The basic principle behind passive fire protection is compartmentation. This compartmentation is achieved through fire separations and firecells.
Some short definition can be found in the Building code:
Firecells are any space including a group of contiguous spaces on the same or different levels within a building, which is enclosed by any combination of fire separations, external walls, roofs, and floors.
Floors, in this context, include ground floors and those in which the underside is exposed to the external environment (e.g. when cantilevered). Note that internal floors between firecells are fire separations.
Fire separation is any building element which separates firecells or firecells and safe paths and provides a specific fire-resistance rating.
We can conclude that fire separation is the method for protecting buildings from the spread of fire into adjoining areas for designated periods by the introduction of fire-resisting elements (e.g. walls, floors, doors, ducts). These time periods are defined in the Building Regulations.
Buildings may be subdivided into firecells (also called fire compartments) designed to contain the fire and prevent its spread for a specified time period. Firecells are separated using construction elements that are fire-resistance rated for at least the specified period required for the firecell (typically for 30, 60, 90 or 120 minutes).
The period of time that the element acts as a barrier to the spread of fire. It is intended to prevent it from developing into a much larger fire, to provide people in the same building sufficient time to evacuate, and to limit the damage caused to the property. Fire-resistant construction must be located to completely separate the intended firecell from the rest of the building.
In such cases where a fire separating element includes an opening (i.e. door, internal window, penetrating duct or access panel), the opening must have the same fire rating as the element in question to maintain the performance of the fire-rated construction.
Where the fire separation meets unrated construction, such as a suspended ceiling, it should be continued and extended to reach another fire separation or the main boundaries of the building envelope (such as floors, external walls and ceiling/roof).
The figure below illustrates the concept of firecells and fire separations.
Source: Guide to Passive Fire, BRANZ