The basics of device circuit breakers | Extended functions
3.4 With or without current limiting
Depending on the application , the following decisions have to be made : Current-limiting technology has the benefit that it never allows more current than the amount up to the predefined limit to flow in the load circuit . The limit is generally between 1.25 to 2.0 times the nominal current . This also prevents the power supply from being too severely overloaded . To the power supply , even a hard short circuit appears only as a slight overload . There is also no need to lay over-dimensioned cables . In some cases , however , current-limiting technology allows too little current at startup time . However , that is exactly what is needed for a trouble-free load startup , e . g ., in order to create the motor starting torque .
Device circuit breakers without current-limiting technology ensure that the full amount of current is allowed through in the load direction at startup time . This allows even heavier loads to start up faster , e . g ., motors with high starting torque or capacitive loads . Intelligent current analysis procedures are used for this that can differentiate between a true hard short circuit and high inrush currents . This ensures a safe shutdown before critical system operation conditions arise .
3.5 Electrical isolation
In many industries , such as the process industry , actual electrical isolation is required . This is used to ensure that there is no longer any physical connection to the power supply in cases such as shunting for maintenance purposes . The current path is broken via complete electrical isolation in the event of a fault .
This requirement is not a challenge for thermal and thermomagnetic device circuit breakers . Due to its working principle , when a MOSFET is used as the switching element in an electronic circuit breaker , it is unable to do this . At the moment of actuation , it is very high-impedance and cuts the current flow off almost completely without truly opening the contact .
A combination of relays and MOSFET can also provide a robust and long-lasting solution for achieving electrical isolation . The load current is routed over the MOSFET during the shutdown and restart process . This protects the contacts of the electrical isolation element ( relay ) from contact burn-through or fusing . In the switched-on operating state , however , the load current is routed virtually loss-free over the relay .
18 PhoENix CoNtACt