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.
Cables follow the typical construction pattern of conductor, insulation, bedding, armour and outer sheath. Conductors are either copper or aluminium.
Cable conductors can take various shapes. IEC 60228 "Conductors of insulated cables", identifies three main conductor shapes:
The shaping of conductors takes place during the manufacturing process and can result in improvements on conductor dimensions and a.c. resistance.
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.
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.
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 underlying metallic layer and armour are of different materials. Applied between laid up cores and armour of low voltage cables.
Oversheath - outer covering of the cable.
Given the large variety of cables available, it is not possible to cover all constructions. With myCableEngineering we use the following typical pattern:
Note: potentially there are other construction elements (water blocking tape, woven fabric tape, for example). Any effect of these on calculations is negligible. Construction elements are ignored for cables without these (no armour in unarmoured cable for example).