A man in a metallic cage is not affected even when the cage is highly charged due to what is known as the Faraday cage effect. In a state of equilibrium, charges are as far from each other as possible. When charge is introduced into a conductive sphere, which can either be hollow or solid, the charge moves due to the forces of repulsion between like charged particles and arranges itself on the outer shell of the sphere. They are the farthest from each other when arranged in this way.
Due to this the charge inside a metallic sphere is always zero. This is also the reason why a person in a charged metallic cage does not receive a shock. This is the reason behind people advised to stay inside their cars during a lightning storm. Even if lightning strikes the car the person inside is not affected by it.