Plenum Test
A "Standard For Test Method For Fire and Smoke Characteristics of Cables
Used in Air-Handling Spaces" was developed by Underwriters' Laboratories
(UL) to classify cables for this NEC requirement. This test is performed
in a 25 foot Stein Tunnel test furnace (also specified in ASTM E-84 test
for building materials). Designed to match the rigors of the application,
this test is quite demanding.
The Steiner Tunnel a 25-foot, horizontal test chamber was
developed by UL engineer Albert J. Steiner to evaluate the flame spread
and smoke development properties of building materials. About 20 years
ago, UL first used this apparatus to test plenum cable. This work became
the basis for UL's development of UL 910.
In the UL-910 test, a single layer of 24 foot lengths of cable are supported
by a one foot wide cable rack, which is filled with cables. The cables
are ignited by an 88 K W (300,000 BTU/hr) methane flame. Flame spread is
aided by a 240 ft/min draft. During the 20 minute test, flame spread is
observed through small windows spaced one foot apart. Smoke is measured
by a photocell installed in the exhaust duct.
To qualify, cables must have a flame spread of less than 5 feet beyond
the end of the 4 1/2 foot ignition flame, a peak optical density of 0.5
maximum (33% light transmission) and a maximum average optical density
of 0.15 (70% light transmission).
Riser
A riser is a vertical pathway or space between floors. Cables within
risers must be rated in order to prevent the spread of fire between floors.
NEC Type OFNR cables pass the UL 1666 flame test This test is not a stringent
as the UL 910 test. Plenum cables can be placed in risers; riser rated
cables must never be placed in plenums. Riser rated cables can be used
in general purpose applications in place of cables with lower ratings.
Riser Test
Underwriters' Laboratories (UL) has established a fire test facility
for the purpose of listing cables that meet NEC requirements.
This test chamber is an eight by four simulated shaft, twelve feet high
between the source of ignition and the floor above. A very large propane
burner, 145 K W (495,000 BTU/hr) is ignited for a period of 30 minutes.
Flames must not extend above the 12 foot mark if the cable is to be UL
Classified for this duty.
Conduit
Intrabuilding conduit runs can be in celings, walls or under floors,
with certian limits, as conduit systems are very inflexible. Conduit systems
should be used only when workstation outlet locations are absolutely permanent,
no flexibility is required, and densities are low.
A plenum is defined as any space used as part of an air-handling system.
This includes heating/air-conditioning ducts and air returns, which frequently
include the space between suspended ceilings and the floor above in modern
office buildings. The National Electric Code (NEC) requires that "exposed
cables (those not in conduit) in plenums be listed as having adequate fire-resistant
and low-smoke producing characteristics ..."