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

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_

_NI T TRANSFORM ER

GENERATOR

GENERATOR TRANSFORMER AND 1W CO NNECTIONS

Station VHF and UHF links

All cables from the microphones at the unit control deks out to the transmitters covering the various areas of the station should be in STFP cable. The aerial cable from the transmitter is co-axial and is not available in a fireproof construction it must therefore be routed in a low risk area.

Direct wire telephones

Direct wire telephone circuits are routed point-to-point using STFP cables.

Fire fighting equipment

In order that diesel pumps and other mechanical plant necessary for the continuous operation of the fire fighting equipment shall remain in service when involved in a fire, any cabling on that plant or required to start it in case of fire should be in STFP cable.

Likewise, circuits carrying essential alarms from the fire fighting equipment to the control room should be in STFP cable.

Fire detectors

Fire alarm initiating devices, e.g., 'break glass' devices, smoke detectors and heat detecting cables should be cabled in STFP cable only in the areas that they are protecting. Outside these areas, the circuits should be routed in the multipair cable network. This is on the basis that the alarm circuits are all self-monitoring.

Nuclear alarms

All detector and audible circuits associated with nuclear alarms should be cabled using STFP cables only.

6 Cable support systems

6.1 Introduction

In modern power stations, large quantities of steel framework type structures are used to support the cables on their paths, both horizontally and vertically, across the power station. In the horizontal case, these cables may be routed in purpose built cable tunnels, in cable 'flats' or in general plant areas. In the vertical case, the cables may be running in purpose built 'risers' (sometimes referred to as cable 'flumes'), or si mply through slots in floors/ceilings, again in plant areas. Virtually all of the cables used in a power station, both power and control, will be installed on these steel support structures for at least part of their length. In addition, the following installation methods are also used for some cables:

•Buried direct in the ground.

•Installed in ducts.

• Routed in concrete troughs.

These three methods, by their very nature, tend to be used outside the main station buildings; they are dealt with in more detail in Section 7.5 of this chapter. Additionally, a very small number of cables will be installed either in proprietary electrical trunking or conduit. This applies chiefly to lighting and small po wer circuits such as socket outlet ring mains. Cables for such circuits will be installed in accordance with the edition of the IEE Wiring Regulations [18] valid at the time of installation. The result will therefore be si milar to any other industrial electrical installation. Further consideration to such circuits is not, therefore, given here.

6.2 Design philosophy

A generic design of cable support steelwork has been employed by the CEGB in all of its most recent power stations. The main considerations in arriving at this design were as follows:

•The diversity of locations across a power station where cables, and hence their support steelwork, are installed. At one extreme, these may be purpose built cabling areas such as cable flats, at the other they may be plant areas such as the boiler house or turbine hall.

•The huge differences in quantities of cables which need to be installed in these differing physical locations. Main cable routes like cable tunnels may carry as many as 1500 cables, whereas 'tail end' routes may simply consist of two cables (one power and one control) being routed to, for example, a li mit switch.

•The considerable variation in cable sizes and weights

which need to be accommodated. At one extreme, a 4-core 300 mm 2 power cable has an overall diameter of around 70 mm and weighs 7 kg/m. At the other, a 2-pair multipair control cable has an overall diameter of 12 mm and weighs only 0.3 kg/m.

•The need for the support steelwork to accommodate the installation requirements of different cable types. Power cables, for example, require a free flow of air for cooling purposes and single-core power cables need additionally to be firmly anchored to restrain the large bursting forces generated under fault conditions. (These requirements are addressed later in this section.)

Clearly it was considered an uneconomic proposition, both in terms of design and manufacture, to use cable support assemblies unique to each different application and therefore the idea of using a large 'construction kit' evolved as the means of providing the most flexible solution.

Fin. 6,54 Cross-section through a Cl channel

•A C2 channel, formed from two Cl channels placed back-to -back and spot welded at 150 mm intervals

(see Fig 6.55).

•A C3 channel, which has a depth of only 21 mm, i.e., half that of the basic Cl channel (see Fig 6.56).

•A C4 channel, which is similar to a Cl channel except

that tangs are pressed out from the rear face of the channel at 300 mm intervals (see Fig 6.57). This type of channel is called a 'concrete insert channel' and is used to provide a convenient means of fixing cable support steelwork to the power station civil structure. The channel is temporarily fastened to

PLASTIC FOAM

NOTE:

THE MAIN CHANNEL SECTION

UTILISES THE Cl CHANNEL SECTION

THE INSERT TANGS SHOWN ON THIS

DRAWING ARE TYPICAL ONLY

FIG. 6.57 Isometric view of a C4 channel

the inside surface of the shuttering prior to concrete pour, special polystyrene inserts being used to prevent ingress of concrete into the channel itself. The result is an open channel fixing point, flush with

the concrete wall or ceiling. The tangs in the rear of the channel will be embedded firmly in the concrete. This type of concrete insert channel provides a very cheap method of providing fixings to the civil structure but it does have its drawbacks. These are discussed in greater detail later in this chapter.

These basic channel sections are used in conjunction with a range of standard bracket formations and fixings to form a support framework on which the cable carriers are located. The brackets and channels are connected together using special fixings known as sprung channel nuts (sometimes referred to as `zebedees'), and standard hexagonal head set screws (usually M12) of grade 8.8 steel (as defined in BS3692 [20]). The design of the sprung channel nut is illustrated in Fig 6.58 and the basic composite assembly is shown in Fig 6.59.

Fio. 6.58 Isometric view of sprung channel nut

BRACKET

CHANNEL

FIG. 6.59 Channel nut/screw assembly in a Cl channel

The main advantage which this type of connection has, is that the support frameworks can be erected at site without the need for drilling, giving major savings in assembly time and manpower requirements. The channel nuts are simply inserted sideways into the channel between the lips on the open face, depressed, and twisted into place. The spring holds the channel nut in place whilst the assembly is being effected, hence freeing the operative's hands for other assembly jobs. This means that a reliable connection can be made very quickly and that a great degree of

installation flexibility is obtained, since a conne can be made at any point on a channel.

These reductions in assembly time and the m an.. power needed, produce considerable consequential installation cost savings.

It is worth noting that a version of the basic channel nut is also available without the spring. It is necessary to use this type of nut in conjunction with the C3 channel as the limited depth prevents the use of the spring.

Because no drilling is required to form this type of connection, it follows that for loads parallel to th e major axis of the support channel, the basic load. carryingcapacity of the connection is being derived from a friction grip. To ensure adequate performance of this frictional grip, the top surface of the chann el nuts have specially serrated grooves milled into These grooves, which can be seen in Fig 6.58, are 0. matically aligned with the inturned lips on the si.:pport channel and consequently, when the fixing screw is tightened, these bite into the underside of the channel lip. This creates a very high-strength friction grip. The actual load-carrying capacity of the connection parallel to the channel major axis will therefore be directly dependent upon the tightening torque applied to the fixing screws. For grade 8.8 M12 fixings, the CEGB specifies that a torque of 65 Nm be applied and this gives a slip resistance capability of typically 2700 kgf. For cable steelwork design purposes, however, the maximum permissible static load on a fixing is taken to be 1200 kgf allowing for a factor of ,,afety.

One design of channel section which is widek ailable in the UK, actually takes this type of con. :ion further by having a sawtooth formation on the Jerside of the lips. This mates with a similar for .:tion on the top surface of the channel. In this type of connection, a high tightening torque is not so critical in obtaining the required load-bearing performance, as the interlocking teeth provide the resistance to movement. This design of steelwork is, of course, more expensive than the plain type. Since the required level of performance can be obtained without the need for a channel lip sawtooth formation, the CEGB does not feel it necessary to require this feature in its specification document.

The standard types of brackets which are used in the construction of the support frames have been chosen to be simple and hence cheap. The bulk of the bracket designs are simply bent from 6 mm thick plate with holes punched in them to accept the fixing screws. Typical examples of these brackets are illustrated in Fig 6.60. They are all manufacturers? standard catalogue items. Welding will only be employed in the construction of the bracket if it is:

•Essential to form the required shape of bracket.

•If it is necessary to add strength to the bracket, for example the addition of a gusset to add strength, as illustrated in Fig 6.61.

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