Medium-voltage switchgear serves as the hub where electricity is actually distributed. Here, power arriving from the transmission grid or from local generation sources – including wind power and other renewable energy installations – undergoes centralised collection before being routed to individual consumers, transformers and other sections of the network. A significant part of the power infrastructure relies on MV switchgear to deliver electricity to the right place at the right time.
From the perspective of an engineer designing surge protection, an MV switchgear assembly is both a focal point of risk and the place where protective measures matter most. This is where line, transformer and metering bays converge, and a fault in a single bay can jeopardise the entire installation. In this article, we will discuss the types, construction and rated parameters of medium-voltage switchgear, and examine how its design features shape the requirements placed on surge arresters.
MV Switchgear – Role in the Electricity Distribution System
A medium-voltage switchgear assembly is a functional unit comprising several bays that combine circuit breakers, busbars and protection systems to safely switch, meter and provide electrical protection for MV network circuits. Depending on its function, a given bay may be a line bay (connecting overhead or cable lines), a transformer bay (supplying MV/LV transformers), a metering bay (fitted with current and voltage transformers), or a control bay, interconnecting different sections of the busbars.
MV switchgear is used in a wide variety of contexts: as secondary distribution switchgear in medium-voltage substations supplying housing estates and residential buildings, in the main switchyards of large industrial facilities, and as primary distribution switchgear in power stations, forming part of the primary distribution system for power generated by the plant’s own generators. The growing number of distributed renewable energy installations connecting to the grid has also increased demand for MV switchgear at the point of connection. Each of these applications imposes slightly different requirements regarding the number of bays, rated parameters and operational safety class.
Types of medium-voltage switchgear – Air and gas insulation
The key criterion for classifying MV electrical switchgear is the type of insulation medium used between live parts and earthed parts or the enclosure. The choice of insulation technology affects the switchgear’s dimensions and required insulation clearances and, indirectly, the method used to connect surge protection devices.
Air-insulated switchgear (AIS)
Air-insulated switchgear (AIS) utilises atmospheric air as the primary insulation medium between busbars, internal components and the metal enclosure or internal partitions. It is a traditional, tried-and-tested solution known for its high reliability and minimal maintenance requirements – this kind of switchgear work, such as inspecting insulating components and electrical connections, does not require specialist gas-handling tools. One drawback is that air-insulated switchgear requires a larger footprint than gas-insulated solutions, owing to the lower dielectric strength of air insulation.
Gas-insulated switchgear (GIS)
Gas-insulated switchgear (GIS) traditionally uses sulphur hexafluoride (SF₆) as the insulating and arc-extinguishing medium – a synthetic gas with very high dielectric strength, but also a global warming potential many times that of carbon dioxide (CO2). From 1 April 2026, its use in new MV installation projects will be prohibited. This does not mean that development of the technology is grinding to a halt, however – quite the opposite: gas-insulated switchgear is currently the fastest-evolving segment of the switchgear market. Manufacturers are focusing on replacing SF6 with alternative, non-fluorinated gases – including mixtures based on pure air or nitrogen, often paired with vacuum circuit breakers – that retain similar insulating and switching performance whilst having a far smaller climate impact. Alongside these gas-based alternatives, some manufacturers are also developing solid-insulated switchgear (SIS), which does away with gas altogether in favour of solid dielectric materials.
Thanks to the gas’s high dielectric strength, GIS switchgear has a more compact design than air-insulated equivalents, while offering comparable rated parameters. This makes it particularly well suited to space-constrained applications, such as residential and commercial buildings in city centres, underground substations, and industrial platforms. The enclosed design also ensures high resistance to the ingress of water and environmental contaminants, regardless of external weather conditions. These space-saving benefits become even more pronounced at higher voltage levels, where the size advantage of gas insulation over air insulation grows substantially – one of the key reasons GIS technology dominates in high-voltage applications. Some modern GIS units are also fitted with partial discharge sensors, providing early warning of insulation degradation before a fault develops.
Design of MV switchgear – Key components
Regardless of the insulation technology used, a typical MV switchgear assembly consists of several standardised structural elements, often combined in a modular design that simplifies future extension of the switchgear line-up. The condition and configuration of these elements directly affect operational safety and how the switchgear interacts with surge protection systems.
Busbars and circuit protection devices
Busbars connect the individual bays within the switchgear assembly, enabling current flow between them. Circuit breakers, disconnectors and isolators are used to switch circuits on and off and to isolate electrical equipment. In MV switchgear, circuit breakers are typically vacuum or SF6 types, in contrast to the air circuit breakers typically found in low-voltage switchgear. These electrical connections must meet the required tightening torque and contact resistance specifications; a poor connection generates additional heat and can lead to overheating at the joint.
Insulating and metal partitions
Insulating and metal partitions separate the individual functional compartments within the switchgear – the busbar compartment, the switching equipment compartment and the cable compartment – limiting the risk of short-circuit damage spreading between them. The partition class (PM – metal, PI – insulating) determines how well personnel are protected when working on adjacent compartments that remain live. Compartments built this way are commonly classified, following terminology used by the International Electrotechnical Commission (IEC) and widely adopted across the electrical industry, as metal-clad or metal-enclosed switchgear.
Enclosures and degree of protection
The switchgear enclosure determines its resistance to the ingress of solid objects and water, expressed as an IP rating in accordance with the PN-EN 60529 standard. Requirements are generally less stringent for indoor switchgear than for free-standing units used in outdoor applications, which are directly exposed to the elements.
Rated parameters of MV switchgear
Selecting a switchgear assembly for a specific installation requires consideration of several rated parameters that are also relevant to the surge protection designer – primarily in relation to insulation coordination and the short-circuit withstand capacity of the surge arresters used.
- Rated voltages – the switchgear’s rated voltage (Ur) and rated insulation level (lightning impulse withstand voltage and power-frequency withstand voltage) must match the voltage of the network to which the switchgear is connected.
- Rated continuous current – the maximum current that the switchgear’s current path can carry continuously without exceeding the permissible temperature rise.
- Rated short-time withstand current – the root-mean-square (RMS) value of the short-circuit current the switchgear can withstand for a specified duration (usually 1 s or 3 s) without damage.
- Rated peak withstand current – the peak value of the first half-wave of the short-circuit current, which determines the dynamic loads acting on the busbars and equipment during a short circuit.
The switchgear’s short-circuit parameters should always be checked against the rated short-circuit current of the surge arresters used – in the event of an internal fault in the arrester, the switchgear itself must safely carry the resulting short-circuit current until the circuit protection devices operate.
Operational safety – IAC class and resistance to internal arcing
A short circuit inside the switchgear can generate extremely high arc energy, producing what is known as an internal arc. The effects of an internal arc – a sudden rise in pressure, high temperature, and the ejection of hot gases and particles – pose a direct threat to personnel in the vicinity of the switchgear.
The switchgear’s resistance to internal arcing is defined by its IAC (Internal Arc Classification) class, confirmed through type testing at independent, accredited testing bodies. The IAC classification indicates which sides of the enclosure are protected (A – operator access compartment, F – front wall only, etc.), along with the current level and short-circuit duration for which that protection is guaranteed. This is one of the key parameters when selecting switchgear for facilities where operator safety is critical – industrial plants, public buildings, or substations located near residential buildings.
The IAC class is supplemented by the LSC (Loss of Service Continuity) category, which specifies which parts of the switchgear remain accessible for maintenance work without the need to de-energise adjacent bays. A higher LSC category (e.g. LSC2B) allows for safer and more flexible operation of a switchgear assembly comprising multiple bays.
MV Switchgear and Surge Protection – What Determines Effectiveness
An MV switchgear assembly is a point where many sections of the network converge – overhead lines, cables and connections to transformers – making it a natural focal point of overvoltage risk. Each line bay of the switchgear is a potential entry point for an overvoltage wave propagating from the external network.
Insulation coordination with the surge arrester’s protection level
The switchgear’s rated lightning impulse withstand voltage (LIWV) must retain an adequate margin over the protection level (Upl) of the surge arrester installed at the connection point. This margin should take into account both the distance between the surge arrester and the switchgear terminals, and the inductance of the connection cables – the greater the distance and the longer the connections, the greater the required initial margin.
Surge arresters on line and transformer bays
In practice, surge arresters are most commonly installed on the line bays of the switchgear assembly – at the entry point of an overhead or cable line – and on transformer bays, directly at the MV terminals of the transformer. To protect the electrical equipment inside the switchgear (switching apparatus, current and voltage transformers), the surge must be limited before it reaches the interior of the bay – which is why installing the arrester as close as possible to the switchgear’s entry point is critical.
Switchgear short-circuit rating and surge arrester selection
As mentioned earlier, in the event of internal damage to the surge arrester, a short-circuit current may flow through its varistor column. The surge arrester’s rated short-circuit current must be matched to the short-circuit parameters of the switchgear to which it is connected – otherwise, a damaged arrester significantly increases the risk of an internal arc forming within the switchgear compartment.
Selection of surge arresters for MV switchgear – What to check
When selecting surge arresters to protect medium-voltage switchgear, the designer should take the following factors into account:
- Uc and Ur – chosen based on the network’s neutral earthing method and realistic transient overvoltage (TOV) scenarios for the system in question.
- Insulation coordination margin – matching the arrester’s Upl value with the switchgear’s LIWV value, taking into account the installation distance and the inductance of the connections.
- Short-circuit capacity – matched to the rated short-time withstand current of the switchgear to which the arrester is connected.
- Installation location – as close as possible to the line bay entry or transformer terminals, rather than at any point along the connection.
- Environmental conditions and enclosure type – for indoor switchgear, surge arresters without a weatherproof enclosure may be used; for outdoor switchgear, appropriate weather resistance is required.
The criteria above should always be checked against the technical documentation for the specific switchgear in question – different manufacturers may use different busbar configurations and rated parameter values, even at the same mains voltage.
Surge arresters for MV switchgear – Protektel PROXAR
The PROXAR (Protektel) range of surge arresters includes variants suitable for installation on the line bays of air-insulated switchgear, as well as for indoor substation applications. Key features of the PROXAR data sheets include comprehensive selection data – UTOV, Qrs and Wth parameters for individual duty classes, as well as short-circuit capacity – letting you confirm that your selection complies with a specific switchgear’s rated parameters without further enquiries to the manufacturer.
If you are designing surge protection for medium-voltage switchgear and require assistance with selecting surge arresters, the Protektel team is on hand at every stage of the project. Our expertise begins before the purchase – and it’s the selection stage, above all, that determines the reliable operation of the entire installation.
