Photo 1 – Electric cables installed on cable tray
UPDATE March 18, 2011: As part of the re-construction process of this blog, I will gradually upload photographs of real installations of electrical works.
The photos will be attached to the relevant existing posts when that is possible.
If the original article is in the form of specifications like the one below, having pictures and images in the middle of it may spoil the structure of the article. In that case I may send a new post to load the photographs.
Then I will create a hyperlink in the existing post to connect to the new post and the photos.
Of course the new post will also be hyperlinked back to the original post so the readers will not lose their way trying to come back to the article he/she was previously reading.
The uploading takes time and I will do it gradually. In the process, the article will look half-completed and definitely not nice-looking.
However, I set up this blog to give readers contents. Lots of contents. If you are looking for a nice-looking blog, this is definitely not a right place.
SCOPE
The scope of this section is to set out the materials, requirements, methods, workmanship, standards and regulations in connection with the cable installation works.
TYPE OF CABLES
PVC insulated armored cables (PVC/SWA/PVC)
Cables shall be PVC insulated, steel wire armored, PVC sheathed overall and shall be manufactured and tested in accordance to the specifications of BS 6346. They shall be of the 600/1000 volts grade with high conductivity copper conductor.
XLPE insulated cables
Cross-linked polyethylene insulated cables shall be suitable for 1000 volts with high conductivity copper conductor. They shall be manufactured and tested in accordance with to the specifications of B.S. 5467.
For 11000/ 33000 volts applications, the cable shall be manufactured and tested according to the specifications of B.S. 6622:1985.
For normal operation, XLPE insulated PVC sheathed (XLPE/PVC) cables shall be employed. For underground applications XLPE insulated, steel wire armored, PVC sheathed (XLPE/SWA/PVC) cables shall be used.
Paper insulated cables
The manufacturing of mass-impregnated, non-draining type paper-insulated and lead-alloy sheathed (PILC) cables shall fully comply with the requirements of BS 6480. Unless otherwise specified, PILC cables shall be insulated for 1000 volts with high conductivity copper conductor.
For underground distribution, PILC cables with steel tape armored and served overall (PILCSTA&S) shall be employed.
Mineral insulated cables
Mineral insulated copper sheathed (MICC) cables shall be manufactured and tested in accordance to BS 6207 and shall be of the heavy-duty type of 1000 volts. The cables consist of high conductivity copper conductors, embedded in pressure packed magnesium oxide insulation within a robust, ductile, seamless copper sheath.
For situations corrosive to copper and underground installation, MICC cable with PVC sheath shall be used.
CABLE HANDLING
The most important point to observe in handling cables is that great care must be exercised at all times.
Every precaution should be taken to avoid dropping a drum of cables. Dropping, even from a short height, will flatten the layers of cable nearest to the barrel of the drum due to the weight from the outer layers. Similar distortion will also occur if the drum falls on its sides.
When rolling the drum into position it is essential that the drum rolls smoothly in the direction of the arrow painted on the side of the drum. If this instruction is not followed, slack cables will tend to accumulate towards the inner turns and may result in damages to the cables.
Wooden battens around the cable drum should be very carefully removed. Suitable tools should be used for this purpose.
When a drum is in position, it should be mounted on jacks and disposed so that the cable is pulled off from the bottom and not over the top.
It is preferable to mount the drum at one end of the cable run as close as possible to the edge of the cable trench so that the cable can be pulled off in a continuous manner on rollers in trench and is in its final position when the last turn leaves the drum. This procedure is not always possible because of the excessive length and weight of the cable run or because of obstruction or pipes under which the cables have to be threaded.
In such cases it may be necessary to position the drum at some other point along the cable run and lay off the cable on the ground near the drum in a series of loops, one above the other in the form of a figure eight, crossing the cable back and forth on itself.
When the whole length has thus been removed from the drum the inside end of the cable will be on top and can be pulled along towards its final position on rollers in the same manner as if the cable was coming off the drum itself. Whichever procedure is adopted great care must be taken at all times to ensure that cable is not twisted and that the turns are well above the minimum bending radii of cable. The radii of cable below 22 kV may be taken as follows:
For sizes up to 50 mm overall diameter – Min bending radii: 12 x diameter
For sizes 50 mm and above overall – Min bending radii: 20 x diameter
Wire cable stockings with an eye at one end should always be used with pulling ropes. Ropes should never be tied directly to the cable ends.
RE-DRUMMING OF CABLES
Owing to deterioration of cable drums, it is sometimes necessary to transfer cables to another drum. Steps to do this should be taken immediately when signs of the drum deterioration become evident and on no account must the transfer be delayed until the drum is on the point of collapse as so often happens.
Both drums should be jacked off the ground and the cable slowly wound from one drum to the other with the cable bending in the same direction on the new drum as on the old one.
Great care must be taken to ensure that the radius of the new drum barrel is not less than the minimum bending radius of the cable. As arrow should be painted on the side of the new drum to indicate the direction in which the drum was turned during rewinding operations i.e. the direction in which the drum should be rolled. The words “REWOUND CABLE” should also be clearly painted in Red on both sides of the drums.
The re-drumming of cables will only be carried out under the direct supervision of the S.O.’s Representative.
INSTALLATION OF CABLES
General
All cables shall be terminated in cable boxes or sockets of suitable size for the cable employed without excessive clearance.
The spacing of the cable supports shall be determined by the size of cable and in the case of cables of less than 25 mm in overall diameter the spacing shall not exceed 760 mm.
All cables shall be supported in such a manner as to unsure that they do not sag after erection and their means of support shall be to the approval of the S.O.’s Representative. Fixing shall be made with raw bolts or other patent fixing devise of design approved by the S.O.’s Representative.
Where cables enter or leave cable pipes or ducts, the entries shall be sealed effectively by means of close fitting solognum impregnated split wooden plugs and a mixture of compound and transformer oil or other approved method in order to prevent the ingress of water or dirt.
All cables passing through interior walls or floors shall be sealed effectively to the approval of the S.O.’s Representative by means of asbestos cement after the cables have been pulled through in order to prevent accumulation of moisture and the ingress of debris, sand and vermin.
All cables shall, where they pass through floors or otherwise in such positions vulnerable to damage by mechanical or other means shall be protected by short length of steel pipe suitable bushed to prevent abrasion of the cable.
Straight and tee joints
Straight and tee joints in any of the cables installed under this contract will only be permitted in very exceptional circumstances and only with the S.O.’s Representative approval in writing. The cost of such straight and tee joints, if permitted by the S.O.’s Representative shall be born by the contractor unless such joints are arising from unavoidable limitations in manufacturing lengths or from alterations in routes after initial approval.
Cable pulling
Winching of cables through ducts shall only be carried out with the approval of the S.O.’s Representative in which event a pulling eye shall be attached to the conductors.
A cable sheath stocking may be employed on cables where undue stress in the sheath is likely to occur.
Underground cable boxes
Straight through and tee underground joint boxes shall comprise a good quality, cast iron, split type box complete with nuts and bolts and accommodating a lead sleeve or case suitable for plumbing to the cable sheath. Boxes shall be free from blowholes and provided with compound filling holes and plugs. Internal and external box surfaces shall be painted with an approved preservative compound.
Boxes shall be provided with armor clamps and provision shall be made for bonding the steel wire armoring across the joint and on to the lead sheath or case. The bond shall comprise a tinned copper conductor and in no case shall it have a cross section area less than that of the largest conductor.
Lead sleeves shall be formed from solid drawn tubes and lead cases for three and four way joints shall be made from lead sheet halves rebetted and soldered together on completion of the joint. Lead sleeves and case shall be free from porosity and impurities, of adequate thickness, complete with filling holes and lead seals.
Core numbers printed on papers shall be strictly observed when jointing and such numbers shall be maintained throughout the system. Papers numbered ‘0’, ‘1’, ‘2’, and ‘3’ shall be identified as neutral, red, yellow and blue phases respectively and in the case of two core cables number ‘1’ shall be the phase conductor and ‘0’ the neutral.
Crossing of cores in boxes shall be avoided wherever possible but connections shall be consistent with the foregoing requirements.
Sealing end boxes
Box shall be of the split type with effectively sealed joint surfaces and manufactured from good quality cast iron free from blow holes with screwed plug filling and venting holes of ample dimensions and of such design as to permit continuous filling in one operation in order to avoid the formation of voids.
Box shall be painted internally and externally with an approved preservative compound and shall be complete with brass cone shaped wiping gland, armor clamp, fixing lugs and earth continuity bond across the box from armor clamp to equipment frame.
The top plate shall be tapped for conduit entry or fitted with bushing insulators as appropriate to the situations.
Where means of cable support is not provided within the equipment, or in any situation, the contractor shall provide adequate cable support below the sealing box in order to relieve the joint and the box of stress.
Where it is not possible to fix cable to the framework or supporting members specifically supplied for the purpose in or on apparatus, then the contractor shall supply and install to the satisfaction of the S.O.’s Representative such support fabricated from galvanized steels as may be necessary.
Boxes shall be of such design that they are suitable for attachment to the equipment served and shall allow cable conductors to be formed into the equipment terminals without distortion.
PVC or XLPE insulated armored cables
Jointing of PVC/SWA/PVC or XLPE/SWA/PVC cables shall be carried out by accredited and fully experienced jointers and evidence of this shall be produced to the satisfaction of the S.O.’s Representative before jointing is started.
All terminal sealing boxes, cable cores shall be carried through unbroken to apparatus terminals and cores shall sweat solid where they pass through cast resin.
All joint boxes, jointing materials and tools shall be of the type recommended ans manufactured by the cable supplier.
All joints which are buried in the ground shall be compound filled. The design of the box and the composition shall provide an effective seal to prevent moisture gaining access to the conductor ferrules and armor clamps.
Provisions shall be made for earthing the wire armor to the main electrode at the supply and by means of a metallic bond of adequate conductance, and the bonding connection should be as short and straight as possible.
The wire armoring shall be maintained electrically continuous and careful attention shall be paid to the design of all bonding clamps in joints and terminations to ensure that the resistance across a clamp is not higher than that of the equivalent length of the complete wire armor of the cable.
The conductance of the carcase of cast iron is normally sufficient for this purpose but, where it falls short, an auxiliary metallic bond shall be included.
Compression type glands for the terminations of cables will normally be included with the terminating boxes. Marshalling and other terminating boxes supplied under this contract, however, shall include the terminating cable glands.
The design of compression glands shall be such that the cable is not twisted when the gland is tightened. They shall provide facilities for the efficient bonding and termination of the armor wires and shall project at least 20 mm into the terminating box so that any condensation collected on the inner surfaces of the boxes cannot flow down between the cable cores. Where anti-condensation heaters are not fitted, drain holes shall be provided. It is possible to erect and dismantle any cable compression gland without the use of special tools.
Paper insulated cables
Jointing of PILC cables shall be undertaken only by by an accredited and fully experienced jointer and evidence of this shall be produced to the satisfaction of the S.O.’s representative before jointing is started.
At terminal sealing boxes, cable cores shall be carried through unbroken to terminal apparatus.
Every cable joint shall be started and finished during the same day and whenever cables are to be joined in the open during wet weather conditions, suitable precautions shall be taken to preclude ingress of moisture into the cable joint.
Joint and sealing boxes shall be thoroughly flame warmed on the outside of the box before pouring compound and compound temperature shall be checked frequently by the use of a suitable scaled and protected thermometer to ensure that the pouring temperature does not exceed the manufacturer’s recommendations and that overheating does not occur.
Whenever PILC cable is cut during the course of insulation, the open ends shall be sealed immediately, unless jointing is to follow, by means of a lead cap wiped to the sheath to form an air tight seal. Under no circumstances should PILC cables be sealed with insulating tapes.
PILC cables must be tested for moisture before jointing is commenced. Samples of paper both from the layers nearest to and furthest from the conductor should be immersed in transformer oil or paraffin wax, this will be immediately detected by bubbling. Samples of paper should be tested singly and should not be touched by hand but gripped in a pair of tweezers.
Testing of insulation resistance
An insulation resistance test should be made to each length of cable laid before jointing is commenced.
MICC cables
Insulation and continuity tests shall be carried out before and after MICC cable is installed using a 500 volt “megger” and this must result in an “infinity” reading, no lower value will be accepted. A blow lamp may be used for drying out cut cable ends if it is impracticable to cut to waste in which event the cable should be brought to cherry red heat at about 600 mm from the end and moisture carefully driven towards the cut end.
Minimum bending radius shall not be less than six times the cable diameter and saddle spacing not less than 60 times the cable diameter and nor greater than 760 mm whichever is the less.
An unsupported loop shall be left in the cable at terminations subject to vibration.
It is essential that the greatest care shall be taken to ensure that hands and all materials, particularly the compound are perfectly dry and clean when terminating MICC cables. The length of tails shall be kept to a minimum.
Dirt and metallic particles in the compound and any loose traces of dielectric left at the face of the sheath after stripping shall be carefully removed, prior to sealing.
Cold sealing compound shall be forced down one side of the pot and up the other at the same time “overfilling” in order to avoid trapping air at the base of the pot and to ensure that when the sealing disc is entered before crimping a completely solid insulation barrier is effected.
Electricians employed on termination of MICC cables shall be fully instructed and experienced in the stages of the operation and work will be stopped, if in the opinion of the S.O.’s Representative insufficient care and/or skill is not being exercised particularly in this phase of the work.
TESTING
The contractor shall be responsible for the testing of the cables and certifying that they are safe before the supply is switched on. Tests shall include continuity, phasing out and insulation resistance between conductors and between conductors and sheath employing 500 volts “megger” tester of other approved type tester. Voltage tests to prove the soundness of the protective servicing and continuity test to prove the soundness of the wire armoring. A copy of test results shall be submitted to the S.O. before the cables are energized.
FIRE RESISTANT CABLES AND INSTALLATION
Fire resistant cables shall be of 300/500 volts and 600/1000 volts and suitable for surface wiring and installation in conduit or duct.
Cables shall both fire resistant and flame retardant to IEC 331 and IEC 332 respectively. The cables shall also be tested for resistance to fire and water and mechanical shock to categories C, W and Z of BS 6387. The outer sheath shall be halogen free under IEC 754 Part 1. Test certificates to substantiate the compliance shall be submitted.
Cables shall be made of stranded, soft annealed copper conductor to IEC 228, mica tape fire barrier, insulation of cross-linked polyethylene with continuous conductor temperature of 90 degree Centigrade conforming to IEC 502 and outer sheath made of flame retardant low smoke and non-halogen polyethylene. Core identification of cable shall be by color of the insulation according to IEC 502. The maximum resistance of the conductors at 20 degree Centigrade shall not exceed that specified in IEC 228. The cables shall be FUJIKURA brand or equivalent.
Cables shall be suitable for installation by conventional methods without requiring expensive tools, seals or terminations. The accessories shall be non-hygroscopic, maintenance free and insensitive to vibrations. When installed on the cable trays, the spacing of saddles and the arrangement of cables shall be in accordance with the relevant BS or IEC standards. Minimum bending radius shall not be less than 6 times of the outer diameter of the cables.
The cables shall comply fully with the requirements of the Fire Department and relevant approval letter shall be submitted.
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RELATED ARTICLES: 1) Intro to XLPE armored cables;
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