Service manual

STP 11-25R13-SM-TG
T - 3
b. For pulsed electrical discharges (as from a capacitor), the distinction between categories (2)
and (3) above do not exist, and the electrical energy (not current) determines the possibility of ventricular
fibrillation. The currents passing through the human body which cause the various physiological
responses are provided in Table T-1. The current path through the body influences the thresholds of
these effects. Skin contact resistance determines the actual magnitude of current flow, resulting from
accidental contact of live electrical circuit components. Current magnitude is determined by Ohm's Law;
e.g., I = E/R, where I = current through body, E = applied voltage to body contacts, and R = total
resistance to current flow. This resistance is the sum of two resistances in series, contact resistance, and
internal body resistance. Internal body resistance is a constant value, varying between 200 to 500 ohms
depending on current path, whereas contact resistance may vary over several orders of magnitude
between 1 and 100 ohms. High voltages (600 volts cited in one reference) may cause breakdown of
contact resistance, in which case contact resistance becomes negligible.
Table T-1. Quantitative Effects of Electric Current on Humans
T-11. Current Path Effect.
The stimulating effect of current through the heart deranges heart action, causing ventricular
fibrillation without damage to the cardiac tissues, but resulting in death within a few minutes unless the
fibrillation is arrested. A current just below the threshold for ventricular fibrillation is a maximum to which
a person may safely be subjected. Based on numerous animal tests, this maximum current is
approximately 0.1 ampere for a duration of 1 second or more as applied to a pathway between an arm
and a leg. However, the species and size of the animal during these tests and the current pathway
affected the threshold fibrillation current. For instance, whether the pathway was from one arm to one leg
across the chest or from chest to arm and head to leg, approximately the same threshold current could be
expected. The pathway between two arms would be expected to give somewhat higher threshold
current. For the pathway from leg to leg the proportion of current reaching the region of the heart was so
small that fibrillation did not take place even at currents of 15 amperes or greater. Such currents
probably would burn the victim unless the contacts were good and the shock was of short duration.