Electrical Notes

  Cable Selection Parameters:  (1) Voltage of Cable: The Nominal voltage is to be expressed with two values of alternative current Uo/U in V (volt) Uo/U : Phase to earth voltage Uo : Voltage between conductor and earth U : Voltage between phases (conductors) ( i ) Low-tension (L.T.) cables — upto 1000 V (ii ) High-tension (H.T. ) cables — upto 11,000 V (iii ) Super-tension (S.T.) cables — from 22 kV to 33 kV (iv ) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV (v ) Extra super voltage cables — beyond 132 kV   A low-voltage system usually has a solidly earthed neutral so that the line to earth voltage cannot rise higher than (line volts) ÷ √ 3. Cables for low-voltage use are insulated for 600V rms score to earth and 1000V rms core to core. High-voltage cables used in Shell installations are rated 19000/3300V or 3810/6600V or 6600/11000V, Phase/Phase. In selecting the voltage grade of cable, the highest voltage to earth must be allowed for. For example, on a norma...

  (9) Armoring: Code:  IS: 7098 / IS: 3975 / IEC:60502 / BS:6622/BS:7835. Material:  metallic or non-magnetic Alumimium, Steel wire/strip. Used for  : LV, MV & HV Cables The armor provides mechanical protection against crushing forces. Armor also can serve as an Earth Continuity Conductor (ECC). The armoring type could be: Mechanical protection of the cable is provided by a single layer of wire / Strip strands laid over the bedding. Steel wire / Strip is used for 3-core or 4-core cables, but single-core cables have aluminum wire armoring. When an electric current passes through a cable it produces a magnetic field (the higher the voltage the bigger the field). The magnetic field will induce an electric current in steel armor (eddy currents), which can cause overheating in AC systems. The non-magnetic aluminum armor prevents this from happening. Magnetic Material’s armoring for 3Ph System:  With 3-core or 4-core cables the vector sum of the currents in the co...

  (5) Insulation Screen: Code:  IS:7098/IEC:60502/ BS:6622/BS:7835 Material:  Extruded thermo set semi-conducting compound, Carbon paper and carbon loaded polymer. Used for :  Cable from 6 to 30kV (MV & HV Cables) Purpose: An extruded layer of semi conducting is applied over the insulation layer to insure that the electric stress is homogeneous around the insulated core. The semi conducting layer shall be firmly bonded to the outer layer of the insulation layer. The Purpose of Insulation screen is same as Conductor Screen. The Purpose of Insulation Screen is to reduce voltage stress at the interface between the conducting and insulating component A cylindrical, smooth surface between the insulation and Metallic shield Insulation screen is a layer of black cross linked semi conductive compound of approx 1mm thickness and is either fully bonded to the insulation layer, or can be “cold strippable” by hand. When terminating or jointing the cables, it is necessary to ...

  Cable Construction: Parts of Cable: Conductor (For LV/MV/HT Cables) Conductor Screen (For MV/HT Cables) Filler & Binding Tapes (For LV/MV/HT Cables) Insulation (For LV/MV/HT Cables) Insulation Screen (For MV/HT Cables) Separation Tape (For MV/HT Cables) Bedding (Inner Sheath) Metallic Sheen (For MV/HT Cables) Armor (For LV/MV/HT Cables) Outer Sheath (For LV/MV/HT Cables) Water Blocking Tapes –Optional (For MV/HT Cables) Insulation Tapes–Optional (For MV/HT Cables) (1) Conductors: Code:  IS:8130 / IEC 60228/ BS 6360 Material:  Class 2 – Annealed Plain / Tinned Copper / Aluminum. Used for :  LV ,MV & HV Cables Purpose: Usually stranded copper (Cu) or aluminum (Al) is used. Copper is denser and heavier, but more conductive than aluminum. Electrically equivalent aluminum conductors have a cross-sectional area approximately 1.6 times larger than copper, but half the weight. The size of the copper / Aluminum conductor forming one of the cores of a cable is expres...

  (I)  No of Multi core Cables less than 2000 volts in the Cable Tray   (A) 4/0 AWG/Kcmill (120 Sq.mm) Cable or Larger Cables: The ladder cable tray:  Tray must have an inside available width equal to or greater than the sum of the diameters of the cables, which installed in a single layer. Solid bottom cable tray:  the sum of the cable diameters is not to exceed 90% of the available cable tray width.   (B) Cables Smaller Than 4/0 AWG/Kcmill (120 Sq.mm)   Ladder Type Cable Tray:  The total sum of the cross-sectional areas of all the cables to be installed in the cable tray must be equal to or less than the allowable cable area for the tray width, as per following Table. Solid Bottom Cable Tray:  The allowable cable area is reduced by 22%. Inside width of Cable Tray Allowable Cable Area Sq.inch (Sq.mm) 6 inch (152.5mm) 7 Sq.inch (4516 Sq.mm) 9 inch (228.6mm) 10 Sq.inch (6451 Sq.mm) 12 inch (304.8mm) 14 Sq.inch (9032 Sq.mm) 18 inch (457.2mm) 21...