reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

Ihe CEG13's standard component range also includes a number of brackets and fixings which are used for xtaching the steel frameworks to the civil structure.

dn this context, the civil structure will include not only load-bearing walls, floors and ceilings but also

, tructural steelwork such as columns and rolled steel mists (RSJs).)

The basic type of civil structure fixing method, the C4 concrete insert channel, has already been introduced and described. Whilst this is a very simple and Lnexpensive method of obtaining a fixing point from a structural wall or ceiling, its use relies upon quite detailed cable installation design information being available at an early stage in the power station construction

1/411 2 CONCRETE ANCHOR

CHANNEL BRACKET

FR,. 6.62 Surface-mounted Cl channel using a single fixing anchor per fixing point

The surface mounting of Cl channels is more expensive than using concrete inserts for the following reasons:

•It requires additional components, most notably the concrete anchors themselves, which are quite expensive.

FIG. 6.63 Surface-mounted Cl channel using two anchors per fixing point

Care should always be taken when drilling structural concrete walls or ceilings to ensure that reinforcing bars are not damaged.

In addition to using lengths of channel to provide fixing points on the civil structure, the CEGB's stand-

ard also includes a number of floor/ceiling baseplate brackets. The two basic designs of these baseplates are shown in Figs 6.64 and 6.65 which are intended for use with Cl and C2 channels respectively. These baseplates are fixed to the floor or ceiling using proprietary concrete anchors and the upstands provide the take-offs for the channels. Again, they are designed to be very simple and hence inexpensive, with environmental protection being provided by hot-dip galvanising. For the more elaborate cable steelwork assemblies, which are described in the next subsection, special designs of floorplate are provided in the standard range of components. These have a wide variety of upstand configurations which combine installational flexibility with stable load bearing capacity. They remain, however, more specialised forms of the basic brackets shown in Figs 6.64 and 6.65.

1.1(,. 6.64 Basepiate !or a CI channel

FIG. 6.65 Baseplate for a C2 channel

Many physical situations on a power station dictate the need for cables to be routed to plant which is remote from concrete walls or ceilings. In these cases, the structural steelwork is often used to provide fixing points for the cable steelwork. A specially designed beam clamp, known as a BI5 clamp (see Fig 6.66), is used for this application which provides a convenient method of fixing from the flange of a RSJ. The beam clamps are used in pairs in conjunction with

FIG. 6.66 A B15 heavy duty beam clamp

lengths of CI or C2 channel. The completed

,h o rt

i.semblv, shown in Fig 6.67, has a load-bearing ca- n,ibilitv of 1200 kg which makes it suitable for the or complete cable steelwork assemblies. it

UK, 6.67 Application of a B15 beam clamp

Two other standard components are also used to provide fixings from structural steelwork where smaller load - carrying capacities are required or simply if a bracing point is being provided. The first of these is a duty beam clamp which again provides a fixing

from the flange of a RSJ. This type of bracket, shown Fig 6.68, is usually used to provide a bracing point

for the box section bridges described in the next sub- , e,:tion. The second is a hook bolt fixing which is

to gain a cable steelwork fixing point from

..iaLlding rails, The application of this type of fixing is shown in Fig 6.69.

Fit,. 6.68 A light duty beam clamp

6.69 Application of cladding rail hook bolts

When fixing cable steelwork to the structural steelwork, it is obviously very important to check that the structural steelwork itself is capable of supporting the intended cabling loads safely (bearing in mind that it may also be carrying other equipment loads such as pipework). It is therefore essential to ensure that co-operation and co-ordination exists between the civil and electrical design disciplines when determining cable supporting steelwork routes. The same civil structure loading considerations must also be observed when providing fixings from concrete walls and floors, although the civil design often includes an allowance for loads such as these.

The other large group of components which make up the complete system are the cable carriers.

Since the bulk of cable routes in a power station are horizontal, the most commonly used cable carriers are those which support the cables along these horizontal routes. The cables are supported on so-called 'ladder racks' which are formed by welding sections of standard C3 channel together. Essentially, the ladder rack consists of two C3 side rails joined by C3 channel 'rungs' at 300 mm intervals as illustrated in Fig 6.70.

Three standard widths of ladder rack are available, 600 mm, 450 mm and 300 mm, their use depending on the quantity of cables to be carried. This basic format of cable carrier was selected because it was seen to offer the following advantages:

•The ladders offer a flat surface on which to lay the cables. This permits them to leave and join the ladder without having to negotiate obstacles such as upturned lips.

•The open channel ladder rungs and side rails provide convenient points at which to cleat the cables.

•When compared with other cable carrier types, such as perforated trays, ladder racks offer high strength with low self-weight. A given design of support frame can therefore carry a higher cable weight.

•The ladder type format allows free access of air to the cable hence providing cooling by natural circulation. Cable ratings will therefore be higher when using this type of carrier.

•The ladder racks are completely compatible with the support steelwork and, since they are made from standard channel, can be supplied by the same manufacturer as the 'construction kit'.

A small cost is paid for the above advantages in that the ladder type carriers are slightly more expensive than their perforated tray counterparts. This is chiefly due to the welded content of the ladders which, for the majority of manufacturers, is still done manually. Attempts have been made to introduce automatic welding processes with varying degrees of success but automation will, in the long run, reduce costs.

The ladders are fabricated in standard six metre lengths, protection being provided by hot-dip galvanising after fabrication. The ladders are constructed such that alternate rungs face upwards. The upward facing rungs provide cleating points for the cables, whilst the downward facing rungs provide the facilities for the installation of linear heat detecting cable (see Section 8 of this chapter for more detail). Great care is exercised during fabrication to ensure that the tops of the rungs are level with the tops of the side rails. This is important since any mismatch could leave burrs or weld-bead protrusions which could damage cable sheaths.

Similarly, care is taken to ensure that any galvanising 'spikes' are removed by fettling immediately after the racks are removed from the galvanising baths.

The standard sizes of ladder rack which have been chosen are derived from a 'modular' basis for deter-

mining cable weight-carrying capacity. The modular concept stems from the computer-aided cable routing program which the CEGB employs in managing cable installation contracts (see Section 14 of this chapter),

A module consists of a 75 mm x 75 mm area of available ladder space (75 mm of support width x 75 mm of support depth). The GDCD Standard 197 steelwork is designed to be capable of supporting a cable loading of 7 kg/module/metre run of support. Therefore, a 600 mm wide ladder rack, the largest of the standard rack widths, when allowing for the nonusable space at the side rails, is capable of accommodating seven modules worth of cables, or, 50 kg/m rounding up. For a 450 mm wide ladder rack, the corresponding figure is 35 kg/m and for a 300 mm rack, it is 20 kg/m.

These loading figures give the maximum permissible cable loading for the given ladder size on a module by module basis. In practice, the cable cleating philosophy used by the CEGB (see next section for details) may mean that the ladder rack is physically full before the maximum permissible total cable load has been reached. Another factor which plays a big part in determining the total cable load on a rack is the total combustible mass, comprising the cable insulation and sheathing materials. This is important from a fire propagation point of view.

The maximum permissible combustible mass loading on a cable ladder is again expressed as a weight per metre run of carrier and the limiting value will vary dependent upon cable type, cable size, insulation material and installation method. When the automatic cable routing program has allocated cables to carriers it therefore checks three items:

• Combustible mass loading.

riG. 6.73 Cantilever arm arrangement for a vertical cable route

is used in dedicated cable areas such as tunnels and flats and it will also be found in plant areas, suspended from concrete ceilings, fixed to structural steelwork and anchored to concrete walls using concrete insert channels. The type of fixing arrangement used will essentially be determined by the quantity of cables to be installed.

For major cable routes such as cable flats, the cantilever arms will normally be fixed to C2 channels spanning from floor to ceiling, being fixed at both using the standard baseplate bracket shown in Fig 6.65. Such an upright may be loaded with stacked arrays of cantilever arms on both sides and the resulting assembly, shown in Fig 6.74, is known colloquially as the 'double-sided Christmas tree' assembly. The support uprights could equally be a Cl channel or a C4 concrete insert channel. In the former case, the Cl channel must be fixed firmly to a concrete wall or some other rigid bracing point. The latter type of upright is particularly useful for cable tunnels where cantilever arms can be fixed to both sides of the tunnel, leaving a central walkway for installation and access purposes. The fact that by using concrete insert channels the cantilever arms can be mounted flush to the wall saves valuable space. Cable tunnels are particularly appropriate areas for using concrete insert channels since the required positions of the upright may be accurately determined at a very early stage in the design work.

Where such stacked arrays of ladder racks are used, a standard minimum vertical spacing of 305 mm is

750r1rn

4 .

C2 CHANNEL

0

Fic. 6.74 'Double-sided Christmas tree' ladder rack assembly

maintained between ladders. This is sufficient to gain access to install the cables and also to them. The resulting gap also provides enough s, tion to assist in preventing the propagation of a fire from one tray to another.

Where cables are required to leave routes, the der racks are always installed on the 'next size up' cantilever arm. Thus, a 450 mm ladder rack would normally be installed on a 600 mm cantilever arm while a 600 mm wide ladder is placed on a 750 mm cantilever arm (there is no 750 mm wide ladder). The ladders are always placed to the outboard ends of the cantilever arms leaving a 150 mm gap which permits cables to leave the ladder at the rear in order to change direction and/or level. The alternative, which would be to allow cables to change levels at the front of the ladders, would produce an unsightly installation and would also dictate that future cables installed on the route would have to be threaded behind the cables leaving the racks, which would be inconvenient. The use of oversize cantilever arms is in fact mandatory for the 'double-sided Christmas tree' support as, without them, the horizontal clearance between the racks on either side of the upright could not be made large- enough to reduce the risk of a fire propagating horizon tally from one tray to the other.

In practice, the only areas where oversize cantilever arms are unlikely to be found are cable tunnels. Here,

Fici. 6,76 Swivel cantilever arm

not simply fixed flat against the ladder, as this would require a positive fixing to be made. Instead, specially designed 1-brackets installed at 600 mm spacing are used to carry the cables. These J-brackets (shown in Fig 6.80), are fabricated from a short length of C3 channel with two 6 mm thick plates welded to it at either end, one larger than the other with a fixing hole punched in it. The length of the C3 channel used is either 150 mm or 75 mm (corresponding to two and

one modules-worth of cables respectively). With the J-brackets in place, there is sufficient clearance simpl y to lift the cables into them in much the same way as they are placed on the cantilever arms.

The longer of the two end plates of the J - bracket is in fact 150 mm long. This means that it is possible

to stack four of these brackets and fix them to a 600 mm wide ladder rack, the top J-bracket being anchored into the ladder rail whilst the other three are fixed to the ladder rung as illustrated in Fig 6,81. If large 1-brackets are used, this gives a theoretical total cable carrying capacity for this arrangement of eight modules (or 56 kg/m), hence illustrating that edge-mounted ladder rails can provide significant cable carrying capacity without protruding great distances from the wall.

Where cables pass through plant areas such as the turbine hall basement, it is often the case that there are many obstructions present at ground level, such as pipework and equipment mounting plinths, which pose considerable problems for the cable steelwork layout designer. The problem has two facets; firstly, there is the difficulty of finding a clear route for the cables to follow without the need for excessive bends in the steelwork runs, and secondly, there are the physical problems encountered when installing cable steelwork in congested areas, often requiring complex scaffold arrangements to provide access.

U•BOLTS TO BUTT

AGANST BEAM FLANGE

5.78 Nieihod of fixing cable racking to underside of beams

Cable support systems

LENGTH TO SUIT BEAM AND LADDER RACK WIDTHS

CHANNEL TO OVERHANG BIS CLAMP BY 25r1m

Flu. 6.79 Alternative methods of fixing cable racking to underside of beams

In order to get round this problem, the CEGB has Jo eloped a number of cable support bridge and tower ,i ,emblies. The use of these bridge/tower combina- : wls addresses both of the difficulties identified above.

Iir%tly, it is possible to design clear, uninterrupted cable

-111IS free from the obstructions at ground level, the

ers themselves providing a convenient method of Jropping the cables down to the items of plant which !hey serve. Secondly, it is often possible to carry out rtain amount of 'pre-assembly' of these bridges And towers in site workshops. This reduces the time 'cquired for construction in the congested plant areas,

1 c,wng access free for other contractors.

the box section bridges are formed by bolting to-

lengths of standard ladder rack. Two basic JcNigns of bridge are used, termed light duty and heavy

both of which are capable of carrying the same 1 , erzht of cable. The distinguishing feature between heal is that the maximum allowable span for the heavy

Jut, bridge is 9 m whereas it is only 7.5 m for the light lit!, bridge.

From Fig 6.82, it can be seen that the sides of the L ht duty cable bridge are formed from ladder rack mm, 450 mm or 600 mm wide, depending upon 'Ile number of cables to be carried. These ladders are Joined at the top and bottom by steel bracing straps

FIG. 6,80 J-bracket

at 600 mm intervals, the bottom strap being staggered with the top for additional rigidity. Standard J-brackets are mounted on the side ladder racks to provide supports for the cables in the normal way. For the heavy duty cable bridge, the bracing straps are replaced by

3C0r, PACK

RACK

7SerrnORISOrn.:1 i BRACKETS

FIG. 6.81 J-bracket assembly fitted to ladder rack

300 mm ladder racks as shown by Fig 6.83. These 300 mm ladders are joined to the side ladders using right angle brackets at 1 m intervals. It is important to note that the 300 mm ladders are provided to make the structure more rigid. They are not there to provide extra cable carrying capacity.

Once assembled, these bridges are supported o n box-section towers. These towers are themselves formed by bolting togethei sections of ladder rack at site but away from the job face. The towers are in fact identical to the heavy duty bridges, 300 mm ladder rack being used for all four sides. Tower heights of up to 6 m (the standard length of ladder rack), are permissible.

The standard selection of steelwork components in eludes a range of specially designed gusset and crud. form brackets to facilitate the connection of the bridges to the towers. Some examples of these are shown in Fig 6.84, which illustrates a right angle connection of two bridges to a tower. There are also more elaborate designs of bracket which facilitate angled take-offs of bridges from their support towers.

In the main applications for these large cable bridges, the support towers will generally be found fixed to concrete floors using welded baseplate brackets and proprietary concrete anchors. It is however possible to mount the towers on support stools, which are in turn welded to structural RS.1s. This welded fixing is not a preferred solution since it removes some of the flexibility from the steelwork system. It is also considered good engineering practice to reduce on-site welding of structural steel to an absolute minimum.

In areas such as boiler houses, there are often many cases where the use of welded mountings is unavoid-