When the binary output values are read from the protection relay, the last written value is returned. However, while controlling an object, the final status change of the object should be verified from the corresponding binary input object, if available.
In case of double-point control, the IEC 61850 standard defines a value attribute named stSeld to be always available for the control object. In case of SBO, this value shows if the control object is actually selected, for example, if it is already selected by another controlling source. This value is found in the DNP3 memory map as a regular DNP3 binary input. The stSeld value also goes on (“1”) and off (“0”) during a direct control operation. The stSeld value is always held in true state (“1”) by the protection relay until the control operation is terminated.
In case of a long lasting control operation, for example, a motor controlled disconnector, the stSeld value can stay true for several seconds after the control operation is acknowledged and started. A new control operation cannot be started by the protection relay until the previous one has finished.