Protection against Electric Shock

THE FUNDAMENTAL rule of protection against electric shock is:

• live parts, such as energized conductors, must not be accessible, and
• conductive parts which are accessible, such as metal enclosures of equipment or metal pipes, must not be hazardous-live

These two conditions must be achieved both in normal conditions (no faults on the electrical system) and under single fault conditions (such as a fault from a live conductor to a metal casing).

Protection under normal conditions

Protection under normal conditions is achieved by basic protection, formerly known as protection against direct contact. Protection under single fault conditions is achieved by fault protection and was previously referred to as protection against indirect contact.

Basic protection is defined as:

Protection against electric shock under fault-free conditions

Basic protection is provided to protect persons or livestock coming into direct contact with live parts.

A live part is defined as:

A conductor or conductive part intended to be energized in normal use, including a neutral conductor but, by convention, not a PEN conductor

Figure 1 illustrates a person coming into contact with live parts.

Protection under fault conditions or fault protection is defined as:

Protection against electric shock under single fault conditions

Fault protection provides protection against persons or livestock coming into contact with exposed-conductiveparts which have become live under single fault conditions.

An exposedconductive- part is defined as:

Conductive part of equipment which can be touched and which is not normally live but which can become live when basic insulation fails

Figure 2 illustrates how a person could receive an electric shock under single fault conditions. The person in Figure 2 is in contact with the metal enclosure of an item of Class I electrical equipment which has become live under fault conditions. The potential of the metal enclosure is higher than that of the main earthing terminal of the installation (and that of Earth) because of a potential difference created by the passage of fault current through the impedance of the circuit protective conductors and the means of earthing.

Protective measures

A protective measure must consist of provision of basic protection and provision of fault protection, which normally are independent. For example, in the case of automatic disconnection of supply, basic protection is provided by insulation and barriers and enclosures while fault protection is provided by protective earthing, protective bonding
and automatic disconnection of supply. Basic and fault protection are independent.

More information

For more information please refer to the 17th Edition of the Wiring Regulations. Also, help is at hand, in the form of a new edition of IEE Guidance Note 7 (Special Locations) available soon.