The main applications for 3VA circuit breakers in DC installations or networks are:
Public transport systems, e.g. electric cars, underground rail networks and streetcars
Wind power plants
3VA1 molded case circuit breakers as line protection devices with thermal trip units and magnetic trip units with short-circuit protection are suitable for use in DC networks.
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