This specification specifies the Medium Voltage cables in electrical construction. The contractor shall supply, install, test and commission the 13.8 KV cable network as shown and indicated on the contract documents. Conductor sizes shall be metric as shown, conductor sizes with cross-sectional areas smaller than specified will not be accepted. The cable installation shall include all trench work, sleeves, ducts, and all necessary fixings and cable terminations at both ends of the cable. The contractor shall supply and install cable marker posts to indicate the route of all underground cables. Markers shall be provided at every bend along the route and at sites of crossing roads or pipelines.
Medium Voltage (MV) cables in electrical construction are one that can operate in a voltage range between 1kV and 100kV and is defined by the International Electrotechnical Commission (IEC).
The medium voltage cable shall be standard circular compact copper conductors, XLPE insulated, copper tape screened, assembled together with non-hygroscopic polypropylene fillers, covered with tapped bedding unarmoured and PVC sheathed. Cables shall comply with IEC 502.
APPLICABLE STANDARDS FOR MV CABLES IN ELCTRICAL CONSTRUCTION
The design of MV cables in electrical construction for use in the 13.8 KV network shall be based on IEC specification 502. Contractor may propose cable to other International Standards for the review of the Architect/Engineer and approved by the Client.
For each lot of cable the Contractor shall supply a certificate of original and test issued by the manufacturer stating its original, date of manufacture and standards to which it complies. In the absence of such documents, the Architect / Engineer reserves the right to require tests, at the Contractor’s expense, to be performed by an official laboratory on samples taken from lots of cables delivered on the site.
BASIS FOR MV POWER CABLE SELECTION
The contractor shall check cable ratings indicated on drawings against final equipment ratings and system fault levels when known. Any required change in the cross-sectional area of the cable proposed shall be brought to the attention of the Engineer in charge. Products for the use of this work shall be obtained from the Approved Cable Company.
CONSTRUCTION OF MV POWER CABLE
Medium voltage cables construction shall be armoured, 15 KV XLPE insulated copper conductor cables having the following characteristics:
1. Conductor: Plain circular compacted copper conductor to IEC 228 Cl. 2.
2. Conductor screen: Extruded semi-conductive compound.
3. Insulation: XLPE (cross-linked polyethylene extruded and cross-linked by silanc curing process rated 90 deg. C.
4. Insulation screen (non metallic part): Graphite layer covered with a lapping tape or extruded semi conductive material.
5. Insulation screen (metallic part): Copper tapes 0.10 mm thickness.
6. Bedding: Tapped bedding.
7. Armour: Galvanized steel tapes applied helically over the PVC bedding.
8. Outer sheath: PVC Type ST2 To IEC 502 Color Red.
MEDIUM VOLTAGE CABLE INSTALLATION
The contractor shall be required to carry out necessary field surveys and acquaint himself with the routing of all other service lines.
Medium voltage cables shall be laid directly on the ground on 10 cm minimum soft sand, and the ground will be backfilled properly avoiding any aggregate with sharp edges for damaging the cable insulation. The contractor shall provide 5 cm thick cement tiles above the backfill then HT marker for safety.
The cable ladder shall be bounded to the ground at both ends.
All cables shall be marked with their respective cable numbers all along their routes at intervals not greater than 2.5 meters.
Spliced and joints on MV cables are not permitted.
Radius of cable bends shall be greater than 12 times the outside diameter of the cable.
FIELD TESTS OF MEDIUM VOLTAGE CABLES
All high voltage cables furnished under contract shall be tested by the contractor in the presence of Engineer to determine if properly installed and provide satisfactory performance. The contractor shall provide all test equipment required.
Cable testing shall be a minimum of but not limited to the following:
1. every cable shall be tested prior to its connection to site voltage, in the following manner.
2. The contractor shall test all sub-main cables using DC voltage injection equipment and a digital voltmeter. The test for cables shall follow the following procedure.
2.1 Insulation resistance test.
2.2 Apply test voltage for 2 minutes on each of the following :
- Phase to phase.
- Phase to neutral.
- All conductors to earth.
All conductors not under test shall be bonded to earth. The Contractor shall record the leakage current at the time of all tests.
2.3 Apply a reduced AC voltage at 60 Hz and record the charging current.
3. All final connections and small wiring shall be tested using a 1000 V megger and each reading tabulated and submitted to the supervising officer.
4. The test voltage shall be Dc 2 E + 2 KV where “E” equals the line (phase to phase) voltage that the cable will see in normal operation. It will be acceptable to test LV cable while connected to their source, if the source may be readily isolated.