Electric Shock

An electric shock can kill or seriously injure and is one of the hazards that electrical safety legislation is intended to protect against.

Definition of an Electric Shock

“A dangerous physiological effect resulting from the passing of an electrical current through a human body or livestock.”

- IET Wiring Regulations

The severity of an electric shock is largely dependant on the amount of energy delivered to the body. This is a function of the magnitude of the current and the length of time the current flows. A current exceeding 30mA is likely to be fatal unless it is interrupted in a very short time.

Graph of the effect of current vs time as defined in IEC 60479-1
Graph of the effect of current vs time as defined in IEC 60479-1
  • AC-1: Imperceptible
  • AC-2: Perceptible but no muscle reaction
  • AC-3: Muscle contraction with reversible effects
  • AC-4: Possible irreversible effects
  • AC-4.1: Up to 5% probability of ventricular fibrillation
  • AC-4.2: 5-50% Probability of fibrillation
  • AC-4.3: Over 50% probability of fibrillation


Voltage

Voltage does not give a good indication of the likely severity of an electric shock as current flow depends on the resistance of the human body. The majority of the body's resistance comes from the skin and this can range from 1 MΩ when wet to 100 MΩ when dry.

Exposure to voltages less than 50V a.c. is generally considered low risk in electrical safety terms. An electric shock at 50V a.c. is unlikely to be fatal, however it can still be painful and may cause a related accident from a reaction to the shock.

Voltages greater than 450V a.c. are especially dangerous. At this point the resistance of the skin can break down which significantly reduces the body's overall resistance thereby causing a substantial increase in current.

Current Path

The severity of an electric shock also depends on the path the current takes through the body. Many serious electric shocks occur when the current flows from hand to hand, because the current path is through or near the heart. Hand held appliances present a particular risk as the appliance is gripped in one hand and it is possible to make contact with an earthed surface with the other.

Let-Go Threshold

If the source of the current is held in the hand, it may cause muscles to contract making the person unable to let go voluntarily. 99% Of adults will not be able to let go with a current greater than 22 mA.

Electrical Burns

Current flow though the body can also burn body tissue. Electrical burns differ from thermal burns as they are usually internal, below the surface of the skin. This is especially severe when vital internal organs are damaged.

Electrical Flashover (Arc Flash)

A large proportion injuries sustained from electrical accidents are due to flashover rather than current flow through the body. Electrical flashover is most often caused by accidentally creating a short circuit between two conductors, such as dropping a tool on a live bus bar. Being in the proximity of a flashover is extremely dangerous. The temperature of an arc can reach 19,000 °C, resulting in severe burn injuries. Hearing can be damaged from the resulting shock wave and the high intensity ultraviolet radiation can damage eyesight.


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