1.6 ELECTRICAL SHOCK HAZARD
One of the most complete analyses of occupational electrical injuries in the United States are two papers by Jim Cawley [37, 38]. On an average, one person is electrocuted in work places every day in United States. There are a number of ways the exposure to shock hazard occurs. The resistance of the contact point, the insulation of the ground under the feet, flow of current path through the body, the body weight, the system voltage, and frequency are all important. A dangerous consequence is a heart condition, known as ventricular fibrillation, resulting in immediate arrest of blood circulation. Currents as small as a few milliamperes through the heart can cause disruption of electrical signals that the heart uses to perform its functions. Voltages as low as 50 V can cause fibrillation and can result in death.
The following synopsis of tolerable currents is from IEEE Standard 80, Guide for Safety in AC Substation Grounding [21]:
At 50 or 60 Hz, a current of 0.1 A can be lethal. The human body can tolerate slightly higher 25 HZ current and five times the DC current. At frequencies of 3000–10,000, even higher currents are tolerated. The most common physiological effect, stated in terms of increasing current, are: threshold of perception, muscular contraction, unconsciousness, fibrillation of heart, respiratory nerve blockage and burning [22], and IEC 604791 [23].
The perception level is 1 mA. Currents in the range of 9–25 A may be painful and may make it difficult or impossible to release energized objects. In the range 60–100 mA, ventricular fibrillation, stoppage of heart, or inhibition of respiration might occur, causing injury or death. As shown by Dalziel and others [24], the nonfibrillating current of magnitude IB at durations ranging from 0.03 to 3.0 seconds is related to energy absorbed by the body, given by:
(1.2)
where ts is the time duration of the current in seconds, and SB is an empirical constant related to the energy through the body. Thus, reducing the arc flash incident energy through fast fault clearance times also reduces SB.
Based upon the Dalziel and Lees’ studies [25], it is assumed that 99.5% of all persons can safely withstand, without ventricular fibrillation, the passage of current IB, given by:
(1.3)
Dalziel found that SB = 0.0135 for a body weight of 110 lbs (50 kg). Then:
This gives 116 mA for 1 second and 367 mA for 0.1 second. For 70 kg weight, SB = 0.0246 and k = 0.157. These values are adopted in IEEE Guide 80 [21]. Fibrillation current is assumed to be the function of body weight (Figure 1.6).
Other researchers have suggested different values of IB. In 1936, Fwerris et al. [26] suggested 100 mA as fibrillation threshold; this value was derived by extensive experimentation at Columbia University. Some more recent experiments suggest the existence of two thresholds: one for shock duration less than one heartbeat period, and the other for the current duration longer than one heartbeat period. For a /50 kg body weight, Biegelmeier [27] proposed threshold levels of 500 and 50 mA, respectively. Other studies were carried out by Lee and Kouwenhoven [28]. Figure 1.7 shows a comparison of Equation (1.4) and Z-shaped body current time developed by Biegelmeier.
Figure 1.6. Fibrillating current (ma) rms, versus body weight. Source: Reference [21].
1.6.1 Resistance of Human Body
For DC and AC 50 or 60 HZ currents, the human body can be approximated by a resistance. For the calculation of this resistance, the current path is considered from:
one hand to both feet
from one foot to another foot.
The internal resistance of the body is approximately 300 Ω, while the body resistance, including skin range from 500 to 3000 Ω. Based on Dalziel tests, using saltwater to wet hands and feet to determine let-go currents, hand-to-hand contact resistance is 2330 Ω, and hand-to-feet resistance equals 1130 Ω. Thus, the IEEE Guide for Safety in AC Substation Grounding considers that hand and foot contact resistances are zero, that glove and shoe resistances are zero, and a value of 1000 Ω is taken that represents the body from hand-to-feet and also from hand-to-hand resistance.
Figure 1.7. Ventricular fibrillation curves, current versus time. Source: Reference [21].
NFPA 70E states that energized parts operating at less than 50 volts are not required to be de-energized to satisfy an “electrical safe working condition.” It further lays down that considerations should be given to the capacity of the source, any overcurrent protection between the source and the worker, and whether the work task related to the source operating at less than 50 volts increases exposure to electrical burns or to explosion from an electric arc.
Reference [29] contends that 50 V is inadequate and calculates the maximum and minimum body resistance for path from arm-to-arm and arm-to-leg of the order of 300–500 Ω. IEC standard 604791 [23] recommends shock voltages of less than 50 V in some situations. Some jurisdictions, for example, in France, the safe voltage limit is accepted as 35–50 V. However, NFPA 70E qualifies the 50 V limits by additional cautionary statements as indicated above.
Table 1.2 provides resistance values for 130 cm2 areas of various materials. It is customary to overlay the natural soil with high resistivity materials to increase the step and touch potentials in utility substations [21]. For the grounding systems in industrial electrical distributions, generally, the concept of higher soil resistivity layers to increase step and touch potentials can be applied for the grounding installations around buildings, tanks, substations, fences, and motor and transformer pedestals.
Figure 1.8 from IEC standard [23] illustrates the time–current zones for AC currents of 15–100 Hz, and Table 1.3 provides the physiological effects. IEC considers that hand-to-hand body impedance for 125 V is between 850 and 2675 Ω, and grasping a conductor or faulty electric device rated 120 V can result