2008-05-20
THE FUNDAMENTAL rule of 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.
| Related links |
 IEE |
|
Related contacts
|
The Institution of Engineering and Technology
Michael Faraday House
Stevenage
Herts
SG1 2AY
UK
Tel: +44 (0)1438 313 311
Fax: +44 (0)1438 765 526 |
| For latest electrical industry news, register for Voltimum's free newsletter. |
|
|
Source: IET |
Back