MV Switchgear Buyer Guide: Sizing and Selection Basics

What defines the right size for your installation

Sizing medium-voltage switchgear begins with the electrical data sheet, not the equipment catalog. Procurement teams often rush to compare enclosures and breakers before confirming the system constraints. That approach leads to over-specification or, worse, equipment that fails under fault conditions.

Start with the maximum operating voltage. The switchgear must carry that voltage continuously. Identify the system voltage class first. If the supply is 10 kV, the equipment must be rated for that class. If the supply is 35 kV, 10 kV equipment is out of the question. The voltage class determines the insulation thickness, the clearances inside the enclosure, and the type of insulation medium. A mismatch here is not a minor adjustment. It is a fundamental design error that can lead to flashover during routine operation.

Next, identify the rated short-circuit current available at the switchgear location. This value depends on the transformer or generator upstream and the cable impedance. If the available fault current exceeds the breaker rating, the equipment will not clear the fault properly. The fault current is not a static number. It changes as you move along the feeder. It is highest at the source and decreases as the cable length increases. The engineer must calculate the available fault current at the exact point where the switchgear will be installed. This calculation includes the impedance of the supply transformer, the cable resistance and reactance, and any other sources of fault current on the bus.

Also list the maximum load current for each circuit. The breaker rating must cover the normal load plus any planned expansion. A common mistake is sizing breakers exactly to the nameplate load. In practice, engineers allow a margin for transient inrush currents and future motor additions. A 25 percent margin is a frequent starting point, but the final figure requires a load study. Consider a motor circuit. The motor may draw five times its rated current for a few cycles during start-up. If the breaker is sized too tightly, it will trip every time the motor starts. This nuisance tripping damages the motor, disrupts production, and wastes the operator’s time. The margin must account for this inrush. It must also account for the possibility that the plant adds another motor or a heating load in the future.

The load profile determines the thermal rating. A factory running motors, compressors, and heaters has a different thermal signature than a data center with steady IT loads. Check the demand factors. If the load varies significantly by hour of day, the breaker may not need a higher continuous rating, but it must handle the peak without nuisance tripping. Look at the thermal time characteristic of the breaker. It defines how much current the breaker can carry for how long before it trips. A steady load of 800 A will heat the breaker differently than a cyclical load that peaks at 1200 A for ten minutes and drops to 400 A for the rest of the hour. The thermal rating must match the actual duty cycle.

Which MV switchgear types fit which applications

There are two main families of medium-voltage switchgear: air-insulated and gas-insulated. Air-insulated switchgear is more common in industrial plants, distribution centers, and substations. It uses air or SF6 gas for insulation and interruption. The enclosures are larger, but the equipment is generally less expensive and easier to maintain. The components are visible. A technician can see a loose connection, a burnt busbar, or a damaged insulator during a routine walk-through. This visibility makes preventive maintenance straightforward. You can tighten a connection, clean a contact, or replace a fuse without opening a sealed compartment.

Gas-insulated switchgear uses SF6 or similar gas inside sealed compartments. The footprint is smaller. The maintenance intervals are longer. The equipment is more expensive to buy and more complex to install. It suits dense urban substations, rooftop installations, and sites where space is limited. The sealed design protects the internal components from dust, moisture, and contamination. This is a significant advantage in harsh environments. However, the sealed nature means that internal faults are harder to detect. A leak in the gas compartment can go unnoticed for months. The equipment relies on gas pressure monitoring systems to alert the operator to a leak. If the monitoring system fails, the insulation strength degrades silently.

Selection depends on the site. A plant with a large electrical room and easy access to the equipment will usually choose air-insulated switchgear. The room is often at ground level. The equipment is on the floor or on a raised platform. Technicians can work around the units. A building with a small rooftop mechanical room will likely need gas-insulated units. The room is small. Access is limited. The equipment must fit in a tight space. Gas-insulated switchgear allows a higher density of breakers and busbars in the same footprint.

Another factor is the voltage class. Standard medium-voltage systems operate at 10 kV to 35 kV. The equipment must match the system voltage. If the system is 35 kV, 12 kV equipment will not work. Check the phase-to-phase and phase-to-ground ratings. The phase-to-phase rating is the voltage between two conductors. The phase-to-ground rating is the voltage between a conductor and the earth. These values must be checked against the system specifications. A system at 10 kV phase-to-phase has a phase-to-ground voltage of approximately 5.8 kV. The equipment must be rated for both.

How to verify short-circuit capability

Short-circuit rating is the most technical part of the selection. The breaker must clear the maximum fault current for the required time without damage. This rating is usually stated in kA, such as 31.5 kA or 50 kA. The rating is not just a number. It is a promise of performance. It means the breaker has been tested to clear that current without internal damage. The test involves simulating a fault in a laboratory. The breaker is connected to a large inductive source. The source is charged to a high voltage. The fault is initiated. The breaker must open the contacts and interrupt the current. The contacts must not weld. The insulating gas must not break down. The internal parts must not be destroyed by the electromagnetic forces.

The available fault current is calculated by the design engineer. It depends on the source impedance and the cable length. If the switchgear is located close to a large transformer, the fault current will be higher than at the far end of a long feeder. The calculation uses the short-circuit impedance of the transformer. It uses the cable impedance. It accounts for the system voltage. The result is a current value. This value must be compared to the breaker rating. If the available fault current is 35 kA, a 31.5 kA breaker is insufficient. The breaker will not clear the fault. The contacts will burn. The insulating gas may explode. The busbars may deform.

Ask each bidder for the type test certificate. The certificate shows the current the equipment cleared during factory testing. A 31.5 kA rating means the equipment survived a 31.5 kA test current. If the plant calculation shows 35 kA available, the 31.5 kA equipment is not sufficient. Do not accept a vendor’s verbal assurance. The certificate is the proof. It is a document issued by a testing laboratory. It contains the test date, the test current, the duration of the test, and the results. It is a legal document. It is part of the contract.

Also check the breaking capacity of the busbars. The busbars inside the switchgear must carry the fault current for the same duration as the breaker. If the busbars are rated lower than the breaker, the assembly fails. The busbars are the conductive paths inside the enclosure. They connect the breaker to the cables. During a fault, the busbars carry the same current as the breaker. They are subjected to the same electromagnetic forces. If the busbars are too thin, they will heat up. They may melt. They may break. The busbar rating must be at least as high as the breaker rating. Check the specification sheet for the busbar material and thickness. Check the test data for the busbar assembly.

What to check in the insulation coordination

Insulation coordination matches the switchgear voltage rating to the system voltage. A 24 kV switchgear can operate on a 15 kV system. It cannot operate on a 24 kV system unless the rating is higher. The voltage rating is the maximum continuous voltage the equipment can handle. It is not the maximum voltage the equipment can ever see. The system voltage fluctuates. The generator may speed up slightly. The grid frequency may vary. The voltage can rise above the nominal value. The equipment must handle this variation without stress.

Check the withstand voltage. This is the AC voltage the insulation withstands for a set duration. For example, a 24 kV equipment may have a withstand voltage of 42 kV. The system phase-to-ground voltage must be below the withstand rating. The withstand test is performed in a laboratory. The insulation is subjected to a high voltage for a specific time. If there is no breakdown, the insulation passes. The test voltage is much higher than the operating voltage. It is a safety margin. It ensures that the insulation will not fail under normal operating conditions.

Dielectric strength is another check. It is the maximum voltage the insulation can handle during a fault. If a fault occurs, the insulation must not break down. The dielectric strength must exceed the maximum transient overvoltage the system can produce. Transient overvoltages are short spikes in voltage. They can be caused by lightning strikes, switching operations, or capacitor discharge. These spikes can reach several times the normal system voltage. If the insulation is not rated for these spikes, it will break down. The switchgear will fail.

Lightning arresters and surge protection devices are part of this picture. If the site is exposed to lightning, the transient overvoltage may be higher than the normal system voltage. The switchgear insulation must handle that surge. The surge protection devices limit the voltage to a safe level. But the switchgear must still be able to handle the residual voltage. Check the site’s lightning risk. If the site is in an open area, the risk is higher. If the site is shielded by buildings or trees, the risk is lower. The insulation coordination must reflect this risk.

How to evaluate maintenance and lifecycle costs

The purchase price is only part of the total cost. Maintenance access, spare parts, and downtime matter. A cheaper unit that is hard to maintain may cost more over ten years. Consider the labor cost. If a routine inspection requires two technicians to work for six hours, that is a significant cost. If the inspection can be done by one technician in two hours, the cost is lower. The physical design of the enclosure affects this. Look at the door openings. Can a technician reach the breaker handle and the busbar connections without removing panels? Some enclosures require full disassembly for a routine inspection. That increases labor time and downtime.

Spare parts availability is a real concern. If the breaker is a discontinued model, the plant may wait months for a replacement. Ask the vendor for the expected service life and the parts availability policy. The service life is the period the manufacturer guarantees the equipment will perform. It is usually ten to twenty years. The parts availability policy states how long the manufacturer will supply spare parts. It should cover the service life. It should also cover the period after the service life ends. If the vendor does not provide a parts policy, that is a red flag. It means the plant may be stranded.

Documentation quality affects handover. A good vendor provides a single-line diagram, a maintenance manual, a spare parts list, and a commissioning procedure. If the documentation is thin, the plant will spend extra time learning the equipment. The single-line diagram shows the electrical connections. It is the map of the system. The maintenance manual tells the technician how to open the unit, how to inspect the contacts, and how to replace parts. The spare parts list identifies the components that are most likely to fail. It helps the plant stock the right spares. The commissioning procedure guides the installation team through the startup process. It ensures that the equipment is tested before it is put into service.

Criteria for comparing bids

Use the table below to compare vendors. It forces a structured comparison beyond price.

Criterion	What to look for	Why it matters
Short-circuit rating	Match or exceed the plant fault current calculation	Ensures the breaker clears faults without damage
Voltage class	Rated for the system phase-to-phase voltage	Prevents insulation failure under normal operation
Insulation coordination	Withstand and dielectric voltage above system transients	Protects equipment during lightning and switching surges
Maintenance access	Door openings and panel removal ease	Reduces labor time for inspection and repair
Spare parts policy	Vendor commitment to parts availability for 10+ years	Avoids long downtime when components fail
Documentation	Single-line diagram, manual, parts list, commissioning plan	Supports commissioning, training, and future maintenance

A decision checklist before issuing an RFP

Use this checklist to confirm readiness.

1. Confirm the maximum operating voltage and phase-to-ground voltage.
2. Complete the load study and identify the maximum continuous current for each circuit.
3. Calculate the maximum short-circuit current at the switchgear location.
4. Select the switchgear type based on space, budget, and maintenance preference.
5. Define the required short-circuit rating and insulation coordination.
6. List the must-have features, such as metering, protection relays, and remote control.
7. Request type test certificates and application calculations from each bidder.
8. Review maintenance access and spare parts policies.
9. Check the vendor’s installation and commissioning support.
10. Shortlist the top two or three vendors for a technical interview.

The checklist keeps the selection process disciplined. It prevents the team from reacting to the first bid price. It forces a comparison of technical fit.