Electricity is the flow of electrons from atom to atom.3, 4 Movement of electrons is comparable to the way water is passed along in a bucket brigade. Electrons, which comprise the current, are passed along from atom to atom. Amperage is the term used for the rate of flow of electrons. Every time 6.242 x 1015 electrons pass a given point in 1 second, 1 ampere of current has passed. The current is what can kill or hurt a victim of an electric injury. One ampere is roughly equivalent to the amount of current flowing through a lighted 100-watt light bulb.
In most materials, a number of electrons are free to move about at random until a driving force termed voltage propels them to move in one direction. A large voltage exerts a greater force, which moves more electrons through the wire at a given rate of time. Electric voltage of 380 volts or less is considered low voltage and above 380 volts, high voltage. High voltage is generated at the power plant and is transformed down to approximately 120 volts for most wall outlets in homes.
Resistance
Resistance of the human body has been likened to that of a leather bag filled with an electrolyte fluid, with high resistance on the outside and lower inside.5 Skin resistance also varies depending on moisture content, thickness, and cleanliness. Resistance offered by the callused palm may reach 1,000,000 ohms/cm2, while the average resistance of dry normal skin is 5000 ohms/cm2. This resistance may decrease to 1000 ohms/cm2 if hands are wet. Skin resistance is encountered primarily in the stratum corneum that serves as an insulator for the body. The voltage gradient in skin cannot be increased indefinitely and breaks down at low voltages. Exposure of the skin to 50 volts for 6-7 seconds results in blisters that have a considerably diminished resistance.
The dermis offers low resistance, as do almost all internal tissues except bone, which is a poor conductor of electricity. Other factors that affect the flow of electrons are the nature and size of the substance through which it passes. If the atomic structure of the material is such that the force of attraction between its nucleus and outer electrons is small, little force is required to cause electron loss. These substances (eg, copper, silver) in which electrons are loosely bound are termed conductors, because they readily permit the flow of electrons. Materials such as porcelain and glass are composed of atoms that have powerful bonds between their nuclei and the outer electrons. These materials are termed insulators because electron flow is restricted.
Resistance is a measure of how difficult it is for electrons to pass through a material and is expressed in a unit of measurement termed an ohm. The resistance offered to the flow of electricity by any material is directly proportional to its length and inversely proportional to its cross-sectional area. Electricity is transmitted by a high-voltage system, because it allows the same amount of energy to be carried at lower current, which reduces electrical loss through leakage and heating. The relationship between current flow (amperage), pressure (voltage), and resistance is described in Ohm's law, which states that the amount of current flowing through a conductor is directly proportional to voltage and inversely related to resistance.
Current (I) = Voltage (E)/Resistance (R)
Electrons set in motion by the current force (voltage) may collide with each other and generate heat. The amount of heat developed by a conductor varies directly with its resistance. Power (watts) lost as a result of the current's passage through a material provides a measure of the amount of heat generated and can be determined by Joule's law.
Power (P) = Voltage (E) x Current (I)
Because E = I x R (resistance), the above equation becomes P = I(squared) R. Consequently, the heat produced is proportional to the resistance and the square of the current. Commercial electric currents usually are generated with a cyclic reversal of the direction of electric pressure (voltage). Pressure in the line first pushes and then pulls electrons, resulting in alternating current. Frequency of current in hertz (Hz) or cycles per second is the number of complete cycles of positive and negative pressure in 1 second. The usual wall outlet (120 volts) provides a current with 120 reversals of the direction of flow occurring each second and is termed 60-cycle current. Frequency of alternating current can be increased to a range of millions of cycles per second. In direct current, electron travel is always in the same direction.
Alternating current
Alternating current has almost entirely superseded direct current, since it is cheaper and can be transformed easily into any required voltage. Most machines in industry and appliances in the home use alternating currents; therefore, workers and consumers are mainly at risk from this current. Direct current usage is primarily restricted to the chemical and metallurgical industries, ships, streetcar systems, and some underground train systems.6, 7
Electric arc
Contact with high-voltage current may be associated with an arc or light flash.8 An electric arc is formed between two bodies of sufficiently different potential (high-voltage power source and the body, which is grounded). The arc has an intense, pale-violet light and consists of ionized particles that are driven by the voltage pressure between the two bodies and are present in the space between them. Temperature of the ionized particles and immediately surrounding gases of the arc can be as high as 4000°C (7232°F) and can melt bone and volatilize metal. As a general guide, arcing amounts to several centimeters for each 10,000 volts. Burns occur where portions of the arc strike the patient. The electric arc remains the cause of most high-voltage electrical burn injuries. Because of its high frequency, the electric arc has become the basis for many standard safety precautions.
Effects of electricity on the body
Effects of electricity on the body are determined by 7 factors: (1) type of current, (2) amount of current, (3) pathway of current, (4) duration of contact, (5) area of contact, (6) resistance of the body, and (7) voltage.9 Low-voltage electric currents that pass through the body have well-defined physiologic effects that are usually reversible. For a 1-second contact time, a current of 1 milliampere (mA) is the threshold of perception, a current of 10-15 mA causes sustained muscular contraction, a current of 50-100 mA results in respiratory paralysis and ventricular fibrillation, and a current of more than 1000 mA leads to sustained myocardial contractions.
Humans are sensitive to very small electric currents because of their highly developed nervous system. The tongue is the most sensitive part of the body. Using pure direct current and 60-cycle alternating current, the first sensations are those of taste, which are detected at 45 microamperes. When subjected to 60-cycle alternating current, the threshold of perception in the hands of men and women, which is usually a tingling sensation, is approximately 1.1 mA. Using pure direct current applied to hands, the first sensations are those of warmth in contrast to tingling, detected at 5.2 mA.
Skin offers greater resistance to direct current than alternating current, thus 3-4 times more direct current is required to produce the same biologic effect elicited by alternating current. With increasing alternating current, sensations of tingling give way to contractions of muscles. The magnitude of the muscular contractions enhances as the current is increased. Finally, a level of alternating current is reached for which the subject cannot release the grasp of the conductor. The maximum current a person can tolerate when holding a conductor in one hand and still let go of the conductor using the muscles directly stimulated by the current is termed the "let-go" current. This tetanizing effect on voluntary muscles is most pronounced in the frequency range of 15-150 Hz.
Such strong muscular reactions may cause fractures and/or dislocations. Numerous reports of bilateral scapular fractures and shoulder dislocations and fractures in electric accidents attest to this occurrence. As the frequency increases above 150 Hz, the potential for this sustained contraction is lessened. At frequencies from 0.5-1 megacycle, these high-frequency currents do not elicit sustained contractions of the skeletal muscles. For 60-cycle alternating current, the let-go threshold for men and women is 15.87 mA and 10.5 mA, respectively. The lower value for women may result from their generally somewhat poorer muscular development compared to men.
Electrical accidents involving power frequency (50-60 Hz) and a relatively low voltage (150 V/cm) occasionally can result in massive trauma to the victim. Skeletal muscle and peripheral nerve tissue are especially susceptible to injury. Historically, Joule heating, or heating by electrical current, was viewed as the only mechanism of tissue damage in electrical trauma. Yet in some instances, Joule heating does not adequately describe the pattern of injury observed distant to the sites of contact with the electrical source. These victims exhibit minimal external signs of thermal damage to the skin, while demonstrating extensive muscle and nerve injury.
Recently, electroporation of skeletal muscle and nerve cells was suggested as an additional mechanism of injury in electrical burns. This mechanism is different from Joule heating, even though it is influenced by temperature. It is the enlargement of cellular-membrane molecular-scale defects that results when electrical forces drive polar water molecules into such defects. Experimental studies have documented that electroporation effects can mediate significant skeletal muscle necrosis without visible thermal changes.
High-voltage accidents
The national electric code defines high-voltage exposure as greater than 600 volts. In the medical literature, high-voltage exposure is judged as greater than 1,000 volts. In high-voltage accidents, the victim usually does not continue to grasp the conductor. Often, he or she is thrown away from the electric circuit, which leads to traumatic injuries (eg, fracture, brain hemorrhage). The infrequency with which sustained muscular contractions occur with high-voltage injury apparently occurs because the circuit is completed by arcing before the victim touches the contact. Currents that cause subjects to "freeze" to the circuit despite their struggle to be free are frightening, painful, and hard to endure, even for a short time.
Turning off power source
Consequently, a witness of the accident must turn off the power source as soon as possible. If this is not possible, the victim must be disengaged from the electric current. Wearing lineman's gloves, trained electricians must separate the victim from the circuit by a specially insulated pole. Looping a polydacron rope around the injured patient is another method of pulling him or her from the electric power source. Ideally, the first responder should stand on a dry surface during the rescue.
Muscular contractions
Tests using gradually increasing amounts of direct current produce sensations of internal heating rather than severe muscular contractions; however, sudden changes in the magnitude of direct current produce powerful muscular contractions. At the instant of interruption of the direct current, the subject occasionally falls back a considerable distance; the impact of the fall may cause a fracture. As the alternating current strength increases above 20 mA, a sustained contraction of muscles of respiration of the chest occurs.
Normal respiration returns after the current has been turned off, provided that the duration of current flow is less than 4 minutes. If sustained contractions last longer than this time interval, death from asphyxiation occurs, unless the current is stopped and mouth-to-mouth ventilation on the breathless patient is started. The pathway of current flow, involved in tetanic contractions of the muscles of respiration, is usually arm to arm or arm to leg and does not pass through the respiratory center located in the medulla of the brainstem. This center is injured in executions in the electric chair, leading to permanent respiratory arrest.
Treatment at the scene
When current flow increases above 30-40 mA, ventricular fibrillation may be induced. Numerous factors can influence the magnitude of electric current required to produce ventricular fibrillation. found to be of primary importance are duration of current flow and body weight. The threshold for ventricular fibrillation is inversely proportional to the square root of the shock duration and directly proportional to body weight.
When the heart is exposed to currents of increasing strength, its susceptibility to fibrillation first increases and then decreases with even stronger currents. At relatively high currents (1-5 amps), the likelihood of ventricular fibrillation is negligible with the heart in sustained contraction. If this high current is terminated soon after electric shock, the heart reverts to normal sinus rhythm. In cardiac defibrillation, these same high currents are applied to the chest to depolarize the entire heart.
If disconnecting the victim from the electric circuit does not restore pulses, the first responder must start cardiopulmonary resuscitation to restore breathing and circulation. (Click to complete a Medscape CME activity on minimally interrupted cardiac resuscitation.) Ideally, when they arrive at the scene of the accident, paramedics will continue this resuscitation. Field intervention should include advanced life support treatments delivered under the direction of a physician at the hospital base station using telemetered communication. Telemetered monitoring of these patients is recommended throughout transport to the advanced life support hospital facility.
These life-threatening consequences of low-voltage electric burns usually occur without any lesions of the skin at entrance and exit points of the current. An absence of local lesions indicates that the surface area of contact (current density) is large and that the heat is insufficient to produce a thermal injury; however, the smaller the surface area of the contact, the greater the density of the current and the more energy is transformed into heat that can cause local burn injury.
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