation
Short shocks (T in seconds)
High voltage surges
* Energy in watt-seconds
Table retrieved from the "Deleterious Effects of Electric Shock," by Professor Charles Dalziel,
||Threshold of sensation
||Cannot let go
||Extreme breathing difficulties
Above is the original table for this page.
Regarding how to properly handle the ground (aka earth ground) and neutral
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