Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 11)

1 Load characteristics and utilization categories
The characteristics of the load to be switched or controlled determine the loading of the
switchgear and correct selection of the latter for the respective application. In particular the
loading of contacts by current and voltage when circuits are made and broken is of high
significance. Thus the making and breaking current under resistance load corresponds to the
continuous operational current while for example squirrel-cage induction motors draw a multiple
of the rated operational current when they are switched on and accelerate.
1.1 Utilization categories simplify the selection of devices
In order to make the choice of devices easier, utilization categories are defined in the standards
for low-voltage switchgear (IEC 60947-1, -2, -3, -4, -5, -6) that take into account the intended
application and hence the associated loading of the various low-voltage switchgear types, such
as contactors, disconnectors, circuit breakers and load switches (
Tab. 1.1-1 ). The rated
operational currents or rated operational powers are listed in the technical data for the devices –
usually for various rated operational voltages. For the sake of universal applicability the data is
usually stated for several utilization categories for one and the same piece of switchgear. For
project engineers, the selection of devices is basically reduced to the comparison of perform-
ance data of the switchgear for the respective utilization category with the ratings of the load
and the choice of a device which meets or exceeds the ratings of the load.
When the rated operational voltage U
e
and the rated operational current I
e
are stated for a
certain utilization category, the required making and breaking capacity for the item of switchgear
is defined. Thus in general no further agreements between users and manufacturers are
required. The selection of a suitable device and comparison of products is thus facilitated.
The test regulations in the IEC standards define the test parameters for the individual utilization
categories. Manufacturers are obliged to carry out tests according to these standards. This
ensures the suitability of the tested devices for the respective application and frees the user
from getting “bogged down” in technical details.
The conditions for application in practice may differ considerably – in a favorable as well as
adverse sense – from these standardized conditions. Examples are heavy-duty starting, high
frequency of operation, especially long equipment life span. In such cases, the users and
manufacturers must agree the permitted loads. In the catalogs as well as in the RALVET
electronic documentation, the corresponding performance data are stated for the most common
special applications.
Because of the very high and cost-intensive expenditures for testing, data for the most important
and common utilization categories are usually provided. In cases going over and beyond this
consultation is required.
LVSAM-WP001A-EN-P - April 2009 1-1