3-30 C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUALWIRING CHAPTER 3: INSTALLATION3Regular contact inputs limit current to less than 3 mA to reduce station battery burden. In contrast, contact inputs withauto-burnishing allow currents up to 50 to 70 mA at the first instance when the change of state is sensed. Then, within 25to 50 ms, this current is slowly reduced to 3 mA as indicated. The 50 to 70 mA peak current burns any film on the contacts,allowing for proper sensing of state changes. If the external device contact is bouncing, the auto-burnishing starts whenexternal device contact bouncing is over.Another important difference between the auto-burnishing input module and the regular input modules is that only twocontact inputs have common ground, as opposed to four contact inputs sharing one common ground (see the ContactInput and Output Module Wiring diagrams). This is beneficial when connecting contact inputs to separate voltage sources.Consequently, the threshold voltage setting is also defined per group of two contact inputs.The auto-burnish feature can be disabled or enabled using the DIP switches found on each daughter card. There is a DIPswitch for each contact, for a total of 16 inputs.Figure 3-26: Auto-burnish DIP switchesThe auto-burnish circuitry has an internal fuse for safety purposes. During regular maintenance, check the auto-burnishfunctionality using an oscilloscope.3.3.6.4 Use of contact inputs with active impedanceContact inputs can be susceptible to parasitic capacitance, caused by long cable runs affected by switching surges fromexternal circuits. This can result in inadvertent activation of contact inputs with the external contact open. In this case, GErecommends using the contact I/O module with active impedance circuit.Active impedance contact input can tolerate external cable capacitance of up to 0.2 μF, without entering the ON state formore than 2 ms. The contact input debounce time can still be set above 2 ms for added security to prevent contact inputactivations caused by external transient ON states.An active impedance contact input is normally in Low impedance mode during OFF contact state (non-activatedcondition). During Low impedance state, contact input impedance is maintained at 10 K Ohms impedance to allow fastdischarge of the stray capacitance of the long cables.When the contact input voltage exceeds the set threshold, active impedance maintains 10 K Ohms impedance. If voltagestarts rapidly decreasing, this indicates that stray capacitance is being discharged through the contact input. If, however,voltage stabilizes above the set threshold, the input impedance is switched to High impedance mode of 100 K Ohms. Thisvalue reduces the input current to <3 mA, and contact input switches to the ON state (operated state).The figure shows the active impedance contact input V-I characteristic. Different thresholds with their correspondingcharacteristics are shown by color. The contact input is in the ON (operated) state if the input voltage is to the right of thecolored threshold band (+/-10% tolerance), and the contact input is in the OFF (non-activated) state when input voltage isto the left of the band. A contact input is in LOW state during non-operated system condition, and actively switches toHIGH state upon detection of input voltage above the settable threshold.
