The harmful effects of electric shocks depend mainly on seven factors:
- The voltage level of the gripped or touched equipment
- Amount of current passing through the person’s body
- Resistance of the person’s body
- Pathway of the current inside the body
- Duration of the shock
- Frequency of the source
- Ground resistance
The voltage level of the gripped or touched equipment
Most electric shocks occur at 100–400 V because it is readily available to everyone, are high enough to produce a significant current in the body, and can cause muscles to contract tightly to the energized object. A higher voltage, in the kilovolts range, is even more lethal because it causes high currents to pass through the person, but the access to this voltage is normally limited to professionals such as linemen. The general public rarely comes in contact with high-voltage wires, unless a wire is downed, a person climbs a high object and touches the wire, etc. Therefore, the number of deaths from high voltage is much less than that from low voltage.
When a person touches a high-voltage wire, fierce involuntary muscle contractions may throw the person away from the hazard. However, it may cause the person to fall from the high elevation of the power line to the ground.
Amount of current passing through the person’s body
Contrary to popular belief, it is the current, not voltage, that causes death. However, the current is a function of the voltage and the impedance of the body as described by Ohm’s law. Electric currents can interfere with the normal operation of the heart and lungs causing the heart to beat out of step and the lungs to function irregularly. Moreover, electric currents passing through tissues produce thermal heat, which is a form of energy that is proportional to the square of the current. This thermal energy E can permanently damage tissues and organs in the body
E = I² Rt
I= is the current in the body
R= is the body resistance
T= is the time duration of the current
Resistance of the person’s body
The higher the body resistance, the lower the current flowing through the person. Body resistance is highly nonlinear and is a function of several factors such as the hydration condition of the body, skin condition, and fat concentration. Palm resistance, for example, can range from 100 Ω to 1 MΩ depending on the skin condition of the person. Dry skin tends to have higher resistance, and sweat tends to lower the resistance. Nerves, arteries, and muscles are low in resistance, while bones, fat, and tendons are relatively high in resistance. The IEEE established the ranges for body resistances shown in Table 1. These numbers can be used to roughly estimate the current through the body. However, the variability in human body resistance could make the results inaccurate for people with body resistances outside the specified range.
Because of the wide range of human resistances, it is hard to analyze the electric safety for a specific individual. Instead, the community has decided to use common values for body resistance. These values are shown in Table 2. A 1000 Ω between two hands or two feet is a good number to use.
Pathway of the current inside the body
A current passing through the skin is not as harmful as the current passing through vital organs. Fatal currents often pass through the heart, lungs, and brain. A mere 10 μA passing directly through the heart can cause cardiac arrest. At lower currents, the heart muscles could beat out of step, resulting in insufficient blood being pumped throughout the body. A current in the spinal cord may also alter the respiratory control mechanism.
Duration of the shock
The longer the duration of the shock the higher the likelihood of death. This is because sensitive organs such as the heart and lungs will eventually stop functioning and the increased thermal heat inside the body can permanently damage muscles. Keep in mind that when the current is above the let-go threshold, the person is incapable of releasing his or her grip on the wire and the shock duration is therefore long.
Charles Dalziel carried out research on the time–current relationship for primary shocks. He, and other researchers, obtained his results by experimenting on animals of weights and organ sizes similar to humans. Although inconclusive, the data of these experiments are the best available information to date. Based on these studies, Dalziel developed the following empirical current duration formula for ventricular fibrillation (VF)
I= is the current in mA that induces VF
T= is the time duration of the current in seconds
K is a constant that depends on the weight of the test subject: for people weighing less than 70 kg. (154.lb), K = 116; and for people weighing more than 70 kg, K = 157 Figure 1 is a graph of Dalziel’s formula. As seen in the figure, it takes a very short time to induce ventricular fibrillation For instance, if the current is about 80 mA, it takes about 4 s to kill a person over 70 kg, while a person weighing less than 70 kg may survive for just 2 s In either case, the survival duration is very short for such a small current.
Frequency of the source
The frequency that impacts humans can be divided into two categories:
- Non-ionizing frequency (0–100 PHz), which includes power line frequency, radio, microwave, and infrared frequencies.
- Ionizing frequency (>100 PHz), which includes x-rays and gamma-rays.
From the electric safety point of view, we are focusing on the low end of the non-ionizing frequencies (up to 10 kHz) In this range, research indicates that the let-go level is almost a parabolic function with respect to the frequency as shown in Figure 2. Unfortunately, at 50–60 Hz, humans are very vulnerable to electric shock. At the DC level or the high-frequency range (3–10 kHz), our tolerance is relatively high.
Table 3 shows additional data for the 10 kHz shocks. Notice that while the perception threshold for men is 1.1 mA at 60 Hz, it is 12 mA at 10 kHz (more than 10 times). The let-go threshold for men at 60 Hz is 16 mA while it is 75 mA at 10 kHz (almost six times). This is why in some rare cases a human may survive a lightning strike.
If a person touching an energized conductor is standing on insulating material, the current through his body is insignificant. However, if he is standing on the ground with bare feet, a current will pass through his body on its way to the ground. The magnitude of this current depends on the resistances in the system as well as the resistance of his body and the ground resistance under his feet.