Applications
3.5 Use of 3VA UL molded case circuit breakers in IT systems according to IEC 60947-2
3VA molded case circuit breakers with UL and IEC certification
148 Manual, 01/2019, L1V30435333-01
Use of 3VA UL molded case circuit breakers in IT systems
according to IEC 60947-2
3.5.1
The main advantage of IT systems is that they safeguard continuity of service by electrical
installations and are therefore deployed predominantly in consumer installations where
availability of electrical power is a priority. As a general rule, circuits are interrupted only if
two insulation faults occur simultaneously.
Preferred applications for IT systems are therefore:
● In buildings with rooms used for medical purposes
● For operation of mobile generators
● In certain industries, e.g.:
– in the chemical industry
– in the oil industry
– in the steel industry
– in mining
The IT system is a low-voltage power supply network with increased failure-safety in the
event of ground faults. In an IT system, live phases are not connected to ground. A ground
fault does not cause system shutdown. DIN EN 60204-1 (VE 0113) therefore stipulates that
an insulation monitoring device must be installed in order to indicate ground faults. In the
unusual event that a fault on the load side coincides with a second fault on the line side, the
full phase-to-phase voltage is connected across one contact of the breaker. The Siemens
3VA UL molded case circuit breakers for line protection, with thermal-magnetic and
electronic trip units, are suitable for use in IT systems.
Accordingly, the molded case circuit breakers meet the requirements of standard IEC /
DIN EN 60947-2 Annex H up to a maximum voltage (U
e, max.
) of 690 V AC.
Selection criteria for 3VA UL molded case circuit breakers
The circuit breakers are always dimensioned and selected irrespective of the system type in
which they will be deployed.
The circuit breaker is always selected according to the maximum potential short-circuit
current in the IT system. The device is selected in accordance with the relevant I
cu
values of
the 3VA UL molded case circuit breaker.
If the system operator takes measures to ensure that a double-phase-to-ground fault cannot
develop on the line side or load side of the circuit breaker, the breaking capacity I
cu
/I
cs
remains unchanged in IT systems. If this cannot be ensured, the values in accordance with
the standard IEC 60947-2 Annex H apply for single-pole short circuits.
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