What Tests and Maintenance Are Required for MV XLPE Cables?

Maintenance is required because the outer sheath, terminations, cable joints, screen connections and main insulation can all weaken over time under mechanical, environmental and electrical effects. MV XLPE cables carry energy at medium-voltage level, so weaknesses that develop in the cable body, termination areas, joint areas or screen-grounding arrangement should be detected at an early stage rather than after a fault. In medium-voltage cable systems, problems often appear not only inside the cable but also in accessories and application details, which is why regular checks matter. If maintenance is neglected, outer sheath damage, moisture ingress, loose screen connections and insulation aging can progress unnoticed, and fault and outage risk increases. The purpose of maintenance is therefore to reduce outage and equipment damage risk, preserve energy continuity and extend cable life by catching developing defects before they turn into failures.

Depending on the application, the sheath test, VLF withstand test, tan delta measurement, partial discharge assessment, thermal inspection and visual and mechanical accessory checks can be performed on MV XLPE cables. The sheath test evaluates the dielectric integrity of the outer sheath, while the VLF withstand test verifies that the cable can withstand a defined voltage level. Tan delta gives information about general aging and dielectric loss behavior, and partial discharge measurement helps locate point weaknesses in terminations, cable joints and local insulation defects. Thermal inspection reveals looseness and increased contact resistance at terminations and connection areas. Visual and mechanical checks cover the route, outer sheath, terminations, cable joints and the screen-grounding arrangement. Test selection should be based on condition assessment rather than only pass-fail logic, so the methods are chosen according to the cable and its history.

The sheath test shows whether the dielectric integrity of the outer sheath has deteriorated. It is an important maintenance tool for understanding the condition of the outer sheath, especially on directly buried routes, routes passing through ducts and routes exposed to mechanical risk. On these routes, sheath testing helps detect outer sheath damage early, before it develops into a larger problem. A damaged outer sheath does not always cause an immediate outage, but over time it can prepare the ground for moisture ingress, screen corrosion and larger faults. By revealing sheath damage at an early stage, the test allows future problems such as moisture ingress to be identified beforehand and the risky line section to be prioritized in the maintenance plan. This is why the sheath test is treated as a basic check for the outer protection layer of MV XLPE cables.

The VLF test is used because it is one of the basic methods for verifying that a medium-voltage cable can withstand a defined voltage level. In MV XLPE cable systems, it is one of the commonly used approaches for withstand verification and condition assessment, and it offers a practical test approach especially in field applications. However, it is not correct to depend on a single test when assessing insulation condition. VLF is often combined with tan delta measurement, which provides information about the general aging and loss behavior of the insulation, and with partial discharge measurement when the defect is at an early stage. Used together, these methods give a stronger picture than pass-fail logic alone. The importance of VLF also increases on lines where termination or joint work has been carried out, since verification tests before re-energizing should not be neglected.

Tan delta provides information about the dielectric loss behavior and general aging of the cable insulation. It is especially valuable for monitoring general insulation health, and when non-homogeneous deterioration, water-tree-like aging effects or increased dielectric loss exist in the insulation, tan delta results can provide meaningful data for the maintenance plan. The measurement does not explain everything on its own; however, when used together with the VLF withstand test and other field observations, it gives a strong idea about the general condition of the cable. In this way, tan delta contributes to a condition assessment approach rather than a simple pass-fail decision. It gives valuable data to the maintenance plan about insulation aging or deterioration tendency, helping to prioritize risky line sections and decide whether more detailed methods such as partial discharge measurement are needed for a given cable.

Partial discharge measurement is important because it is a powerful diagnostic method especially for terminations, cable joints and local insulation defects. An important part of MV XLPE cable faults originates from point weaknesses, and workmanship errors, contamination, voids, screen cutback defects or accessory installation problems can produce PD behavior. If a cable defect is at an early stage, PD measurement can be a much stronger diagnostic tool than withstand testing alone for understanding the location and character of weak points. It therefore provides a significant advantage in critical feeders, in cases of recurring faults and in suspicious conditions after a new installation. Because it targets exactly the accessory and application details where many field faults concentrate, PD measurement helps identify workmanship errors and point weaknesses before they grow into failures, and its findings feed directly into the maintenance decision and trend tracking for the line.

In the field, many problems appear at cable terminations, cable joint areas, screen-grounding connections and mechanically stressed outer sheath points. Terminations and cable joints are the most sensitive areas in MV XLPE cable systems, because many faults originate not from the cable body but from incorrect termination application, weak cable joint workmanship or errors in the screen connection. Darkening, cracks, tracking, surface contamination, leakage or local heating marks in these areas should not be considered small defects. On the outer sheath, crushing, cuts, abrasion marks, chemical effects, rodent effects and suspected moisture ingress along the route are common weak points, since outer sheath damage is often the first sign a cable gives before an electrical fault. The screen-grounding arrangement is also critical, as a loose screen connection or broken screen conductor can affect fault behavior and system safety.

A thermal camera shows looseness or increased contact resistance at an early stage, especially at terminations, cable lugs, RMU or cubicle connections and points where cable screen grounding exists. In MV cable systems the full cable body often cannot be visually monitored, so thermal inspection is very valuable for the accessible connection points where problems tend to concentrate. When a temperature difference appears between similar phases at the same point, this is a strong sign that detailed inspection is required there. Increased contact resistance in termination and connection areas can therefore be detected before it develops into a fault. Because these connection areas are exactly where much of the fault risk sits in an MV XLPE cable system, thermal inspection complements visual checks, sheath testing and PD measurement, and its findings should be recorded and compared with previous observations as part of trend tracking.

The cable route is part of maintenance because outer sheath damage, mechanical pressure, excavation effects, incorrect fixing and environmental deterioration often develop along the route rather than only at the ends. During maintenance the route where the cable is laid, whether cable trench, tray, conduit crossing or directly buried section, should be inspected as far as possible, and later excavation, construction, additional installations or heavy equipment crossings should be evaluated separately. The mechanical protection arrangement also belongs in the plan: cable trays, cable clamps, bending radii, wall penetrations, mechanical protectors, cable labels and warning arrangements should be checked. Especially in underground applications, loss of marking, excavation risk close to the surface or route changes can weaken cable safety over time. Because the cable should not be evaluated only from its ends, route and mechanical observations are carried out together with termination, joint and screen checks.

Record keeping is important because MV cable faults are often understood more clearly not from a single measurement, but from change over time. Sheath test results, tan delta data, PD measurement reports, thermal images, route observations and accessory checks should be stored regularly so that findings can be compared with previous measurements. Repeated small deviations on the same line may be a warning sign of a future major fault, and this only becomes visible when results are archived and trended. In this way, changes in sheath test, tan delta, PD and thermal findings over time allow developing faults to be detected early and risky line sections to be prioritized. Trend tracking therefore strengthens the maintenance decision, turning individual test results into a clear picture of how a specific cable, termination or joint is behaving and whether intervention is needed.