What are the Tests and Maintenances That Should Be Done on RMUs?

RMUs are compact switchgear equipment that ensures energy continuity in medium voltage distribution systems. For this reason, the tests and maintenance that need to be done on RMUs are not only done to see if the device turns on and off; The main goal is to maintain both switching security and distribution continuity. A seemingly minor interlock error in the field, a loose connection, low gas pressure, a faulty auxiliary contact or a waterlogged cable compartment can turn into much larger disruptions and safety risks over time.

The first step in maintenance is always safety. Before working on the RMU, the correct maneuver sequence must be applied, the relevant section must be de-energized, voltage-free must be confirmed and, if necessary, grounding steps must be completed. Especially if the cable compartment is to be accessed, it must be verified that the relevant separator and earth separator positions are correct. Because in RMUs, safety depends not only on the robustness of the device but also on the correct operating sequence.

Visual inspection is the basis of maintenance. The outer body should be checked for impact marks, paint damage, rust, door misalignment, deterioration in seals, lack of labels, problems with door locks and general exterior surface dirt. In outdoor RMUs, the effects of rain, humidity, UV, dust and environmental contamination should be evaluated more carefully. Even if the corrosion that starts on the body seems minor, it can affect the mechanical integrity and level of protection in the long run.

Cleaning is an important step in RMU maintenance. The front panel, cable compartment surroundings, door inner surfaces, low voltage compartment and device surroundings should be cleaned for dust, insects, plant residues, traces of moisture and foreign objects. Particularly in some manufacturer maintenance lists, heavy dust, water ingress and contamination from animals or insects on the inside of the door are also emphasized. Such accumulations can affect both auxiliary circuits and insulation safety over time.:contentReference[oaicite:1]{index=1}

Mechanical operating testing is an essential part of RMU maintenance. The load disconnector, disconnector, earth separator and breaker mechanisms, if any, should be operated in a controlled manner several times and it should be observed whether they move properly. Symptoms such as sticking in the arm movement, not fully reaching the final position, excessive stiffness, abnormal sound or retraction indicate the need for mechanical maintenance. If there is motorized operation, motorized on-off and trip mechanisms should also be tested separately.:contentReference[oaicite:2]{index=2}

Interlock systems are the heart of RMU security. Door interlocks, earth separator and door relationship, key locks, mechanical bolts and all mechanisms that prevent the equipment from being operated in the wrong order should be tested one by one. In some RMU designs, it is clearly stated that the grounding switch must be in a certain position in order for the door to open and close. Such interlock logics are not just design details; It is a direct part of personnel safety in the field.:contentReference[oaicite:3]{index=3}

In RMUs that use gas insulated or sealed gas enclosures, the gas status should be evaluated separately. Not every design is the same; Some models may not require routine gas maintenance. However, this does not mean that there will be no control. It should be monitored to see whether the pressure gauge is in the correct area, especially with the green-red zone logic. If the indicator is in the red zone, the equipment should not be operated and an authorized service evaluation may be required. The best approach to gas RMU maintenance is to follow the pressure and service logic in the model's manufacturer's manual.:contentReference[oaicite:4]{index=4}

The cable compartment and cable connections require special attention in RMU maintenance. Cable headers, plug-in connections, shield-grounding ends, cable lugs and phase arrangement should be inspected visually. It is recommended to separately check cable connections for damage, signs of discharge and water ingress in the manufacturer's maintenance lists. If there is water accumulation in the cable tray or trench, not only drainage but also the additional mechanical stress on bushings and cable terminations must be considered.:contentReference[oaicite:5]{index=5}

Grounding connections are one of the basic safety issues in RMU maintenance. The main ground busbar, body grounding, screen grounding points and door equipotential connections, if any, should be reviewed. Looseness that seems small from the outside can seriously affect the behavior in case of failure. Therefore, during maintenance, not only the main current path but the entire protective conductor structure must be verified.

Electrical verifications may vary by model and application. In some RMU types, contact resistance measurement has an important place in the commissioning and periodic maintenance approach. This check at the microohm level is valuable to detect looseness or contact distortion in the main current path. If there are significant differences between phases or values ​​above the limits, detailed examination is required. In addition, secondary wiring insulation resistance testing is one of the important methods used to see the health of the auxiliary circuit.:contentReference[oaicite:6]{index=6}

The secondary circuit, auxiliary contacts and motorization structures should be tested separately. It should be evaluated whether the auxiliary contacts produce correct position information, whether the operating circuit is healthy if there is a motorized operation, whether the relay or fault indicator connections behave correctly, and whether there is any damage caused by rust or condensation in the low voltage compartment. Rust and surface oxidation in a low voltage compartment, especially in humid environments, can be an important warning sign.:contentReference[oaicite:7]{index=7}

In RMUs with voltage indicator, phase suitability and cable testing facilities, these features are also part of the maintenance. In some designs, cable fault control, voltage presence verification and phase conformity testing can be performed with the cable test device. The presence of such equipment strengthens not only the main device but also the possibility of field diagnostics during maintenance. However, these operations must be carried out according to the operating procedure of the relevant model.:contentReference[oaicite:8]{index=8}

Thermal inspection is a very efficient tool in RMU maintenance. Looseness or increased resistance can be detected early when cable headers, terminal connections, plug-in contact areas, busbar transitions, auxiliary panel connections and fuse areas are thermally examined. What is important here is not a single temperature value, but the evaluation of the differences between phases and similar cells. Thermal anomaly is often the first sign of a growing problem.

At the end of maintenance, all findings should be recorded regularly. Gas gauge status, door and interlock tests, contact resistance results, secondary circuit tests, thermal images, observed corrosion areas, and corrective actions taken should be kept in the maintenance history. Because RMU problems often do not occur in a single day; develops over time. If trend monitoring is done, recurring malfunctions, weakening mechanisms and environmental effects can be seen earlier. In summary, the tests and maintenance that need to be done on RMUs are; It consists of visual control, cleaning, mechanical and interlock verification, pressure gauge monitoring in gas types, cable and grounding connection checks, contact resistance and secondary wiring tests when necessary, as well as thermal inspections. If RMUs, MV cable connections, transformer feeding structures and field security will be evaluated together in your facility HV/MV testing, maintenance and repair with LV/MV/HV project design and consultancy services can support the technical decision process.