Final answer:
The starting switch of a resistance start motor opens when the motor reaches 75% to 80% of its full speed to prevent overheating and damage.
Step-by-step explanation:
The starting switch of a resistance start 4 pole, 60 Hz single phase motor should open when the motor reaches approximately 75% to 80% of its full speed. This is because the starting winding of a single-phase motor is only used to provide the necessary starting torque and is not intended for continuous operation. Once the motor is close to its operational speed, the centrifugal switch or electronic relay will open to disconnect the starting winding to avoid overheating and damage.