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Impedance Calculation

myCableEngineering Calculation Method

Calculation of Impedance is reasonably involved.  Other notes give an insight into the individual calculations.  

With myCableEngienering, the calculation will typically follow the following procedure

  1. d.c. resistance - is determined using IEC 60278.  For any cases in which IEC 60227 is not suitable, the resistance is obtained using CENELEC CLC/TR 50480. See also Conductor Resistance.
  2. temperature adjustment - the calculated d.c. is adjusted so that is value is accurate relative to the operating temperature of the conductor. 
  3. a.c. resistance - the a.c. resistance is obtained by applying skin and proximity effect factors as defined in IEC 60277. See also Conductor Resistance.
  4. positive and zero sequence impedance - are calculated using the methods give in IEC 60909 and resistance values calculated above. See also Impedance.

Resistance, Reactance

Positive and zero sequence impedance calculated above is in complex form.  The real part of the impedance gives the resistance and the imaginary part the reactance.

Capacitive Reactance

Currently, we do not calculate capacitive reactance.  This will likely be added as the application grows and develops the need for this calculation.

Impedance Equations

Cable Cores and Phase

Within myCableEngineering, the type of end load (regarding phases) a cable can be used on is dependant on the number of cores.

Single Core d.c., single phase a.c., three phase a.c., three phase + neutral a.c.
Two Core d.c., single phase a.c.
Three Core d.c., single phase a.c., three phase a.c.
Four Core three phase a.c., three phase + neutral a.c
Five Core three phase a.c., three phase + neutral a.c

Impedance

Impedance equations are presented in details in Impedance.  Depending on the type of cable, the number of cores and system type we use the following equations:

IEC 60609 Equations (Unscreened Cables)
  d.c 1ph 3ph 3-ph+n
None, TN-S, TT        
Single Core Cable 10/11 10/11 10/11 10/11
Two Core Cable 10/11 10/11 - -
Three Core Cable 10/11* 10/11* 10/11 -
Four Core Cable - - 10/11* 10/11
Five Core Cable - - 10/11* 10/11*
TN-C-S        
Single Core Cable 10/14 10/14 - 10/14
Two Core Cable 10/24 10/24 - -
Three Core Cable 10/24* 10/24* 10/11 -
Four Core Cable - - 10/11* 10/24
Five Core Cable - - 10/11* 10/24*
IT        
Single Core Cable 10/∞ 10/∞ 10/∞ 10/∞
Two Core Cable 10/∞ 10/∞ - -
Three Core Cable 10/∞ 10/∞ 10/∞ -
Four Core Cable - - 10/∞ 10/∞
Five Core Cable - - 10/∞ 10/∞

Equations Z1/Z0 - for a.c. cables (refer Impedance)

IEC 60609 Equations (Screened Cables)
  d.c 1ph 3ph 3-ph+n
None, TN-S, TT        
Single Core Cable 15/16 15/16 15/16 15/16
Two Core Cable 10/32 10/32 - -
Three Core Cable 10/32* 10/32* 10/32 -
Four Core Cable - - 10/32* 10/32
Five Core Cable - - 10/32* 10/32*
TN-C-S        
Single Core Cable 15/27 10/27 10/27 10/27
Two Core Cable 10/28 10/28 - -
Three Core Cable 10/28* 10/28* 10/32 -
Four Core Cable - - 10/32* 10/28
Five Core Cable - - 10/32* 10/28*
IT        
Single Core Cable 15/∞ 15/∞ 15/∞ 15/∞
Two Core Cable 10/∞ 10/∞ - -
Three Core Cable 10/∞ 10/∞ 10/∞ -
Four Core Cable - - 10/∞ 10/∞
Five Core Cable - - 10/∞ 10/∞

Equations Z1/Z0 - for a.c. cables (refer Impedance)
* - indicates spare core can be used for additional earth conductor

Note: For armoured cables, the armouring is always considered as part of the earth return path. If the user opts to ignore cable armour, the equations for unscreened cable equations are used. The effect of any return path through the earth itself is considered. If multiple spare cores are available, only one can be used as an earth return path.  For any spare core being used as an earth conductor, it's impedance is added in parallel with that obtained from the IEC equations.

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