M.J. Dyer,
DLC, AMIEE, JP(Building Services Consultant)
INTRODUCTION
The decision as to which cable management system to adopt for any particular installation needs to be a practical one. With the rapidly developing range of innovative materials available to manufacturers, this aspect of electrical instal- lation design has become increasingly complex in recent years. Whereas the tradi- tional methods of cable management remain (mostly) still available, and appropriate, to the designer many new systems exist and must be taken into con- sideration in the design process. The cables to be accommodated may comprise low voltage (l.v.) cables supplying building services and points of utilisation within the building itself, or passing through it, communications cabling (including telephones, data, fibre-optic networks, etc.), and security wiring (including fire alarms, emergency lighting and occupancy detection wiring at both mains and extra low voltage (ELV) levels). Thus consideration has to be given at an early stage to the segregation of services by the type of service provided, and a philoso- phy of arranging appropriately grouped cables will evolve. This leads to the initial derating of power cables where they will need to be grouped and possibly en- closed, for the purpose of the regulations. The routes along which each of the cable types will run must be established in principle and a layout of the distribution services thus built up. Input at this stage from the architectural drawings will allow both a space allocation and a system type decision to begin to emerge. The latter can be specified for the main cable runs, based on the number and physical size of the cables for each ‘leg’ of such a network. Where the runs for the various types of electrical services are similar, a single cable system with segregation for the various categories is indicated, whereas if one ‘class’ of services (e.g. communications or security) take widely differing routes from that of the power cables, then com- pletely separate cable management systems for the various categories may be a more efficient solution.
Having come to a preliminary conclusion as to the main distribution of cabling, the designer must turn his or her attention to the final distribution cables at the points of utilisation, whether that be at work stations in an office environment or at machines or process plants in a factory (or indeed at any other position where electrical power or lighting is required, or data is to be processed or accessed). In many new-build environments the final cable distribution will be highly visible and
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if care is not taken it may become intrusive; thus a more aesthetically influenced decision is called for here. Distribution cabling is an engineering service only, whereas final circuitry has become an environmental component and must be so considered. The range of decorative products available for these tasks indicates the importance, and thus price, placed on such works by the architectural teams and clients. In the following sections there is no attempt to differentiate between ‘main’
and ‘final’ distribution of cables, but the reader is cautioned to define these terms in his or her own mind and then add them mentally to each variation of the cable management system which is available.
The use of the building for which the design is being prepared is also an important factor to consider. Most large projects fall into the categories of indus- trial, commercial, or retail/hotel/conference buildings, and this will tend to dictate the degree of mechanical protection required as well as the appearance criteria and the requirements of accessibility, later, to the cabling. There may also be require- ments to specifically prevent access, or at least unauthorised access, in some environments where security is paramount. Whichever category the building or installation may be eventually placed in, the importance of the data and security cabling cannot be overemphasised. In the current environment these services are no longer secondary to the power cabling, but the sheer volume of them makes design consideration critical. They must not only be physically accommodated with considerable provision for future growth, but the requirements of electromagnetic compatibility (EMC) for such cables must be studied prior to allocating installation space, and methods, for them. The method of construction of the building will also influence the decision as to which method of cable management is most appropri- ate; the creation of ‘fire zones’ with fire resisting floors and walls and the provision (or otherwise) of risers, suspended floors and/or ceilings, etc., all affect the designer’s choice.
A client may have strong ideas or even requirements relating to the relative pri- ority to be attached to these aspects of the system design, in which case matters are straightforward to deal with. However, in speculative construction work or in build- ings where the usage may well change several times within the expected life and probably within the expected ownership of the building, things are less clear. This has led to much construction work being undertaken on a ‘core and fit-out’ basis to separate the changeable requirements from the fixed core of the building, but this option is not available in traditional construction contractual arrangements. It is then necessary to make a fundamental decision regarding the priorities, before design selection can take place; for example, the physical protection aspect may be vital, and in a factory the physical robustness of the cable enclosures, or the placing of such enclosures out of the range of mechanical impact (from, say, forklift trucks in a warehouse), may take precedence. In a commercial building with considerable multipurpose office accommodation, the availability of all the various electrical services at each and every work station will probably be the overriding requirement, and so a system capable of this multicompartment operation will be required. In every case the possibility of adding cables to an installation after it is theoretically completed must be considered.
DECISION MAKING
The increased quantity of cables involved in buildings has led to a relatively greater importance in the design and selection of cable management systems. One of the interesting results is that whereas there used to be a considerable national prefer- ence in such systems, there is now a much more uniform international approach to the question of installing cables in a well managed way. Cables form a very large physical part of an electrical installation but are not required to be visible or acces- sible except for maintenance and alteration purposes. With the increase in IT work and hence data and telephony cabling in offices and factories, the volume of cables has already increased considerably and can be expected to continue to do so. Thus the need to ‘manage’ them has become proportionately more important.
A number of parameters require the attention of the designer when addressing cable management. It is to be assumed that the electrical requirements have already been determined, and that an electrical engineering design therefore exists. The cable types will probably have been selected and the points of utilisation will cer- tainly be known. Certain types of cable such as steel wire armoured (SWA) and mineral insulated copper covered (MICC) cables mitigate against most systems of cable management (except for tray and ladders in very large installations) as they are designed for direct fixing to structural surfaces, but all other cable types will require to be physically enclosed and ‘managed’ in some way. The building struc- ture will already exist, or in the case of new development work will at least have been designed, and this will affect the system selected. There may or may not be specific provision made by the architect for risers and/or cable access areas and it is not uncommon for distribution points to be determined by the provision of space rather than by the logical need for such a position. Whatever the physical arrange- ment of the building, the designer must provide an efficient (and that also means a cost efficient) method of controlling the cable installation work involved. Through- out the length of the cables they must all be secured and segregated as may be required. The method selected may be continuous throughout the whole of the installation, or may vary from place to place within the area served by the installa- tion, but in any case it must, above all, be suitable for the requirements of both the environment in which it is located and the user’s convenience.
DEFINITIONS
Cable management implies the enclosure, segregation, marshalling and mechanical protection of the cables within the building, and hence one must identify every cable and cable type which is required to implement the electrical design.
Design implies the provision of the current (as specified) requirements, also allowing for the expansion of the installation in the future. This consideration requires both space provision within the enclosures selected, and also ensuring that subsequent access to them will be possible and practical when cables have to be added for possibly innovative new services. Although rewiring is always something that has exercised the minds of those responsible for electrical installations in build- ings, it is now the extension of the installation that is more important. With improved
materials in the construction of cables offering longer and longer service life, and the increased sophistication of electrical circuit protection devices precluding cata- strophic failure of cables, rewiring is required infrequently. However, changes to requirements brought about by new technologies, increasing demand, and oppor- tunities for data exchange networks (and by differing commercial practices due to these and other factors), do mean that in many cases extra cables are going to be required. Thus all cable management arrangements must be suitable for cables to be added to the installation with relative ease, and systems should be designed with expansion being given priority in size selection. This might mean providing appar- ently oversized trunkings, etc. at the outset, or it might mean (particularly with modular enclosure systems) leaving provision for the addition of new cableways in the future. These can often be attached to the original trunkings or trays, provided that space has been left at the design stage to accommodate such growth, and pro- vided that the mechanical considerations of the original installation were adequate for any imposed new loadings. In any case the allocation of such space is often a contentious issue between the design disciplines in the team, and requires careful monitoring by the responsible engineer. Problems may occur at distribution boards and points and are likely to become critical at data distribution centres. The modular nature of contemporary designs of power distribution boards provides for relatively simple extension provision, whereas the limited space frequently granted in data distribution enclosures is a real problem as demand for electronic capacity grows, technologies requiring differing electronic assemblies at the distribution points are introduced, and hence more space is required than is available. For this reason it is now good practice to provide the data hubs inside dedicated rooms or cupboards, whereas the power boards can be located in normal circulation areas.
Enclosure implies that cables will not merely be directly fixed to the building sur- faces. Although some forms of cable installation do not involve the actual enclosure of the cables (see above), in general when the term is used at least part enclosure combined with support is implicit.
Segregation is defined by the regulations and by EMC requirements and direc- tives, and is of increasing importance in the selection of systems. More will be said about this later in this chapter.
Marshalling of cables, with particular emphasis on accessibility of the correct cable at the correct point of utilisation, is tied closely to the mechanical protection element of the system design. Both have very similar methods of attainment, but the robustness of the materials required will depend as much on the anticipated environment as upon the properties of the cables to be protected.
TYPES OF SYSTEM
The principal systems available fall into three categories. Many installations will use a combination of such methods at various points throughout the building, but the systems selected need to be compatible with each other as well as suitable for the client’s requirements. The principal categories are conduit and trunking, underfloor systems, and cable tray or basket systems. Each of these has its subdivisions, and within each of the subdivisions the individual manufacturers in the field offer an
array of solutions to each and every circumstance, but the principles are the same and are restricted to the main categories. This chapter is not concerned with detail- ing the use of cable tray and ladders for large and heavy industrial cable instal- lations, as the practice is well established and is strongly indicated wherever considerable quantities of already mechanically protected cables (MICC, SWA) require to be fixed to a surface. Such methods are cable fixing rather than cable management techniques, and as such properly belong in the trade skills area of installation technology. Likewise the direct fixing or clipping of cables to a surface can be a satisfactory and adequate method of installation, provided that the neces- sary criteria of the electrical installation and regulations are complied with, but details are not thought to be needed in a work of this nature.
UNDERFLOOR SYSTEMS
It should be understood that the generic term ‘underfloor’ applies to products designed for such use but which may well be used in other environments where the protection is appropriate. Ducts and risers, and within suspended ceilings are typical spaces where parts of so-called underfloor systems may be appropriately used.There are two main types of such systems: those designed for incorporation into the floor structure itself, such as trunking systems for casting into the screed, etc., and those designed to be installed below false (computer) floors in commercial buildings. The principles of the two systems are the same but the strength of the construction is different, as is the detailing of the designs. Systems for incorporation into the floor structure itself are customarily of metallic materials, often galvanised steel, and need to be of high mechanical strength to withstand the rigours of building site condi- tions at first fix stage. The spine of such systems is usually flat trunking with a top flange intended to eventually become flush with the floor screed. To this, thin but robust covers are fitted. A family of flush outlet boxes is necessary to accommodate the various accessories required, and fittings such as tees and elbows are required to construct the cable runs on site. As the trunking now invariably needs to be multicompartment, great care is needed to arrange the compartments at junctions and crossovers, and this may be an insurmountable difficulty in some circumstances.
Utilisation points can be provided above floor level by the use of power-poles, and conduit can also be teed off the trunking prior to the screeding of the floor. While still a widely used system in new buildings, careful planning is needed at the design stage, not only with the multicompartment arrangements but also with respect to the termination of the trunking(s) at distribution boards, etc.
The more recent developments are all in the area of systems intended for use beneath the raised floor, and laid on top of the structural floor. This method of con- struction is referred to as ‘suspended flooring’ or ‘computer flooring’. Whereas this was traditionally confined to specialist technical areas such as computer rooms and control suites, it is now widely used in commercial buildings due to the flexibility it adds to the building service provisions. The downside of increased height between the structural slabs in a multistorey building is often accepted as the price the archi- tect must pay for good service provision within. The void so formed between the structural and the functional floor provides flexibility and accessibility of a high
order for developing the cable systems throughout the life of the building. Systems for this application are numerous and consist of trunking and conduit similar to con- ventional products, but with adaptations specifically appropriate for the environ- ment proposed. Plastics material is very popular but metallic systems are available if mechanical, or more likely EMC, protection is important in a specific application.
Usually a number of parallel trunkings are run side by side to provide the segre- gated circuits, and thus crossovers and junctions become straightforward to under- stand. Conduits, usually of the flexible type, can be glanded off to outlets, etc. as required. The trunking is usually fixed to the structural slab floor and access is given by removing the tiles of the suspended floor with specialist tools provided by the flooring contractor. Boxes for accommodating accessories in the floor tiles may be from the flooring specialist or the electrical manufacturer, but as they are connected to the trunking by flexible conduit, compatibility will not become a problem. Figure 4.1 shows installed underfloor trunking with flexible feeders to desk sockets.
Underfloor voids are useful spaces for the installation of cables that are passing through the area as part of the distribution system of the whole building. Cable tray or basket may be provided for these runs, and all the horizontal legs can be accom- modated in such spaces with great savings to the electrical contractor, once the floor- ing system has been agreed. Communication and data cabling in such voids is perfectly acceptable but must be treated with the same professionalism as all other
Fig. 4.1 Installed underfloor trunking. Note the segregated circuits.
cable installations. The rapid growth in the quantity of such cabling has led in some instances to the loose installation of them below computer floors in commercial buildings. This technique, where the cables are loosely bunched by means of cable ties and then laid on the structural floor, has its origins in the control rooms of the broadcasting and telecommunications industries. The adoption of the technique in the electrical installation industry came about as a result of specialist telecom con- tractors taking work in the construction industry due to their specialist knowledge of specific types of cabling. However, the system is not satisfactory here and will not meet the requirements of regulations and standards (and therefore of insurance companies), regarding a ‘suitable and workman-like’ installation. Whereas in tech- nical businesses the control and maintenance of such cables can confidently be left in the hands of engineering personnel, in the commercial world access will be made by unskilled persons and the integrity of such cables will be invalidated. The proximity of power distribution cabling then introduces a degree of risk that is not acceptable in the electrical industry.
CABLE TRAY AND CABLE BASKET
The next main type of cable management system is tray, of which basket is a rela- tively recent variation. I have referred already to cable tray as particularly appro- priate where large numbers of cables have to be run for considerable lengths, usually in both vertical and horizontal directions. It has been thought of as an industrial product but new designs have made it particularly suitable for commercial installa- tions. The metallic (often slotted) construction of heavy-duty cable tray makes it appropriate for the support of heavy cables such as SWA mains or large numbers of MICC circuits in an industrial site. Other types and even categories of cables can use the same tray for support when appropriate. Cable ladders are a variation for vertical runs between horizontal trays, and find economic uses in power stations and factories with particularly dense cable populations. The overall technique allows excellent simple mechanical support with accessibility and little of the problem from grouping factors due to bunching of cables. Secondary trays may be installed along- side the main tray to house the final distribution and/or data cables if segregation is a problem or specific EMC considerations prevail; otherwise the same tray can often house various cables.
Plastic coated tray is very effective in hostile conditions or outdoors, and is now widely available. The hot dipped galvanised tray, which was previously the standard specification item for such work, is expensive and difficult to install without preju- dicing the weatherproof qualities of the construction. It is now seldom used unless specified by a particular client.
Inside industrial buildings, if there are not a great number of heavy cables to install, some less expensive system of support is indicated, and indeed cables may often be quite appropriately clipped directly to structural surfaces for no other reason than to avoid the cost and size of a traditional tray installation.
In many applications, the use of cable basket provides a good solution to the above difficulties and at a cost-effective price. Basket is a system of cable support materials which includes all the shapes and accessories expected in a trunking