What Tests and Maintenance Are Required for RMUs?

RMUs are compact switchgear equipment that provide energy continuity in medium-voltage distribution systems. Therefore, the tests and maintenance required for RMUs are not performed only to see whether the device opens and closes; the main purpose is to preserve both switching safety and distribution continuity. A small-looking interlock error, loose connection, low gas pressure, faulty auxiliary contact or water ingress in a cable compartment can turn into much larger outages and safety risks over time. For related context, see What Is an RMU? What Does It Do, How Does It Work and What Parts Does It Include?.

The first step of maintenance is always safety. Before working on an RMU, the correct switching sequence should be applied, the related section should be de-energized, absence of voltage should be confirmed and grounding steps should be completed when required. Especially if the cable compartment will be accessed, the positions of the related disconnector and earthing switch must be verified. Because safety in RMUs depends not only on the device being healthy, but also on the correct operating sequence. For related context, see What Is an MV XLPE Cable? What Does It Do, How Does It Work and What Structure Does It Have?.

Visual inspection is the basis of maintenance. The outer body should be checked for impact marks, paint damage, rust, door misalignment, seal deterioration, missing labels, cover lock problems and general surface contamination. In outdoor RMUs, the effects of rain, moisture, UV, dust and environmental contamination should also be assessed more carefully. Even corrosion starting on the body may affect mechanical integrity and protection level in the long term. For related context, see What Is Metal-Clad Switchgear? What Does It Do, How Does It Work and What Features Does It Have?.

Cleaning is an important step in RMU maintenance. The front panel, cable compartment surroundings, inner door surfaces, low-voltage compartment and the area around the device should be cleaned of dust, insects, plant residues, moisture traces and foreign objects. Some manufacturer maintenance lists especially emphasize heavy dust, water ingress and animal or insect contamination inside doors. Such accumulations can affect both auxiliary circuits and insulation safety over time. For related context, see What Tests and Maintenance Are Required for Power Quality Analyzers?.

The mechanical operation test is an indispensable part of RMU maintenance. The load break switch, disconnector, earthing switch and circuit breaker mechanisms, if present, should be operated several times in a controlled way to observe whether they move properly. Signs such as sticking in handle movement, not reaching the final position, excessive stiffness, abnormal sound or rebound indicate the need for mechanical maintenance. If motorized operation exists, motorized opening-closing and trip mechanisms should also be tested separately.

Interlock systems are the heart of RMU safety. Door interlocks, the relationship between the earthing switch and door, key locks, mechanical slides and all arrangements preventing incorrect operating sequence should be tested one by one. In some RMU designs, the earthing switch must be in a specific position for the door to open or close. These interlock logics are not just design details; they are a direct part of personnel safety in the field.

In gas-insulated or sealed gas enclosure RMUs, gas condition should also be evaluated separately. Every design is not the same; some models may not require routine gas maintenance. However, this does not mean no check should be performed. It should be monitored whether the pressure indicator is in the correct zone, especially with the green-red zone logic. If the indicator is in the red zone, the equipment may need to be kept out of operation and evaluated by authorized service. The most accurate approach in gas RMU maintenance is to follow the pressure and service logic in the manufacturer's manual for that model.

The cable compartment and cable connections require special attention in RMU maintenance. Cable terminations, plug-in connections, screen-grounding ends, cable lugs and phase arrangement should be visually inspected. Manufacturer maintenance lists recommend checking cable connections for damage, discharge signs and water ingress. If there is water accumulation in the cable duct or trench, not only drainage but also additional mechanical stress on bushings and cable terminations should be considered.

Grounding connections are one of the basic safety headings in RMU maintenance. The main grounding busbar, body grounding, screen grounding points and door equipotential bonds, if present, should be reviewed. Looseness that appears small from the outside can seriously affect behavior during a fault. Therefore, during maintenance, not only the main current path but the entire protective conductor structure should be verified.

Electrical verifications may vary according to model and application. In some RMU types, contact resistance measurement has an important place in commissioning and periodic maintenance. This micro-ohm level check is valuable for understanding whether there is looseness or contact deterioration in the main current path. If there are clear differences between phases or values above limits, detailed inspection is required. In addition, insulation resistance testing of secondary wiring is an important method used especially to see auxiliary circuit health.

The secondary circuit, auxiliary contacts and motorization structures should also be tested separately. It should be evaluated whether auxiliary contacts produce correct position information, whether the operating circuit is healthy if motorized operation exists, whether relay or fault indicator connections behave correctly and whether there is rust or condensation-related damage in the low-voltage compartment. Especially in humid environments, rust and surface oxidation in the low-voltage compartment can be an important warning sign.

In RMUs with voltage indicator, phase conformity and cable test capability, these features are also part of maintenance. Some designs allow cable fault checks through a cable test device, verification of voltage presence with a voltage indicator and phase conformity testing. The existence of such equipment strengthens not only the main device but also the field diagnostic capability during maintenance. However, these operations must always be carried out according to the operating procedure of the relevant model.

Thermal inspection is a very efficient tool in RMU maintenance. When cable terminations, terminal connections, plug-in contact areas, busbar transitions, auxiliary panel connections and fuse areas are thermally inspected, looseness or increased resistance can be detected early. The important point here is not a single temperature value, but evaluating differences between phases and similar cubicles. A thermal anomaly is often the first sign of a growing problem.

At the end of maintenance, all findings should be recorded regularly. Gas indicator condition, door and interlock tests, contact resistance results, secondary circuit tests, thermal images, observed corrosion areas and corrective actions should be kept in the maintenance history. Because RMU problems usually do not occur in a single day; they develop over time. If trend tracking is performed, recurring faults, weakening mechanisms and environmental effects can be seen earlier. In summary, the tests and maintenance required for RMUs consist of visual inspection, cleaning, mechanical and interlock verification, pressure indicator tracking in gas-insulated types, cable and grounding connection checks, contact resistance and secondary wiring tests when required and thermal inspections carried out together. If RMUs, MV cable connections, transformer feeder structures and field safety 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.