• With basically zero current. The circuit is open on one side and has a small extension.The capacitive current is small (for example, < 5A) and power transformers withinrush current are not allowed.• To connect or disconnect a parallel circuit carrying load current. The switchingvoltage across the open contacts is thus virtually zero, thanks to the parallel circuit(for example, < 1% of rated voltage). Paralleling of power transformers is notallowed.Grounding switches are allowed to connect and disconnect grounding of isolated points.Due to capacitive or inductive coupling there may be some voltage (for example < 40%of rated voltage) before grounding and some current (for example < 100A) aftergrounding of a line.Circuit breakers are usually not interlocked. Closing is only interlocked against runningdisconnectors in the same bay, and the bus-coupler opening is interlocked during a busbartransfer.The positions of all switching devices in a bay and from some other bays determine theconditions for operational interlocking. Conditions from other stations are usually notavailable. Therefore, a line grounding switch is usually not fully interlocked. The operatormust be convinced that the line is not energized from the other side before closing thegrounding switch. As an option, a voltage indication can be used for interlocking. Takecare to avoid a dangerous enable condition at the loss of a VT secondary voltage, forexample, because of a blown fuse.The switch positions used by the operational interlocking logic are obtained fromauxiliary contacts or position sensors. For each end position (open or closed) a trueindication is needed - thus forming a double indication. The apparatus control functioncontinuously checks its consistency. If neither condition is high (1 or TRUE), the switchmay be in an intermediate position, for example, moving. This dynamic state maycontinue for some time, which in the case of disconnectors may be up to 10 seconds.Should both indications stay low for a longer period, the position indication will beinterpreted as unknown. If both indications stay high, something is wrong, and the state isagain treated as unknown.In both cases an alarm is sent to the operator. Indications from position sensors shall beself-checked and system faults indicated by a fault signal. In the interlocking logic, thesignals are used to avoid dangerous enable or release conditions. When the switching stateof a switching device cannot be determined operation is not permitted.11.5.3 Configuration guidelinesThe following sections describe how the interlocking for a certain switchgearconfiguration can be realized in the IED by using standard interlocking modules and theirinterconnections. They also describe the configuration settings. The inputs for delivery1MRK 506 334-UUS A Section 11Control347Application manual
