Premium Membership ♕

Learn from experienced engineers and study specialized technical articles, guides & courses. Save 50% on courses with the Enterprise Plan.

Home / Technical Articles / The basics of primary medium voltage switchgear

Primary medium voltage switchgear

Primary medium voltage switchgear represents an important part within the primary distribution substation functionality. The switchgear works as a connection node between the outgoing distribution feeders and the in-feeding power transformers. The most common construction with the switchgear is an indoor-mounted metal-enclosed one.

Basics Of Primary Medium Voltage Switchgear
Basics Of Primary Medium Voltage Switchgear (on photo: ABB's MV air-insulated metal-clad switchgear, type 'UNISAFE'; credit:

The rated voltage, current and short circuit withstand ranges for secondary switchgears typically reach up to 36 kV, 1250 A and 50 kA respectively.

The same switchgear constructions are used in primary distribution by the utilities and in heavier industrial and power plant applications.

On a 36 kV voltage level also outdoor air-insulated constructions are used to some extent. These switchgears are built at site utilizing individual standard components. This gives the possibility to construct customer-specific solutions like busbar arrangements that are not available with factory-built metal-enclosed switchgears.

Example of primary medium-voltage switchgear - UniGear ZVC feeder unit
Figure 1 – Example of primary medium voltage switchgear – UniGear ZVC feeder unit

With the release of the IEC 62271-200 standard, new definitions and classifications of Medium Voltage switchgear have been introduced.

One of the most significant changes is that classification of switchgear into metal-enclosed, compartmented and cubicle types has been abandoned. The revision of switchgear classification rules has been based on the user’s point of view, in particular on aspects like service and maintenance of the switchgear, according to the requirements and expectations for proper management, from installation to dismantling.

In this context, Loss of Service Continuity (LSC) has been selected as a fundamental parameter for the user.

Switchgear construction

The following concentrates on factory-built metal-enclosed indoor switchgears. The primary switching devices typically include switch disconnectors, fused switch disconnectors, contactors and circuit breakers, either fixed or withdrawable. The current and voltage measurement can be done either with traditional instrument transformers (CTs and VTs) or with sensors.

Certain combinations are also possible, depending on the individual switchgear manufacturer.

Two main construction principles exist, namely the air-insulated one and the gas-insulated (SF6) one. The choice between these two alternatives is a result of evaluation of different aspects and differentiating factors during the switchgear’s life cycle.

This evaluation can typically include the following viewpoints:

  • Space requirements
  • Service requirements (tools and knowledge)
  • Reliability and availability
  • Installed base
  • Operational safety
  • Life cycle costs
  • Performance ratings

Switchgear consists of a number of cubicles. The basic construction of each cubicle is chosen to meet the intended use. This means that for example the power transformer in-feed cubicle can have different primary and secondary devices from what the outgoing line feeder cubicle has.

The switchgear manufactures provide a selection of typical cubicle constructions. By combining these typical cubicles, one can create switchgear suitable for the intended use.

Example of typical cubicles for single-busbar solution
Figure 2 – Example of typical cubicles for single-busbar solution

The number of different cubicles within the switchgear is coupled together by the busbars. In an air-insulated construction, these busbars are placed in a metal-enclosed compartment surrounded by normal air. With the gas-insulated construction, the busbars are placed in a hermetically sealed metal enclosure under pressurized insulating gas (SF6).

Depending on the manufacturer, the busbar compartment can be divided into cubicle-wide sections.

Typical busbar arrangements supported by the different switchgear manufacturers are the single-busbar and double-busbar arrangements.

Example of typical cubicles for double-busbar solution
Figure 3 – Example of typical cubicles for double-busbar solution

Protection, Control and Metering

The protection relays are externally powered advanced Intelligent Electronic Devices (IEDs), also referred to as Feeder Terminals. In a modern primary switchgear, the bay-dedicated functions like protection, control and measurement are carried out with Feeder Terminals.

The Feeder Terminal performs the as signed protection functions, carries out the local and remote control of switching devices, gathers and processes and displays measured data and indicates the status of the switching devices.

Air-insulated primary switchgear equipped with Feeder Terminals (type 'UniGear ZS1')
Figure 4 – Air-insulated primary switchgear equipped with Feeder Terminals (type ‘UniGear ZS1’)

The horizontal communication between feeder terminals in each cubicle provides the possibil- ity for station level automation and gateway connections to upper level systems for complete primary distribution network real-time control and monitoring.

Recommended literature to study //

UniGear with UFES and Relion relays

Reference // Distribution Automation Handbook by ABB

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. muhammad Elbif
    Aug 15, 2023

    hello sir ..i hope u have a good day.actually i have some customers asking me if u have used switch gears..ABB VD4 1250A .31.5 sc …ref 615
    plz if u have any answers..thank you

  2. Sergio Miranda T.
    Jul 25, 2020

    Muy buen articulo, como siempre Edvard…un gran acierto, un agradecido saludo desde Chile.

  3. Alan Bascon
    Jun 29, 2019

    This article is very informative. Hopefully will continue to share and educate our fellow practitioners.

  4. stanley kolowa
    Mar 08, 2019

    Edvard, this article was very good. I am working on the technical specifications for the design of a 11kV switch-gear and this article has helped a lot. I will be interested in reading more of these articles and posts. thanks.

  5. choi85
    Aug 18, 2018

    Pages very good.

    Mar 26, 2018

    Hello loved your article. Would love to learn more about MV SWITCHGEAR. Do you have a course on MV switchgear? Or more information on MV switchgear?

    • Edvard
      Mar 26, 2018

      Thank you. There are dozen articles and technical guides related to MV switchgears. I would recommend using search function (top-right corner).

  7. Ahmed wahbi
    Dec 15, 2015


Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

Enter Captcha Here :

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge