Thermal = electromagnetic long time delay overcurrent release circuit

DK-5A, 5D, and SDb control boxes all use thermal electromagnetic overcurrent releases, and their circuits are shown in Figure 6-80. The trip
unit has overload long-time delay and short-circuit instantaneous action protection functions . In the figure, TA is the fast saturation current transformer, F is the shunt release
coil, SBi is the opening button, FR is the thermal relay, and QF is the circuit breaker and auxiliary contact.