Variants - Siemens 3VA1 100 A Manual

Features specific to direct current
Short-circuits are interrupted in DC networks by a method similar to the one applied to high
alternating currents, i.e. by current limiting. A high arc voltage develops in molded case
circuit breakers, which forces a current zero when the arc voltage exceeds the value of the
recovery voltage. The current rise, however, is determined by the time constant and not the
make time or power factor.
Overloads are interrupted by a completely different principle to the method applied in AC
networks. With direct currents, there is no periodic zero crossing and thus also no period of
time without magnetic energy in the circuit during which the conditions for extinguishing the
arc are favorable. The arc is not extinguished until the arc voltage exceeds the line voltage
and the current reduces to zero. In other words, a high arc voltage must be developed in
order to interrupt the direct current.
The rated operational current values are generally the same for DC and AC applications.
With DC applications, the breaking capacity depends only on:
● The arc extinction method used
● The line voltage
3.3.2

Variants

The same thermal-magnetic trip units used as line protection devices in AC installations are
also available for protecting DC installations.
However, a correction factor must be applied to the magnetic trip unit.
Example:
3VA1 160 A, TM240 ATAM:
If the switching device is to trip instantaneously in response to an overcurrent of 1200 A, the
parameter I
I = 1200 A x 0.7 = 840 A.
i
For further information on trip units and details about setting parameters, please refer to
chapter "Line protection applications of 3VA molded case circuit breakers (Page 105)".
It is not possible to use electronic trip units for this application.
3VA molded case circuit breakers
Manual, 04/2015, A5E03603177010-02
3.3 DC network applications of the 3VA molded case circuit breaker
for instantaneous short-circuit protection (I) must be set to:
i
Applications
133