Final answer:
The spark plug high tension wire should be disconnected when working with high-power electric circuits to prevent electrical shock or injury.
Step-by-step explanation:
When working with high-power electric circuits, it is advised to disconnect the spark plug high tension wire. This is necessary to prevent electrical shock or injury to the person working on the circuit. The high tension wire carries a high voltage electrical current, and disconnecting it ensures that the circuit is not energized while it is being worked on.