Protection against electric shock

Introduction

Electric shock

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.

Protection measures are described in sections 1 to 8.

Electrical fires

Electrical fires are caused by overloads, short circuits and earth leakage currents, but also by electric arcs in cables and connections.

Protection measures are described in Protection against electrical fire risks.

Danger relative to electric shock

When a current exceeding 30 mA passes near the heart 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 61140, 60364, IEC 60479, IEC 61008, IEC 61009 and IEC 60947 series.

IEC publication 60479-1 updated in 2016 defines four zones of current-magnitude /time-duration, in each of which the pathophysiological effects are described (see Fig. F1).

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 61140, IEC 60364, IEC 60479, IEC 61008, IEC 61009 and IEC 60947 series.

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: Ventricular fibrillation unlikely to happen
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

Standards and regulations distinguish two kinds of dangerous contact:

• contact with live parts
• contact with conductive parts in fault conditions

and corresponding protective measures:

• Basic protection
• Fault protection

The fundamental rule of protection against electric shock is provided by the document IEC 61140 ("Protection against electric shock – Common aspects for installations and equipment")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.

Contact with live part (Direct contact)

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

The protection to be implemented in these circumstances is called "Basic Protection".

Fig. F2 – Contact with live part (Direct contact)

Contact with conductive parts in fault conditions (Indirect contact)

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

The fault current raises the exposed-conductive-part to a voltage liable to be hazardous as it generates a touch current through a person coming into contact with this exposed-conductive-part (see Fig. F3).

The protection to be implemented in these circumstances is called "Fault Protection".

Fig. F3 – Contact with parts in fault conditions (Indirect contact)