# Fictitious Dimensions

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

## Fictitious Method

**Conductors **- them fictitious diameter, *d _{L}* (irrespective of shape or compactness) is given by:

CSA mm^{2} |
d mm _{L} |
CSA mm^{2} |
d mm _{L} |
---|---|---|---|

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:

${D}_{c}={d}_{L}+2{t}_{i}$

and for cables with semi-conducting layers:

${D}_{c}={d}_{L}+2{t}_{i}+3.0$

where:

*D _{c}* - core diamter in mm

*t*- is the nomiman insulation thickness, in mm

_{i}**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:

${D}_{B}={D}_{f}+2{t}_{B}$

where:

*D _{B}* - diamter or inner covering/bedding in mm

*t*- bedding thickness in mm

_{B}= 0.4 mm for

*D*<= 40 mm

_{f}= 0.6 mm for

*D*> 40 mm

_{f}**Concentric condcutors and metallic screens** - increase in diamter due to screens is given by:

CSA of cocentric conductor or metallic screen mm^{2} |
Increase in diamater, mm | CSA of cocentric conductor or metallic screen mm^{2} |
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:

$\text{cross-sectionalarea}={n}_{t}\times {t}_{t}\times {w}_{t}$

where:

*n _{t}* - number of tapes

*t*- nominal thickness of individual tape in mm

_{t}*w*- nominal width of indvidual tape in mm

_{t}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:

$\text{cross-sectionalarea}=\frac{n{}_{w}\times {d}_{w}^{2}\times \pi}{4}+{n}_{h}\times {t}_{h}\times {W}_{h}$

where:

*n _{w}* - number of wires

*d*- diameter of wire in mm

_{w}*n*- number of counter helix

_{h}*t*- thickness of coutner helix in mm (if greater than 3 mm)

_{h}*W*- width of counter helix in mm

_{h}**Lead sheath** - fictitious diamter of lad shearth , *D _{pb}* is given by :

${D}_{pb}={D}_{g}+2{t}_{pb}$

where:

*D _{g}* - fictitious diameter under lead sheath in mm

*t*- thickness of lead sheath in mm

_{pb}**Seperation sheath** - fictitious diameter *D _{s}* given by:

${D}_{s}={D}_{u}+2{t}_{s}$

where:

*D _{u}* - fictitious diameter under separation sheath in mm

*t*- thickness of separation sheath in mm

_{s}**Lapped bedding** - fictitious diameter *D _{lb}* given by:

${D}_{lb}={D}_{ulb}+2{t}_{lb}$

where:

*D _{ulb}* - fictitious diameter under lapped beding in mm

*t*- thickness of lapped bedding in mm

_{lb}**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:

${D}_{x}={D}_{A}+2{t}_{A}+2{t}_{w}$

where:

*D _{A}* - diameter under armour in mm

*t*- thickness or diameter of armour wire, in mm

_{A}*t*- thickness of counter helix (if any) in mm (if > 3 mm)

_{w}- dobuble tape armour:

${D}_{x}={D}_{A}+4{t}_{A}$

where:

*D _{a}* - diameter under armour in mm

*t*- thickness of armour tape in mm

_{A}