Metal Enclosed Switchgear: A Comprehensive Guide

Metal-enclosed switchgear places primary switching and protective equipment inside a metal enclosure. Depending on its class and rating, an assembly can contain circuit breakers, switches, fuses, busbars and current transformers. It controls and protects utility, commercial or industrial circuits at the specified voltage. The equipment nameplate and governing standard identify the exact class; the metal enclosure alone does not.

What is Metal Enclosed Switchgear?

Metal-enclosed switchgear is a factory-assembled group of switching, interrupting, bus and connection equipment enclosed on the sides and top by metal, apart from required ventilation, viewing or pressure-relief openings. IEC 62271-200 covers AC metal-enclosed assemblies above 1 kV through 52 kV. In North American practice, IEEE standards divide the equipment into more specific classes.

IEEE C37.20.3 covers metal-enclosed interrupter switchgear that may contain interrupter switches, power fuses, circuit breakers, metering and protection. IEEE C37.20.2 separately covers metal-clad switchgear with drawout electrically operated circuit breakers and grounded metal barriers around major primary compartments. The internal arrangement therefore depends on the selected class and product design. Common design points include:

• Primary buses, switching devices and cable terminations are arranged in sections or compartments suited to the rated voltage, current, fault duty and maintenance access.
• Mechanical or electrical interlocks can prevent an unsafe operating sequence, such as opening a fuse door while its associated switch is closed. They do not replace isolation, lockout/tagout or an absence-of-voltage test.
• Access can be from the front, rear or both sides. Drawout positions, shutters and test facilities are product-specific, so operators must follow the manufacturer’s instructions and site procedures.

A metal-enclosed interrupter design can suit a distribution system when its ratings and protective devices match the application. Possible benefits include:

• A switch-and-fuse design can have a lower purchase cost than a drawout circuit-breaker lineup for some simple feeder or transformer-protection duties.
• Maintenance can be straightforward when the assembly uses sealed or low-maintenance switching devices, but the required interval still depends on the product, service duty and environment.
• Current-limiting fuses can reduce the peak current and energy passed during faults within their limiting range. They must be coordinated with the load, transformer and upstream protection.
• Manufacturers can supply different sections, metering, controls, bus ratings and enclosure types within the limits of the tested assembly.

Selection also involves trade-offs. The following points are not universal disadvantages, but they must be checked for the proposed installation:

• Air-insulated equipment generally needs more clearance and floor area than a compact gas-insulated design with comparable ratings.
• A standard metal enclosure is not automatically arc-resistant. Internal-arc performance needs a stated and tested classification, installation arrangement and operating condition.
• Dust, moisture, salt, condensation, vermin and corrosive contaminants can shorten service life unless the enclosure rating and environmental controls suit the site.

Types and Designs of Metal Enclosed Switchgear

Metal-enclosed assemblies are built for different standards, voltage classes, interrupting devices, insulation systems and locations. The following sections describe several related designs, but they should not be treated as interchangeable categories.

Metal Enclosed Indoor Switchgear

Metal-enclosed indoor switchgear is installed inside a building or substation room whose temperature, humidity, contamination and access meet the equipment specification. Some metal-clad products use horizontal drawout circuit breakers with primary disconnects that separate as the breaker is racked away from its service position. Other metal-enclosed products use fixed switches and fuses, so drawout features are not universal.

A drawout breaker may have connected, test and disconnected positions. In the connected position, its primary and secondary circuits are engaged. The test position normally separates the primary circuit while retaining the secondary control connection. The disconnected position separates both, subject to the product design. Position labels and permitted operations must be confirmed from the instructions; a racked-out breaker does not prove every conductor in the cubicle is de-energised.

Gas Insulated Medium Voltage Switchgear

Gas-insulated medium voltage switchgear places some or all primary conductors in sealed gas-filled compartments. Legacy and current products may use sulfur hexafluoride (SF6) for insulation, while some newer products use fluorinated-gas-free mixtures. Many medium-voltage designs use vacuum interrupters for fault interruption, so the insulating gas does not necessarily quench the breaker arc.

The compartment count and switching arrangement vary by product. A lineup may combine vacuum circuit breakers or load switches with busbar, cable, metering and earthing functions. Fixed primary equipment can still include disconnecting and earthing positions. The one-line diagram, interlock scheme and ratings define what each position does. Compared with air-insulated equipment of similar duty, gas-insulated switchgear can offer:

• A smaller footprint because the controlled insulating medium supports reduced internal clearances.
• Tested arc-resistant construction on models that carry the relevant classification. Gas insulation by itself does not establish that rating.
• Less exposure of primary parts to humidity, dust, salt and vermin when those parts are inside a sealed pressure system.

Its trade-offs can include:

• A higher initial equipment cost, offset in some projects by a smaller building footprint or reduced site work.
• Special procedures, trained personnel and suitable recovery equipment when a gas compartment needs inspection or repair. Sealed-for-life products may have different maintenance requirements.
• Environmental and regulatory obligations for equipment containing SF6. Leaks during installation, service or decommissioning matter because SF6 is an exceptionally potent and long-lived greenhouse gas.

Metal Enclosed Outdoor Type Medium Voltage Switchgear

Outdoor metal-enclosed medium-voltage switchgear uses a weather-rated enclosure and may include space heaters, ventilation, seals, pressure relief and corrosion protection. These details depend on the enclosure type, climate and site conditions rather than a single roof shape. Cable entry, drainage, working clearances and access must be coordinated with the foundation and installation drawings. Outdoor equipment still needs inspection for water ingress, condensation, contamination, damaged seals and corrosion.

Unitized Power Centers

Unitised power centres are integrated substations rather than one universal switchgear class. A package can combine a medium-voltage switch or protective section, a dry-type or liquid-filled transformer, low-voltage breakers, metering, protective relays, controls and grounding equipment. Commercial and industrial sites use them to transform and distribute power in a coordinated factory-built assembly.

An integrated package can provide project benefits when its components and site interfaces are engineered together:

• Factory assembly can reduce field connections and installation time, although transport, foundations and final cable work still require planning.
• Coordinated drawings and factory tests can reduce interface errors and unwanted voltage drops caused by unsuitable conductor selection or connections.
• Safety features can be specified across the package, including barriers, grounding, protection, remote operation or an arc-resistant classification where required.
• Modular sections can allow planned expansion, but additions must remain within bus, fault, thermal and physical design limits.
• Transformer efficiency, conductor losses, harmonics and power factor must be assessed from the actual load study; enclosure integration does not reduce them automatically.

OEM Medium Voltage Switch Components

Original equipment manufacturers can buy rated medium-voltage components for use in tested metal-enclosed assemblies. These can include load-interrupter switches, power fuses, electronic fuse systems, fuse mountings, holders, indicators and operating mechanisms. A component rating does not by itself establish the rating of the completed assembly. The OEM must assess insulation, temperature rise, short-circuit duty, interlocks, grounding, enclosure and required conformity tests.

Modular components can support an engineered product when the OEM controls the complete design:

• Ratings and operating mechanisms can be selected for the circuit duty and system configuration.
• Published component test data can support the assembly design, but it does not replace testing required for the finished equipment.
• Compatibility must be demonstrated through the applicable assembly standard, drawings, clearances and verified interface ratings.
• Documentation should identify spare parts, inspection intervals, operating limits and technical support for the installed configuration.

Conclusion

Metal-enclosed switchgear groups buses, connections and switching or protective devices within a metal enclosure. The useful classification comes from the applicable standard and design. Before choosing equipment, confirm:

• The rated voltage, continuous current, short-circuit duty, internal-arc requirement and environmental service conditions.
• The switching device, protection scheme, compartment access, interlocks and safe isolation method.
• The installation clearances, cable entry, grounding, pressure relief, inspection and maintenance requirements.

A sound comparison uses project-specific costs and duties:

• Compare purchase, installation, outage, inspection, spares and end-of-life costs across the required service period.
• Check reliability claims against the chosen protective device, service history and operating environment.
• Specify only options that remain inside the tested ratings and documented configuration of the assembly.

Common construction choices include:

• Air-insulated indoor switchgear for controlled rooms where the required working and maintenance clearances are available.
• Gas-insulated equipment for compact layouts or contaminated environments, with the insulating medium and handling obligations stated.
• Outdoor assemblies with the enclosure, heaters, ventilation and corrosion protection selected for local conditions.

Related packaged arrangements include:

• Indoor metal-clad switchgear with drawout circuit breakers and grounded metal barriers between major primary compartments.
• Gas-insulated medium-voltage switchgear with fixed primary equipment and sealed insulating compartments.
• Outdoor metal-enclosed switchgear built and rated for its exposure and access arrangement.
• Unitised power centres that integrate a medium-voltage section, transformer and low-voltage distribution equipment.
• OEM assemblies built from rated components and verified as complete equipment under the applicable standard.

Only qualified people should specify, operate or maintain this equipment. Use the approved single-line diagram, nameplate data, manufacturer instructions and site safety procedure. Interlocks do not replace de-energisation, lockout/tagout and voltage testing. Further background articles are available at electrical4u.com .