Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 153)

5 Control circuits
5.1 Utilization categories
IEC 60947-5-1 defines the requirements for electromechanical devices for control circuits. In the
utilization categories AC-12 to AC-15 and DC-12 to DC-14 reference applications are defined
for switchgear in control circuits that facilitate device selection (
Tab. 5.1-1 ; see also Tab. 1.1-1
in Section 1.1 ).
Kind of
current Category Typical applications Relevant
IEC product
standard
Alternating
current AC-12
AC-13
AC-14
AC-15 Control of resistive loads and solid state loads with
isolation by optocouplers
Control of solid state loads with transformer isolation
Control of small electromagnetic loads (
≤72 VA)
Control of electromagnetic loads (>72 VA)
60947-5-1
Direct
current
DC-12
DC-13
DC-14 Control of resistive loads and solid state loads with
isolation by optocouplers
Control of electromagnets
Control of electromagnetic loads having economy
resistors in circuit
60947-5-1
Tab. 5.1-1
Utilization categories for control circuits in accordance with IEC 60947-5-1 ed. 3.0
Copyright © IEC, Geneva, Switzerland. www.iec.ch
The standard also defines the test conditions for the individual utilization categories so that the
ratings are defined and are comparable in accordance with the utilization category. In addition to
the utilization category, the ratings of a contact include the rated voltage and the rated current or
the rated apparent power. When evaluating devices, this data should be compared with that for
the load to be switched.
The performance data according to the utilization category specify the maximum load capacity
of the switching components. In applications with electronic devices – because of the low level
of voltage and current – contact reliability is the main criterion for selection, i.e. the reliability
with which the low signal levels are switched. See Section
5.3.5 .
5.2 Control voltages
5.2.1 Alternating voltage
Both alternating and direct voltage can be used as control voltages. In the case of alternating
voltages, IEC 60204-1 (Safety of Machinery– Electrical equipment of machines) stipulates that
the control voltage must be supplied via transformers with separate windings. When several
control transformers or a control transformer with several secondary windings are used it is
recommended to arrange the circuits so that the secondary voltages are in phase. Transformers
are not mandatory for machines with only one (1) motor starter and/or a maximum of two control
devices. The maximum rated voltage is 277 V, common preferred values are 110 V and 230 V.
Also 24 V is increasingly used.
Among the good reasons for using control transformers is that in the case of a short-circuit in
the control circuit the prospective short-circuit current is limited by the impedance of the control
transformer and hence welding of the control contacts can largely be avoided. This is also
reflected in the fact that the short-circuit tests for control contacts in accordance with
IEC 60947-5-1 are carried out at a prospective short-circuit current of 1000 A.
The selection of the control voltage has among other things an influence on the size of the
currents flowing. In this regard, special attention should be paid to the pick-up currents of large
magnetic loads (for example of large contactors). Switching contacts and conductor cross
LVSAM-WP001A-EN-P - April 2009 5-1