Cable selection algorithm · U_m > 36 kV

HV cable system selection algorithm

Above 36 kV a cable is not selected on its own — the cable, its accessories and its bonding are rated and qualified as one system. This is that sequence: which standard applies, the bonding decided before the rating, the full thermal model, the screen fault current, and the test evidence that has to exist behind it.

13 steps · clause and formula numbers cited at every node · no standard text reproduced

Above 36 kV the object being selected is a cable system, not a cable: the cable, the joints, the terminations and the bonding are qualified and rated together. IEC 60840 applies above 30 kV (U_m 36 kV) up to 150 kV (U_m 170 kV); IEC 62067 applies above that up to 500 kV (U_m 550 kV), and adds a prequalification test of the complete system.

Two things reorder the algorithm compared with MV. The bonding scheme has to be decided before the rating is calculated, because solid, single-point and cross-bonding give completely different sheath loss factors (IEC 60287-1-1, 2.3). And the thermal environment becomes an input that has to be surveyed: dielectric loss is no longer negligible (2.2), mutual heating between circuits dominates T4, and the risk of the soil drying out around the cable is a design case, not a footnote (IEC 60183, 4.3.2 d).

The algorithm

Flowchart of the HV cable system selection algorithm above 36 kV to IEC 60840, IEC 62067, IEC 60183, IEC 60287 and IEC 60853 HV CABLE SYSTEM, U_m > 36 kVCollect the system, route and thermal dataIEC 60183, 4.2 and 4.3.1 to 4.3.4- nominal and highest system voltage; lightning and switching overvoltagewhere U_m is 300 kV or more- type of earthing, and the earth-fault duration where the neutral is noteffectively earthed- continuous, cyclic and emergency currents, with the load curve; possibleforced cooling- symmetrical and asymmetrical fault currents to earth and between phases,and their maximum duration- route length and profile, laying arrangement, and how the metalliccoverings are bonded- soil thermal resistivity along the route, measured or assumed, plus themeteorological data needed to judge the risk of drying out- ductbank geometry, manhole spacing, proximity of other circuits and heatsources1Place the circuit in its voltage standardIEC 60840, Clause 1; IEC 62067, Clause 1; IEC 60183, 5.3 and 5.4, Table 1- above 30 kV (U_m 36 kV) up to 150 kV (U_m 170 kV): IEC 60840- above 150 kV (U_m 170 kV) up to 500 kV (U_m 550 kV): IEC 62067- U_m not less than the highest system voltage; U_p from IEC 60071-1 againstthe line insulation level, the protective levels and the cable and linesurge impedances2Choose the cable system designIEC 60840, 4.2 to 4.4 and Clause 5, Tables 1 and 2; IEC 62067, 4.2 to 4.4- insulation and its maximum conductor temperature in normal operation, fromTable 1 of the applicable standard- metal screen or sheath design: the standard's own note is that in allcases it should be able to carry the total fault current- a radial water-impermeable barrier is recommended for cables in ground,floodable galleries or water; consider a longitudinal barrier too- oversheath type ST1 or ST2 (PVC), ST3 or ST7 (PE), by the mechanical,thermal and fire duty3Decide the bonding scheme firstIEC 60183, 4.2 f); IEC 60287-1-1, 2.3- solid (both-ends) bonding: no sheath voltage to manage, but circulatingcurrents cut the rating- single-point bonding and cross-bonding: the screen loss largelydisappears, but sheath voltage limiters and an earth continuity conductorbecome part of the design- the scheme sets the sheath loss factor, so it has to be fixed before therating is calculated, not after4Take a trial conductor size and formationIEC 60183, Clause 6 a) to e); IEC 60228- standard size; trefoil or flat, with the spacing and depth of the trenchsection- electric stress at the insulation surface constrains how small theconductor may be- above 1 600 mm², treat skin and proximity effects explicitly and confirmthe a.c. resistance by measurement- the economic optimum over the life of the link often decides between twoworkable sizes (IEC 60287-3-2)5Calculate the rating properlyIEC 60287-1-1, 1.4.1.1, 2.1 to 2.4; IEC 60287-2-1, 4.1.2 to 4.2.7; IEC 60287-3-1- a.c. conductor resistance with skin and proximity effects (2.1)- dielectric loss W_d = ω·C·U₀²·tan δ — at HV this is no longer negligible;Table 3 gives the U₀ above which it must be counted (2.2)- sheath and screen loss factor for the chosen bonding, and armour loss(2.3, 2.4)- thermal resistances T1 to T4, including mutual heating from the othercircuits (IEC 60287-2-1)- cyclic and emergency ratings from IEC 60853-2 and IEC 60853-3 where theload curve allows6Conductor temperature within the Table 1 limit?IEC 60840 or IEC 62067, Table 1; IEC 61936-1, 6.2.9.17yesRoute safe against soil drying out?IEC 60183, 4.3.2 d); IEC 60287-3-1 for soil data8yesConductor withstands the short-circuit?IEC 60949, Clauses 2 and 39yesScreen withstands the earth-fault current?IEC 60949, Clause 6; IEC 60840, 4.3, note10yesCheck the thermo-mechanical and pulling designIEC 61936-1, 6.2.9.2 and 6.2.9.5; IEC 60183, 7.1- thermal expansion relieved by snaking, flexible connections or expansionterminations, or the force carried by the equipment- pulling tension and sidewall bearing pressure against the manufacturer'slimits; joint bay positions- electrodynamic force between single-core cables on short-circuit; cleatdesign and spacing- crossings and proximity: clearance, induced voltage on pipelines andtelecom, mutual heating11Specify and qualify the accessoriesIEC 60840 and IEC 62067, type approval and Clause 13 prequalification; IEC 60183,7.2 and 7.3; IEC TS 60815-1; IEC 62271-209- type-test the cable system, not the cable alone; check the stated range ofapproval covers your cross-section and construction- above U_m 170 kV a prequalification test of the complete system applies(IEC 62067, Clause 13)- terminations: creepage for the pollution class, clearances increased above1 000 m, GIS and transformer terminations to IEC 62271-209- jointing is a workmanship-critical operation — the standard asks fortrained, qualified jointers12Decide on monitoringIEC 60183, Annex A- distributed temperature sensing to confirm the rating actually achieved- partial-discharge or sheath-current monitoring on the accessories andbonding13RECORD THE GOVERNING CRITERIONIncrease the size, space the circuits,improve the backfill, or force-coolengineered thermal backfill, wider spacing and deeperburial pull in different directions — depth raises T4;forced cooling is a design commitment, not a fixnoRate on the dried-out resistivity, or use astabilised backfilla trench that dries out around the cable moves the soilresistivity far from the assumed value; a rating builton an assumed 1,0 to 1,5 K·m/W and no survey is theclassic HV design failurenoIncrease the conductor size, or reduce theclearing timeI = ε·I_AD; above 36 kV take the permitted finaltemperature from the manufacturer's qualified design —IEC 60986 covers only 6 kV to 30 kVnoIncrease the screen cross-section, or add anearth continuity conductorwith single-point bonding the whole earth-fault currenthas one return path; the screen temperature limit is setby the material in contact with itno
Spine down the left is the path a compliant design takes. Every branch to the right is a failed check and the change it forces, with the dashed arrow showing where the algorithm restarts. The table below carries the same content in text.

The steps in words

The same algorithm as a table: what is checked at each step, the criterion, and the clause it comes from.
#Check or actionCriterionReference
1Collect the system, route and thermal dataAt HV the thermal survey of the route is part of the input data, not an assumption.IEC 60183, 4.2 and 4.3.1 to 4.3.4
2Place the circuit in its voltage standardThe applicable test standard, U_m and U_p — settled before any conductor is sized.IEC 60840, Clause 1; IEC 62067, Clause 1; IEC 60183, 5.3 and 5.4, Table 1
3Choose the cable system designScreen able to carry the full fault current, and a water barrier wherever the route can be wet.IEC 60840, 4.2 to 4.4 and Clause 5, Tables 1 and 2; IEC 62067, 4.2 to 4.4
4Decide the bonding scheme firstBonding scheme, plus the SVLs and earth continuity conductor it implies.IEC 60183, 4.2 f); IEC 60287-1-1, 2.3
5Take a trial conductor size and formationSize, formation, spacing and depth as one trial arrangement.IEC 60183, Clause 6 a) to e); IEC 60228
6Calculate the rating properlyConductor temperature at the design load, from the full IEC 60287 model for the real trench section.IEC 60287-1-1, 1.4.1.1, 2.1 to 2.4; IEC 60287-2-1, 4.1.2 to 4.2.7; IEC 60287-3-1
7Conductor temperature within the Table 1 limitengineered thermal backfill, wider spacing and deeper burial pull in different directions — depth raises T4; forced cooling is a design commitment, not a fixIEC 60840 or IEC 62067, Table 1; IEC 61936-1, 6.2.9.1
8Route safe against soil drying outa trench that dries out around the cable moves the soil resistivity far from the assumed value; a rating built on an assumed 1,0 to 1,5 K·m/W and no survey is the classic HV design failureIEC 60183, 4.3.2 d); IEC 60287-3-1 for soil data
9Conductor withstands the short-circuitI = ε·I_AD; above 36 kV take the permitted final temperature from the manufacturer's qualified design — IEC 60986 covers only 6 kV to 30 kVIEC 60949, Clauses 2 and 3
10Screen withstands the earth-fault currentwith single-point bonding the whole earth-fault current has one return path; the screen temperature limit is set by the material in contact with itIEC 60949, Clause 6; IEC 60840, 4.3, note
11Check the thermo-mechanical and pulling designExpansion, pulling force, short-circuit force and crossings all resolved on the drawings.IEC 61936-1, 6.2.9.2 and 6.2.9.5; IEC 60183, 7.1
12Specify and qualify the accessoriesCable system type-tested and, above U_m 170 kV, prequalified — with the range of approval covering this design.IEC 60840 and IEC 62067, type approval and Clause 13 prequalification; IEC 60183, 7.2 and 7.3; IEC TS 60815-1; IEC 62271-209
13Decide on monitoringMonitoring scope agreed, or a written decision not to monitor.IEC 60183, Annex A

What actually governs the size

Running the algorithm is mechanical. Knowing which check will bind before you start is what makes it quick — and what tells you whether a schedule someone else produced was ever checked at all.

CaseWhat binds
Almost every buried linkT4 and the soil. Mutual heating, depth and thermal resistivity decide the rating; the conductor size is often the smallest of the levers available.
Single-core circuits, which is all of themThe bonding scheme, decided first. It sets the sheath loss, the sheath voltage, whether SVLs and an earth continuity conductor are needed, and the length of an electrical section.
Circuits at 132 kV and aboveDielectric loss. W_d = ω·C·U₀²·tan δ is a fixed loss that exists at no load and cannot be designed away — Table 3 of IEC 60287-1-1 gives the U₀ above which it must be counted.
Earth faultsThe screen. Under single-point bonding the whole earth-fault current has one return path, and above 36 kV the permitted screen temperature comes from the manufacturer's qualified design — IEC 60986 stops at 30 kV.
The procurement stageThe evidence. The type-test range of approval must actually cover the cross-section and construction offered, and above U_m 170 kV a prequalification test of the complete system applies (IEC 62067, Clause 13).

Standards this algorithm is built from

StandardWhat it supplies here
IEC 60840:2011Cable systems above 30 kV (U_m 36 kV) up to 150 kV (U_m 170 kV) — test methods and requirements
IEC 62067:2011Cable systems above 150 kV (U_m 170 kV) up to 500 kV (U_m 550 kV), with prequalification
IEC 60183:2015Guidance for the selection of high-voltage a.c. cable systems
IEC 60287-1-1, -2-1, -3-1Rating equations and losses, thermal resistances, soil and ambient data
IEC 60853-2 and -3Cyclic and emergency ratings
IEC 60071-1:2011Insulation coordination — the impulse withstand level U_p
IEC 60949:1988+A1:2008Thermally permissible short-circuit currents
IEC 61936-1:2010+A1:2014Installation requirements for cables in power installations above 1 kV

The clause and formula numbers above were read from the standards themselves. What is not reproduced anywhere on this site is the text of a standard: if you calculate for a living, buy the document from IEC. How each engine here is checked against the standards' own published values is on the validation page.

Calculators that carry out these steps