Is there a legal way that complies with the USA-NFPA fire code, to put regular non-plenum network devices above a drop ceiling in a plenum airspace?
Background:
In many places in the United States, a licensed electrician is not required to install low-voltage cabling such as network and phone cables into residential or commercial buildings, nor is wiring inspection required. However, it is still important for unlicensed installers to follow electrical and fire safety rules outlined in the National Fire Protection code book.
For example it is not acceptable to install general purpose network electrical devices directly in a "plenum airspace", which is where the open air above a drop/suspended ceiling is used as an air return for the building ventilation. There is risk of a smoldering plastic fire in the hidden space, which is both fueled by the moving forced air fanning the flames, and the burning fumes are sucked into the building air circulation and can potentially unknowingly poison, injure, or kill building occupants.
Plenum rated network devices and plenum rated cables are specifically designed for plenum airspace use, with special expensive low smoke and self-extinguishing plastics.
EDIT: Further discussion for the inexperienced:
Wikipedia: Plenum space
https://en.wikipedia.org/wiki/Plenum_space
(Yeah, um, I created this article back in 2012. The illustrations are my own work.)
Article: What Lurks in the Ceiling, Mar 24, 2011
Electrical Construction and Maintenance Magazine
http://ecmweb.com/low-voltagecommunications/what-lurks-ceiling
Questions:
- If a non-plenum device is partially or completely sealed off, such as
putting the device in a covered metal enclosure with an electrical
outlet inside, and the enclosure is itself installed by a licensed
electrician for the device, is this sufficient to comply with the
code? If a device failure and electrical fire were to break out, the
metal enclosure would likely contain the fire, limit burning, and
limit spread. - Alternately, if that is not sufficient, would a gasketed metal
enclosure with cable clamps or rubber bushings to limit air exchange
through the punched cable openings be acceptable? This would further
choke off the air circulation for any potential device overheating
and fire so that it rapidly uses up its air supply in the enclosure,
and go out. - If an approved enclosure for a non-plenum device is available and
workable, is it possible to cool the non-plenum device in some
approved manner that limits air exchange and potential fire spread,
such as a sealed air-to-air heat exchanger? (Also known as a fan
blowing air in a circle inside, and a second fan blowing air on the
enclosure shell on the outside.)
Best Answer
Vent the Device into the Room's Airspace
If you seal off the device from the "plenum airspace" and vent it into the room, then, from the point of view of smoke, fire, and air handling the device is no longer in the plenum but rather in the room. This way the device can physically occupy the space above suspended ceiling without requiring a flame-test rating for plenums.
Duct the Return Air
Usually the "plenums" where I am inclined to install equipment are in the return air path. I have found that it is often a straightforward matter to duct the return air from the room to the air handler. There is good, easily-installed flex duct that I have used for this purpose.
By ducting the return air, the volume above the ceiling ceases to be a plenum thereby relieving the plenum-rating requirements. This also has the added benefit of improving indoor air quality because you no longer have that giant plenum filled with all manner of awful things off-gassing and throwing particulate into your air supply.