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• Moisture is a major cause of electrical
equipment and material failures. Breathing due to
temperature cycling results in the conduits
accumulating relatively large amounts of moisture.
The conduits then pipe this moisture into the
electrical equipment enclosures which over a period
of time results in the deterioration of the equipment
insulation systems and their eventual failure. Also,
moisture may become a factor in the corrosion
failure of some of the critical electrical equipment's
metallic components. Conduit seals are not effective
in blocking the movement of moisture. The conduit
systems may be designed to reduce the moisture
problems but not to completely eliminate it. Few
designers go into the design detail necessary to
reduce the effects of moisture in the conduit
systems. Tray cables do not provide internal
moisture paths as do conduits.
• In the event of external fires in industrial
installations, the damage to the tray cable and cable
tray is most often limited to the area of the flame
contact plus a few feet on either side of the flame
contact area. For such a fire enveloping a steel
conduit bank, the steel conduit is a heat sink and the
conductor insulation will be damaged for a
considerable distance inside the conduit.
Thermoplastic insulation may be fused to the steel
conduit and the conduit will need to be replaced for
many feet. This occurred in an Ohio chemical plant
and the rigid steel conduits had to be replaced for
90 feet. Under such conditions, the repair cost for
fire damage would normally be greater for a conduit
wiring system than for cable tray and tray cable. In
the Ohio chemical plant fire, there were banks of
conduits and runs of cable tray involved. The cable
tray wiring systems were repaired in two days. The
conduit wiring systems were repaired in six days and
required a great deal more manpower.
• In the event of an external fire, the conduit
becomes a heat sink and an oven which decreases
the time required for the conductor insulation
systems to fail. The heat decomposes the cable
jackets and the conductor insulation material. If
these materials contain PVC as do most cables,
hydrogen chloride vapors will come out the ends of
the conduits in the control rooms. These fumes are
very corrosive to the electronic equipment. They are
also hazardous to personnel. A flame impingement
on a cable tray system will not result in the fumes
going into the control room as there is no
containment path for them. They will be dispersed
into the atmosphere
.
IN MOST CASES AN OBJECTIVE
EVALUATION OF THE REQUIREMENTS
FOR MOST HIGH DENSITY WIRING
SYSTEMS WILL SHOW THAT A CABLE
TRAY WIRING SYSTEM PROVIDES A
WIRING SYSTEM SUPERIOR TO A
CONDUIT WIRING SYSTEM.
Abandoned Cables
Easily identified, marked, or removed - all
possible from an open Cable Tray System
For the 2002 National Electrical Code, several
proposals were submitted to the NFPA to revise the
1999 NEC
®
for Articles 300, 640, 645, 725, 760,
770, 800, 820, and 830 to require all abandoned
cables to be removed from plenum spaces.
The purpose of the proposals is to remove the
cables as a source of excess combustibles from
plenums and other confined spaces such as raised
floors and drop ceilings. All of the Code Making
Panels agreed that this should be acceptable practice
except Code Making Panel 3, which oversees
Article 300.
Because Article 300 is exempt from this
requirement only low-voltage and communication
cables are affected.
Each Article adopted a definition of abandoned
cables and the rule for removal. The general
consensus is that abandoned cable is cable that is
not terminated at equipment or connectors and is
not identified for future use with a tag. Please refer
to each individual NEC
®
Article for specifics.
Having to tag, remove, or rearrange cables within
an enclosed raceway can be a time consuming and
difficult job. Without being able to clearly see the
cables and follow their exact routing throughout a
facility, identifying abandoned cables would be very
difficult and expensive.
With the open accessibility of cable tray, these
changes can be implemented with ease. Abandoned
cables can be identified, marked, rearranged, or
removed with little or no difficulty.
8
Cooper B-Line, Inc Cable Tray Manual