IEC 60840 Guide: HV XLPE Cables from 66 to 132 kV — the Questions Engineers Ask

Above 30 kV, a power cable stops being a catalogue product and becomes a project. Each circuit has its own route, its own bonding scheme and its own accessories, and the cable is judged together with them. The reference for this voltage range is IEC 60840, which covers power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV). In practice, most of these HV XLPE cables are single-core designs for 66, 110, 132 and 150 kV networks.

This IEC 60840 guide does not repeat the standard's requirements or test values. It works through the questions design engineers and tender teams actually ask when a 66 kV or 132 kV enquiry arrives.

What is IEC 60840 for, and where does it start?

IEC 60840 sits directly above IEC 60502-2. The boundary is the 30 kV rated voltage: a 19/33 kV cable (Um = 36 kV) is still medium voltage, while a 36/66 kV cable is the first common HV class. Above 150 kV, IEC 62067 takes over for extra-high-voltage systems.

The important shift is in what the standard qualifies. IEC 60502-2 is mainly a cable standard. IEC 60840 qualifies a cable system: the cable together with its terminations and joints. A type test is performed on an assembled loop of cable and accessories, and the approval belongs to that combination.

Does IEC 60840 tell you how thick the insulation must be?

No — and this is the single biggest difference from the MV standard. IEC 60502-2 gives insulation thicknesses per voltage class. IEC 60840 leaves the insulation thickness to the manufacturer. What the manufacturer actually designs is the electrical stress at the conductor screen and at the insulation screen, and the thickness follows from that choice and the conductor diameter.

That has three practical consequences:

• Two compliant 132 kV cables can have different insulation thicknesses. A manufacturer with clean materials and a well-controlled extrusion line can work at higher stress and offer a smaller, lighter cable. Another may choose a more conservative design.
• The type test defines your design envelope. The standard explains how a successful type test extends to other conductor sizes and voltages, and the electrical stresses of the tested cable are central to that. Design a new size at a stress above what was tested, and you are outside your approval.
• Higher-stress designs need more proof. For designs that work at higher electrical stress, the standard calls for a long-duration prequalification test of the complete system before type approval counts.

In other words, insulation thickness at HV is a strategic decision tied to your test history, not a value you look up.

What does the standard actually decide?

IEC 60840 fixes the rules that make a cable system comparable across suppliers:

• Rated voltages expressed as U0/U (Um), and the requirement that the cable and accessories match them.
• Materials and their properties for the insulation, semi-conductive screens and oversheath, verified by sample and type tests.
• The test regime: routine tests on every production length (including a voltage test and a partial discharge measurement), sample tests on selected lengths, type tests on the system, prequalification where it applies, and electrical tests after installation.
• Requirements for optional features, such as longitudinal water blocking and radial water barriers, once the purchaser asks for them.

What does it leave to the purchaser and the manufacturer?

This is where HV tenders are won or lost. The standard deliberately leaves the following open:

• Conductor size and type. Large HV conductors are often segmental (Milliken) designs to reduce the AC resistance caused by skin effect. Whether the tender wants copper or aluminium, compacted round or segmental, enamelled wires or not, has a big effect on cost and rating.
• Metallic screen or sheath. Copper wire screens, lead sheaths, smooth or corrugated aluminium sheaths and bonded aluminium laminates all appear in HV practice. The choice is driven by the earth-fault current and its duration, the need for a radial water barrier, weight and local habit. The standard verifies the result; it does not choose for you.
• Oversheath. Polyethylene is common for mechanical and moisture performance. Many utilities also ask for a semi-conductive outer layer so the oversheath can be tested after installation, or for a flame-retardant sheath in tunnels and substations.
• Installation and bonding. Laying formation, depth, backfill, and whether the screens are solidly bonded, single-point bonded or cross-bonded are project decisions. They control screen losses and the IEC 60287 current rating far more than any catalogue entry.

Can an IEC 60502-2 type test cover a 66 kV cable?

No. The two standards have different scopes and different test regimes, and IEC 60840 requires a system type test with accessories. A manufacturer moving from 33 kV into 66 kV needs a new qualification programme, not an extension of the MV approval. The same applies in the other direction: an HV system approval says nothing about your MV range.

Can I use another supplier's accessories with my cable?

Only if that combination is covered. Because IEC 60840 qualifies the system, a type test report for your cable with one accessory supplier's joints does not automatically cover a different supplier's joints. Utilities usually ask for evidence that the exact cable and accessory combination offered has been tested. Tender teams that quote cable and accessories from separate sources without checking this are taking a real risk.

What changes in the cable design itself?

Moving from MV to HV changes how each layer is treated (see our layer-by-layer guide):

• The insulation system is extruded as one. Conductor screen, XLPE insulation and insulation screen are applied in a single triple-extrusion pass, and cleanliness of the interfaces becomes critical. Many purchasers specify dry curing and super-clean compounds.
• Water is the long-term enemy. Moisture in XLPE under high stress leads to water trees. That is why a radial water barrier — a metal sheath or laminate — is standard on most HV cables, often combined with water-swellable tapes for longitudinal blocking.
• Thermal behaviour dominates sizing. With high currents and single-core circuits, losses in the conductor and screen, the bonding arrangement and soil thermal resistivity decide the conductor size. Our insulation materials guide explains why XLPE's operating temperature is what makes these ratings possible.
• Weight and drum length become logistics constraints. A large single-core HV cable is heavy, and the length per drum often decides where the joints go — and therefore how the cross-bonding sections are laid out.

Typical tender mistakes with HV XLPE cables

1. Writing "132 kV" instead of U0/U (Um). It leaves room for different interpretations of the rated and maximum voltage.
2. No earth-fault data. Without fault current and duration, the metallic screen or sheath cannot be sized correctly, and offers become impossible to compare.
3. No bonding arrangement. Solid bonding versus cross-bonding can change the current rating, and therefore the conductor size, significantly.
4. Assuming insulation thicknesses are comparable. Two offers with different thicknesses can both be compliant. Compare design stresses and type test coverage, not just diameters.
5. Ignoring accessory compatibility. A cable type test without the accessories actually offered does not prove the system.
6. Forgetting tests after installation. If the oversheath must be tested on site, the design needs to support it.

A tender checklist for IEC 60840 cables

Before you design or price an HV cable, make sure you know:

1. Rated voltage U0/U (Um) and network frequency.
2. Conductor material, size and construction, including any segmental or enamelled-wire requirement.
3. Earth-fault current and duration for the metallic screen or sheath.
4. Type of metallic sheath and whether a radial water barrier and longitudinal water blocking are required.
5. Oversheath material, semi-conductive layer and any fire-performance requirement.
6. Installation conditions: laying formation, depth, backfill, ducts, tunnels or troughs.
7. Bonding arrangement and section lengths.
8. Accessories in scope and whether a system type test with that combination exists.
9. Prequalification and type test evidence required, and how your existing approvals cover the offered design.
10. Maximum drum length and weight for transport and site handling.

How CablePLM handles IEC 60840 cables

CablePLM includes IEC 60840 in its standards library for single-core XLPE cables from 36/66 to 76/132 kV. The calculation method follows the standard's logic: you set the rated voltage and the design stresses, and CablePLM builds the construction layer by layer — conductor, screens, insulation, metallic sheath and oversheath — with dimensions, weights and a full bill of materials. It calculates IEC 60287 current ratings for the installation and bonding you specify, plus the short-circuit capability of conductor and metallic sheath.

Each design produces an engineering datasheet and a customer version, with every value marked as standard data or estimate. The Qualification & Quality add-on tracks which designs are covered by your type tests, the AI Tender Agent helps extract fault levels, bonding and test requirements from tender documents, and Tenders & Projects keeps the HV project and its revisions in one place.

Want to see your HV range designed this way? Book a demo.