Saturday, October 29, 2016

Basic Safety Provision for Electrical Installation According to BS 7671


I. Protection against Direct Contact


BS 7671 Provides

1. Insulation of live parts
The insulation is intended to prevent any contact with live parts.

NOTE 1:
  • Live parts shall be completely covered with insulation, which can only be removed by destruction.
  • For factory-built equipment, the insulation shall comply with the relevant standards for the electrical equipment.
  • For other equipment, protection shall be provided by insulation capable of durably withstanding the stresses to which it may be subjected in service such as mechanical, chemical, electrical and thermal influences. 
  • Paints, varnishes, lacquers and similar products alone are generally not considered to provide adequate insulation for protection against electric shock in normal service. 

NOTE 2:
  • Where insulation is applied during the erection of the installation, the quality of the insulation should be confirmed by tests similar to those, which ensure the quality of insulation of similar factory-built equipment.

2. Barriers or enclosures
Barriers or enclosures are intended to prevent any contact with live parts.

3. Obstacles
Obstacles are intended to prevent unintentional contact with live parts but not intentional contact by deliberate circumvention of the obstacle. 

4. Placing out of reach
Protection by placing out of reach is intended only to prevent unintentional contact. 

5. Additional protection by RCD’s
The use of residual current devices (RCDs) with a rated operating residual current not exceeding 30 mA, is recognized as additional protection in case of direct contact in the event of failure of other measures of protection or carelessness by users.

II. Protection against direct contact


BS 7671 provides

Persons and livestock shall be protected against dangers that may arise from contact with exposed-conductive-parts in case of a fault. This protection can be achieved by one of the following methods:

1. Preventing a fault current from passing through the body of any person or any livestock;

2. Limiting the fault current which can pass through a body to a value lower than the shock current;

3. Automatic disconnection of the supply in a determined time on the occurrence of a fault likely to cause a current to flow through a body in contact with exposed-conductive-parts, where the value of that current is equal to or greater than the shock current.


NOTE
  • In connection with the protection against indirect contact, the application of the  method of equipotential bonding is one of the important principles for safety.

Source:

BS 7671 


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