Mediumvoltage Switchgear Key to Power System Safety

At the core of every electrical power system lie the unsung guardians of power transmission and distribution – switchgear. These sophisticated components serve as protective sentinels, performing critical functions of protection, control, and isolation within electrical networks. Selecting the appropriate switchgear is paramount to ensuring safe, reliable, and efficient power system operation. This article examines the essential aspects of medium voltage switchgear to facilitate informed decision-making.

Electrical switchgear fundamentally represents an integrated assembly of circuit protection devices, including circuit breakers, fuses, and switches, working collectively to protect, control, and isolate electrical equipment. These protective components are housed within metal structures, with one or more such assemblies constituting a switchgear lineup or assembly. Switchgear finds extensive application in utility transmission and distribution systems, as well as in medium-to-large commercial and industrial facilities. While IEEE standards govern electrical switchgear in North America, IEC standards prevail in Europe and other global regions.

The primary function of electrical switchgear involves distributing power to various sections of a facility and the electrical loads within those sections. Simultaneously, it safeguards personnel and equipment by maintaining system currents within safe operational limits.

Gas-insulated switchgear (GIS) employs sealed enclosures filled with sulfur hexafluoride (SF6) gas or SF6 mixtures with other insulating gases. This gas-filled sealed configuration enables compact, low-profile installations. Compared to similar air-insulated switchgear, the use of gaseous insulation permits reduced clearances between current-interrupting components. GIS designs undergo testing per ANSI standards C37.60 and C37.72. The IEEE C37.20.9 standard for gas-insulated switchgear was published in summer 2019; prior to this, GIS equipment adhered to International Electrotechnical Commission (IEC) performance standard 62271.

Defined by IEEE C37.20.3, metal-enclosed switchgear incorporates circuit protection devices including circuit breakers, power fuses, and fused switches, along with control and metering equipment. These components may be installed in common compartments without the separate barriers or segregation required in metal-clad switchgear. Metal-enclosed switchgear serves commercial and many industrial facilities with incoming electrical services exceeding 480/600V.

Per IEEE C37.20.2, metal-clad switchgear refers to medium voltage electrical switchgear structures where all electrical components – including incoming bus, outgoing bus, instrumentation, and main circuit breakers or switches – are enclosed within separate metal compartments. This configuration provides enhanced safety, robustness, and maintenance accessibility. Rated for 5 kV to 38 kV applications, metal-clad switchgear features draw-out circuit breakers for simplified maintenance, making it particularly suitable for industrial facilities, as well as power generation and transmission equipment.

Defined by IEEE C37.74, pad-mounted switchgear is specifically designed for underground distribution systems rated 5 kV to 38 kV that require above-ground operation. Featuring outdoor-rated, low-profile, and tamper-resistant construction, these units are ideal for utility distribution, feeder sectionalizing, and circuit protection applications. Switches, fuses, and vacuum interrupters provide load protection, fault isolation, and outage minimization. Pad-mounted switchgear can accommodate up to six ways in a common insulated sealed tank, with insulation options including air, SF6 gas, fluid, solid-dielectric air technology, and solid materials.

Also defined by IEEE C37.74, vault or underground switchgear serves 15 kV to 38 kV distribution systems requiring switch operation from vault or underground locations – whether dry or prone to water ingress. This configuration permits manual or relay-operated switch operation from ground level, utilizing vacuum interrupters for load protection and fault isolation. Insulation methods include SF6 gas, solid-dielectric air technology, and solid materials.

• Type 1 – Requires arc resistance only on the equipment front
• Type 2 – Requires arc resistance around the entire equipment perimeter
• Type 2B – Maintains arc resistance around the entire perimeter even with instrument or control compartment doors open
• Type 2C – Requires arc resistance between adjacent compartments within the equipment assembly and around the entire perimeter

Conventional electrical switchgear manufactured to IEEE (North America) or IEC (Europe and elsewhere) standards provides relatively safe environments for equipment and personnel during normal operation. However, traditional switchgear isn't designed to withstand the tremendous energy released during electrical fault events. Arc-resistant certified switchgear is engineered to safely contain arc-flash energy and redirect it away from operators, typically through pressurized chambers that channel the energy to safe release areas without endangering personnel or equipment.

ANSI/IEEE C37.20.7 defines arc testing standards, specifying two accessibility levels for switchgear assemblies. Type 1 provides protection only at the equipment front, while Type 2 offers protection on all sides. Additional suffixes define arc performance between control compartments and switchgear vertical sections: Suffix B indicates normal operation involves opening doors/covers on compartments specifically identified as low-voltage control or instrument spaces; Suffix C requires isolation of internal arc fault effects between all adjacent compartments within the assembly and around the entire perimeter; Suffix D addresses installations where certain external surfaces remain inaccessible and don't require Type 2 design.

Remote operating mechanisms enable operations like disconnecting, testing, and connecting metal-clad switchgear circuit breakers and auxiliary compartments from distances typically spanning 25-30 feet.

Medium voltage switchgear offers various enclosure options suitable for both indoor and outdoor applications. Outdoor enclosures are available in both walk-in and non-walk-in configurations for certain voltage classes.

Most medium voltage switchgear requires front and rear access for installation and maintenance. Designs permitting only front access can be installed directly against walls, offering significant space savings compared to conventional switchgear.

The insulation medium refers to the environment within switchgear enclosures that protects live components (such as bushings, bus bars, etc.) from accidental arc faults. While air represents the most common insulator, gaseous and liquid insulation provide higher dielectric strength, enabling more compact switchgear structures. Liquids additionally offer cooling benefits.

• Air – The most economical and prevalent insulator, but with the lowest dielectric strength properties, requiring physically larger, more robust equipment to withstand arc effects.
• Gas Insulation – Offers significantly improved dielectric strength compared to air, with sulfur hexafluoride (SF6) being the most common switchgear insulating gas. Electrical contacts are sealed within tanks pressurized with SF6 gas, eliminating contactor maintenance requirements.
• Solid-Dielectric Air Technology – Utilizes insulating, non-conductive materials to provide structure and insulation for fault interrupters, bus bars, and high-voltage components in sealed tanks filled with low-humidity air. The combination of non-conductive material and air gaps delivers low dielectric loss, high mechanical strength, and resistance to thermal/chemical degradation. The tank's deadfront construction eliminates partial discharge and conducts fault current to ground.
• Liquid Insulation – Provides improved dielectric properties over air along with cooling effects. Various liquids serve as electrical insulators in switchgear, transformers, and other equipment. Selection criteria include fire resistance and environmental considerations.

Switchgear interrupting devices that break current flow include overcurrent protective devices (fuses, circuit breakers) and switches:

• Air Switches – Utilize air as the dielectric medium, typically offering lower interrupting capacity than oil or vacuum switches but greater economy and visible disconnect capability.
• Fuses – Interrupt excessive current by melting designed wires or strips rated for specific time/temperature characteristics. In medium voltage applications, fuses often pair with switches to provide overcurrent protection plus circuit opening/closing capability.
• Oil Switches – Switchgear devices immersed in oil-filled enclosures, commonly used in pad-mounted switchgear where oil insulation enables compact, low-profile construction.
• Vacuum Circuit Breakers – Feature arc interruption and extinction within sealed vacuum bottles, enabling rapid arc quenching that reduces arc energy. These devices can interrupt higher voltage faults than air circuit breakers with significantly reduced space requirements.
• Vacuum Fault Interrupters – Serve dual roles as overcurrent protection devices and load-break switches, eliminating separate fuse/switch requirements.
• Vacuum Switches – Electrical switches where current interruption occurs within sealed vacuum bottles, enabling rapid arc extinction. These space-efficient switches accommodate higher voltages than air or oil switches.

When selecting medium voltage switchgear, these critical electrical parameters require careful consideration:

Typically specified in symmetrical current terms, this rating indicates the maximum current an overcurrent protective device (usually a vacuum circuit breaker) can safely interrupt without damaging itself or the switchgear. Peak and asymmetrical ratings also commonly apply to medium voltage overcurrent protective devices. Note that interrupting ratings apply only to the actual overcurrent protection devices interrupting circuits under fault conditions, not to the switchgear assembly itself. Medium voltage vacuum circuit breakers typically offer interrupting ratings ranging from 25 kAIC to 63 kAIC symmetrical (referred to as breaking current in IEC equipment).

This rating specifies the maximum current the switchgear can safely withstand (pass through) without sustaining damage. It reflects bus bracing capability, ensuring bus integrity when conducting high currents resulting from downstream faults. Upstream switchgear must exceed the worst-case current passing through it during downstream faults to prevent damage to upstream equipment in the fault current path. Per ANSI standards, typical short-circuit (or withstand) ratings range from 25 kA to 63 kA symmetrical for 2-second ratings, and 40 kA to 101 kA asymmetrical for 10-cycle ratings. Alternative terms include short-circuit withstand rating or withstand rating.

This parameter indicates the maximum current the switchgear's main overcurrent protective devices and main bus can continuously carry without initiating tripping or causing equipment damage. Medium voltage switchgear typically offers continuous current ratings from 600A to 4000A.

ANSI and IEEE standards define these voltage classifications:

• Low voltage: Up to 600V
• Medium voltage: 600V to 69 kV
• High voltage: 69 kV to 230 kV

While ANSI/IEEE standards also define extra-high and ultra-high voltage classes, NEC 2014 expanded the low-voltage definition to include up to 1,000V.

Medium voltage switchgear is classified by its maximum service voltage. For example, 15 kV switchgear (maximum voltage rating) typically serves various actual voltages including 12.47 kV, 13.2 kV, 13.8 kV, and 14.4 kV.

Selecting appropriate medium voltage switchgear is fundamental to ensuring power system safety, reliability, and efficiency. By understanding switchgear types, key electrical parameters, and relevant standards, stakeholders can make informed decisions when choosing optimal solutions for their electrical systems. The selection process should carefully consider specific application requirements, environmental conditions, and budget constraints, with professional electrical engineering consultation recommended to ensure long-term suitability.