What Tests and Maintenance Are Required for MV Cable Terminations?

Maintenance is required because MV cable terminations are among the most sensitive points of a medium-voltage cable. The cable itself is produced in a controlled way at the factory, while the termination is often prepared and installed in the field, which makes it more exposed to workmanship-related weaknesses. The fact that many faults in medium-voltage cable systems are concentrated in termination and joint areas clearly shows why this maintenance is so important. Over time, surface deterioration, loose connections, screen errors or stress control defects can develop under electrical stress and environmental effects and lead to serious faults. Maintenance therefore aims to detect these weaknesses early, not just to check whether the accessory appears healthy. By verifying surface health, the stress control structure, the screen-grounding chain and connection safety together, termination maintenance reduces fault risk and helps preserve energy continuity and termination safety.

Visual inspection, surface cleaning, crack and tracking inspection, screen-grounding verification, mechanical connection checks, thermal inspection and, when required, VLF, tan delta and PD assessment together with the cable system can be performed. Visual inspection looks for cracks, cuts, swelling, dirt, carbon marks, tracking, erosion, UV aging and water or oily contamination on the termination body. Cleaning removes surface dirt that can raise leakage currents, using a method that does not damage the silicone or polymer surface. Screen-grounding verification confirms the metallic screen is terminated correctly and mechanically secure, while mechanical connection checks cover lugs, bolted tightness and connector surfaces. Thermal inspection detects looseness and heating at lug and terminal areas, and partial discharge assessment reveals point weaknesses. Sealing condition is also checked to prevent moisture ingress. These checks are chosen according to whether the termination is indoor, outdoor, plug-in or newly installed.

The stress control area is important because electric field intensity naturally tends to increase where the cable screen ends, and the most critical design function of an MV termination is to make this transition safe. This is the most stressed part of the termination, so it requires special attention during maintenance. The check evaluates whether the stress control element has moved from its place, whether deterioration has occurred on its surface, and whether tracking or local discharge signs exist in the screen cutback area. A defect here does not stay local; a problem in the stress control region can raise the field stress the termination has to handle and produce partial discharge behavior over time. Because this area carries the highest electrical stress in the whole termination, it is one of the areas that most directly affects termination reliability and is treated as a priority in the maintenance plan.

Problems are most often seen in the screen cutback area, the stress control area, lug connections, sealing points and plug-in contact areas. The screen cutback and stress control region is critical because electric field intensity increases where the cable screen ends, and tracking or local discharge can start there. Lug and terminal connections are common weak points because looseness raises contact resistance and leads to heating over time. Sealing points matter because any gap or material deterioration where the termination meets the cable outer sheath can allow moisture ingress, and moisture is one of the main causes of long-term faults in medium-voltage terminations. On plug-in and separable terminations, incomplete seating, surface contamination and interface deterioration can affect both electrical performance and partial discharge behavior. Because the termination is prepared in the field, many of these faults are workmanship-related and develop gradually, which is why these areas receive the most attention.

A thermal camera shows looseness, increased contact resistance and abnormal heating at lug and terminal areas at an early stage. When lug connections, terminal surfaces, the contact points of plug-in terminations and screen grounding ends are thermally inspected, developing problems can be detected before they cause a fault. A clear temperature difference between phases is a strong sign that detailed inspection is required at that point. Abnormal temperature increase on the termination body itself should also be considered, because it can indicate internal deterioration or surface discharge behavior rather than a simple connection problem. Thermal inspection is therefore a very valuable, non-intrusive tool in MV cable termination maintenance, especially where looseness in mechanical connections would otherwise go unnoticed until heating becomes severe. Its findings are recorded and compared with previous images and with similar phases so that trends can be followed over time.

Partial discharge measurement is important because a workmanship-related void, stress control defect, contamination or local insulation weakness in the termination area can produce partial discharge behavior. Since the termination is prepared and installed in the field, exactly these kinds of point defects are common, and PD measurement is a powerful diagnostic method for detecting them early. Some weaknesses that cannot be understood by visual inspection alone can be revealed much more clearly with a PD-based assessment, which is why it is especially valuable on outdoor terminations and critical feeders. In practice, terminations are often not tested in isolation; VLF, tan delta and PD measurement are applied to the whole cable system so that accessory areas such as terminations and joints are also assessed. The importance of PD measurement increases after termination replacement, major repair or commissioning, when field workmanship most needs to be verified.

Some checks are specific to the termination, but in most cases the termination is evaluated together with the rest of the cable system rather than in isolation. Visual inspection, cleaning, stress control checks, mechanical tightness and sealing control are carried out directly on the termination itself. However, tests such as VLF withstand, tan delta and partial discharge measurement are usually applied to the whole cable system, which lets accessory areas such as terminations and joints be assessed indirectly or directly at the same time. The purpose of this holistic approach is to judge not only the general condition of the cable but also the termination areas where much of the fault risk sits. The importance of these system-level tests increases especially after termination replacement, major repair or commissioning, so that both the cable and its terminations are verified before the line is re-energized.

Yes, there is a clear difference between indoor and outdoor termination maintenance. In outdoor terminations, the effects of rain, UV, salt, industrial contamination and surface sealing become more important, and the influence of wind, vibration and cable weight on the termination should also be evaluated. In indoor terminations, surface darkening, dust accumulation and tracking signs are the main things searched for, and mechanical strain where the cable passes close to the cubicle or transformer needs attention. Cleaning matters in both cases, because dirt combined with moisture increases surface leakage currents and raises the risk of tracking, erosion and flashover over time. Sealing control is especially critical on outdoor terminations and terminations rising from underground, since moisture is a main cause of long-term faults. So while the core checks are shared, the environmental risks and the points that require the most attention differ between indoor and outdoor terminations.

On plug-in and separable terminations, contact surfaces, full seating condition, the screened body structure, connection torques, interface cleanliness and any signs of surface discharge are checked especially. In socket-type connections, incomplete seating, surface contamination, interface deterioration or improper installation can affect both electrical performance and partial discharge behavior, so looking only at the external appearance is not enough. For this reason, manufacturer installation dimensions, torque values and seating accuracy should also be part of the maintenance decision rather than a visual judgement alone. Thermal inspection of the contact points and screen grounding ends helps reveal looseness or increased contact resistance early, and a PD-based assessment can expose interface weaknesses that visual inspection misses. Because these terminations rely on a precise mechanical interface, correct seating and clean, properly torqued contacts are central to keeping the connection electrically sound and free of partial discharge over time.

Record keeping is important because MV cable termination faults often develop not suddenly, but through signs that grow over time. At the end of maintenance, visual defects, thermal images, PD findings, tan delta results, torque checks, cleaned areas and replaced parts should all be archived regularly so that each termination has a history. When this data is stored, changes in surface condition, connection heating and test results can be compared from one maintenance visit to the next. In this way, progressing deterioration in a specific termination can be noticed before it turns into a failure, and trend tracking makes weakening points visible early. Because so many termination problems are workmanship-related and slow to develop, keeping records turns individual observations into a clear picture of how each termination is behaving, which strengthens the maintenance decision and helps prioritize which terminations need attention first.