Key takeaway: “Primary” and “secondary” describe the switchgear’s application in the distribution network—not the primary and secondary windings of a transformer. Both categories can operate at the same medium-voltage level, and no single rating or construction feature is sufficient to classify every installation.
Primary and secondary medium-voltage switchgear perform the same fundamental job: they switch, isolate, protect and control electrical circuits above 1 kV. The difference lies mainly in where the assembly is used, how much power it carries, which switching duties it must perform, how protection is organised and what level of operational flexibility is required.
This distinction matters because product literature often presents primary switchgear as the higher-performance category, while ring main units (RMUs) and compact switchgear are commonly labelled secondary distribution. That description is useful, but it is not an absolute quality ranking and it is not a formal IEC 62271-200 nameplate classification.
Primary and secondary do not mean transformer winding sides
The most common misunderstanding is to assume that primary switchgear is installed on the primary side of a transformer and secondary switchgear on the secondary side. In distribution terminology, that can be exactly the opposite.
- The 20 kV switchboard immediately downstream of a 110/20 kV power transformer is electrically on the transformer’s secondary side, but it is normally called primary-distribution MV switchgear.
- The 20 kV RMU feeding a 20/0.4 kV distribution transformer is electrically on that transformer’s primary side, but it is normally called secondary-distribution MV switchgear.
The labels therefore follow the distribution stage, not the transformer terminal. Terminology can vary between utilities and countries, so project definitions and the network operator’s practice must always be checked.
Where each category sits in the network
Primary-distribution switchgear
Primary MV switchgear is typically installed at a main substation where a transmission or sub-transmission system feeds the medium-voltage network. It may also be used in a large industrial plant, data centre, mine, steelworks, railway supply system, generation plant or renewable-energy collector substation where the electrical duty resembles a utility primary substation.
Typical functions include:
- power-transformer incomers;
- high-capacity outgoing feeders;
- bus couplers and bus-section circuit-breakers;
- busbar and feeder metering;
- transformer, feeder, motor and generator protection;
- sectionalised or double-busbar operation;
- remote control, interlocking and SCADA integration.
Secondary-distribution switchgear
Secondary MV switchgear is normally installed closer to the final load. Its classic application is the local MV/LV distribution substation: two ring feeders maintain network continuity, while a transformer feeder supplies the local distribution transformer.
Typical applications include:
- compact and prefabricated substations;
- urban and rural ring networks;
- commercial buildings and infrastructure;
- small industrial substations;
- wind-turbine and solar-plant collector stations;
- customer intake and transformer substations.
Compactness, low installation cost, simple operation and minimal maintenance are usually stronger design priorities than complex busbar arrangements or extensive protection flexibility.
Primary vs secondary MV switchgear at a glance
| Engineering criterion | Typical primary distribution | Typical secondary distribution |
|---|---|---|
| Network position | Main HV/MV substation or high-demand industrial MV system | Local MV/LV substation, ring network or load-side distribution point |
| Typical voltage range | Often 12–36 kV; sometimes 40.5 kV | Often 12–24 kV; 36/40.5 kV products also exist |
| Typical busbar current | 1,250–2,500 A and above | 400 or 630 A; some designs reach 1,250 A |
| Typical short-time withstand | 25–31.5 kA; 40 kA or higher in demanding systems | 16–20 kA; 25 kA is also available |
| Typical duration | Frequently specified for 3 s | Often 1 s, but 3 s designs are common |
| Switching device | Full circuit-breaker functions dominate | Load-break switches and switch-fuse combinations are common; circuit-breakers are optional or application dependent |
| Protection | Dedicated CTs/VTs, numerical relays and complex selectivity | Fuse protection or simpler/self-powered relays; advanced relays are increasingly available |
| Busbar arrangement | Single or double busbar, sectionalisers and bus couplers | Normally single busbar or ring-main architecture |
| Design priorities | Performance, availability, protection, flexibility and maintainability | Compactness, simplicity, cost and ease of installation |
The main engineering differences
1. Continuous-current and thermal duty
A primary switchboard may carry the total output of a large power transformer. Incomer and bus-section panels can therefore require 2,000 A, 2,500 A or more, while a traditional RMU ring feeder is commonly rated 630 A.
Higher continuous current affects conductor size, contact resistance, temperature rise, enclosure ventilation and allowable ambient conditions. The basic loss relationship is P = I²R; manufacturers reduce resistance and redesign the current path as ratings increase, but thermal management still becomes a major design and type-testing challenge—especially in compact GIS.
2. Short-circuit withstand and interruption
The declared fault current must be read together with voltage, duration and peak current. A 31.5 kA rating at 36 kV represents a much larger short-circuit power than the same current at 12 kV:
S″k = √3 × Un × I″k
- 24 kV and 31.5 kA correspond to approximately 1.31 GVA.
- 36 kV and 31.5 kA correspond to approximately 1.96 GVA.
Short-time withstand current creates thermal stress, while peak withstand and making current create severe electrodynamic forces. Those forces rise approximately with the square of the instantaneous current. The assembly, busbars, supports, contacts, earthing circuit and enclosure all have to withstand the declared duty.
For more detail, see How to Calculate Short-Circuit Current at an MV Switchboard and How to Read MV Circuit-Breaker Ratings.
3. Circuit-breaker, load-break switch and fuse duties
Primary switchgear normally uses circuit-breakers throughout the main protection chain. The relay detects the abnormal condition, the trip circuit releases the mechanism and the circuit-breaker interrupts the fault current within its declared making-and-breaking capability.
Secondary RMUs commonly use load-break switches on ring feeders. A load-break switch can switch declared normal-load and specified capacitive currents and may have a short-circuit making rating, but it is not automatically a short-circuit interrupting device. Transformer protection may instead use current-limiting fuses in a coordinated switch-fuse combination. Modern secondary switchgear can also use vacuum circuit-breakers and numerical relays, so the presence of a circuit-breaker alone does not prove that an assembly is primary.
4. Protection, measurement and control
A primary switchboard frequently needs multiple CT cores, protection-class CTs, revenue or operational metering, voltage transformers, trip-circuit supervision, arc protection, synchronism checking, directional protection, transformer differential protection or busbar protection. Its low-voltage compartment and inter-panel interfaces are consequently larger and more complex.
Secondary switchgear traditionally uses simpler protection because the network section and connected transformer are smaller. Self-powered overcurrent relays, fault indicators and fuses are common. However, automated secondary networks now include motor operators, RTUs, voltage and current sensors, IEC 61850 or conventional SCADA interfaces and remote fault-passage indication. Digitalisation therefore reduces the protection-and-control gap without eliminating the underlying application difference.
5. Architecture, service continuity and accessibility
Primary switchgear often provides larger cable compartments, bus sectionalising, withdrawable or fixed circuit-breaker options, segregated functional spaces and maintenance access to instrument transformers and secondary systems. Secondary GIS frequently places several functions in a compact sealed tank and uses plug-in cable interfaces to reduce site work.
These are design tendencies—not rules. A single-tank GIS can still be primary-rated, while secondary switchgear can be modular, extensible, circuit-breaker based and highly compartmentalised.
IEC 62271-200 evaluates metal-enclosed assemblies through exact rated values and declared features such as Loss of Service Continuity (LSC), partition class and Internal Arc Classification (IAC). “Primary” and “secondary” should not replace those declarations. Read our guide to IEC 62271-200 switchgear classification for the correct engineering boundary.
How to identify whether switchgear is primary or secondary
Use the following sequence rather than relying on a single number.
- Locate it on the single-line diagram. Is it the main MV board distributing the output of an HV/MV power transformer, or is it a local ring/transformer station near the load?
- Identify the intended application. Check the manufacturer’s product description, the utility specification and the project philosophy—not only the product name.
- Review the functional units. Full circuit-breaker incomers, outgoing feeders, bus couplers, sectionalising and dedicated metering point toward primary duty. Two ring switches plus a transformer fuse/CB point toward secondary duty.
- Check all ratings together. Voltage, continuous current, short-time current, duration, peak current and breaking current form one coordinated set.
- Review protection and instrument transformers. Multiple CT cores, VTs and complex relay schemes usually indicate primary-distribution responsibilities.
- Check the busbar and operational philosophy. Double busbars, transfer arrangements and high switching flexibility are stronger primary indicators than voltage alone.
- Verify the assembly evidence. Confirm the exact type-tested configuration, IAC notation, LSC category, partition class, cable interfaces and installation conditions.
Fast practical indicators
- 24 kV, 630 A, 20 kA, two load-break-switch ring ways and one transformer fuse way: almost certainly a secondary-distribution RMU.
- 24/36 kV, 2,500 A busbar, 31.5 kA for 3 s, circuit-breaker feeders, CT/VT panels and a bus coupler: strongly characteristic of primary distribution.
- 24 kV, 1,250 A and 25 kA: a grey zone. The network position, functional units and manufacturer’s intended application decide the answer.
Common exceptions and grey areas
Product capabilities increasingly overlap. For example, Ormazabal describes its cgm.zero24 as secondary distribution at up to 24 kV, 630 A and 20 kA, while its cgm.3 secondary range reaches up to 40.5 kV and 25 kA. This demonstrates why neither 36 kV nor 25 kA automatically means primary distribution.
The following shortcuts are unreliable:
- “GIS means secondary.” Both primary and secondary switchgear can be GIS, AIS or solid insulated.
- “A circuit-breaker panel is always primary.” Secondary RMUs may use circuit-breaker transformer or feeder functions.
- “A load-break switch means secondary.” It is a strong clue, but the complete application still matters.
- “36 kV is always primary.” Secondary-distribution products exist at 36/40.5 kV.
- “A high IAC rating makes it primary.” IAC is a conditional assembly classification, not a distribution-stage label.
- “Primary means withdrawable.” Primary GIS commonly uses fixed-mounted vacuum circuit-breakers.
Why Nuventura emphasises primary switchgear
Nuventura’s emphasis is technically meaningful, but it is also a market-positioning message. The company is not merely presenting a fluorinated-gas-free RMU; it is presenting dry-air GIS for the higher-current and higher-fault duties associated with primary distribution.
At the time of this article’s verification, Nuventura’s official Nu1 data lists:
- 24 kV and 36 kV rated-voltage versions;
- 2,500 A busbar current;
- 1,250 A or 2,000 A feeder current;
- 31.5 kA short-time withstand for 3 s;
- IAC AFLR 31.5 kA for 1 s;
- dry-air insulation with vacuum circuit-breaker technology.
Those ratings sit squarely in the conventional primary-distribution performance range. For comparison, Siemens identifies NXPLUS C 24 Clean Air as primary-distribution GIS up to 24 kV, 25 kA and 2,500 A, while its 8DJH secondary-distribution range is typically centred on a 630 A busbar and feeder, with up to 20 kA at 24 kV. The comparison illustrates an industry pattern, not a universal boundary.
Why dry-air primary GIS is challenging
- Dielectric design: Dry air has lower dielectric strength than SF₆ under comparable conditions. Clearances, field control, interfaces, pressure and insulation geometry must be engineered carefully to retain a compact GIS footprint.
- Thermal performance: Carrying 2,000–2,500 A through a sealed or semi-sealed compact enclosure makes temperature-rise control demanding.
- Short-circuit forces: A 31.5 or 40 kA fault produces high thermal and electrodynamic stress in conductors, supports, contacts and the earthing circuit.
- Internal-arc management: Primary-class fault levels demand robust enclosure behaviour and a verified pressure-relief concept for the declared IAC arrangement.
- Switching and protection integration: Circuit-breakers, disconnectors, earthing switches, sensors or conventional CTs/VTs, interlocks and protection systems must work as one coordinated assembly.
- Type-test applicability: High ratings are valuable only when the exact marketed configuration is supported by relevant assembly and switching-device test evidence.
The EU regulatory transition also increases the commercial value of proven fluorinated-gas-free equipment. Regulation (EU) 2024/573 sets staged restrictions on putting into operation MV switchgear that uses fluorinated greenhouse gases in the insulating or breaking medium: up to and including 24 kV from 1 January 2026, and above 24 kV up to and including 52 kV from 1 January 2030, subject to the regulation’s detailed conditions and derogations. See our separate guide to SF₆ alternatives and the EU F-gas Regulation.
Independent engineering view: Nuventura can legitimately point to primary-class ratings as an achievement. However, “primary” is not proof that a product is automatically safer, more reliable or better for every project. The correct comparison remains the exact rating set, tested configuration, operational requirements, maintainability, lifecycle support and total project risk. An oversized primary GIS is not necessarily the best solution for a simple 630 A ring-main station.
Primary distribution vs primary equipment: a second meaning
Engineers also use “primary” and “secondary” inside an individual switchgear panel:
- Primary equipment/circuit: busbars, circuit-breaker main contacts, disconnectors, earthing switches, main CT conductors, cable terminations and the MV current path.
- Secondary equipment/circuit: protection relays, meters, control switches, trip and close circuits, auxiliary contacts, communication equipment and low-voltage-compartment wiring.
This primary/secondary circuit distinction is separate from the primary/secondary distribution classification. A secondary-distribution RMU contains both primary power components and secondary control circuits.
Selection checklist for a real project
Before selecting or approving an MV switchgear family, document at least:
- network location and required distribution role;
- maximum operating voltage and insulation level;
- continuous current for each feeder and busbar section;
- calculated maximum and minimum short-circuit currents;
- short-circuit duration and peak withstand requirement;
- breaking, making and special switching duties;
- protection, CT/VT, metering and SCADA philosophy;
- single, sectionalised or double-busbar requirement;
- IAC, LSC and partition-class requirements;
- cable quantity, size, termination interface and test access;
- installation room, exhaust path, ambient conditions and altitude;
- maintenance, extension, spare-parts and service strategy;
- applicable standard edition, test reports and permitted design variants.
Frequently asked questions
Is primary MV switchgear always connected to a transformer’s primary winding?
No. A main MV board downstream of an HV/MV transformer is commonly primary-distribution switchgear even though it is connected to the transformer’s secondary winding.
Is an RMU always secondary switchgear?
An RMU is the classic secondary-distribution arrangement, but classification should still follow its intended application, functional units and ratings. The term RMU describes a ring-main function and compact architecture more directly than a formal performance class.
Can secondary switchgear have a circuit-breaker and protection relay?
Yes. Many secondary GIS families offer vacuum circuit-breaker panels and numerical or self-powered relays. The use of a circuit-breaker does not by itself convert the whole system into primary switchgear.
Is primary switchgear defined by IEC 62271-200?
IEC 62271-200 applies to qualifying metal-enclosed assemblies above 1 kV and up to and including 52 kV. It deals with exact ratings, construction and classifications such as LSC, partition class and IAC. “Primary distribution” and “secondary distribution” are application descriptions and should not be used as substitutes for those technical declarations.
Can the same switchgear family be used in either application?
Sometimes. If the ratings, functions, protection philosophy and type-tested configuration meet the project requirements, a product marketed for one segment may be applied in an overlapping duty. Utility approval and manufacturer confirmation may still be required.
Conclusion
The most reliable distinction is simple: primary switchgear distributes bulk MV power from a main substation or equivalent high-demand source; secondary switchgear operates closer to local loads and distribution transformers. Typical ratings and architectures support that distinction, but they do not create hard boundaries.
To classify a real installation, start with the single-line diagram and operational duty, then verify the complete rating set, switching devices, protection philosophy, busbar arrangement and type-test evidence. That method is more accurate than judging by voltage, enclosure type, circuit-breaker presence or marketing terminology alone.
Editorial record
- Written by: LearnSwitchgear Editorial Team
- Technical review: LearnSwitchgear Engineering Review
- Last verified: 16 August 2026
- Scope: IEC-oriented global application terminology; local utility definitions may differ.
Standards basis and official references
- IEC 62271-200:2021+AMD1:2024 CSV — AC metal-enclosed switchgear and controlgear assemblies above 1 kV and up to and including 52 kV.
- IEC 62271-1:2017+AMD1:2021 — Common specifications for AC switchgear and controlgear.
- IEC 62271-100:2021 and AMD1:2024 — Alternating-current circuit-breakers.
- IEC 62271-103:2021 — AC switches and switch-disconnectors above 1 kV up to and including 52 kV.
- IEC 62271-105:2021 — AC switch-fuse combinations above 1 kV up to and including 52 kV.
- IEC 61936-1:2021 — Design and erection of electrical power installations exceeding 1 kV AC.
- Nuventura: Key Differences between Primary and Secondary GIS — Manufacturer explanation of intended applications and typical characteristics.
- Nuventura product and technology data — Published Nu1 ratings used in the case study.
- Siemens NXPLUS C 24 primary-distribution GIS and Siemens 8DJH secondary-distribution GIS — Illustrative manufacturer classifications and ratings.
- Ormazabal cgm.zero24 and Ormazabal cgm.3 — Secondary-distribution examples showing rating overlap.
- Regulation (EU) 2024/573 on fluorinated greenhouse gases — Official consolidated legal source; apply the detailed provisions, conditions and derogations.
Engineering note: Confirm the contracted standard editions, amendments, national adoptions, project specification, network-operator rules and manufacturer instructions. Product ratings can vary by panel function, market and configuration. This independent article does not reproduce or replace a standard, type-test report or project-specific engineering study.