[mythtv-users] mythtv-users Digest, Vol 84, Issue 14

[Gary Buhrmaster]

> Depending on your local codes, running a cable in a return duct may require

> the use of "plenum-rated" cable, which is generally more expensive than 
> standard cable.
> 
> "Plenum-rated" cable is basically fire proof, or at least fire retardant,
and 
> will prevent or delay the wire being a path for the propagation of a fire 
> trough the ductwork.

"Plenum-rated" cables are sometimes known as "low smoke - zero halogen"
cables.

The *primary* purpose of such cables are to insure that when the
fire happens that the cable will not produce toxic fumes that
would prevent humans from exiting the structure safely.  Many
(home) ducts are some combination of aluminum and the actual
wood framing in the structure, and in an intense fire the wood
will burn (along with any cables that are nearby).  But cables
that are *not* plenum rated are typically PVC covered, which
when exposed to flame generates things like dioxin and
hydrocloric acid.  Stuff that can kill you (or at least make
you unable to leave safely).  Since the ducts are designed
to move air, moving the hydrocloric acid fumes to the humans
via the ducts is considered as a bad thing, so codes require
plenum rated wires inside those ducts.  The fire codes have,
as their primary objective, to protect life.  It is usually
a good plan to follow those codes if you place a high value
on your life or the life of those that share your living
quarters.