GE F35 Instruction Manual page 75

3 HARDWARE 3.2 WIRING
5. Calculating the voltage across the contact input with the Burden Resistor, Voltage across the contact Input:
Vresistor = 2 mA * 14 Kohm = 28 V
Vresistor < contact input threshold (84 V) (EQ 3.5)
In conclusion, in this example, the contact input does NOT operate falsely with the Burden Resistor across its input AND
when a battery ground is present.
USE OF CONTACT INPUTS WITH AUTO-BURNISHING:
The contact inputs sense a change of the state of the external device contact based on the measured current. When exter-
nal devices are located in a harsh industrial environment (either outdoor or indoor), their contacts can be exposed to vari-
ous types of contamination. Normally, there is a thin film of insulating sulfidation, oxidation, or contaminates on the surface
of the contacts, sometimes making it difficult or impossible to detect a change of the state. This film must be removed to
establish circuit continuity – an impulse of higher than normal current can accomplish this.
3
The contact inputs with auto-burnish create a high current impulse when the threshold is reached to burn off this oxidation
layer as a maintenance to the contacts. Afterwards the contact input current is reduced to a steady-state current. The
impulse will have a 5 second delay after a contact input changes state.
current
50 to 70 mA
3 mA
time
25 to 50 ms
842749A1.CDR
Figure 3–22: CURRENT THROUGH CONTACT INPUTS WITH AUTO-BURNISHING
Regular contact inputs limit current to less than 3 mA to reduce station battery burden. In contrast, contact inputs with auto-
burnishing allow currents up to 50 to 70 mA at the first instance when the change of state was sensed. Then, within 25 to
50 ms, this current is slowly reduced to 3 mA as indicated above. The 50 to 70 mA peak current burns any film on the con-
tacts, allowing for proper sensing of state changes. If the external device contact is bouncing, the auto-burnishing starts
when external device contact bouncing is over.
Another important difference between the auto-burnishing input module and the regular input modules is that only two con-
tact inputs have common ground, as opposed to four contact inputs sharing one common ground (refer to the Contact Input
and Output Module Wiring diagrams). This is beneficial when connecting contact inputs to separate voltage sources. Con-
sequently, the threshold voltage setting is also defined per group of two contact inputs.
GE Multilin F35 Multiple Feeder Protection System 3-23