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General product information, power cables
 
Electrical values

Resistance

Product pages give for each product type the maximum direct current (DC) resistance of the conductors at a temperature of 20 °C and the alternating current (AC) resistances at the operating temperature.   

AC resistances include all the additional losses taking place in the conductor (skin effect, proximity effect), dielectric losses in insulation, circulating currents in the metallic screen and eddy currents as well as magnetic reversal in the armour. These additional losses have been added to the phase conductor AC resistance using the following assumptions:
· alternating current frequency is 50 Hz
· metallic screens are connected together at both ends of the cable
· in flat formation the free space between cables is equal to the cable diameter
· in trefoil formation cables are touching each other
  
Inductance

Inductances are approximate values. Inductances are given in the form of inductance/phase.
 
For single-core cables the inductance values have been calculated using the following assumptions:
· in flat formation the free space between the cables is equal to the cable diameter
· in trefoil formation the cables are touching each other
 
Operating capacitance

Operating capacitances are average values at the rated voltage, at a temperature of 20 °C and at a frequency of 50 Hz.
 
Especially with PVC insulated cables, the capacitance will increase by 40 % if the temperature of the conductor rises from 20 °C to the maximum permissible continuous conductor temperature.
 
Charging and earth fault current

Charging and earth faults currents are given for the medium voltage and the high voltage cables. Charging and earth fault currents have been calculated using the following assumptions:
· alternating current frequency is 50 Hz
· the voltage of the network is equal to the rated voltage U
· the capacitance of the cable is equal to the given operating capacitance

Current carrying capacity

Each product page contains information about current carrying capacities in given conditions. These values are valid for a separate multi-core cable or for a separate group of three single-core cables without any other heat generating sources nearby.
 
The basic assumptions of current carrying capacities are given in the following sections. If the conditions differ from the above assumed premises, the load current values shall be adjusted with appropriate correction factors or shall be calculated with professional software.

Conductor temperature
 
The current carrying capacities have been calculated in continuous operation at the maximum permissible temperature of the conductor.
 
If XLPE insulated cables are laid underground, it is important to remember that a continuous conductor temperature of 90 °C may dry up the surrounding ground, causing an overload of the cable. For this reason, the continuous conductor temperature should not exceed 65 °C.

 Surroundings 
 
Underground installation:
· ground temperature is 15 °C
· installation depth is 0.7 m for ≤ 36 kV cables and 1.0 m for ≥ 45 kV
· thermal resistivity of the ground is 1.0 Km/W
 
Increased ground temperature, installation depth or thermal resistivity will decrease the current carrying capacity of the cable. Also installation in troughs, pipes, tunnels or in any other special surroundings may decrease significantly the current carrying capacity.
 
In aerial installations the ambient air temperature is 25 °C. The air shall move freely around the installed cables. Higher ambient air temperature will decrease the current carrying capacity of the cable.

Distance between single-core cables

In flat formation the free space between adjacent cables is equal to the cable diameter.
 
In trefoil formation the cables are touching each other.

Metallic screen circuit of single-core cables

The metallic screen circuit is closed meaning the metallic screens are connected together and earthed at least at one end of the cable.
 
Changing the cross-section or the type of the metallic screen may have an impact on the current carrying capacity of the cable.
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