Shock effects and factors affecting damage

Electric shock is the body's reaction to current passing through it. The current disturbs the electrical signals of nerves, muscles and heart and heats the tissues.

Physiological effects

Main effects explained:

1. Nerve stimulation and pain.
2. Tetanic muscle contraction: the hand grips the conductor, so the shock lasts longer.
3. Respiratory arrest: chest muscles and the brain's breathing centre are paralysed.
4. Ventricular fibrillation: the heart quivers and stops pumping blood; death follows in minutes without defibrillation.
5. Burns: contact burns, arc burns and internal burns from $I^2Rt$ heating.
6. Other: unconsciousness, nerve and brain damage, kidney damage, falls and fractures.

Factors affecting the magnitude of damage

1. Magnitude of current: the most important factor; $I = V/R_{body}$.
2. Duration of current: the safe current falls with time. Dalziel's relation for fibrillation threshold:

$$I_b = \frac{0.116}{\sqrt{t}}\ \text{A (50 kg)}, \qquad I_b = \frac{0.157}{\sqrt{t}}\ \text{A (70 kg)}$$

e.g. for 1 s, 116 mA; for 0.1 s, 367 mA (50 kg). 3. Path of current: paths through the heart (hand–hand, left hand–feet) are most dangerous; foot–foot is less dangerous. 4. Body resistance: dry skin can be 100 kΩ, wet or broken skin about 1 kΩ (500 Ω internal). Moisture, sweat, contact area and pressure lower it. 5. Voltage: higher voltage breaks down skin resistance and drives more current; above about 500 V burns become severe. 6. Frequency: 50–60 Hz is most dangerous; DC and high frequencies need larger currents for the same effect. 7. Phase of heart cycle: shock during the T-wave (vulnerable period) easily causes fibrillation. 8. Body weight, age, sex and health: lighter persons, women, children and heart patients have lower thresholds. 9. Contact condition: footwear, floor material and surface layer resistivity (crushed rock) add series resistance. 10. Speed of rescue and first aid: early CPR and defibrillation greatly reduce fatality.