GE T35 Instruction Manual page 72

WIRING CHAPTER 3: INSTALLATION
Figure 3-23: Auto-burnish DIP switches
3
The auto-burnish circuitry has an internal fuse for safety purposes. During regular maintenance, check the auto-burnish
functionality using an oscilloscope.
3.3.6.4 Use of contact inputs with active impedance
Contact inputs susceptible to parasitic capacitance caused by long cable runs affected by switching surges from external
circuits can result in inadvertent activation of contact inputs with the external contact open. In this case, GE recommends
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 for
more than 2 ms. The contact input debounce time can still be set above 2 ms for added security to prevent contact input
activations cause by external transient ON states.
An active impedance contact input is normally in Low impedance mode during OFF contact state (non-activated
condition). During Low impedance state contact input impedance is maintained at 10 K Ohms impedance to allow fast
discharge of the stray capacitance of the long cables.
When the contact input voltage exceeds the set threshold, active impedance maintains 10 K Ohms impedance value. If
voltage starts 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. This value reduces the input current to <3 mA, and contact input switched to the ON state (operated state).
The figure shows the active impedance contact input V-I characteristic. Different thresholds with their corresponding
characteristics are shown by color. The contact input is in the ON (operated) state if the input voltage is to the right of the
colored threshold band (+/-10% tolerance), and the contact input is in the OFF (non-activated) state when input voltage is
to the left of the band. A contact input is in LOW state during non-operated system condition, and actively switches to
HIGH state upon detection of input voltage above the settable threshold.
3-26 T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL