What occurs when the fuse or circuit breaker in a power source is open due to excess current?

When a fuse or circuit breaker in a power source opens due to excess current, it disrupts the flow of electricity in the circuit. This condition is referred to as an 'open circuit', meaning that electricity cannot flow because the path is interrupted. The primary coil in a transformer is responsible for receiving the incoming voltage supply. When the fuse or circuit breaker opens, it creates an open circuit in the primary coil, meaning that the coil cannot draw current from the power source, leading to a loss of electromagnetic induction and thus no output in the secondary coil.

The other options suggest different fault conditions related to the operation of coils in transformers. A shorted primary coil would imply that there is no interruption and that current could flow, which isn't the case here since we are discussing a fuse that has opened. Similarly, a shorted secondary coil indicates a fault elsewhere in the secondary circuit, and an open secondary coil indicates a break in the secondary circuit. However, in this scenario, it is the primary coil that experiences an open condition due to the protective measure of the fuse or circuit breaker opening.