Capacitance of Three Core Cable

In this theory, formula for capacitance of three core to core cable and core to sheath is given.

Capacitance of Three Core Belted Cable

  • Let us consider the three-core belted cable. 
  • As there is potential difference exists between core to core of cable and core to sheath of cable, electrostatic field set up between them. 
  • There is core to core capacitance Cc and core to sheath capacitance Ce exists in the three-core cable. 
  • The core-to-core capacitance is delta connected whereas core to sheath capacitance is star connected.

Core to core capacitance = Cand Core to sheath capacitance = Ce


  • The three cores to core delta connected capacitance is converted into equivalent star connection.
       Ceq = 3Cc
  • Where Ceq = Equivalent star connection of delta connected core to core capacitance
  • Therefore, the total capacitance is equal to parallel combination of core to sheath capacitance and equivalent star connection of core-to-core capacitance of cable. The capacitance between core to neutral
  • CN = Ce + Ceq   = Ce + 3Cc


Measurement of capacitance of cable

Three Cores Bunched Together

  • The three cores of cable are bunched together and capacitance is measured between cores and sheath of cable. 
  • As the three cores are bunched together, there is no capacitance between cores. 
  • There exists capacitance between core and sheath Ce which are in parallel. Let us consider that this capacitor is C1 therefore

       C1 = 3Ce

       Ce = C1 / 3…….. ( 1 )


Capacitance Between Two Cores Is Measured with Third Core Connected to The Sheath

  • The capacitance between two cores is measured with the third core connected to the sheath. 
  • This eliminated one core to sheath capacitance. Let us consider that this capacitance is C2.

C2 = Cc + Cc / 2 + Cs / 2

     = ( Cc + Cc / 2 ) + Cs / 2

     = ( 3Cc / 2 ) + Cs / 2

     = ½ ( 3Cc + Cs )

      = ½ CN


Capacitance Between Two Cores Shorted with The Sheath and Third Core

Let us consider that this capacitor is equal to C3.

C3 = 2Cc + Cs


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