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:

Layer	Symbol		LV	MV
	Diameter	Thickness
Conductor	dL		X	X
Conductor semi-conducting layer	dsc	tsc		X
Insulation	Di	Ti	X	X
Insulation semi-conducting layer	Dsc	Tsc		X
Insulation metallic screen	Ds	Ts		X
Laid up diameter	Df (multicore)		X	X
Collective metallic screen	Dls (multicore)	Tls		X
Separation sheath	Db	Tb	X	X
Armour	Da	Ta	X	X
Sheath	D	Tos	X	X

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