Cable tray
From Wikipedia, the free encyclopedia
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.
Contents |
[edit] Types
- Ladder
- Perforated
- Solid Bottom
- Trough
- Channel
- Wire Mesh
- Single Rail
[edit] Materials used
- Steel
- Aluminium
- Plastic
- Fibreglass reinforced plastic
The choice of materials is a matter of the physical and mechanical properties produced by each, compared against the intended function, as well as the environment, in which the trays are to be installed.
[edit] 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.
[edit] 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 [1] 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.
[edit] References
- ^ 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
