Excitation cct. for polyphase supply network protective devices - has variable characteristic and two current overload monitors feeding phase logic gates

摘要

The excitation circuit for polyphase supply network protective devices has each phase provided with an overload monitor that responds to the overcurrents arising from a short circuit. A second overload stage can be switched over by means of phase monitors from the load current threshold to the short-circuit current threshold. For each phase the first overload stage is coupled to AND-gates, and the second overload stage to OR-gates. The AND- and OR-gates outputs are combined by a multi-input OR-gate whose output controls the respective phase protection device. Each output of an overload stage may be coupled over a NO-gate a third input of an AND-gate of adjacent phase(s) for dealing with two-pole short circuits.