Information Technology Reference
In-Depth Information
return.” In small buildings, plenums can be individually ducted for forced air and
return air. Such plenums rarely contain cabling, and you may be able to use cheaper
nonplenum-rated cable (if you stay out of the plenums).
Large buildings often have individual forced-air ducts to deliver conditioned air to
the workspace, but use vents into the overceiling area to return “used” air to the air han-
dler. In such cases, the entire area above the ceiling grid is classified as plenum space.
The whole purpose of plenum rating cable is to reduce or eliminate the trans-
port of hazardous gases to uninvolved workspaces, in the event of a fire. Cable
placed in the plenum spaces must be tested for flammability and smoke generation
to minimize this hazard.
Riser cable is intended for use in vertical shafts that run between floors. Many
buildings have a series of equipment rooms that are placed vertically in a reinforced
shaft for the purpose of enclosing power distribution equipment, HVAC units, tele-
phone distribution, and other utility services throughout the building. Large build-
ings may even have multiple utility shafts. Cable placed in these shafts must not
contribute to the spreading of fire from floor to floor. However, the smoke genera-
tion requirements are not as important, because the shafts are not plenums.
For that reason, the riser rating is less stringent than plenum. Plenum rated
cable may generally be substituted for uses requiring riser rating.
These special plenum and riser cable requirements are covered in detail in the
National Fire Protection Association's (NFPA's)
National Electrical Code
(
NEC
).
NEC Sections 770 and 800 explain these ratings for fiber-optic, LAN, and commu-
nications cabling.
UV Light Rating
The ultraviolet light from the sun can badly damage cable jackets that are not spe-
cially formulated. This damage will not be immediate but will take place over a
period of years. The cable jacket will become discolored, then cracked and brittle.
It will fail to keep out moisture and contaminants. Moisture penetration may cause
the LAN connection to fail.
UV-rated cable may be obtained from most cable manufacturers. In fact, many
cables already include sunlight resistance as a standard feature. You should check
with your wire manufacturer if you are not certain.
Outside Locations
Cables must be specially designed for use outside a building (Fig. 5.9). The harsh
outside environment of temperature, moisture, light, and stress place unusually
