Section 61(10)(nn) in The West Bengal Factories Rules, 1958
(nn)In any building not provided with automatic fire alarm, a manual fire alarm system shall be provided if the total capacity of the building is over 500 persons, or if more than 25 persons are employed above or below the ground floor, except that not manual fire alarm shall be required in one-storey buildings where with entire area is undivided and all parts thereof are clearly visible to all occupants.