Document MJZ6qq6wE2eBGObm2nLYL6JOV

TABLE 33-B HUMAN RESISTANCE TO ELECTRICAL CURRENT Body Area Resistance (ohms) Dry skin Wet skin Internal body--hand to foot Ear to ear 100,000 to 600,000 1,000 400 to 600 (about) 100 flow, it is measured in ohms Low VOLTAGE is a rather ambiguous term depending upon whether it is being used by a safety engineer, a plant electrician, ora line man For the purposes of this chapter, low voltage is 24 to 600 volts, and "safety" low voltage refers to voltages below 24 volts Electrical Injuries Current flow is the factor that causes injury in electric shock, that is, the seventy of electnc shock is determined by the amount of current flow through the victim (Table 33-A) Expenmental and field data from authontative sources (see references for articles wntten by Charles F Dalziel) indicate that, m general, an alternating current of 100 milliamperes at commercial frequency (60 hz) may be fatal if it passes through the vital organs Similarly, it is estimated that a value of 16 milliamperes is the average current at which an individual can still release himself from an object held by the hand Such current flow may readily be obtained on contact with low-voltage sources of the ordinary lighting or power circuit Because current flow depends on voltage and resistance, these factors are important Other factors affecting the amount of damage done are the parts of the body involved, the duration of current flow through the victim, and the frequency (if alternating current) Resistance to current flow is mainly to be found m the skin surface Callous or dry skin has a fairly high resistance, but a sharp decrease m resistance takes place when the skin is moist (Table 33-B) Once the skin resistance is broken down, the current flows readily through the blood and body tissues Whatever protection is offered by skin re sistance decreases rapidly with increase m voltage High voltage alternating current of 60 Hz causes violent muscular contraction, often so severe that the victim is thrown clear of the circuit Although low voltage also results m mus cular contraction, the effect is not so violent The fact, however, that low voltage often pre vents the victim from freeing himself from the circuit makes exposure to it dangerous Death or injury by electric shock may re sult from the following effects of current on the body 1 Contraction of the chest muscles, which may interfere with breathing to such an extent that death will result from asphyxia tion when the exposure is prolonged 2 Temporary paralysis of the nerve center, which may result in failure of respiration, a condition which often conbnues until long after the vicbm is freed from the cir cuit 3 Interference with normal rhythm of the heart, causing ventricular fibnllabon In this condibon the fibers of the heart mus cles, instead of contrachng in a coordi nated manner, contract separately and at different bmes Blood circulabon ceases and death ensues, since apparently the heart cannot spontaneously recover from this condition It has been esbmated that 100 milliamperes is sufficient to cause ventricular fibnllabon 4 Suspension of heart acbon by muscular contraction (on contact with heavy cur rent) In this case the heart may resume its normal rhythm when the vicbm is freed from the circuit 5 Hemorrhages and destruction of bssues, nerves, and muscles from heat due to heavy current In general, the longer the current flows through the body, the more serious may be the result Considerable current is likely to flow from high-voltage sources, and in gen- 977