What indicates a blown internal fuse in a transformer when testing it?

A blown internal fuse in a transformer is indicated by an open circuit. When you test a transformer and find an open reading, it signifies that the internal fuse has blown, interrupting the circuit. This means that there is no continuity, which is the direct result of the fuse being blown, cutting off the electrical flow.

In contrast, other test results would suggest different conditions. A short circuit would indicate a direct pathway for the current, often resulting in excessive current flow, but does not imply a blown fuse. A normal reading would indicate the transformer is functioning as it should, and low resistance might suggest a fault, but not specifically a blown fuse. Therefore, identifying an open condition while testing is crucial as it confirms that the fuse needs to be replaced or attended to for the transformer to operate correctly again.