Handbook for Sound Engineers

442 Chapter 14

they can be an invitation to disaster if fire breaks out.
Heat, flames, and smoke can spread rapidly throughout
the air duct system and building if the fire is able to feed
on combustible materials (such as cable insulations) in
the plenum. To eliminate this problem and to keep
fumes from entering the air handling system, the NEC
requires that conventional cables always be installed in
metal conduit when used in plenums.

Plenums, with their draft and openness between
different areas, cause fire and smoke to spread, so the
1975 NEC prohibited the use of electrical cables in
plenums and ducts unless cables were installed in metal
conduit. In 1978, Sections 725-2(b) (signaling cables),
760-4(d) (fire-protection cable), and 800-3(d) (commu-
nication/telephone cables) of the NEC allowed that
cables “listed as having adequate fire-resistance and
low-smoke producing characteristics shall be permitted
for ducts, hollow spaces used as ducts, and plenums
other than those described in Section 300-22(a).”

While plenum cable costs more than conventional
cable, the overall installed cost is dramatically lower
because it eliminates the added cost of conduit along
with the increased time and labor required to install it.

In 1981 the jacket and insulation compound used in
plenum cables was tested and found acceptable under
the terms of the NEC and was classified by UL for use
without conduit in air return ducts and plenums. Fig.
14-25 shows the UL standard 910 plenum flame test
using a modified Steiner tunnel equipped with a special
rack to hold test cables.

Virtually any cable can be made in a plenum version.
The practical limit is the amount of flammable material

in the cable and its ability to pass the Steiner Tunnel Test,

shown in Fig. 14-25. Originally plenum cable was all

Teflon inside and out. Today most plenum cables have a

Teflon core with a special PVC jacket which meets the

fire rating. But there are a number of compounds such as

Halar® and Solef® that can also be used.

14.28.6 Power Distribution Safety

Electricity kills! No matter how confident we are we

must always be careful around electricity. Fibrillation is

a nasty and relatively slow death so it is important that

Defibrillators are accessible when working around elec-

tricity. Table 14-37 displays the small amounts of current

that is required to hurt or kill a person.

14.28.6.1 Ground-Fault Interrupters

Ground-fault circuit interrupters (GFCIs) are sometimes

called earth leakage or residual-current circuit breakers.

GFCIs sense leakage current to earth ground from the

hot or neutral leg and interrupt the circuit automatically

within 25 ms if the current exceeds 4 to 6 ma. These val-

ues are determined to be the maximum safe levels before

a human heart goes into ventricular fibrillation. GFCIs

do not work when current passes from one line to the

other line through a person, for instance. They do not

work as a circuit breaker.

One type of GFCI is the core-balance protection

device, Fig. 14-26. The hot and neutral power conduc-

Figure 14-25. Plenum cable flame test, UL standard 910.

Air

duct

Fire

alarm

Air

return

Sound

system

Suspended

ceiling

Structural

support

Floor

Differential

manometer 6 × 6 window–door Removable top

panel

Air supply

250 ft/min

Cable

and tray

Fire

end

Adjustable

shutter

Gas supply for igniting

25 ft length of test sample

Light

To draft source

chamber

Vents for

gas

analysis

PhotoSheet

metal vent

pipe electric

cell

Sheet

metal

vent

pipe

Vent

end

Thermocouples

Ceiling air plenum area

19 windows 2¾ ×11¾