Final answer:
The statement that a crane operator should stay in the cab until power is de-energized is true, promoting safety by avoiding a pathway for electrical current through the operator's body. High-voltage power lines are not insulated, explaining why birds are not electrocuted when perched on them. For additional safety, it is recommended to work with one hand in high-power electric circuits and to use the back of the hand for a final voltage check.
Step-by-step explanation:
If a crane contacts a power line, it is indeed true that the operator should remain inside the cab until the power has been de-energized. This procedure is to ensure the operator's safety because the crane and the power line could create a pathway for electrical current. If the operator steps out and touches the ground while still touching the crane, their body could become a path to the ground for the electricity, potentially leading to serious injury or death. This is similar to the safety inside a vehicle during an electrical storm; a car acts as a Faraday cage, and if it is struck by lightning or an active electrical wire falls on it, the metal body of the car will transmit the electrical charge around the exterior, protecting the occupants inside as long as they do not touch any metal parts connected to the car's exterior.
Regarding high-voltage power lines, it is false that these wires are wrapped in insulating material. They are typically not insulated, which is why birds can perch on them without being electrocuted; they are not completing a circuit to ground. However, if a large bird touches two wires at once, it can create a circuit and be electrocuted. The recommendation to work with one hand when dealing with high-power circuits is a safety precaution to prevent the current from traveling through the heart should an accidental shock occur.
Experts use the back of their hand as a final check for voltage because if there is still voltage present, the muscle contraction will typically pull the hand away from the source, reducing the risk of grabbing onto and being unable to release the charged object.