GUIDELINES FOR CABLE LAYING (PEM ELECTRICAL) DOCUMENT NUMBER: PE-GL-999-507-E002, Rev. 00 ISSUE DATE: 15.05.14 BHARAT HEAVY ELECTRICALS LIMITED POWER SECTOR PROJECT ENGINEERING MANAGEMENT NOIDA
PREFACEE BHEL PEM being the core engineering group forr EPC projects owning the responsibility for developing the cable trays network & providing the cable routing for complete power station (except the island of ESP Area being developed by BHEL-Ranipet) ). The above activity includes detailedd interfacee with BHEL Units for cable layouts to various load centers. The same is normallyy developed by BHEL PEM-Electrical taking care of the optimization, cost & plant esthetics & ergonomics factors. Further the cable routing being carried out by BHEL PEM Electricall through software for PEM Electrical / C& &I / MAX package Cable Schedules along with BHEL unit Cable schedules. Finally Routed Cable Schedules are forwarded to site for laying of various cables in installed cable trays. Off late it has been felt that in orderr to facilitate BHEL Site for better cable laying, an informative document for cable Laying, dressing, clamping etc. is required to be generated as general good practices to be followed by BHEL site through Site Cabling Contractor. This document has been prepared keeping in mind the above approach and for onward sending to all Power Sector Regions for their further use during cable laying through cabling contractor & furtherr feedbackk (if any).
CONTENTS 1.0 GENERAL PHILOSOPHY 2.0 CABLE ROUTING IN CABLE TRENCHES/TRESTLES 3.0 CABLE ROUTING IN SPREADER R/FALSE FLOOR 3.1 SKETCH-1 & 2 SHOWING TYPICAL ARRANGEMENT BELOW FALSE FLOOR 4.0 CABLE ROUTING IN CABLE VAULT 5.0 SPECIAL PRECAUTIONS TO BE TAKEN WHILE LAYING OF CABLES 6.0 PHOTOGRA APHS SHOWING GOOD / BAD CABLING
1. General Philosophy for laying of different type of cables 1.1 Laying and installation of power, control and special cables shall generally conform to IS : 1255. 1.2 Cable trays shall be routed as per various cable trayy layouts and further cables shall be routed as per the latest revision of routed cable schedules furnished by PEM. Site must check the latest revision of cable tray layouts & cable schedules before starting of cable routing. 1.3 Cable trays route numbering shall be painted / marked on all erected cable trays as per latest cable trays layout furnished by PEM before start of cable laying. 1.4 Proper care shall be taken for dressing the cables. While laying the cables, dressing shall be done after each run of cable. It is not advisable to dress the cable after laying all the cables. The cables shall be held in the trays temporarily by binding wire and shall be clamped finally with nylon ties. 1.5 At bends, large size cables (95sq.mm. & above) shall be laid in the outer edge of the bends to provide space for cable to bend. Smaller size cables shall be laid in the inner bend. 1.6 Jumping of cables between trays with no tray connection between them e.g. from horizontal to vertical connection, cables could be supported on angle or site fabricated support. 1.7 Where cabling passes through brickwork or concrete work, mechanical damage shall be provided. suitable local protection against 1.8 For cable trays which are closely placed, inter connection with tray accessories is not possible, site to fabricate connection from straight trays locally. 1.9 Each cable shalll be provided with identification tags indicating the cable numbers in accordance with the cable schedule. Cables shall be tagged at their entrance at every 30 m and exit from any equipment, junction box. The tags shall be of Aluminium with number punched on it and securely attached to the cable. The location of cable joints,, if any, shall be clearly indicated with cable marker with an additional inscriptions "Cable joint" and "Cable Number". 1.10 Single core cables for a. c. three phase circuits, when laid on trays, shall be in trefoil formation (each trefoil with RYB phases formation), with eachh trefoil twice the diameter of cable apart from the next trefoil in the tray. The trefoil formation shall be duly secured to the cable tray by means
of trefoil clamps of nonmagnetic material at every 1000mm interval. 1.11 Multi-core cables above 1100 V grade when laid clamped by means of cable ties. on trays, shall be in one layer, touching and 1.12 Control and instrumentationn cables shall be laid in multi layers, but not exceeding three layers in any section. 1.13 While power, control and instrumentat tion cables shall generally be laid in separate trays, low current carrying power cables (valve/ damper actuator powerr cables) may be laid along with control cables. 1.14 Generally cables shall be placed on trays on the basis of theirr types and horizontal formations: functions as under for a) b) c) d) HT cables: in the top tier(s). LT power cables: in the tray(s) below the HT cable trays. Control cables: in the tray(s) next below to thee LT power cable tray(s) ). Instrumentation cables (screened control cables): in the bottom most tray(s). HT Power, LT Power and LT Control/Instrumentation cables shall be separated from each other by at least 250mm. 1.15 For vertical formations, the outermost tray shall bee considered as the topmost tray. In rare cases, where there is no clear distinction of bottom/ topp trays, the order convenient for linking the horizontal and vertical formations avoiding criss-crossing, or exit of cables shall be followed.
Typical examples of tray numbering are given below: L1 L2 L3 R1 R2 R3 01 02 03 DOUBLE SIDEDD TRAY (HORIZONTAL RUN) SINGLE SIDED TRAY (HORIZONTAL RUN) 03 02 01 01 02 03 01 02 03 SINGLE SIDED TRAY (VERTICAL RUN) SINGLE SIDED TRAY (VERTICAL RUN) SINGLE SIDED TRAY (VERTICAL RUN) HORIZONTAL TRAY 01 02 03 VERTICAL RISER ALONG WALL 01 02 03 WALL EXAMPLE OF CHANGE OF NUMBERING SCHEME FOR ENSURING LEVEL CONTINUITY 1.16 Wherever it is not possible to accommodate cables as per the criteria indicated in the above clauses (due to layout constraints) for very short field runs, control cables may be laid in the same tray with the instrumentation cables with clear minimumm gap of 100mm between the two types of cables. 1.17 To facilitate pulling of cables in GI conduits, powdered soft stone, plastic soap or other dry inert lubricant may be used. However any material harmful to the cable sheaths shall not be used. 1.18 No single core cable shall pass throughh a GI conduit/ pipe or duct singly except DC single core cables. AC single core cables shall pass through GI conduit/ pipe in trefoil formation only, or through PVC pipes. Conduit/pipe occupancy shall not exceed 40% of the conduit/pipe cross-
section area. Pipes/ conduits shall not be used in corrosive areas. 1.19 Wherever site routed is mentioned in cable schedules, cables route as per the above criteria, as directed by site Engineer. may be laid through the shortest 1.20 Reinforced concrete manhole shall be provided in the duct bank, where required, so that cable may be installed without exceeding allowable pulling tensions, cable sidewall pressures or bending radius. 1.21 Cable clamping spacing for cables laid in cable trays shall be generally as under: (a) Trefoil clamps: 1) Horizontal run spacing: 2) Vertical run spacing: 1000 mmm (max.) 1000 mmm (max.) (b) Other Clamps 1. Power cables of sizes indicated under (i) (ii) (iii) (iv) Single core cables: 185 sq mm (when not laid in trefoil formation- e.g. dc circuits) Two core cables: 120 sq mm Three core cables: 95 sq mmm or higher Three & half core cable: 95 sq mmm or higher Horizontal runs: Individually clamped at 5000 mmm Vertical runs: Individually clamped at 1000 mmm interval (max.) interval (max.) 2. Power cables of other sizes Horizontal runs: Vertical runs: Collectively clamped at 5000 mmm Collectively clamped at 1000 mm interval (max.) interval (max.) 3. Control Cables Horizontal runs: Vertical runs: Collectively clamped at 5000 mmm Collectively clamped at 1000 mmm interval (max.) interval (max.)
1.22 Clamp spacing for cables supported directly along structure/ ceiling shall be as under: a) b) c) Clamp spacing 1. In horizontal runs: 10000 mm (max.)) 2. In vertical runs: 750 mm (max.) Spacing between cables: 30 mm (min.) Supports shall also be provided at each bend. 2. Cable routing in cable trenches/trestles 2.1 For trenches with concrete covers, site to ensure that the concrete segment at load end should be provided with pipe for cable outlet. 2.2 Insert plates should be provided in cable trench @ 1.5/1.25/1.0m (for galvanizing tray support) for supporting cable trays for cable routing. 2.3 Wall cut-out or pipe should be providedd in cable trench wall for smooth passage of proper placement of trench cover and avoiding cutting of cables by trench covers. cables and 2.4 Cable should be trench. laid in duct bank/through wall opening while entering a building through cable 3. Cable routing in cable spreader/false floor: 3.1 Cables below spreader / false floor shall be laid providing cable trays / angles supported on stub columns providedd for supporting false floor. Route shall be identified & marked in the drawing to facilitate site to route the cables in proper manner. Routing shall be planned keeping in mind number of risers available for routing of incoming & outgoing cables in the room. This will avoid jumbling of cables & overloading on particular connecting cable riser. Pl. refer attached SKETCH- to avoid 1 & 2. 3.2 Alternatively cables below spreader false / floor mayy be routed tying together in bunches over crowding & spreading over the floor all around.
4. Cable routing in cable vaults/cellar: 4.1 Inside the cable vaults crossing of cables from onee horizontal raceway to another shall be done through the vertical cable trays supported from ceiling to avoid criss crossings in the horizontal raceways. 4.2 Inside the cable vaults, cross connections away from entry points are to be utilised first and then the cross connections near the entry points are too be utilised later. This is required to avoid jamming of the cable trays at entry points by the cables running in the crosss connections. 4.3 Cables should be lifted in the space available along the cable tray support for termination at panels for smooth passage of other cables. 5. Specific precautions to be taken while laying of cables: 5.1 All the cables should be properly laid in cable trays, dressed and clamped at the time of laying itself. For identification of cables aluminum tags at regular intervals through out the run of the cable shall be provided. 5.2 While laying the cable, a roller shall be temporarily installed at an appropriate place avoid any damage to the cable while releasing the cable from the drum. in order to 5.3 Temporarily laid cables should be removed from cable trays to avoid overloading. 5.4 Temporarily laid, dead, unutilized & damaged cables should be removed from cable trays to avoid over loading & relievee tray space. 5.5 Do not stretch the cable tight inside the tray or duct.. 5.6 Single core cables for AC application should be laid in trefoil formation using supplied trefoil clamps & to fix them with cable trays with the help of hardwares. 5.7 Once the cables are laid, welding should always be avoided either directly on the tray or above the tray and the welding shall be prohibited. If absolutely necessary, steel sheet or any other nonflammable material shall be placed over the tray or the cover so that the cable will not get damaged. 5.8 The cables shalll be meggered before laying. Whilee laying cable the instrumentation cables shall not mix with power cables. Care shall be taken to avoid sharp bending and kinking of conductor
damaging insulation and stressing the cable beyondd the pulling force. Cable shall be protected at all times from mechanical damage. 5.9 No cables shall be laid in ducts or trenches where other services such as oil pipes, steam or water pipes are laid. 5.10 In case of deviation by site of erection of cable trayss w. r. t PEM drawings, intimation / discussion with PEM is required. 5.11 Trunk routes in case planned by site to be shared with PEM for proper planning & rectification. 5.12 All cable tray risers from control room / switchgear room designated for various system cablings. are to be marked with paint for 5.13 All over spilled cables outside cable trays are required proper dressing. pieces may be installed for accommodat tion of thosee cables. If required cable tray 5.14 Close co-ordination between site electrical cabling contractor and C& &I cabling required for cable laying. contractor