At a mine processing site a new process section requires power that will be by an underground service.   It will be fed by a 5MVA 22/3.3kV 7.5% Z transformer.  (3ph fault power 66.7 MVA, fault current 11.7kA for 1s). The load will be approx. 1MVA 3.3kV, 175A 3ph mixed load – service transformer, motors, ups, vsu, HVAC, computer.  Expected utilization is at 70% with a .8pf.  During the peak time 50% of the load will have 50% 2min cycle.

Cable route

• A – B From the Substation cable trench to point B.  500m in made up soils.  110’ turn at point B.
• B – C 150m in virgin clay soil.
• C – D 150m in virgin clay soil and 110’ turn at D.
• D – E 200m Inlet crossing.  Trench and surface lay.  Tidal considerations.
• E – F 50m in made up soil and 45’ turn at point F.
• F – G Under railway crossing. (50m) HD mineral trains – State owned.
• G – H Under roadway crossing.  Basic rock built road, heavy truck traffic (30m)
• H – I   Overhead cable tray.  100m.
• I – J   Indoor cable tray to switchboard.

For each of the segments on the drawing state the best choice of cable in respect of design and suitability and give your reasons for that choice.   

Using the cable tables provided in the notes (SA), along with your countries installation standards – such as AS3000, AS3008, AS 2678 for Australia, determine the cable size to be used based on copper conductor, cable depth 1.5m.  Determine the conditions and cable criteria for each of the segments above.  Draw up a specification and plans for this cable installation.  Where ‘wiring rules’ compliance is required, state so.  For instance, minimum depth for HV cable.

Answer rating: 100% (QA)

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