Question

when is the bulb said to be fused?why

Answer 1

Electric bulb is said to be fused when the tungsten filament breaks which is present in bulb.As the filament is broken the bulb doesn't glow.The filament in bulb is made up of tungsten metal.

Answer 2

The bulb is said to be fused when it's filament gets broken.
The filament may break due to the excessive amount of heat or electricity passed through it.