Final answer:
When a test light illuminates only on the feed side of a fuse, this indicates that the fuse is blown and has interrupted the circuit to protect against overcurrent.
Step-by-step explanation:
If the test light glows only on the feed side of a fuse, the fuse is blown. This condition signifies that the fuse has fulfilled its purpose of protecting the circuit by melting its internal filament, thus breaking the circuit to prevent damage from overcurrent. When testing with a test light or multimeter, a good fuse will show continuity or power on both sides, indicating that the fuse is intact and current can pass through. However, when there is no power or continuity on the load side (side leading to the device or load), and there is only power on the feed side (the side receiving power from the source), it means the fuse has broken the circuit and needs to be replaced.