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Physiological effects of shock and safety practices

Short answer

Physiological effects depend on current: 1 mA perception, 9–25 mA let-go limit, 25–60 mA breathing difficulty, 60–100 mA ventricular fibrillation. Good practices include proper earthing, RCD/ELCB, lock-out/tag-out, insulated tools and PPE, safe clearances, crushed-rock surface, authorized personnel, and regular inspection.

What are the physiological effects of electric shock? What are good safety practices to avoid shocks?

An electric shock is the physiological reaction of the body when electric current passes through it. The damage depends mainly on the current (not voltage), its path, duration and frequency.

Physiological effects (50/60 Hz, hand to feet, typical values)

Body currentEffect
about 1 mAThreshold of perception, slight tingling
1–9 mAPainful shock, but person can let go
9–25 mALet-go limit crossed; muscles contract, cannot release grip
25–60 mABreathing difficulty, severe pain, possible unconsciousness
60–100 mA and aboveVentricular fibrillation (heart beats irregularly), usually fatal
above about 1 ACardiac arrest, severe burns, nerve and tissue damage

Other effects: burns at entry and exit points, internal burns, damage to nervous system, fall from height due to reflex action, and later kidney damage.

Dalziel's relation for the fibrillation threshold (used in IEEE 80):

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

where tt is shock duration in seconds (0.03–3 s).

Good safety practices to avoid shock

  1. Proper earthing of all metal frames and enclosures, with low earth resistance, so a fault trips the protection quickly.
  2. Use of RCD/ELCB (30 mA) and fast protective relays/fuses.
  3. Isolation and lock-out/tag-out: switch off, lock, tag and test for "dead" before work; earth the conductors being worked on.
  4. Insulation: use double-insulated tools, insulated mats, rubber gloves and boots of proper voltage rating.
  5. Safe clearances and barriers around live parts; warning signs.
  6. Substation design: earth mat, surface layer of crushed gravel, and equipotential bonding to keep step and touch potentials within safe limits.
  7. Trained and authorised persons only; work permits for HV work.
  8. Avoid moisture: do not touch switches with wet hands; use weather-proof fittings.
  9. Regular inspection and testing of insulation, earthing and protective devices.
  10. First aid knowledge: cut the supply, separate the victim with a dry insulating object, and give CPR.

✓ Answer checked by Claude Opus 5.5 (high effort)

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