The entire pantograph is electrically conductive which is why it is insulated from the roof. There is a large cable which goes to the main transformer, some times through a vacuum breaker. The cable itself is double insulated and protected inside an outer shield ala a coaxial cable. It is housed inside grounded metal conduit so if the insulation fails the current goes to ground harmlessly. Equipment with two pans usually have isolation switches in case a pan were to be damaged. That way the defective pan can be dealt with and the train can still operate.
"I may not have a brain, but I have an idea!" BOB, benzoate ostylezene bicarbonate.