Metal-Clad vs. Metal-Enclosed MV Switchgear

Why the distinction matters

Both assemblies are medium-voltage, typically 5 kV through 38 kV class, and both put switching devices, bus and cable terminations inside a grounded metal enclosure. In the IEEE C37.20 family, metal-enclosed is the broad category; metal-clad is a subset with additional requirements that affect safety, maintainability and cost.

Specifications frequently say metal-clad when the drawings show fused load-interrupter switches, or say switchgear when the intent is a simple fused transformer primary. Either mismatch creates bid confusion and, in the worst case, delivers equipment that does not support the protection and operating practices the owner assumed.

What makes switchgear metal-clad

IEEE C37.20.2 defines metal-clad switchgear by a set of construction features that work together. The main switching device is a removable, drawout circuit breaker, today almost always a vacuum breaker, that can be racked between connected, test and disconnected positions. Major primary compartments are separated by grounded metal barriers, so a fault in a cable compartment is contained away from the main bus and the breaker.

Primary bus and connections are insulated, and the primary disconnect stabs are covered by automatic shutters that close when the breaker is withdrawn, leaving no exposed energized parts in the breaker compartment. Mechanical interlocks prevent racking a closed breaker or closing a breaker that is between positions. The low-voltage control and relay compartment is isolated from the primary compartments.

• Drawout vacuum circuit breakers with connected, test and disconnected positions
• Grounded metal barriers between breaker, main bus and cable compartments
• Insulated primary bus and connections
• Automatic shutters over primary disconnect stabs
• Mechanical interlocks against racking a closed breaker
• Isolated low-voltage control and relay compartment

Metal-enclosed interrupter switchgear

Metal-enclosed interrupter switchgear built to IEEE C37.20.3 uses load-interrupter switches, either air or vacuum, usually in series with power fuses. The switch is rated to make and break load current and to close into a fault, but it is not rated to interrupt fault current; the fuses do that. Some designs use fixed-mount breakers instead of switch-fuse combinations, and these are still metal-enclosed rather than metal-clad because the breaker is not drawout.

Compartment barriers are fewer, bus may be bare rather than insulated, and shutters are not required. Viewing windows to verify blade position, key interlocks that tie switch and fuse access together, and mechanical interlocks preventing the fuse door from opening with the switch closed are the usual safety provisions. Duplex and selector configurations allow one transformer to be fed from either of two sources with a single switch operation.

Protection and operating differences

A metal-clad breaker is operated by protective relays, which means the full set of protective functions is available: phase and ground overcurrent, differential, undervoltage, directional elements and breaker failure schemes. Breakers can be tripped and reclosed remotely, transferred automatically and integrated into SCADA with no physical intervention.

A fused switch has a fixed time-current characteristic and no remote trip. Current-limiting fuses can clear high faults very fast and reduce let-through energy, which is valuable for transformer protection, but a blown fuse on one phase leaves a three-phase load single-phased until someone opens the switch. Fuse replacement requires an outage of that circuit and access to the fuse compartment, and coordination flexibility is limited to fuse selection.

Cost, footprint and maintenance

Metal-clad sections are deeper, heavier and more expensive than interrupter switchgear of the same voltage class because of the breaker, racking mechanism, barriers, shutters and relay compartment. Maintenance practice centers on the breaker: racking it out, inspecting contacts and mechanism, timing tests and keeping a tested spare on hand so a lineup never waits on a repair.

Interrupter switchgear is lower in first cost and simpler to maintain, with spare fuses as the primary consumable. Because the switch is fixed, cleaning, inspection and torque checks generally require de-energizing the section. Where switching is infrequent and the load can tolerate that outage, the simplicity is an advantage rather than a compromise.

Choosing between them

Metal-clad switchgear is the normal choice for main substations, generator breakers, large motor feeders, tie breakers in main-tie-main arrangements and any circuit that needs relay protection, automatic transfer, frequent switching or remote operation. Metal-enclosed interrupter switchgear fits transformer primaries, small unit substations, loop-feed and radial distribution points where switching is occasional and fuse protection is adequate.

Both types can be specified in arc-resistant construction tested to IEEE C37.20.7, and both can carry metering and communications. Applicable listings, standards and design requirements depend on equipment type, configuration, project specifications and jurisdiction, so the one-line diagram and protection philosophy should drive the choice before the budget does.