Steel wire cable trays

In the highly competitive electrical contracting industry there is a constant push to reduce the cost of materials and the time taken to install electrical systems. This has led contractors and consulting engineers to identify installation methods and equipment that replace old commodity-type products while trying not to compromise the quality or performance of the design.

Over the past ten years in the UK, steel wire cable tray, or cable basket as it is sometimes known, has gradually made the transition from a niche data cabling containment method to an all-purpose power, data, bms, security and even modular wiring support system. As traditional sheet steel containment products have become generally more lightweight with the use of lighter gauge steel thickness, heavy duty wire trays have become comparable in load performance giving good strength to weight ratios. This load performance combined with installed cost savings has driven demand for the product in a traditionally slow moving and conservative market. This demand is some years behind continental Europe where the product market is more developed. Wire mesh cable trays have up to 40% of the total containment market in some countries with the product always being considered for routing power cables.

Standards
Wire mesh cable trays are covered under British Standard EN 61537: 2002 which covers all tray and ladder support systems. This standard sets out load testing and performance criteria for containment to allow for meaningful comparison between competing systems.

Load capability
Deflection tables are one of the most reliable ways of determining the performance capability of the solution being considered in the system design. The deflection performance is directly proportional to the rigidity of the system and, in the case of steel wire cable tray, its rigidity is determined by its construction.

Obviously the larger width trays need to be stronger than the smaller trays because the load requirement is greater. Table 1 shows typical weights of armoured cables and demonstrates how many cables can be supported in a heavy duty 450 mm wide steel wire cable tray. The most important factors are the thickness and positioning of the wires and the quality of the weld at each wire intersection.

The lateral wire and the longitudinal side wire diameters determine the strength of the tray and it is generally recommended that on trays over 300 mm wide, these wires should have diameters of no less than 6 mm.

The quality of the weld is another critical factor and this is determined by the node or the cross-section of the intersecting wires. This node is always dependent on the smaller diameter of the intersecting wires, so even if a tray is constructed of thick wires, the strength is limited if they intersect at the key side points with wires of a smaller diameter.

The selection of appropriate tray sizes may be done simply by using a manufacturer's data table or chart.

Installation methods
Many traditional standard specifications demanded that all changes in direction or level of the cable support system should be performed by using pre-formed factory-made fittings. This was to prevent the manufacture of fittings from straight lengths of tray on site.

The cost of factory-made fittings is such that it is very attractive for the contractor to site-produce fittings; however, sheet steel is not easy to work with and fittings made on site are notoriously inconsistent in their quality and performance, especially if the labour is unskilled.

Steel wire cable tray is designed to have all the changes in level and direction created on site. By cropping the wires at the welded intersections, the tray is easily formed into high quality and consistent fittings using just a handful of standard fixings and a set of "side action" bolt croppers. This function also allows for a wider range of fittings to be created than is usually available with traditional pre-formed sheet steel fittings. For example, a "slow" or "radius" bend can be made out of any size tray by adapting the tray to the correct bend radius of the cable. This can significantly reduce the cost of the installation by avoiding the use of oversized pre-formed trays to accommodate the cable at a 90° bend. Any combination of reducing tees or bends or any size of set or riser can be readily made in just a few minutes maintaining a consistent set of fittings.

With traditional sheet steel pre-formed fittings, the tray has to be cut and treated in order to assemble the fittings in the correct place; this requires power tooling at the "work face" and can be quite time consuming as well as resulting in wasteful tray off-cuts. With wire tray, the trays themselves are used to create the fittings. This simplifies the estimation and supply process, and the labour content of creating a fitting in-situ can actually be less than in fitting a pre-formed component.

Customisation of fittings is also easily achieved at low cost to accommodate odd angle bends or unusual lateral twists.

This has great benefits when services are shared or the product is being installed in an area that was previously not accessible. Even the best planned projects sometimes have to be altered at the point of installation due to unforeseen circumstances.

Steel wire cable trays allow air to pass easily over the cables (see figure 1). As the solid surface area is limited to less than 10% of the base it is equivalent to the cables being clipped in free air. This de-rating allows smaller cables to be used in some installations.

Environmental considerations
Storage space on site is increasingly becoming an important factor. Limited space means the contractor has to rely on the manufacturer or distributor to have the correct stock to ensure the project runs smoothly. Creating fittings on site reduces the space requirement as bulky pre-made fittings are no longer stored around the project.

The waste is also dramatically reduced as only a handful of wires are removed from a tray when it is shaped in situ. With pre-formed fittings, sections of sheet steel need to be cut and removed from site and disposed of. No power tools are used with wire trays, therefore reducing the need for a "hot license", and the clean and quiet cutting action may mean that the tray can be installed in sensitive areas such as working hospitals.

Economy
There is continuing pressure to reduce the cost of site installations leading to the increased use of prefabricated solutions or fast-fix products. Steel wire cable trays generally have fast coupling systems and are much easier to handle. They are lighter in weight than traditional sheet steel trays and trunkings, they tend to nest easily and require less labour to transport to site and fix in position.

The material cost of site-made fittings is much less than sheet steel pre-formed components. Manufacturers estimate that savings of up to 30% are possible when compared with the cost of traditional containment products.