Medium-voltage switchgear maintenance checks everything that lets the lineup carry, isolate and interrupt current safely: connections and busbars, insulators and cable terminations, the breaker and its mechanism, the withdrawable truck and shutters, the interlocks, the earthing switch, the instrument transformers, the relay and the auxiliary circuits. It combines in-service inspection (thermography, ultrasound) with electrical tests on the de-energized cubicle.
Why switchgear is maintained
Medium-voltage switchgear is where the plant’s energy is concentrated: it receives the incoming feeder, protects the transformer and supplies the medium-voltage circuits. A fault inside a cubicle is usually an internal arc, with serious equipment damage and risk to anyone nearby. Almost all of these failures have a cause that maintenance can catch in time. It may be a connection that loosens and heats up, a contaminated insulator that starts to show surface discharges, a poorly made cable termination, a mechanism that becomes slow, or an interlock that someone defeated.
It is known as an MV switchgear preventive maintenance checklist. A checklist is useful as long as it is adapted to the type of switchgear and to the manufacturer’s manual.
The switchgear type changes the scope
| Type | What can be serviced | Particularity |
|---|---|---|
| Air-insulated, withdrawable breaker (metal-clad) | Breaker outside the cubicle, plug-in contacts, shutters, busbars | Allows the breaker to be maintained without de-energizing the whole lineup, depending on its loss of service continuity category |
| Air-insulated, fixed-mounted | Everything, but with the section de-energized | Greater exposure during the work |
| SF6-insulated (MV GIS, RMU) | Mechanism, gas indicator, terminations, auxiliaries | The gas compartment is usually sealed and is not opened |
The product standard, IEC 62271-200, classifies switchgear by its loss of service continuity category (LSC), which indicates what stays in service when a compartment is opened, and by its internal arc classification (IAC). These classifications define what can be done with the lineup energized and what cannot.
What is checked, component by component
| Component | What to look for |
|---|---|
| Busbars and connections | Signs of heating, discoloration, bolt torque per the manufacturer |
| Insulators, bushings and cable terminations | Dust, moisture, surface discharge marks (tracking), cracks |
| Breaker | Condition of the mechanism and its lubrication, operation counter, contact wear, vacuum interrupter or SF6 pressure |
| Withdrawable truck and shutters | Enters and leaves without forcing, plug-in contacts clean and with pressure, shutters that close |
| Interlocks | That they prevent the improper operations required by the standard and by the local code |
| Earthing switch | Operation, contacts, position indication |
| Current and voltage transformers | Secondary connections, insulation condition, VT fuses |
| Protective relay | Alarms, recorded events, settings in force, battery and clock |
| Auxiliaries | Anti-condensation heaters, wiring, terminal blocks, voltage presence indicators, lighting |
| Enclosure | Doors, seals, arc relief ducts unobstructed, ground connection |
The truck and the interlocks are explained in withdrawable truck and switchgear interlocks. Interlocks between switching and isolating devices are required by the product standard and, in most jurisdictions, by the local code. For withdrawable equipment, the breaker must not be operable except in the service, disconnected, withdrawn or earthed positions, and it must not be inserted or withdrawn unless it is open.
In-service inspection and de-energized tests
Switchgear maintenance combines two moments.
With the lineup in service: external visual inspection, thermography under load, ultrasound and, in switchgear that allows it, partial discharge measurement. This shows which cubicles need attention at the next outage. See thermography in substations and panels.
With the section de-energized and earthed: cleaning, tightening and electrical tests.
| Test | What it verifies |
|---|---|
| Insulation resistance | General condition of the insulation of busbars, breaker and cables |
| Contact resistance | That the main circuit and the plug-in contacts have no abnormal resistance |
| Operating times | That the breaker opens and closes on time and with pole simultaneity |
| Vacuum integrity | That the vacuum interrupter keeps its ability to insulate |
| On-site withstand voltage | The insulation under a test voltage, when the scope calls for it |
| Instrument transformers | Ratio, polarity, insulation |
| Secondary injection to the relay | That the relay operates with its settings and trips the correct breaker |
Breaker tests have their own guide: medium-voltage circuit breaker maintenance. For RMUs, with their sealed tank, see RMU testing.
Switchgear, protection and arc flash
Switchgear maintenance is tied to the plant’s arc flash study. The incident energy shown on the labels is calculated from the protection clearing time and the breaker opening time. If the breaker has become slow or the relay has a setting different from the one studied, the real energy of an arc is higher than the label value, and the personal protective equipment selected from it is no longer sufficient. That is why operating times and the relay test are not a formality: they support the safety of the person operating the lineup. See arc flash study. For how to plan electrical maintenance during a plant shutdown, see plant shutdown: planning electrical maintenance.
What must be recorded
One report per cubicle, with the equipment identification and the condition found. It includes the result of each test against the manufacturer’s or the standard’s criterion and against the previous test. It also includes the downloaded relay settings, the findings with their priority and what was corrected. That record is what auditors and many local codes expect for substation maintenance, and it is what allows you to decide when to intervene again.
Frequent mistakes
- Cleaning without testing. The cubicle looks good and the breaker is still slow.
- Tightening bolts without a specified torque. An over-tightened bolt also fails.
- Forgetting the heaters. Without them, condensation in humid climates promotes surface discharges.
- Leaving interlocks defeated after an «emergency» operation.
- Not testing the real trip. A relay that issues the command but does not open the breaker because of a defective coil or control cable does not protect.
How we help
At Tiza Group our team performs technical maintenance of medium-voltage switchgear in industrial plants and substations. It includes inspection, tests on the breaker and instrument transformers, protection tests by secondary injection with a real trip, and review of relay settings and events. We deliver one report per cubicle, with results that can be compared over time. This fieldwork is done in Colombia, or elsewhere on request.
See our testing and commissioning and circuit breaker testing, or write to us with the type and number of cubicles in your lineup.
Related guides
- Medium-voltage switchgear: parts, functions and tests
- Withdrawable truck and switchgear interlocks
- Medium-voltage circuit breaker maintenance
- Contact resistance test
- On-site withstand voltage test
- Medium-voltage switch-disconnector and RMU
- Medium-voltage RMU (Ring Main Unit) testing
- Substation maintenance: what it includes
- Plant Shutdown: Planning Electrical Maintenance
References
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEC 62271-100, High-voltage alternating-current circuit-breakers.
- ANSI/NETA MTS, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems.
Frequently asked questions
What is checked in medium-voltage switchgear maintenance?
The condition of busbars and connections, the cleanliness and insulation of insulators and cable terminations, the breaker (mechanism, contacts, vacuum interrupter or SF6 pressure), the withdrawable truck and shutters, the interlocks, the earthing switch, the current and voltage transformers, the protective relay, the heaters and the auxiliary wiring.
What tests are performed on medium-voltage switchgear?
The usual ones are insulation resistance, main circuit contact resistance, breaker operating times, vacuum integrity or gas verification depending on the technology, withstand voltage where applicable, instrument transformer tests, and a relay test by secondary injection with a real trip to the breaker.
Does SF6-insulated switchgear need maintenance?
Its gas compartment is usually sealed for life and is not opened, but the rest is maintained: operating mechanism, gas pressure or density indicator, interlocks, cable terminations, earthing, relay and auxiliary circuits. The scope is defined by the manufacturer's manual.