GE Multilin B90 Low Impedance Bus Differential System 3-193 HARDWARE 3.2 WIRING3USE 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 surfaceof the contacts, sometimes making it difficult or impossible to detect a change of the state. This film must be removed toestablish circuit continuity – an impulse of higher than normal current can accomplish this.The contact inputs with auto-burnish create a high current impulse when the threshold is reached to burn off this oxidationlayer as a maintenance to the contacts. Afterwards the contact input current is reduced to a steady-state current. Theimpulse will have a 5 second delay after a contact input changes state.Figure 3–17: CURRENT THROUGH CONTACT INPUTS WITH AUTO-BURNISHINGRegular 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 to50 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 startswhen 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 Inputand 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.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 3–18: AUTO-BURNISH DIP SWITCHESThe auto-burnish circuitry has an internal fuse for safety purposes. During regular maintenance, the auto-burnishfunctionality can be checked using an oscilloscope.842749A1.CDR50 to 70 mA3 mA25 to 50 mscurrenttimeCONTACT INPUT 1 AUTO-BURNISH = OFF= OFFCONTACT INPUT 2 AUTO-BURNISHCONTACT INPUT 1 AUTO-BURNISHCONTACT INPUT 2 AUTO-BURNISH= ON= OFFCONTACT INPUT 1 AUTO-BURNISHCONTACT INPUT 2 AUTO-BURNISH= OFF= ONCONTACT INPUT 1 AUTO-BURNISHCONTACT INPUT 2 AUTO-BURNISH= ON= ON842751A1.CDRNOTE
