Log-log graph of the effect of alternating current
Iof duration
T passing from left hand to feet as defined in IEC publication 60479-1.
[18]
AC-1: imperceptible
AC-2: perceptible but no muscle reaction
AC-3: muscle contraction with reversible effects
AC-4: possible irreversible effects
AC-4.1: up to 5% probability of ventricular fibrillation
AC-4.2: 5-50% probability of fibrillation
AC-4.3: over 50% probability of fibrillation
The lethality of an electric shock is dependent on several variables:
- Current. The higher the current, the more likely it is lethal. Since current is proportional to voltage when resistance is fixed (ohm's law), high voltage is an indirect risk for producing higher currents.
- Duration. The longer the duration, the more likely it is lethal—safety switches may limit time of current flow
- Pathway. If current flows through the heart muscle, it is more likely to be lethal.
- High voltage (over about 600 volts). In addition to greater current flow, high voltage may cause dielectric breakdown at the skin, thus lowering skin resistance and allowing further increased current flow.
- Medical implants. Artificial cardiac pacemakers or implantable cardioverter-defibrillators(ICD) are sensitive to very small currents.[19]
- Pre-existing medical condition. [20]
- Age and Sex.[21]
Other issues affecting lethality are
frequency, which is an issue in causing cardiac arrest or muscular spasms. Very high frequency electric current causes tissue burning, but does not penetrate the body far enough to cause cardiac arrest (see
electrosurgery). Also important is the pathway: if the current passes through the chest or head, there is an increased chance of death. From a main circuit or power distribution panel the damage is more likely to be internal, leading to
cardiac arrest.[
citation needed] Another factor is that cardiac tissue has a
chronaxie (response time) of about 3 milliseconds, so electricity at frequencies of higher than about 333 Hz requires more current to cause fibrillation than is required at lower frequencies.
The comparison between the dangers of
alternating current at typical power transmission frequences (i.e., 50 or 60 Hz), and
direct current has been a subject of debate ever since the
War of Currents in the 1880s. Animal experiments conducted during this time suggested that alternating current was about twice as dangerous as direct current per unit of current flow (or per unit of applied voltage).
It is sometimes suggested that human lethality is most common with
alternating current at 100–250 volts; however, death has occurred below this range, with supplies as low as 42 volts.
[22] Assuming a steady current flow (as opposed to a shock from a capacitor or from
static electricity), shocks above 2,700 volts are often fatal, with those above 11,000 volts being usually fatal, though exceptional cases have been noted. According to a
Guinness Book of World Records comic, seventeen-year-old Brian Latasa survived a 230,000 volt shock on the tower of an ultra-high voltage line in
Griffith Park, Los Angeles on November 9, 1967.
[23] A news report of the event stated that he was "jolted through the air, and landed across the line", and though rescued by firemen, he suffered burns over 40% of his body and was completely paralyzed except for his eyelids.
[24]
