CHAPTER 3: INSTALLATION WIRINGB90 LOW IMPEDANCE BUS DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 3-1933.3.5.1 Contact inputsA dry contact has one side connected to terminal B3b. This is the positive 48 V DC voltage rail supplied by the power supplymodule. The other side of the dry contact is connected to the required contact input terminal. Each contact input grouphas its own common (negative) terminal that must be connected to the DC negative terminal (B3a) of the power supplymodule. When a dry contact closes, a current of 1 to 3 mA flows through the associated circuit.A wet contact has one side connected to the positive terminal of an external DC power supply. The other side of thiscontact is connected to the required contact input terminal. If a wet contact is used, then the negative side of the externalsource must be connected to the relay common (negative) terminal of each contact group. The maximum external sourcevoltage for this arrangement is 300 V DC.The voltage threshold at which each group of four contact inputs detects a closed contact input is programmable as17 V DC for 24 V sources, 33 V DC for 48 V sources, 84 V DC for 110 to 125 V sources, and 166 V DC for 250 V sources.Figure 3-15: Dry and wet contact input connectionsThere is no provision in the relay to detect a DC ground fault on 48 V DC control power external output. We recommendusing an external DC supply.3.3.5.2 Use of contact inputs with auto-burnishingThe contact inputs sense a change of the state of the external device contact based on the measured current. Whenexternal devices are located in a harsh industrial environment (either outdoor or indoor), their contacts can be exposed tovarious types of contamination. Normally, there is a thin film of insulating sulfidation, oxidation, or contaminates on thesurface of the contacts, sometimes making it difficult or impossible to detect a change of the state. This film must beremoved to establish 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 has a five-second delay after a contact input changes state.Where a tilde “~” symbol appears, substitute the slot position of the module.
