This question checks a specific fire-safety number from the National Building Code (NBC) of India 2016: the minimum fire resistance rating required for service ducts and shafts. Let's look at each option.
- 45 minutes: This is a relatively short rating, closer to what might apply to some lighter fire-check doors, not to a service shaft that runs through multiple floors of a building.
- 60 minutes: One hour of fire resistance is a common baseline for many secondary elements, but service shafts, being a direct path for fire to move between floors, are held to a stricter standard than this.
- 120 minutes: This equals 2 hours, and this is exactly the minimum fire resistance rating NBC 2016 sets for service ducts and shafts, reflecting how serious a breach in a vertical shaft can be for fire spreading through a building.
- 240 minutes: 4 hours is a longer rating than the code actually requires for this element; such high ratings are reserved for other, more critical structural fire-safety situations.
Because a service shaft can act like a chimney and rapidly carry fire and smoke from one floor to the next if it fails, the code sets its minimum fire resistance well above the lighter 45 or 60 minute ratings but does not go as high as 240 minutes: it lands at exactly 120 minutes.
Let's summarize:
- Service ducts and shafts need a stronger fire-resistance enclosure because they connect multiple floors directly.
- NBC 2016 fixes this minimum rating at 120 minutes (2 hours).
So the correct answer is 120 minutes.