Metal-clad switchgear is medium voltage switchgear in which the main circuit breaker is withdrawable (draw-out), and every major component — breaker, main busbar, cable terminations and low voltage instruments — sits in its own compartment separated by grounded metal barriers. This compartmentalisation limits the effect of an internal fault and lets operators rack the breaker out safely. In North America it is defined by IEEE C37.20.2; under IEC it corresponds to metal-enclosed switchgear with full partitioning (LSC2B, PM) to IEC 62271-200.
Medium voltage power distribution has one piece of equipment at its centre that most engineers have specified, installed, or maintained — but surprisingly few can define precisely when challenged. Metal-clad switchgear is one of those terms that gets applied loosely across the industry, often used interchangeably with metal-enclosed switchgear, cubicle switchgear, or simply medium voltage switchgear. They are not all the same thing, and the differences between them matter considerably when you are specifying equipment for a critical application.
This article explains what metal-clad switchgear actually is under international and American standards, how it is constructed, what types exist, and the specific conditions that make it the right — and sometimes the only appropriate — choice for your power distribution system.
What Is Metal-Clad Switchgear? The Precise Definition
IEEE and ANSI Definition
Under IEEE standard C37.20.2, which governs metal-clad equipment in North America and is widely referenced internationally, metal-clad gear is defined as medium-voltage switchgear where all major components are completely enclosed by grounded metal barriers that have no intentional openings between compartments — except for small openings necessary for wires, wire bundles, or insulated primary conductors to pass through.
The standard applies specifically to metal-clad assemblies containing draw-out electrically operated circuit breakers, with rated maximum voltage levels from 4.76kV up to 48.3kV and main bus continuous current ratings of 1,200A, 2,000A, 3,000A, and 4,000A.
The key phrase in that definition is “completely enclosed by grounded metal barriers.” This is what separates metal-clad from other categories of metal-enclosed switchgear — not just the metal outer enclosure, but the internal compartmentalisation that physically separates every major functional component from every other component using grounded sheet metal barriers.
IEC Definition
Under IEC 62271-200, which governs AC metal-enclosed switchgear for rated voltages above 1kV and up to 52kV, the equivalent term is “metal-clad” (MC) switchgear. The IEC definition aligns closely with the IEEE definition: metal-clad gear is characterised by compartmentalisation where each functional unit is enclosed in a separate grounded metal enclosure. The main bus, the switching device, the cable connection compartment, and the low voltage compartment are all individually enclosed with no direct access between compartments during normal operation.
This is the standard framework used for Chinese-manufactured medium voltage switchgear, including our KYN28-12 metal-clad equipment manufactured at Danli and certified to IEC 62271-200. The compartmentalisation requirements under IEC are directly comparable to those under IEEE C37.20.2, which is why KYN28-12 panels are accepted on international projects governed by either standard set.
What Makes Switchgear Truly Metal-Clad?
The Seven Defining Characteristics
IEEE C37.20.2 sets out specific construction requirements that equipment must meet to qualify as metal-clad. Understanding these requirements explains why metal-clad construction is more capable and more expensive than simpler metal-enclosed alternatives.
The first requirement is complete compartmentalisation. Every primary circuit component — the main bus, the circuit breaker, the cable termination space, and the instrument transformers — must be housed in its own individually enclosed compartment formed by grounded metal barriers. There must be no intentional openings between these compartments large enough to allow contact with live parts in an adjacent compartment.
The second requirement is draw-out circuit breaker mounting. The circuit breaker must be mounted on a moveable element — a truck or trolley — that can be withdrawn from its compartment without disconnecting any primary wiring. The draw-out mechanism must provide at least three defined positions: connected, test, and disconnected. This requirement is fundamental to metal-clad classification and is what makes metal-clad design maintainable in ways that fixed-mounted alternatives are not.
The third requirement is automatic shutters. When the circuit breaker truck is withdrawn from the connected position, automatic metal shutters must close over the stationary primary contacts, preventing inadvertent contact with live primary conductors when the breaker is absent from its compartment. These shutters must remain closed unless deliberately defeated by a mechanical interlock during breaker insertion.
The fourth requirement is grounded metal barriers in front of or as part of the circuit interrupting device. When the circuit breaker is in the connected position, a grounded metal barrier must be present in front of, or forming part of, the device so that the primary circuit elements are not accessible.
The fifth requirement is isolation of secondary devices. Instruments, meters, relays, secondary control devices, and their associated wiring must be isolated by grounded metal barriers from all primary circuit elements. This protects low-voltage control and protection equipment from the effects of primary circuit faults and limits arc flash exposure to the low voltage compartment during operation.
The sixth requirement is that all live parts are enclosed within grounded metal compartments. No live medium-voltage conductors should be accessible during normal operation without defeating multiple mechanical interlocks and opening compartment covers.
The seventh requirement is interlocking. This switchgear must include mechanical interlocks that prevent unsafe operating sequences — closing the circuit breaker while the truck is between positions, moving the truck while the breaker is closed, or accessing the cable compartment while it is energised.
Metal-Clad vs Metal-Enclosed: The Critical Distinction
| Feature | Metal-clad | Metal-enclosed (load interrupter) |
|---|---|---|
| Main switching device | Withdrawable circuit breaker | Fixed load-break switch, often with fuses |
| Compartments | Breaker, bus, cable and LV fully separated by grounded metal | Fewer internal barriers |
| Automatic shutters | Yes, cover primary contacts when breaker is withdrawn | Not required |
| Protection | Relays with CTs trip the breaker on faults | Fuses clear faults |
| Relative cost | Higher | Lower |
| Typical use | Main incomers, critical feeders, large motors | Simple radial feeders, transformer protection |
Why the Difference Matters
The term metal-enclosed covers a broader category of switchgear that includes metal-clad as a subset. Not all metal-enclosed switchgear is metal-clad. This distinction matters because metal-enclosed switchgear that does not meet the full compartmentalisation and draw-out requirements of the metal-clad standard offers a meaningfully lower level of protection for operating personnel and a different fault containment performance.
Metal-enclosed load interrupter switchgear, for example, uses a load interrupter switch rather than a draw-out power circuit breaker as the main switching device. The fault current interruption is handled by current-limiting fuses rather than the switch itself. This construction is simpler, less expensive, and smaller than metal-clad, but it has a significantly lower electrical duty cycle — typically 30 operations under ANSI standards compared to 1,000 operations for metal-clad vacuum circuit breakers. When fault interruption demands are low and the application does not require frequent switching or draw-out maintainability, metal-enclosed load interrupter switchgear is adequate. When fault interruption capability, operational flexibility, and personnel protection are priorities, metal-clad is the correct specification.
Types: Air, Gas and Solid Insulated
Air-Insulated Metal-Clad Switchgear
The most widely used type globally, air-insulated metal-clad gear uses atmospheric air as the primary insulating medium between conductors within each compartment. The KYN28-12 manufactured at Danli is air-insulated metal-clad equipment — the vacuum circuit breaker, main busbars, current transformers, voltage transformers, and cable terminations are all separated by air within their respective grounded metal compartments.
Air-insulated metal-clad design is suitable for indoor installation in the voltage range from 3.6kV to 40.5kV, covers the most common industrial and utility medium voltage distribution voltages, and is maintainable with standard electrical safety procedures without specialist gas handling equipment. It is the standard specification for industrial secondary substations, motor feeder panels, bus coupler panels, metering panels, and capacitor bank control panels across manufacturing, water treatment, oil and gas, mining, and power generation applications worldwide.
Gas-Insulated Metal-Clad Switchgear
Gas-insulated variants of this switchgear use sulphur hexafluoride gas or alternative gas mixtures at elevated pressure as the insulating medium inside the compartments. This allows the physical size of each compartment to be dramatically reduced compared to air-insulated designs at the same voltage rating, because the superior dielectric strength of SF6 means conductor clearances can be much smaller.
Gas-insulated metal-clad gear is used where space constraints make a standard air-insulated lineup impractical — urban substations, offshore platforms, indoor installations in existing buildings with limited room dimensions, and applications where contaminated or high-humidity environments make air-insulated equipment difficult to maintain reliably. The trade-off is higher initial cost, specialist gas handling requirements for maintenance, and the regulatory environment around SF6 that is evolving in many jurisdictions.
Solid-Insulated Metal-Clad Switchgear
A more recent development in medium voltage switchgear technology is solid-insulated construction, sometimes called SIS or C-GIS — compact gas-insulated switchgear using a solid dielectric rather than gas. In solid-insulated designs, the primary conductors, switchgear contacts, and busbars are encapsulated in epoxy resin or similar solid insulating material rather than relying on an air gap or pressurised gas for insulation.
Solid-insulated metal-clad equipment is particularly well suited to humid tropical environments, coastal installations, and applications where maintenance resources are limited, because the sealed solid insulation is completely immune to moisture ingress, condensation, and atmospheric contamination that can degrade air-insulated insulators over time. It also eliminates the SF6 environmental concerns associated with gas-insulated designs.
Key Components Inside the Cabinet
Circuit Breaker Compartment
The circuit breaker compartment houses the draw-out vacuum circuit breaker on its truck mechanism. In the KYN28-12 design, compatible vacuum circuit breakers include the VS1-12 and VH1-12 domestic types and international equivalents including the ABB VD4 series. The circuit breaker truck rolls on guide rails that engage primary disconnecting contacts when the truck reaches the service position and automatically disengage when the truck is withdrawn.
Main Busbar Compartment
The main bus runs horizontally through the top of the lineup, connecting adjacent panels and distributing current from the incoming source to all outgoing feeders. In air-insulated metal-clad design, the busbars are bare copper or aluminium conductors supported on post insulators within the bus compartment. The bus compartment is completely enclosed by grounded metal barriers and is not accessible without removing bolted covers — a process that requires a formal isolation procedure and is not part of routine operation.
Cable Termination Compartment
The cable compartment at the rear bottom of the cabinet provides connection points for the primary incoming or outgoing power cables. Depending on the circuit function, this compartment also houses current transformers for protection and metering, earthing switches for safe isolation of the cable connection before maintenance, and surge arresters for cable protection where specified.
Low Voltage Compartment
The low voltage compartment at the top front of the cabinet contains the secondary equipment — numerical protection relays, control switches, indication lamps, metering instruments, and secondary wiring terminal blocks. This compartment is segregated from all medium voltage sections and is safe to access during normal operation, allowing relay settings to be checked, event logs to be reviewed, and control functions to be operated without any exposure to medium-voltage live parts.
Read More: Switchgear vs Switchboard vs Panelboard
When to Specify It
Applications Where Metal-Clad Is the Right Choice
This switchgear is the correct specification for medium voltage distribution applications where personnel safety, operational flexibility, fault containment, and long-term maintainability are primary requirements. These conditions apply to the following applications.
Secondary substations in industrial facilities — manufacturing plants, steel mills, cement works, mining operations, and petrochemical facilities — where the medium voltage distribution system supplies multiple large motor loads and transformer feeders and where planned maintenance must be performed without full facility shutdowns.
Water treatment plants and pumping stations where the medium voltage supply feeds large pump motors and blowers, the facility must operate continuously, and the draw-out circuit breaker capability allows individual feeder maintenance without interrupting the rest of the system.
Oil and gas facilities where strict electrical safety standards apply, high fault current levels exist close to utility connection points, and the compartmentalised construction of metal-clad gear provides the fault containment performance required under the applicable area classification standards.
Power generation facilities where generator step-up switchgear and auxiliary supply distribution boards must handle high fault current levels, frequent switching operations, and the reliability requirements of base-load or peaking generation service.
Hospitals, data centres, and critical infrastructure where continuity of supply is a contractual or regulatory requirement and the draw-out circuit breaker capability allows maintenance without service interruption.
When Metal-Enclosed Load Interrupter Switchgear May Be Sufficient
For simpler distribution applications — secondary distribution to small commercial facilities, rural utility distribution networks, and applications with low fault current levels and infrequent switching requirements — metal-enclosed load interrupter switchgear with current-limiting fuses may be technically and economically appropriate. The simpler construction, lower cost, and smaller footprint can be justified where the full performance envelope of metal-clad construction is not required by the application.
Governing Standards and Certification
International Standards
Metal-clad equipment manufactured for international markets is governed primarily by IEC 62271-200, which covers AC metal-enclosed switchgear and controlgear for rated voltages above 1kV and up to 52kV. Type tests under this standard include dielectric withstand tests at power frequency and lightning impulse, temperature rise tests under rated continuous current, short-circuit making and breaking tests, internal arc tests where specified, and mechanical endurance tests on the draw-out mechanism and interlocks.
North American Standards
For projects governed by North American standards, metal-clad design must meet IEEE C37.20.2, which covers construction, ratings, testing, and application of metal-clad medium-voltage switchgear from 4.76kV to 48.3kV. ANSI C37.20.2 is the companion standard from the American National Standards Institute. For low-voltage power circuit breaker switchgear, the applicable standard is IEEE C37.20.1 and UL 1558.
Danli Manufacturing and Certification
At Danli, our KYN28-12 switchgear is manufactured in Zhejiang under ISO 9001, ISO 14001, and ISO 45001 certified quality management systems. Each panel is fully assembled and factory acceptance tested before shipment. Test documentation includes insulation resistance measurements, contact resistance measurements on the circuit breaker, secondary wiring continuity checks, interlock functional tests, and operational tests of the draw-out mechanism through its full cycle.
Our standard KYN28-12 lineup is certified to IEC 62271-200 and meets CE marking requirements. We supply compatible vacuum circuit breakers from verified manufacturers, full panel wiring diagrams, component datasheets, operation and maintenance manuals, and spare parts recommendations with every order.
Our engineering team provides technical support from initial specification through commissioning and for the full operating life of the equipment — which for properly maintained metal-clad gear is typically 25 years or more.
Frequently Asked Questions
Metal-clad uses withdrawable breakers and grounded metal barriers between every major compartment, with automatic shutters. Metal-enclosed equipment is housed in a metal enclosure but has fewer internal barriers and usually fixed load-break switches.
It is used in medium voltage systems, typically from about 3.3 kV up to 38 kV in North America and up to 40.5 kV under IEC practice.
Yes. KYN28-12 is a withdrawable, fully compartmented 12 kV cabinet and is a common example of metal-clad construction under IEC 62271-200.
IEEE C37.20.2 defines metal-clad switchgear in North America. Internationally, IEC 62271-200 covers AC metal-enclosed switchgear from 1 kV to 52 kV, with LSC and partition classes describing the level of compartmentalisation.
