What are NO NC?| Difference between NO and NC 

NO and NC are the contacts of relays and contractors that define the states of switches under when their coil is de-energized. Difference between NO and NC is NO contacts are used to make the circuit ON, Whereas NC contacts are used to make the circuit OFF.

What are colorful Aerial marker balls and why are they used?

Aerial marker balls

Aerial marker balls are used for safety purposes only. They are placed on power lines to make the conductor crossings visible to aircraft pilots. They save lives and infrastructure by making transmission lines and communication lines visible to helicopters and low-flying aircraft.