The combined overcurrent and earth-fault relayis a secondary relay device to be connected tothe current transformers of the feeder to beprotected. The three-phase overcurrent unit andthe non-directional earth-fault unit continuously measure the phase currents and the neutral current of the protected feeder. In fault situations these units initiate external auto-reclosefunctions or trip the circuit-breaker, dependingon the selected protective scheme.When a phase current exceeds the starting valueof the low-set overcurrent unit, the unit starts, simultaneously starting the corresponding timingcircuit. When the set operating time has elapsed,a circuit-breaker tripping command is delivered.Correspondingly, the high-set stage of the overcurrent unit starts when its starting value is exceeded, starting its timing circuit and performinga tripping when the set time has elapsed.The low-set stage of the non-directional earthfault unit operates in the same way. Dependingon the protective scheme it either signals, performs a tripping or initiates a function of anexternal auto-reclose relay. The input circuitcomprises a low-pass filter, which reduces theamount of harmonics in the neutral currentbefore the signal is measured.The low-set stage of the overcurrent unit may begiven definite time or inverse definite minimumtime (IDMT) characteristics. When the IDMTcharacteristic is to be chosen six curve types areavailable in the relay. Four of the curves typescomply with BS 142 and IEC 60255 and arenamed normal inverse, very inverse, extremelyinverse and long-time inverse. The two additionalcurves are named the RI curve and the RXIDGcurve. The low-set stage of the earth-fault unit isoperating on a definite time basis.By appropriate programming of the trippingrelay matrix, the starting signals of the overcurrent and non-directional earth-fault modules arereceived as contact functions. This contact information is used e.g. for the blocking of cooperating protective relays located upstreams.The relay comprises one external logic controlinput, which is actuated by a control signal ofthe auxiliary voltage level. The influence on therelay by the control input is determined by programming switches in the measuring module.The control input can be used either for blocking one or more of the protective stages, for resetting a latched output relay in the manual reset mode or for selecting a new group of relaysettings by remote control.