Siemens SENTRON 3VL series, System Manual

Page: 69 / 360

Application planning
6.4 Use in DC systems
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01
69
6.4 Use in DC systems
The SENTRON VL circuit breakers for system protection with thermal overload and magnetic
short-circuit releases are suitable for use in DC networks.
The SENTRON VL circuit breakers with electronic overcurrent trip units are not suitable for
switching direct current.
Selection criteria for circuit breakers
The following are the most important criteria for selecting the optimal circuit breaker for
protecting a DC system:
●
The rated current determines the power rating and the frame size of the circuit breaker
● The rated voltage determines the number of series-connected poles required for
switching off
● The maximum short-circuit current at the connection point determines the breaking
capacity
● The type of network determines the circuit design
Ampacity of current path
The rated current values are the same for both DC and AC applications.
DC switching capacity
In AC circuits, arc quenching is facilitated by the fact that the current flows through the zero
point. These preconditions aren't true for DC.
For this reason, a high arc voltage must be developed to interrupt the direct current.
Therefore, the switching capacity depends on the arc quenching method and the network
voltage. Several switching contacts can be connected in series in order to achieve a higher
arc voltage.
Furthermore, the kind of effects that are to be expected in the event of a ground fault or
double ground fault must also be taken into consideration.