Within myCableEngineering, we consider standard cable constructions.  Cables which vary from this may be simulated using approximations to the standard construction or particular case additions. 

Low Voltage Cable Construction

Cables follow the typical construction pattern of the conductor, insulation, bedding, armour and outer sheath.   Conductors are either copper or aluminium.   

Conductor Shape

Cable conductors can take various shapes.  IEC 60228 "Conductors of insulated cables", identifies three main conductor shapes:

1. Circular
2. Circular compacted
3. Shaped

The shaping of conductors takes place during the manufacturing process and can result in improvements in conductor dimensions and a.c. resistance.

Stranding

Stranded conductors consist of individual wires bound together to form the larger conductor.  Standing improves the flexibility of conductors and reduces the overall inductance.

 IEC 60228 "Conductors of insulated cables",  gives guidelines on the minimum number of wires in a stranded conductor for various cross-sectional areas. 

High Voltage Cable Construction

The construction of high voltage cables is similar to that of low voltage.  Typically conductor and insulation screens are added to prevent air-filled cavities which would lead to electric discharges. 

Where armour is not needed,  it may still be desirable to have a metallic outer screen for functional reasons.    This could consist of tapes or a braid, or concentric layer of wire or a combination of wires and tapes.

Cable Buildup

Conductors - normally copper, aluminium or aluminium allow class 1 or class 2 in accordance with IEC 60228.

Insulation - extruded dielectric (various types in use).

Screening - medium and high voltage cables have a metallic layer surround the cores (either individually or collectively).

- conductor: non-metallic, semi-conducting layer
- insulation: non-metallic, semi-conducting layer and metallic screen
- collective:  semi-conducting inner covering, and metallic screen (overall laid up cores)

Metallic Layers - 

- metallic screen:  wires or tapes
- concentric conductor: electrical resistance dictated by regulation
- metallic sheath: typically lead  or lead alloy tube
 - metallic armour: flat wire, round wire or double tape (sometimes braided)

Separation sheath  (bedding) - applied where the underlying metallic layer and armour are of different materials.    Applied between laid up cores and armour of low voltage cables. 

Oversheath - the outer covering of the cable.

Typical Construction

Given the large variety of cables available, it is not possible to cover all constructions.  However, typically cable construction will have the following pattern:

Note: potentially there are other construction elements (water blocking tape, woven fabric tape,  for example).   Any effect of these on calculations is negligible. 

From the 1st July 2017, cables sold within the European Union need to comply with the Construction Products Regulation 305/2011 (CPR).   From this date, it is mandatory for you to specify CPR CE marked cables.

CPR - how it works

The CPR creates a harmonised standard for the fire rating of cables.  Ratings (class) are classified from A to F depending on the flame characteristics.  Additional properties cover smoke, flaming droplets and acidity.  Cables rated A have the best performance, with those rated F the lowest.

• A_ca
  • high performance, products which practically cannot burn, PCS
• B1_ca
  • no or very little burning (30 kW flame source), FIGRA, FS, H, HRR, THR
• B2_ca
  • very little burning (20.5 kW flame source), FS, THR, HRR, FIGRA
• C_ca 
  • continuous flame spread, limited fire growth limited heat release (20.5 kW flame source), FIGRA, FS, H, HRR, THR,
• D_ca   
  • continuous flame spread, moderate fire growth, moderate release (20.5 kW flame source), FIGRA, H, HRR, THR,
• E_ca   
  • products with a small flame attack, do not cause a large flame spread, H
• F_ca   
  • flammable, H

FIGRA, fire growth rate index, test method EN 50399
FS: vertical flame spread, test method EN 50399
H: vertical flame spread, test method EN 60332-1-2
HRR: the maximum value of heat release, test method EN 50399
PCS: the gross heat of combustion, test method EN ISO 1716
THR: total heat release, test method EN 50399

B1_ca to D_ca include additional tests:  smoke production EN 60134-2, flaming droplets EN 50399, and acidity EN 60754-2.

CPR considers the reaction to fire inline with EN 50575 "Power, control and communication cables. Cables for general applications in construction works subject to reaction to fire requirements", and resistance to fire EN 50577 "Electric cables. Fire resistance test for unprotected electric cables (P classification)".  The CPR fire test takes into consideration five properties:

1. Flame spread
2. Heat release
3. Smoke production, s1 (best, good visibility) to s3 (worse, low visibility)
4. Flaming droplets, d0 (best, no droplets) to d2 (worse, long/persistent droplets)
5. Acidity, a1 (low acid cables) to a3 (standard cables)

The full classification of a cable is the combination of its class and additional properties, for example, C_ca -s1b, d1 a1. 

The CPR cover all cables, at all voltages, including power, control and optical fibre, which are installed in fixed installations.  There are, however, options (special applications), which may be exempted by a countries National Regulations. Countries National Regulations and Authorities may specify particular CPR classifications for different types of installations.  

Labelling and Certificates

Manufacturers are obliged to provide labelling containing mandatory information and a declaration of performance (DoP).

Specifying Cables

You should check your countries regulating authorities for details on the specifications of cables. 

In the absence of guidelines, industry practice would recommend class E_ca or higher, Low Smoke Zero Halogen insulated and sheathed cables. For fire sensitive installations, higher classes should be used.

Centre to centre spacing is frequently used in cable calculations.  This is particularly obvious in the calculation of inductance. Where the spacing varies between cores (for example, in flat configurations), an average spacing is used; the geometric mean distance.

Geometric Mean Distance

Given  known spacing between conductors, the geometric mean distance is given by:

$d=\sqrt[3]{d_{L1L2}{d}_{L1L3}{d}_{L2L3}}$

and 

$d_{LN}=\sqrt[3]{d_{L1LN}{d}_{L2LN}{d}_{L3LN}}$

where:
d             - geometric mean distance between phases, m
d_n            - geometric mean distance between phase and neutral, m
L_XL_X        - spacing (centre to centre) between phases, m
                        L₁L₂        - between L1 and L2
                        L₁L₃        - between L1 and L3
                        L₂L₃        - between L2 and L3
                        L₁L_N        - between L1 and N
                        L₂L_N        - between L2 and N
                        L₃L_N        - between L3 and N

Cable Configurations - Distance Between Conductors

The spacing between cores depends on the cable arrangement and configuration.  The starting point is a set of standard cable/core configurations:

	cables or cores arranged in trefoil $$\begin{gathered} L_1{L}_2=d \\ {L}_1{L}_3 =d \\ {L}_2{L}_3 =d \\ {L}_1{L}_N =d \\ {L}_2{L}_N =\sqrt{3}d \\ {L}_3{L}_N =2d \end{gathered}$$
	cables or cores in flat formation (touching) $$\begin{gathered} L_1{L}_2=d \\ {L}_1{L}_3 =2d \\ {L}_2{L}_3 =d \\ {L}_1{L}_N =3d \\ {L}_2{L}_N =2d \\ {L}_3{L}_N =d \end{gathered}$$
	cables or cores in flat formation (spaced) $$\begin{gathered} L_1{L}_2=d \\ {L}_1{L}_3 =2d \\ {L}_2{L}_3 =d \\ {L}_1{L}_N =3d \\ {L}_2{L}_N =2d \\ {L}_3{L}_N =d \end{gathered}$$
	cable or cores in 4-core arrangement $$\begin{gathered} L_1{L}_2=d \\ {L}_1{L}_3 =d \\ {L}_2{L}_3 =\sqrt{2}d \\ {L}_1{L}_N =\sqrt{2}d \\ {L}_2{L}_N =d \\ {L}_3{L}_N =d \end{gathered}$$
	cable or cores in 5-core arrangement $$\begin{gathered} L_1{L}_2=d \\ {L}_1{L}_3 =\frac{h}{\cos \left({\displaystyle \raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$10$}\right.}\right)} \\ {L}_2{L}_3 =d \\ {L}_1{L}_N =\frac{h}{\cos \left({\displaystyle \raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$10$}\right.}\right)} \\ {L}_2{L}_N =\frac{h}{\cos \left({\displaystyle \raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$10$}\right.}\right)} \\ {L}_3{L}_N =d \end{gathered}$$ where: $r=\frac{d}{2*\sin \left({\displaystyle \raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$5$}\right.}\right)}$ and $h=r \left[1+\cos \left(\raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$5$}\right.\right)\right]$ * angles are in radians

For single core cables, d is the overall diameter of the cable.  For cores of a low voltage multicore cable, d is the diameter of the core over the insulation. For medium voltage cables, d is the diameter over the insulation and including any insulation semi-conducting layer and metallic screen.

The distances given in the table are accurate for circular conductors.  For sector-shaped conductors, using the values above will result in insignificant minor variations against actual geometric mean distances.