|
Understanding Cable Tray Usage With
trilingual summary
Conduit protects
the electrical conductors within, as well as any persons who might
otherwise come in contact with them. However, conduit systems have
many limitations. Chief among them is a lack of flexibility. Adding
or removing circuits; changing the destination of a cable; and
inspecting wiring condition--all are difficult. Developed more than
60 years ago, and better suited to the growing petrochemical
industry, was a wiring system more adaptable to plant and process
changes--the cable tray.
As the name
implies, "tray" is an open structure (most often in a "ladder"
configuration), suspended from overhead supports or wall brackets,
into which groups of signal, control, or power cables are laid, and
can exit or enter at any desired location along the length of the
tray. Other tray configurations include one-piece, channel, and
lattice or wire basket types. For installation either indoors or
outdoors, most trays are of galvanized steel or aluminum, although
fiberglass is available for corrosive environments.
Cable tray
construction and installation are governed by Standards VE 1, VE 2,
and (for fiberglass) FG 1 of the National Electrical Manufacturers
Association, and by Article 392 of the National Electrical Code. The
Code rules dictate placement of cables within trays to minimize
exposure of lower voltage circuitry to faults in higher voltage
conductors. That requires either fixed barriers or (for some
voltages) metal-clad cable. Together with ANSI/NEMA WC 51/ICEA
P-54-440, they also stipulate allowable ampacity for various cable
configurations within a tray.
Because tray
cables are subject to both external fire and fault-generated
ignition, fire detection and extinguishing systems are often
recommended for cable trays. Special sealing methods and materials
are used to prevent spread of a tray fire through wall openings.
Cable insulations have been developed that produce fewer toxic
combustion products.
Other concerns
with cable tray systems include grounding (when sections are properly
bonded together, trays themselves can serve as equipment grounding
conductors within Code limits) and electromagnetic interference
between circuits, especially signal or variable frequency drive
conductors (which may require shielding).
Periodic
maintenance of cable trays should include checks for loose joints
between sections; corrosion; weakened supports; and deterioration of
cable insulation.
From “Understanding
Cable Tray Usage," to
be published in Electrical Apparatus
February 2006
. Visit our online webstore to
order a
copy. © 2006
Barks Publications, Inc. All Rights Reserved.
|
