Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 109)

4 Protection
The protection of persons, domestic animals and property from dangers that result from the
operation of electrical equipment is defined as principal elements of the safety objectives of the
Directive 2006/95/EC of the European Union (Low-voltage Directive). The demand for safe
operation and the avoidance of hazards and damage of all kinds is a prevailing requirement in
low-voltage engineering, whether in avoiding electric shocks, dangerous overheating or the
effects of electric arcs. This applies both for normal operation and in the presence of faults.
Besides the question of avoiding hazards and damage, a significant aspect of protection is to
ensure the availability of electrical equipment and hence it should be seen as a productivity-
ensuring measure. Each malfunction-preventing measure contributes to the safety and availabil-
ity of an item of plant.
In a narrow sense protection of low-voltage devices in main circuits means
- protection of the components of the circuit themselves and
- protection of the load
The protection embraces
- protection against overload and excess temperatures and
- protection against the consequences of a short-circuit or limitation of such conse-
quences by early and timely shutdown, but also
- recognition of impending malfunctions before a protective shutdown actually occurs,
such as for example overloading of a drive, asymmetrical supply or lack of flow of the
medium in submersible pump motors.
While protection primarily aims at the prevention of damage, this is also always connected with
the question of utilization of the equipment. Protective shutdowns before they are actually
necessary, while they may be compatible with the safety objectives, represent a disruption to
operations and can prevent full exploitation of production equipment. In this way, protective
measures always have an economic significance and increased expenditure for high quality
protection can be justified from this point of view.
4.1 Protection requirements
The key protection requirements for low-voltage installations are
ƒ protection against electric shock
ƒ protection against overload / excess temperature
ƒ protection against the consequences of short-circuits and ground faults
The following discussion relates specifically to the protective functions of low-voltage switchgear
in accordance with IEC 60947.
4.1.1 Protection against electric shock
Protection against electric shock is achieved by a multi-level approach:
ƒ Protection against direct contact
ƒ Protection against indirect contact
ƒ Complementary
protection
4.1.1.1 Protection against direct contact
Protection against direct contact with live components is achieved in electrical installations by
encapsulation. IEC 60439-1 stipulates at least protection type IP2X or IPXXB for switchgear
assemblies. Switchgear assemblies to which unskilled persons have access must in accordance
with IEC 60439-3 have a degree of protection of at least IP2XC. National regulations must
always be respected.
LVSAM-WP001A-EN-P - April 2009 4-1