What Tests and Maintenance Are Required for MV Cable Terminations?

MV cable terminations are among the most sensitive points of a medium-voltage cable. Because the cable is produced in a controlled way at the factory, while the termination is often prepared and installed in the field. Therefore, the tests and maintenance required for MV cable terminations are performed not only to check whether the accessory appears healthy, but also to detect electrical stresses, workmanship-related weaknesses and environmental effects that may develop in the termination area at an early stage. 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. For related context, see What Is an MV XLPE Cable? What Does It Do, How Does It Work and What Structure Does It Have?.

The first step of maintenance is always safety. Before working on an MV cable termination, the related line or cubicle should be taken out of service with the proper switching sequence, absence of voltage should be confirmed and the work area should be made safe. The cubicle, RMU, transformer bushing or outdoor connection to which the termination is connected should also be evaluated together. Because the termination is not a standalone part; it is a termination system operating together with the cable, equipment and screen-grounding chain. For related context, see What Is Metal-Clad Switchgear? What Does It Do, How Does It Work and What Features Does It Have?.

Visual inspection is the basis of maintenance. The termination body should be carefully inspected for cracks, cuts, punctures, swelling, dirt layer on the surface, carbon marks, tracking, local erosion, UV aging, water marks or oily contamination. In indoor terminations, surface darkening, dust accumulation and tracking signs are searched for; in outdoor terminations, rain, UV, salt, industrial contamination and surface sealing become more important in addition to these. Even small deterioration in the external appearance of the termination can indicate a larger future fault risk. For related context, see What Tests and Maintenance Are Required for Electricity Meters?.

Cleaning is an important part of MV cable termination maintenance. Especially on outdoor terminations and in dirty indoor volumes, dirt accumulated on the surface can increase surface leakage currents when combined with moisture. This increases the risk of tracking, erosion and flashover over time. Therefore, the termination surface should be cleaned with a suitable method, but the material used must not damage the silicone or polymer surface. Incorrect cleaning practices can shorten termination life instead of performing maintenance. For related context, see What Tests and Maintenance Are Required for Transformer Differential Protection Relays?.

The stress control area requires special attention on MV cable terminations. Because electric field intensity naturally tends to increase where the cable screen ends. The most critical design function of the termination is to make this transition safe. During maintenance, it should be evaluated 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. This area is the most stressed part of the termination.

The screen and grounding arrangement is one of the most critical headings in MV cable termination maintenance. It should be verified that the metallic screen is terminated at the correct point, the screen grounding connection is solid, there is no looseness in the bonding arrangement and screen conductors are held mechanically secure. A loose or broken screen connection can negatively affect not only fault behavior, but also the electric field distribution and operational safety of the termination. Therefore, termination maintenance cannot be considered independently from the screen-grounding chain.

Mechanical connections and lug areas should also be checked separately. The equipment terminal to which the termination is connected, cable lugs, bolted tightness, connector surfaces and plug-in connection elements, if present, should be inspected for tightness and surface health. Looseness in these areas often does not produce an electrical fault at the first stage; however, as contact resistance increases, heating starts and can lead to serious problems in the termination area over time. Therefore, mechanical tightness is not an auxiliary check in MV cable termination maintenance, but a basic requirement.

In indoor MV cable terminations, it should also be observed whether there is mechanical strain in the area where the cable passes close to the cubicle or transformer. Improper cable bending, pulling in a way that places excessive load on the termination or incorrect clamping can strain the termination area over time. In outdoor terminations, the effects of wind, vibration and cable weight on the termination should also be evaluated. The termination must be protected mechanically as well as electrically.

Thermal camera inspection is a very valuable tool in MV cable termination maintenance. Especially when lug connections, terminal surfaces, contact points of plug-in terminations and screen grounding ends are thermally inspected, looseness or increased contact resistance can be detected at an early stage. A clear temperature difference between phases is a strong sign requiring detailed inspection at that point. Abnormal temperature increase on the termination body should also be considered in terms of internal deterioration or surface discharge behavior.

The partial discharge approach is especially important on MV cable terminations. Because if there is a workmanship-related void, stress control defect, contamination or local insulation weakness in the termination area, this can produce partial discharge behavior. PD measurement is a powerful diagnostic method for detecting such point defects early. Especially in outdoor terminations and critical feeders, some weaknesses that cannot be understood by visual inspection can be revealed more clearly with PD-based assessment.

Terminations are often not tested separately from the rest of the cable system; they are evaluated together with the cable. Therefore, in the maintenance approach for MV cable terminations, VLF withstand test, tan delta and PD measurement are often applied to the whole cable system. The purpose here is to assess not only the general condition of the cable, but also accessory areas such as terminations and joints indirectly or directly. The importance of these tests increases especially after termination replacement, major repair or commissioning.

Sealing control also has a critical place in MV cable termination maintenance. There should be no gap, opening or material deterioration that allows moisture ingress in the area where the termination meets the cable outer sheath, in the stress control area and in the external protection layers. Moisture is one of the main causes of long-term faults in medium-voltage terminations. Therefore, sealing condition should be monitored carefully especially on outdoor terminations and terminations rising from underground.

On plug-in and separable terminations, contact surfaces and the screened body structure should also be evaluated separately. In socket-type connections, incomplete seating, surface contamination, interface deterioration or improper installation can affect both electrical performance and partial discharge behavior. On this type of termination, looking only at the external appearance is not enough; manufacturer installation dimensions, torque values and seating accuracy should also be part of the maintenance decision.

At the end of maintenance, all findings should be recorded. Visual defects, thermal images, PD findings, tan delta results, torque checks, cleaned areas and replaced parts should be archived regularly. Because MV cable termination faults often develop not suddenly, but through signs that grow over time. When trend tracking is performed, progressing deterioration in a specific termination can be noticed before a failure occurs. In summary, the tests and maintenance required for MV cable terminations consist of visual inspection, cleaning, stress control and screen-grounding area check, mechanical connection verification, thermal inspection and VLF, tan delta and PD assessments applied together with the cable system when required. If MV cable terminations, cable accessories, RMU-cubicle-transformer connections and maintenance planning in your facility will be evaluated together, HV/MV testing, maintenance and repair and LV/MV/HV project design and consultancy services can support the technical decision process.