Final answer:
The dual voltage switch on a transformer must be operated only when the transformer is de-energized to prevent electrical arcing and ensure safety.
Step-by-step explanation:
The dual voltage switch on a transformer should only be operated when the transformer is de-energized. This means that the transformer must be completely disconnected from any power source, ensuring that there is no electrical current flowing through it. Changing the voltage setting while the transformer is energized can cause serious harm to both the equipment and the operator due to the risk of electrical arcing, leading to equipment failure or personal injury.