CRCS_STOPBLOCKPLTR_CRDCSTRIPCRLANSI05000448-1-en.vsdANDANDAND025ms0-tCoord0050msANSI05000448 V1 ENFigure 246: Simplified logic diagram for blocking scheme12.5.6.2 Permissive under/overreaching schemeIn the permissive scheme the forward directed ground-fault measuring element sends apermissive signal to the other end, if a ground fault is detected in the forward direction.The directional element at the other line end must wait for a permissive signal beforeactivating a trip signal. Independent channels must be available for the communication ineach direction.An impedance measuring IED, which works in the same type of permissive mode, withone channel in each direction, can share the channels with the communication scheme forresidual overcurrent protection. If the impedance measuring IED works in the permissiveoverreaching mode, common channels can be used in single line applications. In case ofdouble lines connected to a common bus at both ends, use common channels only if theratio Z1S/Z0S (positive through zero-sequence source impedance) is about equal at bothends. If the ratio is different, the impedance measuring and the directional ground-faultcurrent system of the healthy line may detect a fault in different directions, which couldresult in unwanted tripping.Common channels cannot be used when the weak-end infeed function is used in thedistance or ground-fault protection.In case of an internal ground-fault, the forward directed measuring element operates andsends a permissive signal to the remote end via the CS output (sent signal). Local trippingis permitted when the forward direction measuring element operates and a permissivesignal is received via the CR binary input (received signal).The permissive scheme can be of either underreaching or overreaching type. In theunderreaching alternative, an underreaching directional residual overcurrentSection 12 1MRK 506 335-UUS AScheme communication520Technical manual
