Final answer:
An open in the Voltage Regulator shunt coil would lead to a higher generator voltage output at cruising RPM, causing an overvoltage relay or circuit breaker to trip.
Step-by-step explanation:
If an open occurred in the VR (Voltage Regulator) shunt coil, the result would be a higher than normal generator voltage output at cruising RPM. This occurs because the VR shunt coil is responsible for diverting excess voltage. Without the shunt coil to regulate, the voltage builds up leading to an overvoltage condition that would eventually cause the protection mechanism, such as an overvoltage relay or circuit breaker, to trip.
Related to an RL circuit, specifically referring to voltage across the source, voltage across the inductor, and voltage across the resistor; if the RL circuit represents the internal wiring of a generator with the shunt coil playing the regulating part, then an open in the shunt coil would imply that the regulation mechanism has failed, and the voltage would rise unchecked.