Office buildings usually have fake ceilings that connect every working space in the campus. This fake ceiling offers space for a variety of uses:
- Air conditioning system
- Different types of pipes
- Network cabling channels
The area above the fake ceiling is called a plenum if it offers air flow through it. On the other hand, a non-plenum area is one that contains air-supply-dedicated pipes that connect to the working space below so the air does not flow freely above the fake ceiling (i.e., it’s a non-circulating air space).
| Note: A special situation is having dedicated pipes for air going into the working space but using free air flow for air coming in from the working space, thus creating a plenum.
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The plenum area becomes very important in the case of a fire because it can make the fire worse by feeding it oxygen in the open space above the fake ceiling. In those environments you have to be very careful about the type of network cabling you use on top of the fake ceiling, as the cables may be affected by a fire and can even act as a facilitator for fire to reach other building areas. To avoid such potential disasters, you need to use a special type of cable in plenum areas. A plenum-rated cable looks just like a standard UTP cable but it has some differences:
- The outer protection jacket, which is usually made of special materials like PVC or FEP, is more resistant to fire
- It is not as flexible as regular cables because of the outer jacket material