What does a reading of 0 on a voltmeter when testing a known power source indicate?

A reading of 0 on a voltmeter when testing a known power source typically indicates that there is no voltage present at the terminals being measured. This scenario suggests there may be an issue preventing the power source from delivering its expected voltage, which in many cases could be due to a component failure, such as a blown internal fuse. When an internal fuse is blown, it interrupts the circuit, resulting in a zero voltage reading because the circuit is incomplete.

If the internal fuse were functioning, it would allow current to flow and the voltmeter would show a positive voltage reading corresponding to the power source's output. Therefore, encountering a reading of 0 volts should prompt further investigation into the internal components of the power source, especially the fuses, to determine if they are the cause of the issue.

In contrast, the other options suggest scenarios where the circuit would still be functional or provide voltage, which would not correlate with a zero reading.