Electrical Apparatus magazine, February 2006 Understanding Cable Tray Usage

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.

Industry news...

Featured Technical Article
February 2006

On this page is a summary of a featured technical article, "Understanding Cable Tray Usage" by Richard L. Nailen, P.E., which was published in two parts, in the January and February 2006 issues of Electrical Apparatus.  Verständnis der Verwendung von Kabelkanälen ...Comprendre l’utilisation des chemins de câble... Respecto al Uso de las Bandejas de Cables  

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