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

Metal clad systems are structures with strong security and partitioning logic in medium voltage switchgear facilities. For this reason, the tests and maintenance that must be done on metal clads are not done only to see whether the device can carry energy. The main purpose is to ensure the safe operation of the separate compartment structure, withdrawable breaker logic, shutter and interlock system, busbar connections and secondary systems for a long time. Because a small mechanical defect or connection weakness in metal clad panels can directly affect both operational safety and energy continuity over time.

The first step in maintenance is always safety. Before working on the metal clad switchgear, the relevant section must be deactivated in the correct maneuvering order, voltage relief must be verified, grounding procedures must be completed and, if necessary, the withdrawable breaker must be placed in the disconnected position. Especially in systems with drawout type cutters, it is of great importance not to leave equipment in an intermediate position, to ensure safe operation of the shutter area and to preserve the door interlock logic. Therefore, maintenance is not only testing but also correct isolation discipline.

Visual inspection is the basis of maintenance. Front covers, cell body, paint surface, signs of rust or corrosion, cabinet alignment, labels, warning signs and mechanical locking elements should be visually inspected. Although the separate compartment structure provides a safety advantage in metal clad systems, dust, moisture, looseness or mechanical deterioration may occur in these compartments over time. Therefore, external appearance should be evaluated not only in terms of cosmetics but also in terms of functional safety.

Compartment cleaning is a critical step in metal clad maintenance. Busbar compartment, breaker compartment, cable compartment and low voltage/auxiliary circuit compartments must be cleaned appropriately. Accumulation of dust, metal particles, insect residue, traces of moisture or foreign objects can adversely affect both insulation security and mechanical movement. If cleaning is neglected, especially in panels that have been in operation for a long time, shutter movement, auxiliary contact behavior and surface insulation may weaken over time.

The drawer cutter is one of the most delicate parts of the metal clad structure. It should be checked whether the breaker moves smoothly between the service position, test position and reserved position, whether there is any hardening or gap in the racking mechanism, and whether the breaker is fully seated in the cell. In addition, primary contact arms, tulip contact structures and moving main connection surfaces should be reviewed for wear, contamination and loss of pressure. In metal clad maintenance, simply turning the breaker on and off is not enough; Its mechanical operation compatible with the cell should also be verified.

The shutter mechanism also requires attention. In metal clad systems, primary contact openings are often protected by metal shutters, and these shutters are opened and closed depending on the location of the withdrawable breaker. During maintenance, the shutters move freely, balanced and safely; There is no deterioration in the spring or lock elements; When the breaker is disconnected, it must be verified that it covers the openings correctly. A cell with a defective shutter system loses one of the most important security advantages of metal clad logic.

The interlock system is the security heart of the metal clad system. Door interlocks, earth separator and breaker relationship, test position locks, shutter access logic, key locks and electromechanical permission chains, if any, should be tested one by one. A door that opens in the wrong order, a breaker that moves at an inopportune moment, or a malfunctioning earth separator mechanism poses a serious site risk. That's why the interlock test is the main part of the maintenance, not an auxiliary part.

Busbar compartment and cable compartment checks are also of great importance in metal clad maintenance. The busbar connections should be examined for looseness, darkening, heating traces, cracks in the insulators, moisture in the cable compartment, deterioration in the headers, and problems with the screen-grounding arrangement. Cable headers, CT/VT regions and terminal connections are among the areas of the metal clad panel that need the most attention in terms of malfunction. Although the separate compartment structure makes these areas more controlled, it does not eliminate the need for maintenance.

Instrument transformers and auxiliary primary elements should also be checked. CT and VT assemblies, secondary connections, grounding points, fuse structures and insulation surfaces should be reviewed. In metal clad systems, the low voltage instrument section is often separate from the high voltage compartments; This is an advantage, but it does not eliminate the need for regular testing of secondary circuits. If the relays are to work correctly, this measurement chain must first be healthy.

Contact resistance measurement is one of the valuable electrical tests in metal clad maintenance. It can be understood whether there is an abnormal resistance increase in the conduction path by making microohm level measurements, especially at the drawer breaker main contacts, primary fixed contact points and busbar transitions. A significant difference between phases indicates a loose connection, contamination, loss of contact pressure, or the need for detailed mechanical inspection. This test alone is not sufficient, but it is a very powerful early warning tool.

Insulation checks are also important. Insulation evaluation should be carried out with appropriate test procedures on the busbar-ground, phase-phase, secondary circuits and, when necessary, auxiliary cabling side. The insulation resistance approach can provide important data to the maintenance plan, especially in metal clad cells that have been deactivated for a long time, exposed to moisture or field intervention. However, the results should always be interpreted together with previous measurements and environmental conditions.

Secondary circuit and auxiliary equipment tests should not be neglected in metal clad maintenance. Protection relays, auxiliary contacts, breaker opening-closing circuits, if there is a motorized racking mechanism, its remote control, indicators, alarm circuits and cable ends must be verified one by one. Because in metal clad panels, many faults arise from the low voltage control chain, not from the primary side. Even if the cell appears to be intact, if the secondary logic is broken, safe operation is not possible.

Thermal camera inspection is very efficient. Looseness or increased contact resistance can be detected early when busbar joints, primary connections, breaker contact areas, CT/VT terminal areas, cable headers and auxiliary panel connections are thermally examined. Since there is compartment separation in the metal clad structure, thermal observations should be made in a planned manner and comparisons between similar cells should be taken as basis. A significant temperature difference between phases is often a sign that requires detailed maintenance.

At the end of maintenance, all findings should be recorded. Movement behavior of the withdrawable breaker, shutter and interlock tests, contact resistance measurements, insulation results, secondary circuit tests, thermal images and observed mechanical defects should be archived regularly. Because problems in metal clad systems often do not occur suddenly; develops over time. If trend monitoring is done, weakening mechanisms, overheating connections and recurring cell problems will be noticed before failure occurs. In summary, the tests and maintenance that must be done on metal clads are; It consists of visual inspection, compartment cleaning, pull-out breaker and shutter/interlock verification, busbar and cable compartment checks, contact resistance and insulation tests, secondary circuit verifications and thermal inspections. If metal clad cubicles, withdrawable breakers, relay systems and MV panel 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.