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, d_L (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 D_c is given by:

$D_c=d_L+2t_i$

and for cables with semi-conducting layers:

$D_c=d_L+2t_i+3.0$

where:
D_c - core diamter in mm
t_i - 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 D_B is given by:

$D_B=D_f+2t_B$

where:
D_B - diamter or inner covering/bedding in mm
t_B - bedding thickness in mm
= 0.4 mm for D_f <= 40 mm
= 0.6 mm for D_f > 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: 

$\text{cross-sectional area}=n_t\times t_t\times w_t$

where:
n_t - number of tapes
t_t - nominal thickness of individual tape in mm
w_t - 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:

$\text{cross-sectional area}=\frac{n{}_w\times d_w^2\times \pi }{4}+n_h\times t_h\times W_h$

where:
n_w - number of wires
d_w - diameter of wire in mm
n_h - number of counter helix
t_h - thickness of coutner helix in mm (if greater than 3 mm)
W_h - width of counter helix in mm

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

$D_{pb}=D_g+2t_{pb}$

where:

D_g - fictitious diameter under lead sheath in mm
t_pb - thickness of lead sheath in mm

Seperation sheath - fictitious diameter D_s given by:

$D_s=D_u+2t_s$

where:
D_u - fictitious diameter under separation sheath in mm
t_s - thickness of separation sheath in mm

Lapped bedding - fictitious diameter D_lb given by:

$D_{lb}=D_{ulb}+2t_{lb}$

where:
D_ulb - fictitious diameter under lapped beding in mm
t_lb  - 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, D_x is given by:

- flat or round wire armour:

$D_x=D_A+2t_A+2t_w$

where:
D_A - diameter under armour in mm
t_A - thickness or diameter of armour wire, in mm
t_w - thickness of counter helix (if any) in mm (if > 3 mm)

 - dobuble tape armour:

$D_x=D_A+4t_A$

where:
D_a - diameter under armour in mm
t_A - thickness of armour tape in mm