Have you ever wondered why some fire alarm cables are designated as plenum-rated or riser-rated, and what these things mean?
The difference, as this short post will expose, is actually quite consequential, and a cable with one rating should not be used in place of another, as you will see.
But let us get to the bottom of why this matters first.
What Is Riser Space? What About Plenum Space?
Riser space is a vertical space in a building, usually behind the walls, that communicates between the various floors – hence the name. The space is primarily left open to allow conduits and utilities to be run between floors. Consequently, riser space allows for communications and electrical utilities to be distributed vertically.
As for plenum space, this is the empty space above a drop ceiling or beneath raised floors that is reserved for HVAC infrastructure. It is set aside for air circulation, but it is also commonly used for networking cables and electrical installations, too.
The fact that plenum space is specifically designed to enhance airflow, this can pose a significant risk if the appropriate electrical cables are not installed in the space.
So, the Differences Between Plenum and Riser-Rated Fire Alarm Cables?
Fire alarm cables that are riser-rated generally utilize the same conductors and have the same electrical capabilities as plenum-rated cables; however, they have a very different insulated jacket.
Riser-rated cables are usually made with a flame-retardant jacket, but it is not as flame-retardant as plenum-rated cable. As a result, riser-rated cable should never be used in plenum space.
As for plenum-rated cable, since it is intended to be used in plenum spaces which allow for open airflow and have few to no fire-restricting barriers in place, the cables are made with special insulated jackets that are particularly resistant to fire.
Many plenum-rated cables are made with FEP, or fluorinated ethylene polymer, that has a high operating temperature range, which is fire-resistant, and which will self-extinguish if it does catch fire. Plenum rated cables also produce little to no smoke when burning.
These designations are crucial not only to protect the network, but to help prevent flame propagation throughout a building, through the plenum space.
As a result of these differences, it is important to understand the nature of the electrical installation and only to use an appropriately-rated cable for each application.
It is also worth a note that sometimes plenum-rated cables are required to be used in non-plenum spaces in order to remain compliant with building codes, as a result of the advanced protection their insulation offers.
As you might imagine, there is usually a sunstantial cost differential between riser and plenum rated cable, with plenum-rated cable costing significantly more in most instances.
(Note: There is another class of communication cables known as CM, or communication multipurpose cables, that are sometimes appropriate for wiring in non-plenum spaces or through non-vertical runs between floors. These, however, do not have the same fire-protective ratings and will produce greater levels of toxic smoke when they are exposed to flames or extreme temperatures, so should only be used where appropriate, and never in place of rated cables)
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