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Troubleshooting the M320 Fuses


The router uses fuses from the Cooper Bussman brand GMT series for the CB, SIBs, and FPCs. The fuses are located in the rear of the midplane behind the power supply in slot PEM0. When the fuse for a component blows, the component stops functioning even though it is installed correctly and the power supplies are providing power to the router.

Figure 1 shows the location of the fuses in the rear of the midplane for each component. (The labels shown in the figure do not appear on the actual fuses—the clear cover on every fuse reads BUSS GMT-X—and might not match the labels on the midplane. Ignore the labels on the midplane.)

Figure 1: Fuses in the Midplane
Fuses in the Midplane

When a fuse has blown but the power supplies are still delivering power to the router, the yellow LED below the fuse lights. The colored indicator bulb inside the fuse becomes visible through the clear cover on the fuse.


The LEDs are each about 0.1in. (2 mm) square. They might be difficult to see when not lit.

A blown fuse can cause a component to fail even though it is correctly installed and the power supplies are functioning. Check for a blown fuse in the following circumstances:

  • The LED that indicates normal operation for the component fails to light.

  • The appropriate CLI show chassis environment command indicates that the component is installed but is not receiving power.