Protection against electric shock

Danger relative to electric shock

When a current exceeding 30 mA passes through a part of a human body, the person concerned is in serious danger if the current is not interrupted in a very short time.

The protection of persons against electric shock in LV installations must be provided in conformity with appropriate national standards statutory regulations, codes of practice, official guides and circulars etc.

Relevant IEC standards include: IEC 60364, IEC 60479 series, IEC 61008, IEC 61009 and IEC 60947-2.

An electric shock is the pathophysiological effect of an electric current through the human body.

Its passage affects essentially the muscular, circulatory and respiratory functions and sometimes results in serious burns. The degree of danger for the victim is a function of the magnitude of the current, the parts of the body through which the current passes, and the duration of current flow.

IEC publication 60479-1 updated in 2005 defines four zones of current-magnitude/time-duration, in each of which the pathophysiological effects are described (see Fig. F1). Any person coming into contact with live metal risks an electric shock.

Curve C1 shows that when a current greater than 30 mA passes through a human being from one hand to feet, the person concerned is likely to be killed, unless the current is interrupted in a relatively short time.

The point 500 ms/100 mA close to the curve C1 corresponds to a probability of heart fibrillation of the order of 0.14%.

The protection of persons against electric shock in LV installations must be provided in conformity with appropriate national standards and statutory regulations, codes of practice, official guides and circulars, etc. Relevant IEC standards include: IEC 60364 series, IEC 60479 series, IEC 60755, IEC 61008 series, IEC 61009 series and IEC 60947-2.

AC-1 zone: Imperceptible
AC-2 zone: Perceptible
AC-3 zone: Reversible effects: muscular contraction
AC-4 zone: Possibility of irreversible effects
AC-4-1 zone: Up to 5% probability of heart fibrillation
AC-4-2 zone: Up to 50% probability of heart fibrillation
AC-4-3 zone: More than 50% probability of heart fibrillation
A curve: Threshold of perception of current
B curve: Threshold of muscular reactions
C₁ curve: Threshold of 0% probability of ventricular fibrillation
C₂ curve: Threshold of 5% probability of ventricular fibrillation
C₃ curve: Threshold of 50% probability of ventricular fibrillation

Fig. F1 – Zones time/current of effects of AC current on human body when passing from left hand to feet

Protection against electric shock

The fundamental rule of protection against electric shock is provided by the document IEC 61140 which covers both electrical installations and electrical equipment.

Hazardous-live-parts shall not be accessible and accessible conductive parts shall not be hazardous.

This requirement needs to apply under:

• Normal conditions, and
• Under a single fault condition

Various measures are adopted to protect against this hazard, and include:

• Automatic disconnection of the power supply to the connected electrical equipment
• Special arrangements such as:
  • The use of class II insulation materials, or an equivalent level of insulation
  • Non-conducting location, out of arm’s reach or interposition of barriers
  • Equipotential bonding
  • Electrical separation by means of isolating transformers

Standards and regulations distinguish two kinds of dangerous contact,

• Direct contact
• Indirect contact

and corresponding protective measures

Direct contact

Two measures of protection against direct contact hazards are often required, since, in practice, the first measure may not be infallible

A direct contact refers to a person coming into contact with a conductor which is live in normal circumstances (see Fig. F2).

IEC 61140 standard has renamed “protection against direct contact” with the term “basic protection”. The former name is at least kept for information.

Fig. F2 – Direct contact

Indirect contact

An indirect contact refers to a person coming into contact with an exposed-conductive-part which is not normally alive, but has become alive accidentally (due to insulation failure or some other cause).

The fault current raise the exposed-conductive-part to a voltage liable to be hazardous which could be at the origin of a touch current through a person coming into contact with this exposed-conductive-part (see Fig. F3).

IEC 61140 standard has renamed “protection against indirect contact” with the term “fault protection”. The former name is at least kept for information.

Fig. F3 – Indirect contact

ru:Защита людей от поражения электрическим током zh:人身电击保护