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

modification, wire wrap techniques as described in SQc[ ion 9.3.2 of this chapter are used.

The marshalling centre itself consists of a termina- tion cubicle or frame which is provided with jumpering facilities. The function of the marshalling facility is to

form into a circuit a number of loops that are brought in by pairs within the network. The circuits are formed

bs means of jumpers installed between the ends of

the multipair cables. Jumpers are terminated using tab Connectors, as described in Section 9.2 of this chapter,

\Ouch provide the facility for breaking a circuit into ik fundamental pairs for testing. Each jumper has

numbered ferrules applied at each end to enable it to be identified.

By way of an example of the use of this system, the signal from an oil pressure switch could be connected into a field marshalling box on terminals 4A and 4B. The signal would then appear on terminals 4A and 4B of the same module in the marshalling centre, which could then be jumpered to the appropriate terminals of the module routed to the alarm cubicle to initiate a 'lubricating oil pressure low' alarm. En a similar manner, jumper connections can he used to complete the circuit between the alarm cubicle and the alarm facia on the desk. This is shown in the form of a loop diagram in Fig 6.48.

Connections into field marshalling boxes from plantmounted devices use screw/clamp terminations and

.7•378 R

6

i

NOTES

KEY TO COLOUR CODE FOR PAIRED CABLE •

P4 • WHITE BIWE

0 • ORANGE

G • GREEN

RN BROWN GY • GREY O - SED

BK SLACK

Y - YELLOW

Arrangements tet. ICI AND .31 SILL HAVE THE SAME COLOUR PAR CODING AS SHOWN IN

Artangement tal

—7 REPRESENTS A PAIR

ALL MODULES TO HAVE WRAPPED CONNECTIONS ON OUTGOING SIDE.

ferrule numbers are applied to cable cores for identification. Indeed this policy is used for all terminations having less than 20-pairs to enable plant changes or disconnection for test purposes.

This type of cable network system was developed to try and overcome some of the programme and technical difficulties, discussed in Section 5.4 of this chapter, that had been encountered with marshalling schemes that did not include jumpering.

Let us consider how a network which includes jumpering can improve programme performance. It is expected that the backbone of the network system which includes field and pair network marshalling boxes, marshalling centres and associated trunk cables will be designed early. This is carried out before detailed circuit knowledge is available by using experience of previous station requirements and taking into account

any particular control function requirements. Control and instrumentation contractors are encouraged to forward at an early date their requirements for the number of outgoing pairs for their major panels, cubicles and desks to enable trunk cables to these items to be scheduled. When circuit diagrams are obtained from manufacturers for sequence, control and instrumentation, interlock and intertrip equipment, etc., the system loop diagrams can be prepared and the cable system completed. At this stage it is only required that the tail end cables from the field network boxes to the plant devices and the necessary jumpers be scheduled. It should therefore be appreciated that the design work has been spread over a greater period by being effectively started much earlier. Furthermore, the process is better able to cope with late receipt of information as well as design changes. Providing steps are taken

io ensure timely civil access for trunk cables and marhalling centres, the site workload can also be started earlier and spread to give a more controlled, levelled and (hence) economical installation task. Because the large majority of terminations are completed in a repetitive colour code order they are less prone to error by the installer, thus reducing the problems that can occur at the commissioning stage. This repetitive type of termination encourages the widespread use of wire map terminations which are fast, reliable and more economic than the use of ferruled wires in screw clamp terminals.

From a technical point of view, this type of network ss stem encourages paired working and hence improved ,iznal-to-noise ratio, as discussed in Section 5.3 of

his chapter. Individual networks can be provided to meet segregation and separation requirements and this, together with computerised cable design methods, can i e the necessary quality assurance.

5.5.2 Switchgear and interlocking equipment

No mention has so far been made of the involvement ot switchgear and hence the use of multicore cables in he network system.

Switchgear at all voltage levels provides a large number of the inputs into a control and instrumen-

[ation scheme in the form of operations, indications and alarms. In the past, equipment for interlocking

and sequence control, and interposing relays for remote control have all been built into individual switchgear

units, a unique specification being required for each panel. This obviously necessitated early finalisation of

Control and instrumentation design in order that these

devices could be built into switchgear in the factory.

This proved to be too inflexible from a programme Point of view. Late information from control and

instrumentation contractors, who were naturally dependent on mechanical plant designs, conflicted with the necessity to clear switchgear designs in time for delivery to site to meet commissioning dates.

A ready solution to this problem is the use of standard switchgear control circuitry, so that no special equipment has to be housed in the switchgear enclosure other than the interposing relays. Having removed 'individual' control and instrumentation components from switchgear, it has to be located somewhere else and for this purpose an 'interlock cubicle' is provided.

The cost, complexity and programme requirements discourages the purchase of a completely designed and factory-built 'interlock cubicle'. The solution is therefore to purchase a cubicle containing nothing initially other than terminal blocks, wiring and plug-in bases. This enables relays, timers, etc., to be plugged in as requirements become known.

When designing the cabling from switchgear, certain circuits such as electrical protection (including VT and CT secondaries) are excluded from the cable network system. CT secondary circuits are excluded from the network because special measures, in the way of terminals with shorting links, are required to prevent the CT being left open-circuited when energised with resultant high potentials which may be a danger to personnel and equipment. However, circuits from interposing CTs may be routed via the cable network. These circuits therefore have their terminals grouped into discrete blocks to enable them to be cabled direct to their destinations using multicore cables.

All remaining switchgear functions are wired to terminals in groups to match standard multicore cable sizes, i.e., 12, 19, 27 and 37 cores. From these terminal blocks, cables are run to identical terminal blocks in the marshalling cubicle and in this manner all the auxiliary functions of the switchgear are extended to

the jumper field. By means of jumper connections, functions can be extended to other equipment. Where extension is not required for a particular function, but circuit continuity is required (i.e., plant interlock in closing coil circuit), then a jumper is used as a shorting link.

The 'module' approach, described in Section 5.5.1 of this chapter for paired cables, is also applied to muiticore cables. This means that core I is always connected to terminal 1, core 2 to terminal 2, etc., and this can form a standard instruction to the electrician. This technique can apply both to the switchgear and the marshalling centre, hence no wiring diagrams or connection schedules are necessary. All that is required is for the group of terminals to be identified by a module number and that this is then related to a cable number.

Interlock cubicles are cabled direct to the marshalling centre, using multicore or rnultipair cables as appropriate, also employing 'module' techniques.

5.5.3 Design of cable network systems

Section 5.5.1 of this chapter used a simplified model to demonstrate the principles of a cable network system. In practice, the structure of the network will be dependent upon the segregation and separation requirements for a given project. These requirements are discussed in Section 2 of this chapter. This means that a separate segregated network will always be required for each unit.

Segregation requirements will allow station services to be mixed with unit services provided that, in the event of any incident, not more than one unit and half of the station's services are affected. For example, one network could be provided for unit I and station A functions with a separate segregated network for unit 2 and station B functions. Whilst segregation is not required between station and unit services it is recommended that consideration be given to separation between marshalling facilities to reduce their size. This is because experience has shown that large marshalling centres can form an 'installation bottleneck' because access space limits the number of electricians that can work in a given area. For the same reason it is recommended that, on a conventional station, at least three marshalling centres are provided per unit. The location of marshalling centres is discussed later. However, a typical arrangement would be to provide such facilities at the control room, boiler electrical and turbine electrical annexes. A typical unit arrangement for a conventional power station is shown in Fig 6.49.

For nuclear projects the unit electrical system may be provided on a 'train basis' (i.e., a separate electrical system associated with each reactor quadrant) and in this case it is necessary to have separate segregated control cable networks for each train. In addition each 'train' may comprise a number of 'sub-trains' (usually two), each requiring separation between cables

and marshalling centres. In this respect a sub-train i s the plant, equipment and cables associated with the essential cooling system. A typical arrangement for an advanced gas-cooled reactor (AGR) is given i n Fig 6.50.

Regardless of the type of station, separate cables are used for analogue and digital signals although common marshalling centres may be used. A diagra m of a marshalling centre for an

jumpering is given in Fig 6.51.

The location of marshalling centres is important. The initial reaction had been to set up a marshalling centre only adjacent to the control room, an evolution of the earlier idea of using small trunk cables as shown in Fig 6.45. However, as the control cable network has developed to accommodate all C and I circuitry, the physical location of marshalling facilities has had ro be closely re-examined. The actual position will der. j on the specific station layout and therefore no h .rd and fast rules can be applied, but when considering this subject the following points should be taken into account:

•In order to reduce the length of multicore cables between switchgear and the marshalling centre, which are relatively expensive as compared with multipair cables, a marshalling centre should be located as close as practicable to the switchgear.

•For the same reasons as given above, the interlock cubicles should be positioned near to a marsha: centre.

•A marshalling centre should be provided in control block. This should he located as do possible to the underside of the desk to allo flexible cables from individual desk modules terminated directly into the marshalling centre

•Where switchboards are located in outbuildings a s mall marshalling centre should be provided, i.e., wall-mounted jumpering boxes.

5.5.4 Application of cable network systems

As soon as the locations of the major items of plant (mechanical, electrical, and control and instrumentation) are known, the trunk cabling of the cable network system should be laid out. This requires that the marshalling centres are located and that an estimate is made of the number of analogue and digital pairs that are required between major plant areas. This is achieved by using experience of previous station requirements and a knowledge of any particular control and data requirements for the station.

When estimates for the trunk routes have been prepared, the number of pairs should be increased by 20% to allow spare pairs for modifications and additions during construction and to achieve 10 0/o spare pairs for future modifications and extensions. It Is considered preferable to provide adequate spare ca-

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systems cable instrumentation and Control