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5. CALCULATION OF LOSSES IN CABLES
5.1 Dielectric Losses
Cables of the same conductor diameter, insulation material and similar construction from different manufacturers will have similar small dielectric losses which may be compared when buying cable during the tender adjudication stage. The larger the conductor diameter, the greater the losses for a given insulating dielectric material. Dielectric losses in XLPE-insulated cables will be appreciably lower than in oil-filled paper-insulated types which have a higher capacitance per unit length. For example, consider circuits containing 3 single core or 3 core 132 kV cables:
OF cables, 240-800 mm2
- Dielectric losses typically 4.5-9.5 W/m XLPE cables, 240-800 mm2
- Dielectric losses typically 0.7-1.2 W/m
5.2 Screen or Sheath Losses
Screen or sheath/metallic layer losses will be proportional to the current carried by the cables and will be approximately the same for standard cables of the same types and size. If the cables are to be installed on systems with high earth fault levels the sheath or metallic layer cross-sections will have to be increased. In particular care should be taken regarding possible future network expansion and interconnections which might involve increasing fault levels over the life time of the cable installation. Losses may be reduced in the case of circuits employing single core cables by single point bonding on short cable routes (,500 m) and cross bonding on longer routes (see SEC. 4.5).
Cable losses may be calculated and compared at the tender adjudication stage from the maximum permissible loss angle value in accordance with IEC Standards and the maximum current carrying capacity of the cable.
Some Tenderers base their calculations on the loss angle value obtained during cable type tests and the specified cable current rating required. This will give results appreciably lower than the permissible maximum value.
Where the costs due to cable losses are to be evaluated this should be specified at the tender or enquiry stages so that the manufacturers can state the actual maximum losses and not the maximum permissible losses. For example, 132 kV circuits containing 3 single core or 3 core 240 mm2 to 800 mm2 standard XLPE or oil-filled cables will have sheath (or screen) losses of the order of 1.0 to 10 W/m.
TABLE 11 Standards Relating to Fire Properties of Cables [coming soon]
6. FIRE PROPERTIES OF CABLES
6.1 Introduction
This is an area of increasing public and legislative concern, and therefore of increasing interest to engineers. There have been major advances in the fire performance of cables in recent years, and TABLE 11 lists some of the relevant standards.
6.2 Toxic and Corrosive Gases
It is recognized that conventional flame retardant cables having sheathing based upon PVC type materials evolve considerable quantities of halon acid gases such as hydrogen chloride upon burning. Such materials are not there fore suitable for use in confined spaces where the public are likely to travel, and moreover the fire in the ENEL power station at La Spezia in 1967 showed that in certain circumstances PVC cables will burn completely and contribute to the spread of a fire. Materials have now been developed for cable oversheaths and bedding which are normally free of halogen based compounds. They consist of a mixture of inorganic filler such as aluminum hydroxide and polymers such as ethylene vinyl acetate, acrylates and ethylene propylene rubbers. Cables manufactured with such materials are known as 'Low Smoke and Fume' (LSF) and have acid gas evolution less than 0.5% in comparison to 25-30% for PVC compounds.
IEC 60754-1 specifies a method of determining the amount of halogen acid gas, other than hydrofluoric acid, evolved during combustion of halogen based compounds. The method essentially measures the existence of halogen acid greater than 0.5%, the accuracy limit for the test. Therefore cables tested having less than the 0.5% limit are generally termed 'zero halogen' or 'low smoke zero halogen' (LS0H).
6.3 Smoke Emission
Normal cable sheathing compounds also give off dense smoke when burned and this is of particular concern in underground transport system installations.
The generation of large amounts of smoke obscures vision and reduces the ease with which the fire service is able to bring members of the public to safety in the event of a fire. LSF cables therefore play an important part in reducing this danger to a minimum. London Underground Limited (LUL) have developed a test of practical significance which has been designed to measure the density of smoke emission from cables and it has now been adopted by British and IEC Standards. This defines the standard absorbance produced across the opposite faces of a test cubicle and is popularly known as the 3 m cube test. Paris Metro (RATP) adopts the French Standard UTE C20 452 on smoke emission which determines under experimental conditions the specific optical density of smoke produced by burning material. This slightly different approach is generally known as the NBS smoke chamber test.
6.4 Oxygen Index and Temperature Index
'Oxygen index' is the minimum concentration of oxygen in an oxygen/nitro gen mixture in which the material will burn. As air contains approximately 21% oxygen it is stated that a material with an oxygen index greater than about 26% will be self extinguishing. In general, a particular oxygen index value offers no guarantee of resistance to the spread of flames. In practice materials having identical oxygen indices may have widely different burning properties especially if base polymers or additives are of different types.
The 'temperature index' of a material is the minimum temperature at which the material supports combustion in air containing 21% oxygen when tested under controlled conditions. The test is useful for the comparison of similar materials but no correlation with flammability under other fire conditions is implied.
Oxygen and temperature indices are to some extent inter-related. The engineer specifying such cable requirements should not pick out the most favorable parameters from different manufacturers' literature and expect them to comply. For example, a high oxygen index using a particular combi nation of materials may result in a slightly less favorable temperature index rating. In some cases where manufacturers have been requested to provide cables with a temperature index of 280 degrees C or above this requirement was only met at the expense of other important parameters such as tensile proper ties and water permeability. Acceptable values of oxygen index and tempera ture index recommended by leading manufacturers and specified for LSF compounds would be:
- Oxygen index equal to or greater than 30.
- Temperature index equal to or greater than 260 degrees C.
6.5 Flame Retardance/Flammability
6.5.1 Single Core Cables
Flame retardant cables meet the requirements of IEC 60332 Part 1. (For European use EN 60332 is identical to the IEC standard). These tests define the cable performance under fire conditions. The tests are carried out on a single length of cable supported vertically in a draught free enclosure with a burner applied to the lower end of the cable. After a specified time the heat source is removed and the cable should not continue to burn after a stated length of time. The extent of charring at the top of the cable is also defined.
6.5.2 Cables in Bunches or Groups
Single cables which pass the test mentioned in SEC. 6.4 above may not necessarily pass the test when grouped together in vertical racks, where propagation of the fire takes place. Propagation of fire depends upon a number of factors, but is in particular a function of the total volume of combustible material in the cable run.
The tests involved in this category attempt to simulate group cabling installation conditions and are generally covered in IEC 60332 Part 3. The IEC standards define three categories for grouped cables, A, B and C which are related to the volume of combustible (organic) material per meter.
LSF power cables manufactured by leading companies should be covered by the IEC Standards mentioned above. An important feature of the construction of all of the cables which relates to flame retardancy is the cable armor. For example, XLPE insulation as a material on its own is not flame retardant. Provision of the cable armor separates the insulated cores from air for combustion even after the sheath has been destroyed.
6.6 Fire Resistance
Fire resistance is the term used to define cables which can maintain circuit integrity for a specified period of time during a fire.
Such cables have to conform to a severe test in which the middle portion of a 1,200 mm long sample of cable is supported by two metal rings 300 mm apart and exposed to a flame of a tube type gas burner. Simultaneously the rated voltage of the cable is applied throughout the test period. Not less than 12 hours after the flame has been extinguished the cable is re-energized and no failure must occur. There are many variations of time and temperature and also impact tests to simulate falling debris and application of a water deluge after the flame has been extinguished.
Two typical types of fire resistant cables are described below:
1. Mineral insulated (MICC or Pyrotenax) cables complemented with an outer LSF covering and rated voltage 500/750 voltage. IEC 60702 (EN60702 in Europe) covers mineral insulated cables but testing in the UK is to BS 6387 (NF C32-070 CR1 in France). The outer LSF covering would be required to meet BS 6724 (NF C32-200) as far as behavior in a fire is concerned. Cable type BS 6387 degrees CWZ has a 3 hour resistance to fire up to 950 deg. C.
2. Lapped mica/glass tape to be covered by an extruded cross-linked insulation, armored, LSF sheathed. Rated voltage 600/1,000 V and to meet BS 6387 types CWZ or lower temperature performances type A/B/SWX.
Trends in the development of building safety regulations in various countries (e.g. the Construction Products Directive (CPD) in Europe) may mean that use of cables with a defined level of fire resistance in all buildings may be mandated in the future.
6.7 Mechanical Properties
Achieving good mechanical qualities in a cable material is finely balanced by the requirement of maintaining good low-smoke/toxic gas emission and reduce flame propagation.
All cable materials must possess reasonable tensile strength and elongation properties with good resistance to abrasion, where the oversheath should not suffer cracks or splits during installation. Leading manufacturers have now formulated compounds for LSF cables which have similar properties to existing standard sheathing materials. Cables must also have acceptable tear resistant properties and where used for cable sheathing provide adequate protection in a wet environment.
Testing requirements for mechanical properties are defined in IEC 60229 (or EN 60811 for European countries; BS 6469-99-2 covers UK tests not covered in 60811).
7. CONTROL AND COMMUNICATION CABLES
7.1 Low Voltage and Multicore Control Cables
A wide range of cables exists for a multitude of specific applications.
Open terminal substation control cables are usually multicore 600/1,000 V
PVC-insulated copper conductor types laid in concrete troughs from the sub station control building to the switchgear. Within a high security substation building LSF cables may be specified both within and between equipment.
Such cable cores are described by the individual conductor cross-sectional area (mm2 ) together with the number of individual strands and associated strand diameter (mm) making up the conductor core. Control cables are generally armored when laid direct in ground. The type of multicore cable screen and armor will determine the flexibility of the cable and associated bending radius. In general steel wire armored cables have a smaller bending radius than steel tape armor types.
Some standard specification data and standard sizes for such control cables are detailed in TABLE 12.
Care must be taken to ensure adequate cable conductor cross-sectional area when selecting sizes for association with substation current relays located some distance from their associated CT. At the same time the traditional practice of standardizing upon 2.5 mm2 cables for substation control and relay cubicles is now outdated and the terminations onto modern low current consumption electronics are often incapable of accommodating such a large conductor size. Insulation levels for certain applications such as pilot wire cables (pilots) associated with overhead line or feeder cable differential protection schemes must also be clearly specified. Such cables may require an enhanced insulation to counter induced voltages from the parallel power circuit ( TABLE 13).
TABLE 12 Multicore Control Cable Technical Particulars [coming soon]
TABLE 13 Standard Multicore Cable Sizes [coming soon]
7.2 Telephone Cables
Telephone cables likely to be encountered by the transmission and distribution engineer are PE- or PVC-insulated and may be specified as unfilled (standard underground situations) or filled (submarine, high humidity environment or cables laid in waterlogged ground) with a gel to prohibit the ingress of water. Major trunk route cables are often installed with a dry nitro gen gas system. The lay of the cable and a standard colour coding scheme assists in the identification of the correct 'pair' or circuit in a multicore telephone cable. For such communication cables, the characteristic impedance is also important since maximum power transfer is achieved when impedance matching is achieved. Maximum loop impedance for a telephone circuit is typically 1,000 Ohms. Attenuation in telephone cables is normally measured in dB assuming a 600-Ohm impedance. Of particular practical interest to power installation and civil services engineers is the pulling strength capability of such relatively small cables in order for the maximum distance between cable draw pits or pulling chambers to be determined in an early stage of the design. Since such 'hard wire' small signal cables are susceptible to electro magnetic interference from adjacent power cables adequate screening must be provided. Where feasible control and communication (C&C) copper telephone type cables should be laid at the following minimum distances adjacent parallel power cables:
HV single core cables .500 mm
HV multicore power cables
.300 mm (add a physical barrier between power and C&C cables if spacing ,150 mm)
External MV and LV power cables
.50 mm (add a physical barrier between power and C&C cables if spacing ,25 mm) Internal MV and LV power cables .50 mm (use separate trunking if this spacing not possible) Some standard sizes and specification data for telephone cables are detailed in Tables 14 and 15.
TABLE 14 Standard Telephone Cable Sizes [coming soon]
7.3 Fiber Optic Cables
7.3.1 Introduction
In order to improve the rate of transmission of information and the amount of information that can be transmitted over a given channel path the widest possible bandwidth must be employed. In order to achieve the necessary band widths higher and higher frequencies have been employed. Power line carrier systems at frequencies of tens of kHz have been used by superimposing a modulated radio carrier on the overhead transmission line phase conductors.
Alternatively, high quality telephone circuits may be installed or rented from the local telephone company. Microwave radio links using radio-frequency coaxial cable feeders (see IEC 60096 _ Radio-frequency cables) between transceivers and aerials are also used to carry more information. The latest development of this trend is for data transmission and digitized speech or other digitized analogue signals to be transmitted over fiber optic cable where the bandwidth is more than adequate at infra red frequencies.
TABLE 15 Telephone Cable Technical Particulars [coming soon]
Some of the advantages of fiber optic cable are summarized below:
- Fast reliable communications over long distances (.1 Gbit/s over 100 km).
- Low transmission loss or signal attenuation.
- High privacy or security since signals carried are immune from remote detection.
- Wide bandwidth availability hence large data handling capacity.
- Signals unaffected by electromagnetic interference (EMI), radio frequency interference (RFI) and lightning noise. The cable may therefore be installed, without special screening, adjacent to power cables and over head lines.
- No dangerous voltages are employed or induced in such cables.
Therefore they may be installed in hazardous environments which require intrinsic safety.
- Complete electrical isolation between terminations is achieved. This avoids voltage gradients and ground loop problems encountered with hard wire cable solutions.
- Small overall cable diameter, light weight and flexible nature makes for easy installation. However, it is very important to note carefully that fiber optic cables must not be handled roughly (excess pulling tension, over clamping to cable tray, etc.) nor without conforming to the correct installation specification.
7.3.2 Fiber Optic Cable Principles
An optical fiber cable consists of a very pure thin optical strand of silica glass material surrounded by an optical cladding of lower refractive index.
The infra red light radiation in the frequency range 1014 Hz passes down the fiber by a series of total internal reflections (Figs. 13 and 14). Single or mono-mode fibers have very dense and exceptionally small (5 µm) internal core diameters. They offer the greatest information carrying capacity of all fiber types and support the longest transmission distances. Multimode fibers have wider cores (50 µm) with either an abrupt or graded refractive index outer profile. The graded outer cladding allows longer transmission distances at higher bandwidths ( FIG. 15).
FIG. 13 Light entering the fiber within the boundary of the acceptance
cone will be propagated through the fiber.
FIG. 14 Mono- and multimode fibers.
A fiber optic cable communication system always consists of a transmitter light source (laser diode, light-emitting diode (LED) or pin diode in order of cost) pulsed by electronic circuitry at the required data rates, the fiber optic cable and a detector at the receiving end which decodes the light pulses back into electronic signals. A transmitter and receiver are built into a single electronic circuit ('chip') for two way duplex communication. 820 nm to 850 nm wavelengths are used for low data rate communication but other wavelengths (1,300 nm, 1,550 nm) may be used for long distance systems. This is lower in frequency than white light radiation and cannot be seen by the naked eye.
FIG. 15 Fiber optic cable constructions.
7.3.3 Optical Budget
The transmission system is given an 'optical budget' as a level of attenuation which should not be exceeded if correct reception by the detector equipment at the remote end of the link is to be ensured. Four factors are involved:
1. Light source power.
2. Fiber loss (dependant upon fiber size and acceptance angle or matching of light source to the cable).
3. Receiver sensitivity.
4. Jointing or splicing, coupling and connector losses.
Consider a 200 µW light-emitting diode transmitter power source and a receiver sensitivity of 2 µW.
Optical power budget (dB)=10 log10 (available infra red light input)/ (required infra red light output)=10 log10 200/2=20 dB.
Fiber optic cable end connector attenuation loss=3 dB.
Fiber optic cable splice loss (3 joints at 2 dB each)=6 dB.
Fiber attenuation (3 dB/km).
Therefore maximum cable length allowable is approximately 3 km before a regenerator is necessary to boost the signal. The receiver bandwidth capability may be traded off against its sensitivity for some applications. Modern joints have reduced losses by a factor of ten in the last 10 years such that good manufacturers should be able to offer splice losses of only 0.2 dB.
7.3.4 Terminology
The following terms refer to the make up and type of cable and fittings:
Armor: Extra protection for a fiber optic cable to improve the resistance to cutting and crushing. The most common form is galvanized steel wire as for power cables.
Bifurcator: An adaptor with which a loose tube containing two optical fibers can be split into two single fiber cables.
Buffer: Material surrounding the fiber to protect it from physical damage.
Cladding: The outermost region of an optical fiber, less dense (lower refractive index) than the central core. Acts as an optical barrier to prevent transmitted light leaking away from the core.
Core: The central region of an optical fiber, through which signal carrying infra red light is transmitted. Manufactured from high density silica glass.
Loose tube: A type of cable in which one or more optical fiber is/are laid loosely within a tube.
Moisture barrier: A layer of protection built into the cable to keep moisture out.
Multimode fiber: An optical fiber which allows the signal carrying infra red light to travel along more than one path.
Primary coating: A thin plastic coating applied to the outer cladding of an optical fiber. Essential in protecting the fiber from contamination and abrasion.
Sheath: The outer finish of a cable. Usually an extruded layer of either PVC or PE.
Single mode fiber: An optical fiber so constructed that light travelling along the core can follow only one path. (Also called 'monomode'.) Step index or step index profile: A measurement shown in diagrammatic form illustrating how the quality of glass used in this type of optical fiber graduates, in clearly defined steps, from the highest to the lowest. The shift from one level of density or refractive index to another causes to light to be totally internally reflected back into the core as it travels along the fiber.
Strain member: Part of an optical fiber cable that removes any strain on the fibers. Commonly used materials include steel and synthetic yarns.
Tight buffered: A cable in which the optical fibers are tightly bound.
The following terms refer to transmission characteristics:
Analogue link: Fiber optic cables cannot be easily used to transmit analog data directly in analogue form because light source variations, bending losses in cables, connector expansion with temperature, etc. introduce distortion. The analogue signal is normally converted to a digital form in an analog to digital (A/D) converter; accurately determined by the number of bits used, multiplexing the digital bits into one stream in a multiplexer (MUX) and using a pulsed transmission approach.
Attenuation: A term which refers to a decrease in transmission power in an optical fiber. Usually used as a measurement in decibels (dB), for example low attenuation means low transmission loss.
Bit/s: Bits per second. Basic unit of measure for serial data transmission capacity (kbit/s, Mbit/s, Gbit/s, etc.).
Bit error rate: The frequency at which the infra red light pulse is erroneously interpreted. Usually expressed as a number referenced to a power of 10 (e.g. 1 in 105).
Dark fiber: Unused or spare fiber perhaps in a multi fiber cable.
Data rate: The capability to accurately transmit data in a specified rate range.
Drop and insert: Simplest extension to a point to point optical fiber link.
Extends the link along its length from one 'drop' point (node) to the next.
Incoming light energy is split between the receiving port at the insert into the link and also to the ongoing output port.
Frequency shift keying: A form of modulation whereby the frequency of an optical carrier system is varied to represent digital states '0' and '1'.
Useful in schemes where 'handshaking' is employed to recognize transmitter and receiver.
Handshaking: A predefined exchange of signals or control characters between two devices or nodes that sets up the conditions for data transfer or transmission.
Minimum output power: The amount of light, typically measured in micro watts, provided into a specific fiber size from the data link's light source.
Modem: A contraction of the term 'MOdulator-DEModulator'. A modem converts the serial digital data from a transmitting terminal into a form suitable for transmission over an analogue telephone channel. A second unit reconverts the signal to serial digital data for acceptance by the receiving terminal.
Multiplexer (MUX): Employed in pairs (one at each end of a communication channel) to allow a number of communications devices to share a single communications channel. Each device performs both multiplexing of the multiple user inputs and demultiplexing of the channel back into separate user data streams.
Photodetector: Device at the receiving end of an optical link which converts infra red light to electrical power.
Pulse width distortion: The time based disparity between input and output pulse width.
Receiver sensitivity: The amount of infra red light typically measured in microwatts or nanowatts required to activate the data link's light detector.
Regenerators: Devices placed at regular intervals along a transmission line to detect weak signals and re-transmit them. Seldom required in a modern fiber optics system. Often wrongly referred to as 'repeaters'.
7.3.5 Cable Constructions and Technical Particulars
Recent standards covering the fibers themselves and the total fiber optic cable make up and accessories are:
IEC 60793 Optical Fibers Part 1 _ Measurement Methods and Test procedures; generic specification establishing uniform requirements for geometrical, optical, transmission, mechanical and environmental properties of optical fibers.
Part 2 _ Product Specifications, Class A (multimode) and Class B mono mode fibers.
TABLE 16 Fiber Optic Cable Technical Parameters [coming soon]
IEC 60794 Optical Fiber Cables Parts 1 & 2 covering Generic and Product Specifications.
IEC 60874 Connectors for Optical Fibers and Cables
IEC 60875 Fiber Optic Branching Devices
FIG. 15 shows a variety of basic fiber optic cable constructions.
TABLE 16 indicates typical technical parameters to be considered when specifying a cable for a particular application. Fiber optic cables may be buried underground using the transmission wayleave between substations. Power cables are typically supplied in 500 m to 1,000 m drum lengths. This is short for fiber links and introduces the need for a large number of fiber optic cable joints at the corresponding power cable joint if the fiber is introduced as part of the overall power cable makeup. In addition trenching operations are expensive and a cheaper technique is to specify the fiber optic cable as part of the overhead line earth cable in new installations. A further even better alternative is to wrap the fiber optic cable around the overhead line earth cable. This allows greatly increased lengths of fiber cable to be installed free of joints. Installation equipment has been developed to cater for such installations without outages.
8. CABLE MANAGEMENT SYSTEMS
8.1 Standard Cable Laying Arrangements
It is necessary to be able to accurately locate buried cables, and indeed all buried services (fresh water piping, gas mains, foul water piping, etc.), in order to avoid damage when excavations are taking place. Reference should be made to drawings that have been regularly updated to reflect the current status of buried services. In addition to studying these 'as built' drawing records digging should only take place after the necessary 'permit to work' authorization for excavations in a specific area or route has been obtained. It is useful for contractors to adopt a standard cable laying arrangement in road verges. FIG. 3 shows such a typical standard arrangement.
Oil-filled cable tanks are best buried in purpose built pits if being installed adjacent to roadways in order to avoid collision damage by vehicles. Figures 12.17a and b show typical above and below ground oil pressure vessel installation arrangements, trifurcating joint and cable sealing ends.
FIG. 16 12.7/22 kV, 3 core, 120 mm2 , XPLE, LSF, straight cable joint
(note other types also exist for this cable).
It is also normal practice to identify the cable route by cable markers attached to adjacent permanent walls or buildings. Such markers detail the distance horizontally from the marker to the line of the cable route and the depth of the cables at that location. Unfortunately, such markers are often removed in the intervening period between one installation and the next so that reliance has to be placed upon existing record drawings and permit to dig systems. Further security to the existing services installation is achieved by placing cable tiles and/or marker tape above the cables. A cross-section of direct buried 132 kV oil-filled cables with both cable tile protection and marker tape is shown in FIG. 5. When further cables are to be placed in the same wayleave as used by an existing installation a mechanical digger can be used to carefully dig down to the depth of the cable tiles and then hand digging used to expose the existing cable area. In this way damage to existing cable outer sheaths is minimized.
8.2 Computer Aided Cable Installation Systems
Integrated computer aided design and drawing (CADD) packages are avail able for the engineer to assist in:
- The optimum routing of cables (shortest route, least congested route, segregated route, etc.).
- Selection of cable sizes for the particular environmental and electrical conditions.
FIG. 17 (a) Typical sealing and arrangement.
FIG. 17 (b) typical oil feed and alarm system arrangement.
Such programs should have the following advantages and facilities:
- Improved ability to respond to change requests.
- Greater speed in locations of services and repairing faults.
- Automatic or computer assisted rapid cable route design and selection in accordance with carrier selection, accommodation, segregation and separation rules.
- Accurate cable sizing for the given route and data base record.
degrees Consistent accuracy in design and drawing quality. Production of a drawing register to hold drawing numbers, sheet numbers, titles, revisions, etc.
- Automatic calculation of material quantities, including glands, termination equipment, cable tiles, cable ties, sand surround, etc. Production of procurement schedules resulting from the design.
- Production by direct printing of cable installation cards which are able to withstand the rigors of on-site handling.
- Overall progress monitoring and control with up to date information on the number of cables scheduled, routed, design approvals, drummed, shipped, installed, etc.
One such program is the CAPICS (Computer Aided Processing of Industrial Cabling Systems) suite primarily intended to assist in major power installations developed by Balfour Kilpatrick Ltd., Special Products Division, Renfrew, Scotland. A typical cable installation card from such a program is shown in FIG. 18 and a design schedule in FIG. 19. Other proprietary systems are available.
CAD systems also improve the quality of buried cable installation information. The drawing files should be arranged in 'layers' such that each layer is used for different buried services (water, telephone, lighting, power, foul water, etc.). The composite drawing then consists of merging the different files for a full services co-ordination drawing. The program should be integrated with a data base to keep records of duct routes, duct occupancy, draw pit locations, cable information, etc. This may then be used to tie in with a 'permit to work' scheme and records updated accordingly as the work proceeds. Examples of the output from such a system based upon AUTOCAD is given in Figs. 20 and 21.
FIG. 19 Design schedule. [coming soon]
FIG. 20 Permit application. [coming soon]
FIG. 21 Terminal.
FIG. 22 Interface drawing, general contract terminal interface points.
8.3 Interface Definition
As an important general principle it is essential to define as accurately as possible the technical and physical interfaces between different subcontracts within an overall transmission and distribution project. This should be done at the earliest stage in the project. Since cable installation works are often carried out by specialist contractors particular attention must be paid to this by the design engineer. Detail down to the supply of termination materials at the interface point must be given so that materials and construction plant orders may be correctly placed in a timely manner. Lack of definition will only lead to inefficiency and costly disputes at a later stage.
In addition to a description of the subcontract package terminal points an interface drawing such as that shown in FIG. 22 is often even more useful.