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<unk>  current as low as 1mA (rms) for 60Hz AC and as low as 5mA for DC. At around 10mA, DC 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<unk>  regulations. The current may, if it is high enough and is delivered at sufficient voltage, 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. If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however. If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs. 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. If an<unk>  circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established. This is known as a microshock. Currents of only 10 µA can be sufficient to cause fibrillation in this case with a probability of 0.2%. Body resistance Voltage 5% 50% 95% 25 V 1,750 Ω 3,250 Ω 6,100 Ω 100 V 1,200 Ω 1,875 Ω 3,200 Ω 220 V 1,000 Ω 1,350 Ω 2,125 Ω 1000 V 700 Ω 1,050 Ω 1,500 Ω 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<unk>  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<unk>  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<unk>  stimulation less than 10 volts, the skin voltage-current characteristic is quasilinear. Over time,<unk>  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<unk>  characteristics. Above 20 volts,<unk>  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 a 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<unk>  caused by electricity. Factors in lethality of electric shock Log-log graph of the effect of alternating current I of duration T passing from left hand to feet as defined in IEC publication 60479-1. 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. * Pre-existing medical condition. * Age and sex. 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. 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 the 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. 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. 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. 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.
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Blunt trauma