Medium-voltage switchgear explained

Medium-voltage switchgear is a coordinated assembly that routes, isolates, measures, and protects power on a distribution system. It includes a bus and connection system, switching or interrupting devices, control and protective equipment, and the enclosure that holds those parts in their tested arrangement. IEEE C37.20.2-2025 describes relaying, metering, controls, instrumentation, and bus inside metal-clad switchgear; IEEE C37.20.3-2023 covers metal-enclosed interrupter assemblies with switches, fuses, breakers, and other devices. A breaker by itself is not a complete switchgear lineup.

A useful mental model is a set of paths. The incoming cable lands at a main section; the bus carries power across vertical sections; each feeder device connects or disconnects a branch. The relays decide when to trip; instrument transformers scale current and voltage for those relays and meters; control power lets the protection and operating mechanisms function. In a 13.8 kV campus lineup, one feeder may supply a transformer near a building while another serves a motor plant. The same bus voltage does not mean the two feeders have the same load current or protection settings.

Start with the one-line, not a photograph of cubicles. A photograph may show six vertical sections, but it cannot tell you which section is the main, which is a tie, where cables enter, how the bus is sectionalized, or whether a breaker has the interrupting rating the fault study requires. Ask for the one-line, nameplate, and as-built drawings before matching an existing assembly to a new site.

“Medium voltage” is context-dependent. Schneider Electric’s North American system-voltage guide places nominal distribution systems such as 4.16, 13.8, 24.94, and 34.5 kV in its medium-voltage discussion, while IEEE breaker and switchgear standards specify rated maximum equipment voltages. The phrase alone is too broad for a purchase request. Put the nominal system voltage, equipment class, BIL, and applicable standard on the same line.

The main device connects a source to a bus section. A feeder protects or isolates a branch leaving that bus. A tie connects two bus sections under a defined operating condition. Those names describe positions in a topology, not distinct breaker technologies. The same breaker family can appear in all three positions while carrying different current-transformer ratios and relay settings.

Take a main-tie-main arrangement. Two sources normally feed separate bus sections with the tie open. If one source is lost, the controls may open the affected main and close the tie so the healthy source carries selected loads. The engineer must decide whether the source and bus ratings permit that transfer and whether a bus fault should block it. Schneider Electric’s distribution-arrangements guide shows how one-source radial and multi-source arrangements change service continuity. The drawing alone does not guarantee that a transfer sequence has been wired and tested.

Feeder isolation is a maintenance question as well as a fault question. With a drawout breaker in a metal-clad cell, the operator can move the breaker from connected to test or disconnected positions using the prescribed procedure. The surrounding bus may remain energized, but the isolated circuit and work area still need the facility’s electrical safety process. IEEE’s metal-clad description identifies drawout breakers and grounded barriers; it does not convert energized equipment into a safe workspace by itself.

One-line labels also hide secondary systems. A breaker may need DC control power to trip during loss of AC supply. An automatic tie needs status inputs and permissives. A feeder relay needs CTs of a suitable ratio and accuracy. If a submittal contains breaker ratings but omits these control and protection interfaces, the lineup may be electrically sized yet unable to operate as the one-line promises.

The first rating is rated maximum voltage, paired with an insulation or basic impulse level. A 13.8 kV nominal system is commonly served by 15 kV class equipment; a 34.5 kV system needs a class above its operating voltage. The complete mapping, including the special choice at 4.16 kV, is in switchgear voltage classes. Schneider Electric’s nominal-voltage list and Eaton’s switchgear ratings show why nominal and nameplate numbers should not be treated as interchangeable.

The second rating is continuous current. It tells you how much load a bus or device can carry under specified service conditions without exceeding its temperature limits. Main bus and each switching device have their own ratings. IEEE C37.20.2-2025 lists metal-clad main-bus ratings of 1200, 2000, 3000, and 4000 A within its stated voltage range. IEEE’s standard abstract does not mean every OEM offers every combination at every voltage or enclosure style. For example, a 2000 A main bus can feed several lower-current branches, but the summed operating case and contingency case must still be checked.

The third is short-circuit duty. The breaker must interrupt the available fault current at its location; the assembly and bus must also withstand the fault for the specified time. IEEE C37.04 lists short-circuit current and insulation capability among assigned breaker ratings. A 1200 A feeder can require a high interrupting rating if it sits close to a strong source. Conversely, a high continuous-current bus rating says nothing about whether its breaker can interrupt that source fault.

Close-and-latch is a distinct making duty. Older application material for breakers with K = 1.0 expresses it as approximately 1.55 times rated short-circuit current in rms asymmetrical amperes, or 2.6 times in peak amperes. ABB’s Power/Vac application guide prints both bases. Never copy “2.6×” into an rms-asymmetrical field. Review the offered breaker’s actual nameplate and standard edition before applying a historical factor to a particular assembly.

Finally, state the environment and service conditions. Indoor versus outdoor enclosure, ambient temperature, altitude, seismic design, cable entry, and accessibility can change the available configuration. The example “15 kV, 2000 A, 40 kA” is therefore a starting line, not a complete specification. Add the system grounding, BIL, main and feeder ratings, and tested assembly arrangement.

Switchgear only protects a system when a sensing path, logic path, and trip path all work. Current transformers supply scaled current to relays. Voltage transformers may supply voltage for metering, directional logic, or synchronization. A relay applies settings to those signals and commands the breaker to open. The control circuit, trip coil, and source of control power are therefore part of the protection chain. A fault-current rating on the breaker does not prove the relay is set to clear the fault quickly or selectively.

A transformer feeder illustrates the difference. It may draw a high inrush current when energized but still need to trip for an internal fault. A setting chosen solely to avoid inrush nuisance trips can delay genuine fault clearing. Schneider Electric’s HVL/cc application guide discusses transformer protection and the need to coordinate device and relay selection. The exact curve belongs to the project study, because the transformer impedance, fuse or breaker type, source, and downstream devices all change the case.

Metering and protection can share measured quantities but do not always need the same transformer accuracy or ratio. Ask for the CT and VT schedule, the relay model and firmware, settings files, and the terminal or communications diagram. A relay order code without its CT ratio is not enough to reproduce the protection study. Likewise, a front-panel display that shows current does not establish that the feeder has a functioning trip path.

During a retrofit, compare as-built wiring with the proposed controls. A new relay may need different auxiliary inputs, output contacts, or control voltage than the old one. A missing trip-circuit supervision contact is an operational gap that a mechanical inspection will miss. For existing gear, request operation counts and maintenance records alongside the protective-device test results; otherwise the electrical duty and the actual ability to clear a fault are being evaluated separately.

“Vacuum” describes the breaker’s interruption chamber, not necessarily the insulation of the whole assembly. In a vacuum circuit breaker, contacts part inside a sealed interrupter; other live parts still need insulation and spacing. A gas-insulated lineup can therefore contain vacuum breakers while using a separate gas around conductors. Siemens’ explanation of vacuum and clean-air GIS describes exactly that split. If a request specifies “vacuum switchgear,” ask whether the buyer also requires SF6-free primary insulation.

The practical comparison is by duty and service model. Check the breaker’s interrupting and continuous-current ratings, mechanical operating sequence, test provisions, contact or interrupter condition indicators, and OEM maintenance instructions. Do not substitute a generic operation-life number for the offered model’s manual. If a spare breaker is needed, confirm the exact cell, racking mechanism, secondary disconnects, control voltage, and interlocks. Two breakers with the same kV and kA label need not be mechanically interchangeable.

Gas-insulated and air-insulated assemblies make different room and service demands. GIS may use a sealed gas vessel; air-insulated metal-clad gear uses its own compartment and spacing arrangement. Siemens’ blue GIS portfolio names Clean Air products with specific voltage, current, and fault ratings. Those product numbers are useful examples of how to compare designs, not a claim that one medium fits all projects. The jurisdiction-specific SF6 rules and gas questions live in SF6-free switchgear explained.

Metal-clad identifies a specific medium-voltage assembly construction under IEEE C37.20.2, with drawout electrically operated breakers and grounded barriers around major primary components. Metal-enclosed interrupter gear is covered by IEEE C37.20.3 and may contain switches, fuses, or breakers. Both can be valid; the one-line and maintenance plan decide which is required. The requirement-by-requirement comparison is in metal-clad versus metal-enclosed switchgear. IEEE C37.20.2 and IEEE C37.20.3 show the two scopes.

Arc-resistant is an additional tested internal-arc behavior, not a synonym for metal-clad. IEEE C37.20.7-2024 is a recommended practice for testing switchgear up to 52 kV for internal arcing faults. IEEE’s C37.20.7 errata and title and Eaton’s application guide identify the test framework and accessibility types. The tested protection depends on the installed doors, covers, and exhaust path. The arc-resistant switchgear guide handles Type 1, Type 2, and the suffixes in detail.

Switchgear may be a standalone service-entrance or distribution assembly, or one section of a close-coupled unit substation. Eaton defines a unit substation as primary equipment, a transformer, and secondary distribution equipment. The transformer and the outgoing section add their own voltage, cooling, short-circuit, and interface requirements. A switchgear quote does not by itself cover that whole package.

For a 34.5 kV campus service stepped to 13.8 kV, one medium-voltage lineup may be upstream of a transformer and another downstream. The upstream and downstream buses do different jobs and can have different fault levels and protection. A buyer who writes “one 34.5 kV switchgear lineup” for the entire path has left the conversion and secondary distribution undefined. The unit-substation guide explains how to define the three-section boundary.

The physical interface is also a design decision. If MV gear is adjacent to a transformer, check bus transition, cable or throat connection, grounding continuity, shipping split, and who supplies each transition. If it is remote, cable length and termination details become part of the installation. These details belong in drawings, because a single-line diagram cannot show whether the room accommodates the actual equipment.

Provide a one-line and say what the lineup must do: receive a source, split a bus, protect named feeders, transfer between sources, or serve a transformer. State nominal system voltage and rated maximum class, BIL, available fault current and study basis, frequency, grounding, bus current, feeder-device ratings, construction standard, arc requirement if any, and indoor or outdoor installation. Add the protection and metering schedule, control-power source, cable entry, dimensions, and maintenance access.

A realistic request might say: “13.8 kV nominal source; 15 kV class; metal-clad per IEEE C37.20.2; main-tie-main; 2000 A main bus; feeder duties per attached schedule; available fault current and X/R per attached study; indoor bottom entry.” That still needs drawings, device and relay choices, and installation conditions, but it gives a supplier a coherent starting point. An RFQ saying only “13.8 kV switchgear” does not distinguish a fused transformer switch from a drawout breaker lineup.

If sourcing an existing lineup, attach the current as-built one-line and acceptability criteria. Record nameplate ratings on the assembled gear and each breaker, bus arrangement, breaker compatibility, CT and VT data, relay files, control voltage, enclosure condition, and test or maintenance history. A candidate that meets the voltage class may still miss the short-circuit duty or room geometry. Ask for a submittal review before treating a photographed lineup as a match.

Pantheon’s electrical catalog provides a place to start a quote request. Send the one-line, device schedule, ratings, and site constraints so we can check sourcing options for that lineup. The project engineer can then review a proposed configuration against the fault study and installation drawings.

Start with a document match. Compare the original nameplate and as-built one-line to the new system’s nominal voltage, rated maximum class, BIL, frequency, bus current, available fault current, and grounding. Then compare every installed breaker or switch with the device schedule. A lineup can have the right main-bus rating but a feeder breaker with too little interrupting duty for the new source. IEEE C37.04 lists the breaker ratings that must be treated separately, while IEEE C37.20.2 describes the assembly scope.

Next inspect the control and protection package. Request relay settings and test reports, CT and VT ratios, control voltage, trip and close wiring, interlock diagrams, and any battery or charger interface. A breaker that closes manually in a yard inspection has not proved its remote trip path will work after installation. If the old site used a different grounding method, the relay functions may need more than a settings copy.

Finally, inspect the physical and service evidence. Ask for maintenance history, operation counts where available, breaker condition checks required by the OEM, bus and insulation test results, cable-compartment condition, and a list of modifications. Compare shipping splits, cable-entry geometry, door swing, and any arc plenum with the new room drawing. The purpose is to find an actual mismatch early, not to treat a generic “tested” stamp as proof of suitability.

An engineer can then classify a candidate as matching, requiring an OEM-approved modification, or unsuitable. That decision should be recorded against the new one-line and fault study. Without the record, the same lineup may be compared repeatedly on just its kV and ampere labels while the unresolved protection or access issue remains hidden.

Using the system voltage as the full rating. A 13.8 kV nominal bus needs an equipment class and insulation level, but it also needs current and fault-duty ratings. The class alone does not make a lineup suitable.

Confusing continuous and fault current. A 1200 A breaker can have a much larger short-circuit interrupting number. Both must be read on the correct nameplate and checked against the study at that location.

Writing “metal-enclosed” when a drawout metal-clad design is intended. The standards cover different construction. Name IEEE C37.20.2 when its drawout and compartment arrangement is required; name C37.20.3 when its interrupter assembly is the design basis.

Treating an arc label as a work permit. An arc-resistant test is conditional on a configuration and installation. OSHA’s arc-flash guidance still directs attention to energized-work hazards and protective practices. The equipment test and the workplace assessment answer different questions.

Leaving the building interface until after procurement. Cable approach, door swing, shipping split, and an arc exhaust route can determine whether an otherwise correct electrical lineup can be installed.