IEC 60183-2015 pdf Guidance for the selection of high-voltage A.C. cable systems

4.3.1 General The following details apply: a) Length and profile of route. b) Details of laying arrangements (e.g. flat or trefoil arrangement) and how the metallic coverings are connected to each other and to earth. c) Special laying conditions, for example cables in water. Individual installations require special consideration. 4.3.2 Underground cables The following details apply: a) Typical ambient temperatures over the year (see IEC 60287-3-1). b) Details of installation conditions (e.g. direct burial, in ducts, mechanical laying, etc.) to enable decisions to be taken on composition of metallic screen or sheath, type of armour (if required) and type of serving, for example anti-corrosion or anti-termite. c) Depth of laying. d) Thermal resistivities and kinds of soil along the route (e.g. sand, clay, made-up ground, special backill), and whether this information is based on measurement and inspection or only assumed. Meteorological data to evaluate risk of soil drying (see IEC 60287 series). e) Minimum, maximum and average ground temperature at the depth of burial. f) Proximity of other load-carrying cables (especially where the link involves several parallel circuits) or of other heat sources, with details. g) Lengths of troughs, ducts or pipe lines, with spacing of manholes, if any. h) Details of ductbanks, if any: number of ducts or pipes. Internal diameter of ducts and pipes. Spacing between individual ducts and pipes, if more than one. Material of ducts or pipes. i) Risk for water ingress and corrosion (the right cable design shall be chosen).
5.2 System categories The following details apply: – Category A This category comprises those systems in which any phase conductor that comes in contact with earth or an earth conductor is disconnected from the system within 1 min. Category B This category comprises those systems, which, under fault conditions, are operated for a short time only with one phase earthed. This period should, in general, not exceed 1 h, but a longer period can be tolerated as specified in the relevant cable standard. – Category C This category comprises all systems which do not fall into category A or B. Reference should be made to the relevant cable standards choosing, for example, between those listed in Clause 2. In a system where an earth fault is not automatically and promptly isolated, the extra stresses on the insulation of cables during the earth fault reduce the life of the cables to a certain degree. If the system is expected to be operated fairly often with a permanent earth fault, it is advisable to classify the system in category C.