User manual

8 ExtroNews 12.1 January/February 2001
insulation on the wires and jacket on the
cable must be capable of withstanding a
specified amount of heat for a specified
amount of time without combustion or
contributing significantly to the
sustenance of a fire. The ideal cable will
not burn at all.
The most common insulation and
jacketing material used on wire and cable
is Polyvinyl Chloride, PVC. PVC has many
attributes that make it a great material for
general-purpose wire. Unfortunately, PVC
is very flammable. When PVC burns, a key
byproduct is hydrochloric acid. The smoke
and residues are very corrosive. While
there are several versions of PVC with
varying characteristics, none are able to
pass the plenum test. Some versions of
PVC and another group of polymers from
the family of plastics called Polyolefin may
attain plenum capability when combined
with certain other polymers that are more
fire resistant. However, maintaining the
safety margins against the plenum flame
test is sometimes difficult. Construction
must be highly controlled and, in some
instances, cable designs that pass the test
one time may not pass on another trial.
The best insulation for fire resistance to
date is also one of the best dielectric
materials for lower loss cables…Teflon®
FEP (fluorinated ethylene propylene).
Teflon FEP is a registered trademark of
DuPont. But, Teflon FEP is much more
expensive to manufacture which explains
the higher cost of plenum rated cables.
The material is tougher and more difficult
to extrude. This is why plenum cables are
not as flexible as PVC.
To obtain a plenum rating, the cable
must pass the Steiner Tunnel Test within
UL 910. The Steiner Tunnel is a specially
constructed fire chamber that positions a
group of cables of the same type and
about 24 feet in length into a horizontal
frame within an air handling plenum. Air
rushes into one end of the plenum. Gas
burners supply a specific level flame
under the cable bundle about 4.5 feet
from the end near the air inlet. While the
flame is applied for a specified period of
time, the length of flame travel along the
cable is monitored as well as the amount
of smoke produced. At the opposite end
of the tunnel, a vent shaft funnels the air
and smoke past photoelectric sensors.
Criteria under which the test results must
comply are:
•
Smoke Peak Optical Density:
less than 0.50
•
Smoke Average Optical Density:
less than 0.15
•
Maximum Flame Propagation:
less than 5.0 feet from point of
application
The Secret Ingredient
Most all of the wire and cable
insulation made in the US depends on
the addition of halogens for fire
retardancy. What are halogens?
Halogens are the elements in group VIIa
on the periodic chart (yes, you will now
use some of that obscure high school
chemistry). The name is of Greek origin,
meaning “salt-bearing.” The naturally
occurring halogens are fluorine (F),
chlorine (Cl), bromine (Br), and iodine (I).
Halogens are nonmetallic and closely
resemble one another. They readily form
bonds among themselves and with most
other elements.
TECHNICALLY SPEAKING...
MP CM CL2 CL3 FPL PLTC
Plenum MPP CMP CL2P CL3P FPLP –
Riser MPR CMR CL2R CL3R FPLR –
General Purpose MP, MPG CM, CMG CL2 CL3 FPL PLTC
Dwelling – CMX CL2X CL3X – –
Cable Family
Cable Marking Designation
Application
Table 2. Cable Marking Designations for NEC Application Categories