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A cable tray system, according to the US National Electrical Code, is "a unit or assembly of units or sections and associated fittings forming a rigid structural system used to securely fasten or support cables and raceways." Cable trays are used to hold up and distribute cables, as an alternative to open wiring or electrical conduit systems. Cable trays are especially useful where changes to a wiring system are anticipated, since new cables can be installed by laying them in the tray, instead of pulling them through a pipe. Cable trays are commonly used for cable managment in commercial and industrial construction.

Types

Several types of tray are used in different applications. A solid-bottom tray provides the maximum protection to cables, but requires cutting the tray or using fitings to enter or exit cables. A deep, solid enclosure for cables is called a cable channel or cable trough.

A ventilated tray has openings in the bottom of the tray, allowing some air circulation around the cables, water drainage, and allows dome dust to fall through the tray. Small cables may exit the tray through the ventilation openings, which may be slots or holes punched in the bottom. Ladder-type tray has the cables supported by a traverse bar, like the rungs of a ladder, at regular intervals on the order of 4 to 6 inches (100 to 150 mm).

Ladder and ventilated trays may have solid covers to protect cables from falling objects, dust, and water. Tray covers for use outdoors or very dusty locations may have a peaked shape to shed snow, ice or dust.

Where a great number of small cables are used, such as for telephone or computer network cables, lighter cable trays are appropriate. These may be made of wire mesh, or may take the form of a single central spine (rail) with ribs to support cable on either side, a little like a fish spine and ribs.

Large power cables laid in tray may require support blocks to maintain spacing between conductors to prevent overheating of wires. Smaller cables may be laid unsecured in horiztonal trays, or can be secured with cable ties to the bottom of vertically-mounted trays.

To maintain support of cables at changes of elevation or direction of a tray, a large number of specialized cable tray fittings are made compatible with each style (and manufacturer) of tray. Horizontal elbows change direction of a tray in the same plane as the bottom of the tray and are made in 30, 45 and 90 degree forms; inside and outside elbows are for changes perpendicular to the tray bottom. Tees, crosses, and other shapes exist. Some manufacturers and types provide adjustable elbows, useful for field-fitting a tray around obstacles or around irregular shapes.

Various clamping, supporting and splicing accessories are used with cable tray to provide a complete functioanl tray system. For example, different sizes of cable tray used within one run can be connected with reducers.

Materials used

Common cable trays are made of galvanized steel, stainless steel, aluminum, or glass-fiber re-inforced plastic. The material for a given application is chosen based on the corrosion resistance required for the location.

Fire safety concerns and solutions

Combustible cable jackets may catch on fire and cable fires can thus spread along a cable tray within a structure. This is easily prevented through the use of fire-retardant cable jackets or intumescent or endothermic fireproofing or fire retardants.

Proper housekeeping is important. Cable trays are often installed in hard to reach places. Combustible dust and clutter may accumulate if the trays are not routinely checked and kept clean.

Plastic and fibreglass reinforced plastic are combustible and their effect is easily mitigated through the use of fire retardants or fireproofing.

Ferrous cable trays expand with the increasing heat from accidental fire. This has been proven by the German Otto-Graf-Institut's Test Report III.1-80999/Tei/tei "Supplementary Test On The Topic Of Mechanical Force Acting On Cable Penetration Firestop Systems During The Fire Test", dated 23 October 1984, to dislodge "soft" firestops, such as those made of fibrous insulations with rubber coatings. The same thing would apply to any silicone foam seals. This is easily remedied through the use of firestop mortars, as shown above, of sufficient compression strength and thickness. Also, some building codes mandate that penetrants, such as cable trays are run in such a manner as to avoid their contribution to the collapse of a firewall (construction), or an occupancy separation.

Fireproofing

Fireproofing and firestopping systems are passive fire protection measures.

The cable trays themselves are only of concern if they are made of plastic. Typically, what is of most concern is the items placed upon the trays. In some cases, the primary concern is to mitigate the fuel load that the cable jacketing represents. As cable jackets burn, smoke is produced, containing chlorine as one of the combustion by-products. Chlorine bonds with airborne moisture, yielding hydrochloric acid, which contributes to corrosion damage after the fire.

Mitigation methods include the use of fire retardants, as well as concealment, panelling and fire-resistant wraps. Cables certified as low smoke-producing or resistant to the spread of fire produce limited amounts of smoke and combustion. The other concern is circuit integrity. Particularly, though not exclusively in nuclear reactors, it is necessary to maintain the operability of cables during a fire, so that critical equipment, such as reactor cores, safety valves, etc. can be shut down, to prevent a catastrophe. Mineral insulated cables are inherently fire-resistant.

Boxes or wraps constructed of fire resistant materials may also be used to isolate heat from the cables. Boxes and wraps must be accounted for in four respects:

• The added weight must be included in support calculations, hanger sizing and spacing, as well as seismic calculations.
• Boxes and wraps inhibit the ability of power cables to disperse operational heat. The hotter the cable, the less energy can be conducted. The difference between the unwrapped and the wrapped (or boxed) conductivity is quantified as a percentage, which is referred to as Ampacity Derating. If, for instance, a certain fireproofing system causes the ampacity to be lowered by 40%, then 40% more cables are needed to conduct the same amount of electricity. Ampacity derating is remedied through the use of purpose-designed "windows" that allow air flow during normal operations, but shut if exposed to the heat from accidental fires.
• Hanger systems may also need to be fireproofed.
• Firestops around penetrating cable trays that have been fireproofed must have been included at the time of test so that compatibility between firestopping and fireproofing can be documented. Chemical compatibility between the two systems should be checked to avoid longterm operational degradation.

Images

• Cable trays can be used in stores and dwellings
• Firestopped cable tray penetration. The cables and the tray are penetrants.
• Cable tray cross barrier firestop test, full scale wall, in Germany as per DIN4102.
• Fire test in Sweden, showing rapid fire spread through burning of cable jackets from one cable tray to another.
• Cable tray with inoperable firestop in Canadian pulp and paper mill. Cable tray with inoperable firestop in Canadian pulp and paper mill.
• Cable tray penetration in Lingen/Ems, Germany. The tray stops short of the fire barrier to prevent damage from heat expansion of the tray during an accidental fire.
• Circuit integrity fireproofing of cable trays in Lingen/Ems, Germany using calcium silicate board system qualified to DIN 4102. Contractor: Signum, Bissendorf, Germany.
• Cable trays in cable spreading room of a fossil fuel power plant, Point Tupper, Nova Scotia, Canada. This picture shows how combustibles, such as dust, cardboard, lumber and debris can build up on cable trays. Good housekeeping even in hard-to-reach places mitigates fire hazards.
• Cable spreading room at Point Tupper.

See also

• Cable
• International Brotherhood of Electrical Workers
• Passive fire protection
• Circuit integrity
• Firestop
• Fire test
• Fireproofing
• Intumescent
• Endothermic
• Conduit (electrical)

References

1. http://www.tnb.com/contractor/docs/cabletray_us_revised_lr.pdf One manufacturer's cable tray catalog, retrieved 2010 Aug 11
2. NFPA 70 National Electrical Code 2008 Edition / Chapter 7 Special Conditions / ARTICLE 725 Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits / IV. Listing Requirements, 725.179

• Electrical components