Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 30)

Fig. 2.2-1
Functional elements of a load circuit
The selection of suitable devices to fulfill the required functions is based on the characteristics
of the load (e.g. rated power and utilization category), the operational requirements (e.g.
switching frequency, required availability after a short-circuit) and the nature of the power supply
(e.g. rated voltage, prospective short-circuit current).
2.2.1 Device types
Various types of device are available for carrying out the switching and protection tasks listed
under
2.2 that are specially designed to fulfill the respective requirements. The various parts of
IEC 60947 (Low-voltage switchgear and controlgear) specify the design, performance and test
features of the devices. The most important features of the main device types are presented
below.
2.2.1.1 Disconnectors (isolating switches)
The disconnector is a mechanical switchgear that fulfills in the open position the requirements
specified for the isolation function (IEC 60947-1). The purpose of the isolating function is to cut
off the supply from all or a discrete section of the installation by separating the installation or
section from every source of electrical energy for reasons of safety. The key factor here is the
opening distance. Isolation must be guaranteed from pole to pole and from input to output,
whether this is by means of a visible isolation gap or by suitable design features within the
device (mechanical interlocking mechanism).
A device fulfills the isolating function stipulated under IEC 60947-1 when in the “Open” position
the isolation at a defined withstand voltage is assured between the open contacts of the main
circuit of the switchgear. It must also be equipped with an indicator device in relation to the
position of the movable contacts. This position indicator must be linked in a secure, reliable way
to the actuator, whereby the position indicator can also serve as actuator, provided that it can
Isolation
Short-circuit
protection
Thermal
protection
Operational
switching
Load
Example
Disconnector (Isolator)
Switch disconnector
Circuit breaker with isolating function
Fuse
Circuit breaker
Fuse (line protection)
Circuit breaker with thermal release
Motor protection relay (thermal, electronic)
Contactor
Load switch
Motor protection circuit breaker
Motor
Heater
Lighting
Capacitor
LVSAM-WP001A-EN-P - April 2009 2-2