CHAPTER 5: SETTINGS CONTROL ELEMENTSG30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 5-2675Figure 5-148: Digital element logicCircuit monitoring applicationsSome versions of the digital input modules include an active voltage monitor circuit connected across form-A contacts.The voltage monitor circuit limits the trickle current through the output circuit (see technical specifications for form-A).As long as the current through the voltage monitor is above a threshold (see technical specifications for form-A), the ContOp 1 VOn FlexLogic operand is set (for contact input 1—corresponding operands exist for each contact output). If the outputcircuit has a high resistance or the DC current is interrupted, the trickle current drops below the threshold and the Cont Op 1VOff FlexLogic operand is set. Consequently, the state of these operands can be used as indicators of the integrity of thecircuits in which form-A contacts are inserted.Example 1: Breaker trip circuit integrity monitoringIn many applications it is desired to monitor the breaker trip circuit integrity so that problems can be detected before a tripoperation is required. The circuit is considered to be healthy when the voltage monitor connected across the trip outputcontact detects a low level of current, well below the operating current of the breaker trip coil. If the circuit presents a highresistance, the trickle current falls below the monitor threshold, and an alarm is declared.In most breaker control circuits, the trip coil is connected in series with a breaker auxiliary contact that is open when thebreaker is open (see figure). To prevent unwanted alarms in this situation, the trip circuit monitoring logic must include thebreaker position.Figure 5-149: Trip circuit example 1
