Water Streams, Power Lines, and Shock: How Serious a Hazard?
BY VYTO BABRAUSKAS
SHOCK, INJURY, OR DEATH {ELECTROCUTION) from the passage of electric current through a human body has been studied for more than a century. The level of response or injury depends on the magnitude of the current and also on the frequency, whether it is direct current (DC), power line alternating current (AC), or AC voltages of higher frequency. Some typical values are shown in Table 1.1 Children are generally more sensitive than adults; thus, guidelines often assume that a child is the person to be protected. With firefighting, however, this assumption is not reasonable. Furthermore, protection against a startle reaction is the most severe level of protection. A startle reaction is described as one in which a person jumps because of a small shock that does not injure the person. No direct electrical injury occurs because of startling; there is some hazard caused by jumping, but it is remote.
The "inability to let go" of an energized conductor that has been accidentally grabbed can cause pain and injury if the current increases to an injurious level. This is also called "muscle tetanization." Thus. Many safety requirements are based on a safety-factor-reduced value of the let-go current, even though this is quite a conservative stance. Matters are further complicated. Since individuals show variation in their responses and standards, bodies typically pick a very conservative level (often the 0.5 percentile) instead of the 50-percentile value. Physiological effects of electric current, furthermore, depend strongly on frequency. The most dangerous frequencies are the 50- or 60-Herz (Hz) power line frequencies. The human body becomes less sensitive to electric current at high frequencies.2 Dalziel3 has published a useful summary of safety guidance on electric shock.