Physiological effects of shock and body current

An electric shock is the effect produced when current flows through the human body. The body reacts to the current, so its effects depend on the magnitude of current, the path, the duration and the frequency.

Physiological effects of electric shock

1. Sensation and pain: small currents stimulate nerves and cause tingling and pain.
2. Muscular contraction (tetanus): current stimulates muscles; the hand grips the conductor and cannot let go, increasing the duration of shock.
3. Respiratory paralysis: current through the chest affects breathing muscles and the brain's breathing centre, causing suffocation.
4. Ventricular fibrillation: current through the heart disturbs its rhythm; the ventricles quiver instead of pumping, and blood circulation stops. This is the main cause of death.
5. Cardiac arrest: large currents stop the heart completely (it may restart after the current is removed).
6. Burns: heating ($I^2Rt$) at the contact points and inside tissue; arc burns from high-voltage flashes.
7. Nerve and brain damage, loss of consciousness, and later kidney failure due to muscle breakdown.
8. Secondary injuries: falls from height and injuries from involuntary movement.

Effects of electric current in the human body (50/60 Hz, about 1 s, hand to feet)

Dalziel's safe body current relation for fibrillation threshold:

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

For example, for $t = 0.5$ s and a 50 kg person, $I_b = 0.116/\sqrt{0.5} = 0.164$ A.

Factors deciding the severity

• Magnitude of current = voltage ÷ body resistance (dry skin up to 100 kΩ, wet skin about 1 kΩ).
• Path: hand to hand or hand to foot (through the heart) is most dangerous.
• Duration: longer time lowers the safe current.
• Frequency: 50–60 Hz is the most dangerous; DC and high frequency are less likely to cause fibrillation.
• Body condition: weight, health, heart condition, moisture and contact area.