## Fictitious Dimensions

Last updated on 2023-03-31 5 mins. to read

Traditionally the thickness of cable coverings has been related to nominal cable diameters by step tables.  As there can be variations in calculated nominal diameter, this can cause variations in thickness of layers for cables of the same design.  IEC 60502-2 'Cables for rated voltages from 6 kV up to 30 kV', introduces the concept of fictitious calculation to overcome these issues.

## Fictitious Method

Conductors - them fictitious diameter, dL (irrespective of shape or compactness) is given by:

CSA mm2 dL mm  CSA mm2  dL mm
1.5 1.4 15 13.8
2.5 1.8 185 15.3
4  2.3 240  17.5
6  2.8 300  19.5
10 3.6 400  22.6
16 4.5 500  25.2
25 5.6 630  28.3
35 6.7 800  31.9
50 8.0 1000  35.7
70 9.4 1200 39.1
95 11.0 1400  42.2
120  12.4  1600  54.1

Cores - for cable cores without semi-conducting layers, the ficititious core diameter Dc is given by:

$D c = d L +2 t i$

and for cables with semi-conducting layers:

$D c = d L +2 t i +3.0$

where:
Dc - core diamter in mm
ti - is the nomiman insulation thickness, in mm

Diameter over laid-up Cores - is given by:

$D f =k D c$

where: k is the assembly coefficient

k = 1 for single core cables
k = 2 for two core cables
k = 2.16 for three core cables
k = 2.42 for four core cables
k = 2.70 for five core cables

Inner Covering/ Bedding - the fictitious diamter DB is given by:

$D B = D f +2 t B$

where:
DB - diamter or inner covering/bedding in mm
tB - bedding thickness in mm
= 0.4 mm for Df <= 40 mm
= 0.6 mm for Df > 40 mm

Concentric condcutors and metallic screens - increase in diamter due to screens is given by:

CSA of cocentric conductor or metallic screen mm2 Increase in diamater, mm   CSA of cocentric conductor or metallic screen mm2  Increase in diameter mm
1.5 0.5  50  1.7
2.5 0.5  70  2.0
4 0.5  95  2.4
6 0.6  120  2.7
10 0.8  150  3.0
16 1.1  185  4.0
25 1.2  240  5.0
35 1.5  300  6.0

Note: if the cross-sectional area lies between two values, that the largest value as the increase in diameter.

- tape screen:

where:
nt - number of tapes
tt - nominal thickness of individual tape in mm
wt - nominal width of indvidual tape in mm

for lapped tape with overlap, the total thickness is twice that of one tape

for longitudinally applied type:
- overlap < 30%, total tickness = thickness of tape
- overlap >= 30%, total thichness - 2 x thickness of tape

- wire screen:

where:
nw - number of wires
dw - diameter of wire in mm
nh - number of counter helix
th - thickness of coutner helix in mm (if greater than 3 mm)
Wh - width of counter helix in mm

Lead sheath - fictitious diamter of lad shearth , Dpb is given by :

$D pb = D g +2 t pb$

where:

Dg - fictitious diameter under lead sheath in mm
tpb - thickness of lead sheath in mm

Seperation sheath - fictitious diameter Ds given by:

$D s = D u +2 t s$

where:
Du - fictitious diameter under separation sheath in mm
ts - thickness of separation sheath in mm

Lapped bedding - fictitious diameter Dlb given by:

$D lb = D ulb +2 t lb$

where:
Dulb - fictitious diameter under lapped beding in mm
tlb  - thickness of lapped bedding in mm

Addition bedding for tape-armoured cables - provide dover inner covering:

- fictitious diameter under addition bedding <= 29mm,  increase 1.0 mm
- fictitious diameter under additional beddign > 29 mm, increase 1.6 mm

Armour - fictitious diameter over the armour, Dx is given by:

- flat or round wire armour:

$D x = D A +2 t A +2 t w$

where:
DA - diameter under armour in mm
tA - thickness or diameter of armour wire, in mm
tw - thickness of counter helix (if any) in mm (if > 3 mm)

- dobuble tape armour:

$D x = D A +4 t A$

where:
Da - diameter under armour in mm
tA - thickness of armour tape in mm