What Tests and Maintenance Are Required for RMUs?

Maintenance is required because RMUs are compact switchgear equipment that provide energy continuity in medium-voltage distribution systems. Maintenance is 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, a loose connection, low gas pressure, a faulty auxiliary contact or water ingress in a cable compartment can turn into much larger outages and safety risks over time. Because an RMU sits at a point where many feeders and transformer connections meet, a problem inside it can affect a wide part of the distribution network. Regular maintenance detects mechanical faults, interlock problems, low gas pressure, loose connections and cable compartment issues at an early stage, before they create safety and continuity risks. In this way it protects distribution continuity, field safety and the equipment itself.

Visual inspection, cleaning, mechanical operation trials, interlock verification, gas indicator checks, cable compartment inspection, grounding connection checks, auxiliary contact and motorization checks, and contact resistance and secondary circuit tests when required can be performed. Visual inspection looks for impact marks, rust, door misalignment, seal deterioration and cover lock problems, while cleaning removes dust, insects, plant residues and moisture from the panel and compartments. Mechanical operation trials confirm that the load break switch, disconnector, earthing switch and any breaker mechanism move properly, and interlock verification checks door and earthing-switch logic. In gas-insulated types the pressure indicator is monitored, and the cable compartment and grounding connections are inspected. Contact resistance measurement checks the main current path, and secondary wiring, auxiliary contacts and motorized elements are tested. Thermal inspection completes the picture. The exact scope depends on the RMU model and its operating procedure.

Not every model is the same, so routine gas maintenance is not always required. Some sealed gas enclosure designs do not need routine gas maintenance, but this does not mean no check should be performed. During maintenance it should be monitored whether the pressure indicator is in the correct zone, especially using 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, because operating it in that condition is not safe. The most accurate approach in gas RMU maintenance is to follow the pressure and service logic given in the manufacturer's manual for that specific model, rather than applying a single rule to all designs. So even where routine gas maintenance is not required, verifying the gas indicator remains a basic part of RMU maintenance and a clear sign of whether the enclosure is still healthy.

The interlock test is important because interlock systems are the heart of RMU safety; they prevent doors and switching elements from being operated in the wrong sequence. During maintenance, door interlocks, the relationship between the earthing switch and door, key locks, mechanical slides and all arrangements that prevent an 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, and these interlock logics are not just design details; they are a direct part of personnel safety in the field. If this structure is faulty, a door could be opened or a switch operated at an unsuitable time, and field safety is seriously weakened. Because safety in an RMU depends not only on the device being healthy but also on the correct operating sequence, verifying every interlock is treated as a main part of maintenance.

In the cable compartment, damage, discharge marks, water ingress, termination and plug-in connection health, the screen-grounding arrangement and mechanical stresses are checked at the cable connections. Cable terminations, plug-in connections, screen-grounding ends, cable lugs and the phase arrangement are inspected visually, and 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 the additional mechanical stress this can place on bushings and cable terminations should be considered. The cable compartment requires special attention because it is one of the areas where faults and safety risks tend to develop in an RMU. By checking connection health, moisture and mechanical loading together, maintenance catches problems such as loose terminations, discharge and water ingress before they grow into an outage, and the findings are recorded for comparison at the next visit.

Contact resistance measurement shows the contact quality in the main current path of the RMU. In some RMU types this micro-ohm level check has an important place in commissioning and periodic maintenance, because it reveals whether there is looseness or contact deterioration in the conducting path. If there are clear differences between phases, or values above the expected limits, detailed inspection is required at that point. High or unbalanced micro-ohm readings therefore act as an early warning that a connection or contact is weakening, even when nothing is visible from the outside. The test is not sufficient on its own; it is interpreted together with visual inspection, thermal findings and the maintenance history. Because a poor contact raises resistance and leads to heating over time, catching an abnormal contact resistance value early helps prevent a developing hot connection from turning into a fault in the main current path.

Secondary circuit tests are performed because, even if the RMU is healthy as main equipment, safe field operation cannot be maintained if the auxiliary side has deteriorated. During these tests it is evaluated whether auxiliary contacts produce correct position information, whether the operating circuit is healthy when 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. Insulation resistance testing of the secondary wiring is also used to see auxiliary circuit health. Because the secondary circuit carries position signalling, control and protection functions, a fault here can prevent correct operation or indication even when the primary equipment is sound. For this reason the secondary side is tested separately as a distinct part of RMU maintenance.

A thermal camera helps detect abnormal heating at an early stage across the RMU's connection points. 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 found before it causes a fault. The important point is not a single temperature value, but evaluating differences between phases and between similar cubicles, because a relative difference is what reveals a developing problem. A thermal anomaly is often the first sign of a growing issue, so thermal inspection is a very efficient tool in RMU maintenance. It is non-intrusive and can be carried out while the equipment is in service, which makes it well suited to the many bolted and plug-in connections in an RMU. The findings are recorded and compared with previous images so that heating connections can be tracked and addressed before they fail.

If possible, an RMU that has not been operated for a long time should be operated several times in a controlled way to verify that the mechanism moves properly, in line with the manufacturer's maintenance approach. The mechanical operation test is an indispensable part of RMU maintenance: the load break switch, disconnector, earthing switch and any circuit breaker mechanism should move without problems. Signs such as sticking in the handle movement, not reaching the final position, excessive stiffness, abnormal sound or rebound indicate that mechanical maintenance is needed. If motorized operation exists, the motorized opening-closing and trip mechanisms should also be tested separately. Exercising the mechanism matters because a switch that is left unused for a long time can stiffen or stick, and this may only become apparent when it is needed. All of this should be done safely, following the correct switching sequence and confirming absence of voltage before any work.

Record keeping is important because RMU problems usually do not occur in a single day; they develop over time. At the end of maintenance, gas indicator condition, door and interlock tests, contact resistance results, secondary circuit tests, thermal images, observed corrosion areas and corrective actions should all be kept in the maintenance history. When this information is archived, gas condition, thermal differences, contact resistance, interlock behavior and auxiliary circuit findings can be compared from one visit to the next. If trend tracking is performed, recurring faults, weakening mechanisms and environmental effects such as corrosion or condensation can be seen earlier. In this way, records turn individual observations into a clear picture of how each RMU is behaving and whether a specific compartment or mechanism is deteriorating. Because so many RMU problems grow slowly, keeping records lets maintenance catch deterioration trends before they turn into an outage or a safety risk.