Switchgear: fundamentals of medium-voltage switchgear

Electrical switchgear refers to a centralized collection of circuit breakers, fuses and switches (circuit protection devices) that function to protect, control and isolate electrical equipment. The circuit protection devices are mounted in metal structures. A collection of one or more of these structures is called a switchgear line-up or assembly.

Switchgear is commonly found throughout electric utility transmission and distribution systems as well as in medium to large sized commercial or industrial facilities. Standards for electrical switchgear are defined by IEEE in North America and by IEC in Europe and other parts of the world.

How does switchgear work?

Electrical switchgear refers to a collection of circuit protection devices (circuit breakers, fuses or switches) mounted in a common, metal enclosure. The circuit protection devices distribute power to various sections of a facility and the electrical loads within those sections. They also provide protection to individuals and equipment throughout the facility by limiting the current flow through the system to safe levels.

Gas insulated switchgear (GIS) has sealed enclosure(s) filled with insulating gas sulfur hexafluoride (SF6) or mixture of SF6 and other insulating gases recently released to the market. The gas-filled sealed enclosure facilitates a compact, low-profile installation. The use of gas as an insulating medium, when compared to similar air-insulated switchgear, allows for the distance between interrupting components to be reduced. Gas-insulated padmounted switchgear is designed and tested in accordance to with ANSI standard C37.60 and C37.72. The IEEE C37.20.9 standard for gas-insulated switchgear was released in the summer of 2019, prior to this GIS switchgear is designed, manufactured, and tested to International Electrotechnical Commission (IEC) performance standard 62271.

Metal-clad switchgear is defined by IEEE C37.20.2 and refers to the construction of medium-voltage electrical switchgear where all electrical components including the incoming bus, outgoing bus, instrumentation and main circuit breaker or switch, are enclosed in separate metal compartments to provide an additional level of safety, ruggedness and ease of maintenance. Rated voltage levels for metal-clad switchgear range from 5 kV to 38 kV. Metal-clad switchgear features draw-out circuit breakers for ease of maintenance and is often applied in industrial facilities and in electrical power generation and power transmission facilities.

Metal-enclosed switchgear is defined by IEEE C37.20.3. Metal-enclosed switchgear contains circuit protection devices including circuit breakers, power fuses and fusible switches as well as control and metering equipment. These devices can be mounted in common compartments and do not require the separate barriers, or compartmentalization required in metal-clad switchgear. Metal-enclosed switchgear is applied in commercial and many industrial facilities where the incoming electrical service is above 480/600V.  

Pad-mounted switchgear is defined by IEEE C37.74. Pad-mounted switchgear is designed for underground distribution systems rated from 5 to 38 kV that are required to be above grade operable. Pad-mounted switchgear’s outdoor rated, low profile and tamper-resistant construction makes it ideal for utility distribution, feeder sectionalizing and circuit protection applications. Switches, fuses and vacuum Interrupters are used to protect loads, isolate faults and minimize outages. Pad-mounted switchgear is available with up to 6-ways in a common insulated sealed tank. Insulation means include air, SF6 gas, fluid, solid-dielectric-in-air technology and solid materials.

Vault or subsurface switchgear is defined by IEEE C37.74. Vault or subsurface switchgear is designed for electrical distribution systems rated from 15 to 38 kV requiring that the switch and its accessories be operable from inside a vault or below-grade location. These locations can be dry or subject to water ingress. Vault or subsurface switchgear can also allow the user to operate the switch from above ground manually or with the use of relays and utilizes vacuum interrupters to protect loads and isolate faults. Insulation means include SF6 gas, solid-dielectric-in-air technology and solid materials.

ANSI/IEEE C37.20.7 classification summary

• Type 1—Must be arc resistant in the front of the equipment only
• Type 2—Must be arc resistant around the entire perimeter of the equipment
• Type 2B—Must be arc resistant around the entire perimeter of the equipment, even with instrument or control compartment doors open
• Type 2C—Must be arc resistant between adjacent compartments within the assembly, as well as around the entire perimeter of the equipment

Conventional electrical switchgear is built to IEEE (North America) or IEC (Europe and other parts of the world) standards and provides a relatively safe environment for equipment and maintenance personnel in normal operating conditions. However, conventional switchgear is not designed to withstand the enormous energy released during an electrical fault condition. Switchgear certified as arc-resistant is designed to safely contain and redirect arc flash energy away from the operator. This is typically accomplished by diverting arc flash energy through a plenum to an area it can be released without danger of harm to personnel or equipment.

Arc resistant testing standards are defined by ANSI/IEEE C37.20.7. This standard defines two levels of accessibility to switchgear assemblies. Type 1 provides protection only when in front of the gear. Type 2 provides protections on all sides. In addition, a suffix is added to define arc performance for control compartments and between vertical sections of the switchgear. Suffix B designates equipment where normal operation of the equipment involves opening the door or cover on compartments specifically identified as low-voltage control or instrumentation compartments. Suffix C designates for equipment where isolation from the effects of an internal arcing fault is desired between all adjacent compartments within a switchgear assembly. And suffix D designates specifically designed for installations where some external surfaces of the equipment are inaccessible and no need exists to use a Type 2 design. Eaton's arc-resistant medium-voltage switchgear options include Type 2, 2B and 2C.  

Additionally, remote racking can be used perform operations such as disconnect, test and connect of circuit breakers and auxiliary compartments of metal-clad switchgear from typically 25-30 feet away.

Medium-voltage switchgear is available in enclosures rated for both indoor and outdoor applications. Outdoor enclosures are available in sheltered-aisle and aisleless configurations for some voltage classes. 

Medium-voltage switchgear typically requires access to both the front and rear of the assembly for both installation and maintenance. Switchgear designed for front-only access can be mounted with the rear surface directly against a wall. This type of front access switchgear provides significant floor space savings as compared to conventional switchgear.

• Air is the most common insulator and the least expensive. However, air also has the lowest dielectric strength properties and require physically larger and more rugged equipment to withstand the effects of an electrical arc
• Gas insulation offers significantly improved dielectric strength compared to air. The gas most commonly used as a switchgear insulating medium is Sulfur Hexafluoride (SF6). The electrical contacts are sealed inside a tank with pressurized SF6 gas. The sealed-tank design also eliminates the need for contactor maintenance
• Eaton’s solid-dielectric-in-air technology uses insulated non-conductive materials to provide structure and insulation to fault interrupters, bus and high-voltage components inside of a sealed tank filled with low humidity air. The combination of non-conductive materials with air gaps provide low dielectric losses, high mechanical strength and resistance to thermal and chemical deterioration of the switchgear. The tank’s dead-front construction is rated to remove partial discharge and carry fault current to ground

• Fluid also offers improved dielectric when compared to air, while also adding a cooling benefit. Although often referred to as oil, various fluids are used for electrical insulation in switchgear as well as transformers and other devices. It is important that the fluid selected is fire-resistant and environmentally friendly. Eaton offers medium-voltage switchgear with E200 fluid, FR3 fluid and mineral oil
  • Mineral oil is a petroleum-based, time-proven insulation and has reliable electrical insulating properties
  • E200 fluid is fire-resistant biodegradable, polyol ester-based, nontoxic low-viscosity fluid with excellent dielectric, thermal and physical properties. The low-viscosity characteristic allows it to be used in VFI switchgear down to -30° C. Its fire point is greater than 300° C (572° F), a requirement for less flammable fluids
  • FR3 fluid is formulated from edible vegetable oils and food grade performance enhancing additives. It does not contain any petroleum, halogens, silicones or any other questionable material. It quickly and thoroughly biodegrades in both soil and aquatic environments. The fluid tested nontoxic in aquatic toxicity tests

• Air switch is a switching device that uses air as a dielectric. Air switches will typically have lower interrupting ratings than oil or vacuum switches, however they are also more economical and provide a visible means of disconnect.
• Fuse is a device that interrupts excessive current flow. The current is interrupted by the melting of an electrical wire or strip designed to melt at a prescribed time/temperature rating. In medium-voltage switchgear applications, fuses are commonly paired with a switch to provide both overcurrent protection as well as the ability to open and close the circuit.
• Oil switch is a switching device that is submerged in an oil-filled enclosure. Oil switches are often found in pad-mounted switchgear where the oil insulation permits construction of a compact, low-profile enclosure.
• Vacuum circuit breaker is a type of circuit breaker where the arc interruption and quenching takes place inside of sealed vacuum bottles. The vacuum allows the arc to be extinguished quickly reducing the arc energy. Vacuum circuit breakers can interrupt much higher voltage faults than air circuit breakers and require significantly less space. Vacuum circuit breakers are featured in Eaton's VacClad metal-clad switchgear.
• Vacuum fault interrupter functions as both an overcurrent protection device as well as a load-break switch eliminating the need for a separate fuse and switch. Vacuum interrupters are featured in Eaton's ISG-SD switchgear and Cooper Power series VFI underground distribution switchgear.
• Vacuum switch is a type of electrical switch where the flow of electricity is interrupted inside of sealed vacuum bottles. The vacuum allows the resulting arc to be extinguished quickly. Vacuum switches require less space than air or oil switches and can be applied at higher voltages. Vacuum switches are featured in Eaton's ISG-SD switchgear.

Interrupt rating is typically specified on a symmetrical current basis and refers to the magnitude of current that the overcurrent protective device (typically a vacuum circuit breaker) can safety interrupt without damaging itself or the switchgear. Peak and asymmetrical ratings are also typical specified values in medium-voltage overcurrent protective devices. Interrupt ratings only apply to the actual overcurrent protective devices that are interrupting the circuit under fault conditions, not to the switchgear assembly itself. Typical interrupt ratings for medium-voltage vacuum circuit breakers range from 25 kAIC to 63 kAIC symmetrical. (For IEC equipment, this rating is often referred to as breaking current.)

The short circuit current rating of switchgear is the maximum amount of current that the switchgear can safely withstand (i.e. allow to pass through it) without it causing damage to the switchgear. This is a measure of the bracing and support of the busbars which allows them to remain intact and free from damage while passing high currents due to faults that occur downstream of the switchgear. For faults in downstream equipment, the upstream switchgear must have a short circuit current rating that exceeds the worst-case currents that will be passing through the switchgear. This ensures that downstream faults do not damage the upstream equipment that is in the fault current path. Per ANSI standards, typical short circuit current (or withstand) ratings range from 25 kA to 63 kA symmetrical for the 2-second rating, and 40 kA to 101 kA asymmetrical for the 10-cycle rating. For electrical switchgear, this rating is also sometimes referred to as short-circuit rating, short-circuit withstand rating or withstand rating.

The continuous current rating of switchgear is the amount of current that the main overcurrent protection device and main bus within the switchgear can carry continuously without initiating a trip or damaging the equipment. Continuous current ratings for medium-voltage switchgear can typically range from 600A to 4000A.

ANSI and IEEE standards define voltage classifications as follows:

• Low-voltage: up to 600V
• Medium-voltage: between 600V and 69 kV
• High-voltage: between 69 kV and 230 kV
• Extra-high voltage and ultra-high voltage classes are also defined in the ANSI/IEEE standards; however, NEC 2014 expanded the definition of low-voltage to include up to 1,000V.

Medium-voltage switchgear is classified by the maximum voltage it can service. For example, 15 kV switchgear (maximum voltage rating) is commonly applied at various actual voltages including: 12.47 kV, 13.2 kV, 13.8 kV and 14.4 kV.