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

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In order to achieve an efficient, aesthetically-con- toured (i.e., light fittings arranged to follow the li ne of desks and panels) lighting scheme in the control room, emergency operational lighting is prol,ideci by supplying the centre AC fluorescent tubes of the special recessed fittings via dedicated inverters from a batterybacked source. A duplicate feeder arrangement is provided with 50°Ai of the emergency fluorescent tubes connected to each feeder. The lamps are interlaced and supplied from alternate emergency AC distribution centres to guard against a single fault, such as failure of an inverter or charger, rendering sections of the control room inoperative. The personnel escape lighting in the control room is provided by 'light spill' from the emergency operational lighting.

13.4 Lighting of special areas

The lighting of some areas within a power station requires special consideration, either because these areas are hostile and hazardous, or the luminaires used are unique to a particular application.

p( 'Hilts within half a second. A timer is provided he low voltage relay to cater for the re-strike time catcd with discharge lamps. The timer delays the of the DC contactor on restoration of the \( .tipply until the AC lighting has returned. Dupli-

batteries and chargers are provided to prevent a ,i0c fault, including a fire, causing the loss of all

c,cape lighting.

Centralised batteries are used for other DC systems switchgear closing and tripping) and the main- •..p..ance procedures for such batteries also apply to the :n,er.=ency lighting batteries and chargers (see Chapter

1. meNency supply equipment).

11iliough the independent DC system has always regarded as uncomplicated and economic, it does I he disadvantage of requiring DC contactors and %oltage relays at each distribution centre. Also rittini2s, which are inefficient, are required

LdLiItLOfl :o the normal AC light fittings. A future ,:lopmerit that is being considered for power station

i)( liAting systems is a hybrid of the independent s.,ierti and the self-contained luminaire system.

• !hls system, continuously-energised AC fittings with :.:L, -4ral inverters are supplied from centralised battery

•% !enis, thus combining the efficiency of AC fluores-

•er tations, a detailed study of the reliability of

integral inverters must be performed to ensure

[he rehability of the new system is equal to that he existing systems.

The luminaires used in these areas are corrosion proof to Section 8.1 of CEGB Standard 127304. To prevent the possibility of hydrogen being trapped adjacent to ceiling-mounted luminaires, luminaires in battery rooms are suspended on chains from the ceiling. The fittings are arranged so that they are above clear floor areas and not suspended directly above the battery cells. The switches and sockets are mounted outside the room.

13.4.2 Hydrogen plant (Division 1 and Division 2 areas)

The choice of lamps and luminaires for these areas is either SON lamps in directional reflectors for the main AC lighting with tungsten lamps in bulkhead luminaires for the emergency lighting, or fluorescent tubes in Division I or Division 2 luminaires for the main AC lighting and tungsten lamps in wellglass luminaires (Division 1 or Division 2) for emergency lighting.

The choice of fittings is twofold because there are two alternative methods of illuminating a Division 1 or Division 2 area:

•By locating no fittings inside the area itself, but providing the light from conventional lamps and luminaires through sealed and toughened glass windows.

•By using special fittings located at low level inside the area itself. These fittings must comply with B54683: 'Electrical apparatus for explosive atmospheres'.

In selecting a suitable lighting system for these explosive areas, the design must comply with the require-

ments of CMS lighting guide for 'Hostile and hazardous environments'.

13.4.3 Central control rooms

The lighting scheme in power station central control rooms (CCR) incorporates recessed air handling fittings using 1800 min warm white fluorescent tubes. The fittings are recessed to eliminate glare (either direct or reflected) from control surfaces including indicator glasses.

The fittings are positioned to enable the lamps to be changed and the fittings maintained while the plant is running.

Maintenance of the control room lighting is effected from below without the use of staging or elaborate access facilities. No light fitting is positioned over a control desk.

The recommended configurations for various areas are:

• Panel vertical surfaces (front) A continuous row of luminaires following the contour of the panels and positioned so that there is no specular glare reflected by the polished surfaces of the panels from the luminaire controlling the light directly onto the panel vertical surfaces.

The recessed luminaires are placed in a continuous line parallel to the main axes of the desks (normally square horseshoe-shaped). They are positioned so that there is no specular glare to affect a seated or standing operator at the desks.

A continuous row of luminaires is provided which follows the contour of the panels.

Variable illuminance by means of thyristor or switched controlled dimmers is provided to meet the varying needs of the personnel working in the control room, particularly the unit control areas. Care is taken to ensure that the minimum levels of illumination specified for the control room are always maintained.

13.4.4 Hazard warning lights

To meet the requirements of the Department of Trade and Industry and the Ministry of Defence, aircraft obstruction lighting is provided on all obstructions (e.g., chimneys and cooling towers) having a height of 153 m or more above ground level at the site of the obstruction. The warning lights are occulting lights with a special light distribution and a particular mounting arrangement. The requirements of the Department of Trade and Industry, the Ministry of Defence and the construction and mounting arrangement of the warning lights are all fully specified in GDCD Standard 34.

In a similar manner, warning lights are required to identify hazards to shipping, such as CW outfall

structures. The present requirement for shipping haz. ard lights is a white flashing light that flashes o nce every five seconds. However, the CEGB closely consults with the relevant Port and Docks Authority on th e latest requirements for shipping hazard lights.

1 3.5 Supplementary heating and minor power systems

The minor power supplies (e.g., small ventilations fans, electrically operated doors, etc., and similar loads up to 30 A) and all heating supplies, are taken from th e minor power fuseboard on each distribution centre, Also, the supply to the 110 V AC power socket fuseboard is taken from the minor power fuseboard, via a centre-tapped 415/110 V transformer.

110 V AC socket outlets are provided in plant area s to supply hand tools, etc., which are used for mai n . tenance. By earthing the centre tap of the 415/110 V transformer output winding and connecting it also to the earth terminal of the apparatus, the line to earth voltages are reduced to 55 V while the Full 110 V supply is available to power the apparatus. As most electric shocks occur between a live part of an equipment and earth, this is a major step in the reduction of the shock risk. The adoption of this system in the construction and mining industries has resulted in dra- matic fall in the number of electrical portii:- . fool

outlets and fuse spur units' specifies the requi,-_.:T . c:nts for 110 V sockets for power stations.

In addition to the 110 V AC socket outlet syosm, a system of three-phase 415 V socket outlets is r ided to supply the mobile plant utilised in power ,„:Lions. The type of plant envisaged is welding equipment, bolt heating equipment, etc.

The use of mobile electrical equipment, irrespective of its size, always introduces additional hazards to a permanent installation.

These risks can largely be identified with:

•Electric shock from the mobile equipment in the event of a fault within that equipment.

•Damage to the trailing cable used to connect the mobile plant to the permanent cabling installation.

Both these risks can be minimised if the correct equipment is chosen both for the remote electrical apparatus and for the trailing cable, and its connections.

Providing that the conductors utilised are enclosed within an earthed metallic screen and this screen Is of adequate capacity to carry any fault currents that could arise from insulation failure of those conduc - tors, then there is no risk of electrocution from contact with the conductors of the installation. Special consideration must be given, however, to the rise ot

`access lighting is left permanently switched on, and the 'inspection' lighting switched when required by means of a contactor controlled by a switch adjacent to the plant covered by that load centre.

` Access' lighting basically covers walkways and exits to allow free and safe movement around items of plant, and utilises approximately 50n of the total complement of fittings. 'Inspection' lighting uses the remaining fittings and pro%. ides supplementary illumination covering plant items,

Because of the nature of the rooms in which fluorescent tubes are used, it is not necessary in some cases (e.g., switchrooms, relay rooms, offices, etc.) to provide illumination at all times. Therefore switches are provided in the sub-circuits to give a simple and flexible arrangment. Switches are 15 A, single-pole, to BS3676: 'Switches for domestic and similar purposes'. Switches are positioned at each entrance to each area with 2-way and intermediate switching as required. En 'industrial' areas, switches are of galvanised steel and surface-mounted. In areas where a 'fair finish' is required, switches are of satin chrome plate and flush-mounted.

Current switching capacity is the maximum steady current which may be interrupted, or prospective current which may be made, with a purely resistive (nonreactive) load. Loads, other than of a purely resistive nature, invariably require derating if working life and, in some cases, safe working are to be maintained. An inductive load may cause high voltage and current surges as the magnetic flux collapses. Such voltage surges may cause flashover and damage to insulation. Therefore, for inductive (fluorescent) loads, suitable s witches are always specified.

13.6.3 AC supplies

An area of plant (e.g., turbine hall) will be divided for the purposes of lighting, heating and minor power system into zones.

Situated as near as possible to the 'load centre' of each zone will be a group of distribution fuseboards known as the 'distribution centre'. Each AC distribution centre is designed to be either floor or wall/column mounted and includes the following:

•One distribution fuseboard for the continuously energised lighting. En the areas where switched lighting is to be provided, one distribution fuseboard for 'access' lights and one distribution fuseboard for `insptetion' lights is required. The inspection fuseboard should be fed via a contactor controlled by a switch located adjacent to the plant covered by the corresponding lamps.

•One distribution fuseboard for minor power supplies.

•One splitter box, to split the incoming cable to feed the lighting fuseboard(s) and the minor power fuseboard.

•One distribution fuseboard for power sockets.

•One 415 V/110 V AC transformer fed from th e minor power fuseboard, supplying the 110 V Ac power socket fuseboard.

A lamp characteristic which should be considered when planning the main AC lighting distribution system i s the stroboscopic effect. The stroboscopic effect is an illusion which makes a moving object appear stationary, or moving in a different manner from that which it is in fact moving. All lamps operating on alternatin g current exhibit some degree of cyclic variation of light output. It is most significant with discharge lamp s which do not employ a phosphor coating. The problem can normally be reduced, or eliminated, by ha v i ng alternate rows of luminaires fed by different phases of the supply, ensuring that critical areas containing unguarded moving machinery receive illumination in roughly equal proportion from each phase.

13.6.4 DC supplies

Each area covered by the DC battery-backed emergency lighting scheme is divided into the same zones as the corresponding AC supplies. A DC distribution centre provides the emergency lighting in that zone. The DC distribution centre is separated from the AC distribution centre to avoid loss of DC supplies in the event of a fire at the AC distribution centre. The DC distribution centres comprise the following:

•One distribution fuseboard for personnel escape and emergency operational lighting.

•An automatic low voltage relay with a manual override, accommodating a three-phase feed from the associated AC lighting distribution fuseboard. Where 'access' and 'inspection' fuseboards are installed, the three-phase feed shall be from the 'access' distribution board.

•One timer linked to the low voltage relay. One DC contactor operated by the low voltage relay in the DC distribution centre.

13.6.5 Cabling

The majority of the cables used for the lighting, heating and minor power systems of modern power stations are of the armoured type described in Section 5.4 of this chapter. This method of cabling is compatible with the main station cabling and is less labour intensive to install than conduit and trunking. However there are exceptions to this, for example, where circuits have to be run buried in plaster to give a 'fair finish', or in areas where a multiplicity of terminations is required. In these cases trunking and conduit may be more economical than armoured cables.

The AC supply cable to any one AC distribution centre from its switchboard is designed to be segte-

The works necessary to bring any one piece of plant 1 0 a stage whereby it can be commissioned.

•Provision within the station layout of adequate accommodation for electrical equipment and cables.

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Cable system design.

Electrical .:ireuit design.

Scheduling of cables.

Routing of cables.

Design clearance of cables, i.e., circuit design completed and terminations specified for each cable and

Ore.

14.3.1 Layout

During the initial development of station layout, came routes are planned. These are based on estimated q uantities and known major plant locations, i.e., tehhouse, control rooms and plant areas. Although [he first consideration in station layout is the economical disposition of plant and minimum civil works' a),ts, cabling space provision must be adequate and

c abling needs catered for.

The most important of these needs is that the routes must be available with unobstructed access as early a, possible. This implies being able to include them in the early phase of civil construction, with minimum dependence on steelwork and plant erection. One way io achieve this is to locate them in basement areas or tunnels. The alternative of locating them below operating floor level, provides an acceptable technical dlternative especially at estuarine stations where tunnels are costly. ft is also important to provide cable routes

hich are as short and simple as possible. To this end, basement cable routes, with electrical plant at around level, should be designed into the layout.

14.3.2 Cable support systems

ables are carried on steelwork and trays installed %%ithin major cable routes. To facilitate design and manufacture, proprietary systems have been developed, iested and approved. Provided access is available, the ihe of these systems should avoid delays in this area.

14,3.3 Information from plant contractors

The inforrrtation concerned is of several types, including:

•Details of numbers and ratings of plant items.

•Circuit and wiring diagrams of equipment.

•Termination details.

These various types of information are required at different times and are associated with different phases of the cabling design task.

14.3.4 Cable systems and electrical circuit design

Cable systems and electrical circuit design involves a tremendous volume of derailed engineering work to ensure that each circuit and wire is cabled and connected correctly.

A large number of staff is involved. The coordination has, therefore, to be very close and responsibilities have to be defined clearly to ensure that efficient working is maintained and design clearance achieved to programme. The introduction of computeraided design techniques into drawing offices for the preparation of all types of electrical diagrams (covering cable block diagrams, schematic diagrams and loop diagrams) has assisted greatly in solving resource problems. Provided that the correct engineering resource has been allocated at the right time, the rate of design clearance of cables and systems is dependent on the progress of the engineering in plant contractors' works and will reflect any plant delays that are occurring.

The development of the CEGB's current design procedures has addressed various difficulties which are described as follows:

Electrical circuit design

The process of circuit design consists of the working out of schematics which have then to be converted into detailed connection diagrams. Sometimes in the past, no permanent record of the overall schematic (or loop diagram) was provided; this led to difficulties at site during installation and commissioning and embarrassed station staffs when maintenance, repair or modification was required. The present process for clearing circuit design and producing final information for cable connections involves the preparation and issue of the following information:

(a) System flow diagrams These are logic diagrams and are prepared for all complex systems, e.g., sequence controls, automatic boiler controls, oil burner control systems, etc.

(b) Schematic diagrams These are drawn for individual plant items or circuits on a disconnected contact logic basis so that the circuits can be understood and specified correctly. Some are drawn by the CEGB to give guidance to the contractors, e.g., switchgear protection and control, but the majority are prepared by manufacturers for the equipment they are supplying.

These are detailed point to point wiring diagrams prepared for all electrical equipment cubicles, panels, control desks, local equipment cubicles, etc., by the equipment

manufacturer. They will be marked up with basic cabling and core information requirements.

These are normally drawn on a subsystem basis and will show all marshalling cubicles, junction boxes and connections to plant items for the circuits involved. They do not generally detail the wire numbers.

These are prepared on a system or subsystem basis to co-ordinate all the connections between the various plant items, initiating devices and equipment, and detail all wire numbers and circuit connection requirements. They are drawn to ensure the circuit schematic design is correctly translated into control system wiring and to provide the permanent record of C and I wiring connections.

(f) Wire and jumper schedules Provide simplified work face information for the cable contractor on site, scheduling the wire connections for each cable and the jumper connections for each system.

The aim is to provide accurate information for fast installation and provide a complete permanent record for the station staff. it meets the stringent requirements for nuclear projects, particularly in the safety consideration of circuit security and segregation.

Cable system design

The design of cable systems has become more constrained by segregation rules, especially in nuclear power stations. These rules are determined by both plant operational availability and by safety considerations (especially in the nuclear case). They have been clearly defined and are handled by the computerised cable design methods now used. In fact the necessary quality assurance can only be guaranteed in this area by the use of computerised techniques and their use is a material factor in establishing the safety case for a nuclear power station.

The design of control and instrumentation system cables must both achieve technically suitable schemes and also make it as easy as possible to achieve installation programmes. The major technical objective is to improve signal-to-noise ratios, while the programme aim is to permit early installation and termination of as much cable as possible. There is also the aim of reducing cost by the use of large multicore and multipair cables.

These aims led to the development of C and I cable system networks using the large trunk multipair cables and marshalling cubicles incorporating jumpering facilities described in Section 5 of this chapter.

Scheduling of cables

This work consists of allocating a cable number to a particular cable, deciding its type and listing it on

the computerised cabling schedules. Cable block diagrams are prepared for power and control cables for the various subsystems and groups usirg estimated quantities for cable cores in the case of control cables. The cables can then be scheduled.

Routing of cables

To enable a cable to be sized and routed, it is essential that the location of each end is identified o n layout drawings. This work is dependent on the final layout of electrical equipment and manufacturers' plant items.

Power cable routing can normally proceed at a reasonable rate, but C and I cable routing is frequently delayed by a lack of plant and C and I information with subsequent heavy peaks of work.

The use of a computer speeds up the process and accurately routes cables in conformity with routing rules including segregation. It produces a precise route card for the cable listing all node points, which is of considerable assistance to the site installation work.

Design clearance of cables

The 'design cleared' marker can only be allocated to a cable when it is scheduled, sized, routed and full termination details are specified for each core. This will then allow the cable to be fully installed, connected and tested, but when delays occur cables are released for installation where the cable is scheduled and routed, but termination details are not complete. This procedure, however, creates extra recording and work.

1 4.4 Installation and contract management information

14.4.1Introduction

The cabling installation process on a power station project can be divided into five parts:

•Electrical equipment that is part of the cable installation, e.g., junction boxes, distribution boards.

•Cable steelwork.

•Cable installation.

•Cable glanding, terminations.

•Cable marshalling and jumpers.

The magnitude and complexity of electrical cabling contracts combined with the segregation and quality assurance requirements of power station design, give rise to the need for a sophisticated system of design and management control.

Total project information (TPI) is a comprehensive computer-based design and management control system, covering the activities both in the design office

d at site, including contract planning aids, work face

an and evaluation. It includes a cable manageinstruction

ment section.

cabling comprises a number of programs which TPIa single project interface with a common data i-ornt Each program maintains specific areas of the

,idta. The programs involved in this section of TP[

,ire as follovss:

•TOPIC2 Cable steelwork/route matrix.

•TOPIC2 Cable scheduling and routing.

•TOPICS Cable marshalling and jumpers.

•TOPIC7 Measurement and stores control.

Dam \sill be prepared using the CEGB KEY/MASTER jata entry system as well as the automatic stripping or data from the CAD system, and engineering detail

from contractors.

Outputs are produced partly by the TOPIC programs and partly using the SOCRATES report generator, and in addition there are management graphics facilities. Outputs can be requested by the users.

On-line interrogation of the data can be achieved us- mg he MANTIS screen displays which run under CICS.

14.4.2 The aims and functions of TPI cabling

De aims and functions are as follows:

•Scheduling.

•Designing.

•Planning.

•Organising.

•Commanding.

•Co-ordinating.

•Monitoring.

•Valuation.

•Reporting.

Scheduling

[he following basic design data is recorded within the

' :ern and forms the nucleus of the cabling area of he database:

• Plant scheduling The areas of plant may be split into separately erectable/commissionable areas based on a hierarchical breakdown of systems, subsystems and groups; the concept being that a subsystem comprises a major commissionable entity. For example, systems are boiler, turbine-generator, etc., subsystems are a FD fan, turbine barring gear, etc., and groups are single drives such as jacking oil pumps, etc.

Design and management techniques

•Equipment scheduling All equipment items that are to be either separately erected or cabled are scheduled as separate items within the equipment of the switchboard. For each item of equipment to be scheduled, the group within which each equipment item falls must also be given at the time of scheduling.

•Cable scheduling All numbered cables must be scheduled in their appropriate group as going from one equipment item to another. The latter information may be either specified as an equipment item or by an English description. The appropriate gland and termination data must also be given before the cable can be design cleared.

•Steelwork The cable carriers (trays, conduits, etc.) that form the cable route matrix must be scheduled for each segment. In addition, tailend steelwork can be designed and scheduled.

14.4.3 Designing

An automatic cable routing and sizing process is an integral part of the overall Tlpf system.

Planning

The key target dates are assigned to commissioning systems based on the project master commissioning programme. This data may either be generated automatically from critical path network programs or explicitly input. Dates can be assigned to the following events:

•Design completion.

•Access for erection.

•Termination release/cable early start.

•Cable start.

•Cable finish.

•Cable latest finish — jumper start.

•Commissioning complete — jumper finish.

The estimated number of cables and jumpers in each system or subsystem may also be input as a basis for the initial plans.

Subsequently the site joint planning team takes into account improved information, obtained from questionnaires as the project proceeds, to review and progressively refine the dates. This is normally done by assigning dates at group level. This is a continuous process from the long term (16 weeks plus) to the short term and immediate work programmes.

Organising

The organisation depends upon the control of resources, both material and labour, to match the current work programme and access. This is done by arranging the cables, terminations and equipment into work packages.

Details of all material orders and deliveries are entered into TPI. In conjunction with the figures derived from progress data, these form the basis of a stock control system which compares quantities with requirements computed from the scheduled quantities.

The labour resources required for the current work programme are computed from the estimated labour content of each task to be performed by the contractor.

This allows the joint planning team to ensure that a consistent level of work is released to the contractor and that the material required is available.

Commanding

The instructions to the contractor (which are not intended to overcast contractual responsibilities) are the working instructions and are issued as follows, standing work released to the contractor is the subject of a work release and is listed on the current schedules.

Work face instructions for all of these tasks are computer produced in the form of work cards bearing all the necessary information to carry out the task. For example, cabling work cards state full details of the cable type, route, glands and methods of installation.

Completed work can be recorded on the work cards which can then be used as computer input documents.

Work cards are produced for the following tasks:

•Steelwork supports.

•Steelwork installation trays/trunking.

•Cable installation.

•Equipment installation.

•Termination instructions.

•Jumper instructions.

The site joint planning team monitors progress on work packages and work cards.

Monitoring

Report venerators are used to summarise and display data in a manner enabling the management team to monitor and analyse design and site activities. During the design phase, the cable route matrix segment usage and control cable core usage can be monitored allowing advantageous design changes to be made prior to installation. During the site phase progress, costs and productivity can all be kept under review.

Valuation

The system will accept measurement information for the following:

•Steelwork quantities and supports installed.

•Route matrix segment lengths.

•On-matrix cable tail lengths.

•Off-matrix cables.

•Cable terminations.

•Jumpers installed.

•Equipment erections.

•Material delivery.

This information will normally be input by the quantity surveyors. Valuations of material delivered, material used and labour content value for either a sp ec ific valuation period or for the whole of the contract to date may be produced. The latter is known as cumulative valuation.

Cable measurement cards are used to assist in the task of cable measurement. They show any outstanding measurement task, i.e., installed and not measured, together with any measurements previously recorded.

Reporting

An important aspect of the aid to management al. forded by TPI is the quality of reporting that is possible. In addition to special ad-hoc enquiries, there are routine reports that can be produced showing the progress and status of the contract works against programme.

Management summaries can be produced showing the total work done on each section of the contract. These may also be produced in graphical form so that current trends can be analysed.

15 References

(11 I EC287: Calculation of the continuous current rating of cables . 1982

[2)GDCD Standard 17: 6350/11000V extruded solid insulation cables: May 1978

[3]BS6346: Specificati,)a for PVC-insulated cables for electricity supply: 1969 (1977)

[4]ERA Report 69-30: Current rating standards for distribution cables: Part 3: Sustained current ratings for PVC insulated cables to 13S6346: 1969 (AC 50Hz and DC)

[5]CERL Report RD/LN 197/75: CURB03 computer program: Three-phase power frequency impedance characteristics of single-core power cables with special reference to current sharing between cables in parallel: 1976

[6]BS2692: Part 1: Specification for current-limiting fuses: 1986

[ 7 1 BS5907: Specification for high voltage fuse-links for motor circuit applications: 1986

1 8 ) BS88: Part 2: Supplementary requirements for fuses of standardised dimensions and performance for industrial purposes: 1975(1982)

( 9 1 IEE Wiring Regulations: 15th Edition 1981: Regulation 43Protection against both overload and short circuit currents

[10]ESI Standard 44-3: Electric motors — specification (3300V and above), 1980

(11]ESI Standard 44-4: Electric motors — specification (415V and below, 0.75 kW and above), 1980

[12]BS5000: Part 40: Motors for driving power station auxiliaries: 1973 (1984)

[13]BS4999: Part 41: Specification for general requirements for rotating electrical machines — General characteristics: 1977

[14]BS4999: Part 101: Specification for general requirements for rotating electrical machines — Tolerances: 1972