FB2 has four circuits, every circuit has fuses on both polarities and is monitored. Every pair of fuses supplies a optoisolator with positive and negative voltage. When both polarities are present the optoisolator will trigger the inverter. When the input of the inverter is triggered, the output will be inactive. If a fuse should break the optoisolator will cut the signal to the inverter, causing the output to be activated and a LED indicates which circuit is broken. All inverter outputs is connected to a OR gates that sends an alarm signal to Main monitoring unit. FB2 also monitors and external fuse.