Power Cables: Parts, Types, Protection and Testing

Short answer

An insulated power cable carries energy at medium, high or extra-high voltage through ducts, trays or buried. Its insulation, almost always XLPE, sits between two semiconducting layers that control the electric field, and its metallic shield carries the fault current and defines the grounding arrangement. Most problems appear at terminations and joints, which is why tests after installation are key.

What a power cable is

An insulated power cable carries energy where an overhead line is not possible or convenient: urban substation feeders, connections between transformers and switchgear, collector networks of solar and wind plants, and the internal distribution of industrial plants. This article covers medium, high and extra-high voltage cables.

Main parts

From the center outward:

Conductor of copper or aluminum, stranded and compacted. In large sizes, segmented conductors are used to reduce the skin effect.
Conductor screen (semiconducting), which smooths the irregularities of the strands and evens out the field.
Insulation, today almost always cross-linked polyethylene (XLPE). There are also EPR cables and, in older installations, paper-insulated or oil-filled cables.
Insulation screen (semiconducting), which confines the electric field within the insulation.
Metallic shield or sheath of copper wires or tapes, or a lead or aluminum sheath at high voltage, which also serves as a moisture barrier. It carries the capacitive current and the ground-fault current.
Water-blocking elements, longitudinal and radial.
Outer jacket of PVC or polyethylene, and armor in industrial or direct-buried cables.

To these are added the accessories: terminations for outdoor or indoor use, plug-in connectors for switchgear, GIS and transformers, and joints. Each accessory breaks the cable geometry and rebuilds the field control with stress cones or stress-control materials.

How it works

The semiconducting screens make the electric field radial and uniform in the insulation. A capacitive current flows through the cable that grows with length and voltage; on long high-voltage circuits it may require compensation with reactors.

The charging current induces voltages in the metallic shields. How they are grounded is a design decision:

Grounded at both ends: there is no induced voltage, but currents circulate in the shields, causing losses and reducing the cable rating.
Single-point grounded: there is no circulating current, but a voltage appears at the open end, which is limited with sheath voltage limiters.
Cross-bonding: on long high-voltage circuits the shields are transposed in link boxes so that the induced voltages cancel out.

Current rating depends on the installation: depth, ducts, grouping and the thermal resistivity of the soil. The reference calculation method is IEC 60287.

Types

Extra-high voltage: XLPE cables above 150 kV (IEC 62067), with a watertight metallic sheath, premolded joints and cross-bonding systems.
High voltage: above 30 kV up to 150 kV (IEC 60840), common in urban feeders and in connections from transformers to GIS.
Medium voltage: from 6 kV to 30 kV (IEC 60502-2), the most numerous in distribution, industry and solar plants, with aluminum conductors in collector networks.
Industry: armored cables, tray cables and cables with low-smoke requirements or hydrocarbon resistance depending on the environment.

Associated protection

The cable is part of the protection zone of the circuit it feeds:

On medium-voltage feeders it is protected with phase and ground overcurrent 50/51 and 50N/51N, often with a core-balance CT for ground. Here is one of the most common mistakes: if the shield connection passes through the CT, it must return back through it.
On high-voltage circuits, line differential is used; see the line differential 87L test. On mixed lines, distance protection must consider the impedance change between the overhead and underground sections.
Automatic reclosing 79 is blocked on underground circuits and conditioned on mixed lines.

Commissioning and maintenance tests

After installation:

Insulation resistance and continuity of conductors and shields.
Phase identification at both ends.
Outer jacket test with DC voltage, to detect mechanical damage from installation.
AC voltage test: at medium voltage, with VLF (very low frequency) per IEEE 400.2; at high and extra-high voltage, with a resonant source per IEC 60840 or IEC 62067.
Partial discharge measurement during the voltage test, which locates defects in accessories (IEEE 400.3).
Verification of the shield bonding system, link boxes and sheath voltage limiters.

In maintenance, tan delta with VLF is used to assess insulation aging, together with partial discharge and thermography of terminations.

Design data and standards that shape the project

The ABB user guide for high-voltage XLPE cable systems (in English, North American edition, revision 3, consulted in a copy hosted by a third party) and the official standard summaries make it possible to set criteria that are rarely explained together.

What each standard covers. According to the summaries from IEC and standards bodies:

IEC 60840 covers the test methods and requirements for cable systems, cables and accessories for fixed installations of rated voltage above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV), in single-core and three-core cables with individual screens. It excludes special cables such as submarine cables. The 2011 edition added a prequalification procedure for cables of high electrical stress tested as a system with their accessories. One of the sources consulted lists an edition later than 2011 that could not be confirmed with IEC: verify the current one before citing it.
IEC 62067 covers from 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV), and IEC 60502 from 1 kV up to 30 kV (Um = 36 kV).
IEC 60287-1-1 gives the formulas for current rating and losses under steady-state conditions, that is, with a constant 100 % load that asymptotically produces the maximum conductor temperature. It applies to all alternating voltages and direct voltages up to 5 kV, and to cables directly buried, in ducts, troughs or steel pipes, with or without partial soil drying, and in air. The current edition is IEC 60287-1-1:2023. Cyclic and emergency ratings are calculated with IEC 60853, and the short-circuit temperature limits for cables above 30 kV are in IEC 61443.
IEEE 575 is the guide for bonding of shields of single-conductor cables from 5 to 500 kV and for calculating induced voltages and currents. The 2014 edition is listed as inactive-reserved and a revision project was approved in 2025.
IEEE 400.2 details VLF field tests for shielded cable with extruded and laminated insulation. The 2024 edition is the current one; it recognizes 0.1 Hz as the most widely used commercial frequency, and includes tables of recommended voltage levels for installation, acceptance and maintenance and of the effectiveness of the VLF test for different defects. Earlier editions were limited to aged cables of 5 to 69 kV.

Shield bonding and current rating. The ABB guide explains that losses depend on what flows in the shields, so the bonding method changes the current rating:

With shields grounded at both ends there are circulating currents under normal conditions; losses are lower with cables in trefoil than with cables laid flat and spaced apart.
With single-point grounding there is no path for circulating currents or for external fault currents; voltage is induced between the shields of neighboring phases and to ground, proportional to the cable length and the current. It suits limited routes, because the permissible shield voltage limits the length.
With cross-bonding the shields are sectionalized and transposed to eliminate circulating currents; there is induced voltage to ground but no significant current, and the maximum appears at the link boxes. It allows a current rating equal to that of single-point bonding, with longer routes, at the cost of separating shields and additional link boxes. Joints need an integrated shield break.

Overload and short circuit. According to the same guide, an XLPE cable can be overloaded up to 105 °C at the conductor, with few and brief emergency events. In a short circuit, the maximum conductor or shield temperature is set by the adjacent material. For XLPE conductors it is 250 °C. For a copper shield it is 250 °C, which with an initial 50 °C corresponds to about 83 A/kcmil for 1 s. For a lead sheath it goes up to 210 °C, about 14 A/kcmil in 1 s. The guide’s formula is considered valid for durations of 0.2 to 5 s. Dynamic forces between conductors must also be verified. The guide includes tables of maximum short-circuit current in the conductor and in the shield by cross-section: the shield is checked with its own table, not with the conductor’s.

Handling on site. For the single-core high-voltage cables in the guide, in standard design (copper wire shield only) the minimum bending radius is 15 times the outer diameter during pulling and 10 times once installed. For special designs (with metallic laminate, lead sheath or integrated optical fiber) it is 18 and 12 times. The guide limits the pulling force to 4.5 lb/kcmil for aluminum conductors and 8.0 lb/kcmil for copper, and the sidewall pressure at bends to 345 lb/ft in direct burial and 515 lb/ft in ducts, accepting higher values depending on the design. These are one manufacturer’s values: always use those of the cable actually supplied.

Factory tests and tests after pulling. The guide lists as routine tests the partial discharge measurement, the voltage test of the main insulation, the electrical test of the jacket and visual inspection. As tests after pulling it lists the DC test of the jacket and the AC voltage test of the main insulation. See on-site withstand voltage test and partial discharge test.

Common failures

Assembly defects in accessories: cuts in the insulation, semiconductor residue, poor cleaning or wrong position of the stress cone. They are one of the most frequent causes of failure.
Mechanical damage during pulling or from later excavations. When the cable fails underground, the point is found with underground cable fault location before digging.
Water trees in old cables with moisture.
Overheating from grouping, unventilated ducts or dry soils.
Corroded or poorly connected shields, with circulating currents or excessive induced voltages.
Core-balance CT installed incorrectly with respect to the shield, causing false trips or failure to trip.

We can support you with your power cables

At Tiza Group we test medium- and high-voltage cables and accessories, and review their protection in substations, industry and solar plants. Field work is done in Colombia, or elsewhere on request; protection review can be delivered remotely worldwide. If you are going to energize a new circuit or had a fault, contact us.

References

IEC 60502-2: extruded insulation cables for 6 kV to 30 kV.
IEC 60840: extruded insulation cables for above 30 kV up to 150 kV.
IEC 62067: extruded insulation cables for above 150 kV up to 500 kV.
IEC 60287: calculation of the current rating of cables.
IEEE 400, IEEE 400.2 and IEEE 400.3: field testing of shielded cables, VLF and partial discharge.
IEEE 48 and IEEE 404: cable terminations and joints.
IEEE 575: guide for bonding of shields and sheaths of single-conductor cables.

Frequently asked questions

Why does the ground protection not see a cable fault even with a core-balance CT?

It is almost always because of the way the shield passes through the CT. If the shield grounding connection goes through the CT, it must return back through it; otherwise the fault current returning on the shield is subtracted from the measurement and the protection is blind or misoperates.

Can an XLPE cable be tested with high DC voltage?

It is not recommended for in-service XLPE cables, because DC voltage can leave trapped charge in the insulation and does not reproduce the stress distribution of AC. For medium voltage, VLF is preferred, and for high voltage an AC test with a resonant source.

Why is reclosing blocked on cable circuits?

A fault in an insulated cable is almost always permanent, and closing onto it again aggravates the damage. That is why reclosing is blocked on fully underground circuits and, on mixed lines, enabled only when the fault is located in the overhead section.