The Railbus Isolator module provides galvanic isolation between the 2/2 and 2/1 sections of the Railbus, thus preventing fault voltages on the general purpose (2/2) section of the Railbus from invading the parts that have IS field wiring. A Railbus Isolator is required only in nodes that contain IS field wiring. The Railbus Isolator mounts on its own carrier and is placed in the line of carriers immediately before the first (2/1) carrier that will carry IS field wiring. It is usually followed by an 8920-PS-DC IS module power supply (on its own carrier) which provides power for the 2/1 I/O modules.