When Used In An Electrical Circuit, The Application May Look Like Diagram B.
When power is applied (L1, L2, and L3), if any fuse becomes open, the associated indicator illuminates. In addition, if a phase is present, but no fuse is in place, the associated indicator will not illuminate. The ferrules of the fuse complete the indicator circuit, so no fuse -> no ferrule -> no circuit -> no light.
Note: If a phase is not present, then the associated indicator would not illuminate, no matter what the status of the fuse.
For DC Circuits: One LED light lit indicates a blown fuse on a DC application. Since DC has a polarity of "+" or "-", only one bulb senses 1/2 of the sine wave for the given DC circuit. The light being lit depends on how the plus and minus are wired.
For AC Circuits: Two LED lights lit indicate a blown fuse on an AC application. Since AC is sinusoidal, the LED senses 1/2 of the sine wave and the other 1/2 causing both lights to be lit for a given AC circuit.