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 in a current flow between 45 and 140 mA.
TABLE 1.2. Resistance of 130-cm2 Areas of Various Materials
Source: Reference [23].
| Material | Resistance in MΩ |
| Rubber gloves or soles | >20 0 |
| Dry concrete above grade | 1 0–5.0 |
| Dry concrete on grade | 0.2–1 0 |
| Leather sole, dry, including foot | 0.1–0.5 |
| Leather sole, damp, including foot | 0.05–0.2 |
| Wet concrete | 0.01–0.05 |
Figure 1.8. Shock hazard categories according to IEC.
Source: Reference [23].
See also Section 2.4.
1.7 FIRE HAZARD
NFPA and National Fire Incident Reporting Systems (NFIRS) statistics of fire hazard can be viewed on websites. These statistics are based upon:
heat source, that is, arcing
contributing factors like electrical failure or malfunction
equipment involved in electrical distribution, lighting, and power transfer.
TABLE 1.3. Time–Current Zones for 15–100 Hz AC Currents for Hand-to-Feet Pathway
Source: Reference [23].
| Zone | Boundaries | Physiological Effects |
| AC-1 | Up to 0.5 mA, curve a | Perception possible but usually no “startled” reaction |
| AC-2 | 0.5 mA up to curve b | Perception and involuntary muscular contractions likely but usually no harmful physiological effects |
| AC-3 | Curve b and above | Strong involuntary muscular contractions, difficulty in breathing. Reversible disturbances of heart function. Immobilization may occur. Effects increasing with current magnitude. Usually no organic damage to be expected. |
| AC-4a | Above curve c1 | Pathophysiological effects may occur, such as cardiac arrest, burns, or other cellular damage. Probability of ventricular fibrillation increasing with current magnitude and time |
| c1–c2 | AC-4.1: Probability of ventricular fibrillation increasing up to about 5% | |
| c2–c3 | AC-4.2: Probability of ventricular fibrillation increasing up to about 50% | |
| Beyond curve c3 |
AC-4.3:
|
