The minimum current a human can feel depends on the current type (
AC or
DC) as well as
frequency for AC. A person can sense electric current as low as 1
mA (
rms) for 60Hz AC and as low as 5mA for DC. At around 10mA, AC current passing through the arm of a human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations. The current may, if it is high enough, cause tissue damage or
fibrillation which can cause cardiac arrest; of AC (rms, 60 Hz) or of DC at high voltage can cause fibrillation. A sustained electric shock from AC at 120
V, 60 Hz is an especially dangerous source of
ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source. However, the potential seriousness of the shock depends on paths through the body that the currents take. The protection offered by the skin is lowered by
perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.
Body resistance The voltage necessary for electrocution depends on the current through the body and the duration of the current.
Ohm's law states that the current drawn depends on the resistance of the body. The resistance of
human skin varies from person to person and fluctuates between different times of day. The
NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body's resistance to 1,000 ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms". The
International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents (the columns contain the distribution of the impedance in the population
percentile; for example at 100 V 50% of the population had an impedance of 1875Ω or less):
Skin The voltage-current characteristic of human skin is non-linear and depends on many factors such as intensity, duration, history, and frequency of the electrical stimulus. Sweat gland activity, temperature, and individual variation also influence the voltage-current characteristic of skin. In addition to non-linearity, skin impedance exhibits asymmetric and time varying properties. These properties can be modeled with reasonable accuracy. Resistance measurements made at low voltage using a standard
ohmmeter do not accurately represent the impedance of human skin over a significant range of conditions. For sinusoidal electrical stimulation less than 10 volts, the skin voltage-current characteristic is quasilinear. Over time, electrical characteristics can become non-linear. The time required varies from seconds to minutes, depending on stimulus, electrode placement, and individual characteristics. Between 10 volts and about 30 volts, skin exhibits non-linear but symmetric electrical characteristics. Above 20 volts, electrical characteristics are both non-linear and symmetric. Skin conductance can increase by several orders of magnitude in milliseconds. This should not be confused with
dielectric breakdown, which occurs at hundreds of volts. For these reasons, current flow cannot be accurately calculated by simply applying
Ohm's law using a fixed resistance model.
Point of entry •
Macroshock: Current across intact skin and through the body. Current from arm to arm, or between an arm and a foot, is likely to traverse the heart, therefore it is much more dangerous than current between a leg and the ground. This type of shock by definition must pass into the body through the skin. •
Microshock: Very small current source with a pathway directly connected to the heart tissue. The shock is required to be administered from inside the skin, directly to the heart i.e. a pacemaker lead, or a guide wire, conductive catheter etc. connected to a source of current. This is a largely theoretical hazard as modern devices used in these situations include protections against such currents.
Lethality Electrocution The earliest usage of the term "electrocution" cited by the Oxford English Dictionary was an 1889 newspaper reference to the method of execution then being considered. Shortly thereafter, in 1892, the term was used in
Science to refer generically to death or injury caused by electricity. 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 shock duration, the more likely it is lethal—safety switches may limit time of current flow. Short high-current pulses, as from capacitors, are usually less dangerous than longer-lasting low-current shocks. • Pathway: If current flows through vital organs, like 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. • Pre-existing medical condition • Age, body mass, and health status • Sex: Women are more vulnerable to electric shock than men. 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 stimulate the nerves strongly 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. 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 frequencies (i.e., 50 or 60 Hz), and
direct current has been a subject of debate ever since the
war of the currents in the 1880s. 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. 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 the
Guinness Book of World Records, 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. 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 sustained burns over 40% of his body and was completely paralyzed except for his eyelids. The shock with the highest voltage reported survived was that of Harry F. McGrew, who came in contact with a 340,000 volt transmission line in Huntington Canyon, Utah. The severity and lethality of electric shocks generally depend on the duration and the amount of current passing through the human body. Frequency plays a role with AC and pulse DC. For example, a high frequency current has a higher ventricular fibrillation threshold than lower frequency. Also, shorter single pulses have higher thresholds than short pulses. Below 10 ms are usually believed to have a primarily charge dependent threshold and shock amplitude. Research shows that for very short electric pulse durations below 100 μs the threshold curve converges into a constant charge criterion independent of peak current or RMS values. Even though the for both muscle and nerve stimulation including the heart and the brain. Heating is primarily determined by the amount of energy and is not related to stimulation. These definitions have been included into the IEC standard 60479-2 in opposite to IEC 60479-1 which addresses longer pulse durations above 10 ms for both DC and AC, which use a current over time duration curve based classification. These principles are used to determine the risks from capacitors, electric weapons, electric fences and other short pulsed low- and high-voltage electrical applications outside the medical field. ==Prevention in points ==