Applications
3.8 IT system applications of 3VA molded case circuit breakers
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
181
IT system applications of 3VA molded case circuit breakers
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
● When mobile generators are in operation
● 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. IEC 60364-4-41 (VDE 0100-410) therefore requires
insulation monitoring to indicate a fault of this kind. 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. Siemens 3VA molded case circuit breakers for
line protection, whether they are equipped with a thermal-magnetic trip unit or an electronic
trip unit, 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 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 molded case circuit breaker.
If the system operator ensures that no double ground fault can occur on the input or output
side of the molded case circuit breaker, the breaking capacity of 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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