Metal-Clad vs Metal-Enclosed Switchgear

In the field "metal-clad" and "metal-enclosed" get used as if they were interchangeable. They are not — they name different IEEE standards with different requirements, and the gap between them is the whole point of this page.

• Metal-clad switchgear is built to IEEE C37.20.2. This is a construction standard: it dictates a specific physical recipe the assembly must follow.
• Metal-enclosed interrupter switchgear is built to IEEE C37.20.3. This is a performance standard: it says what the switch must accomplish, and leaves most of the construction open.
• Metal-enclosed low-voltage power circuit-breaker switchgear is a third, separate standard, IEEE C37.20.1 — the low-voltage cousin, out of scope for a medium-voltage line-up but often confused into the same conversation.

"Metal-enclosed" is the umbrella: it means the live parts sit inside a grounded metal box. "Metal-clad" is a specific, heavily-specified kind of metal-enclosed gear. This is the medium-voltage switchgear distinction that costs the most money to get wrong.

Metal-clad construction is defined by a checklist. To earn the label, a line-up must have all of the following:

• Draw-out (removable) circuit breakers — the breaker rolls out of its cell rather than being fixed-mounted.
• Grounded metal barriers separating every major compartment from the others: breaker, main bus, cable, and instrumentation each sit in their own grounded metal box.
• Automatic shutters that cover the primary stabs — the live connection points — whenever the breaker is racked out, so an open cell does not expose energized metal.
• Insulated primary bus — the main bus is insulated, not bare.
• Mechanical interlocks that prevent racking a closed breaker into or out of its connected position.

Every one of those is a safety and maintainability feature. Together they are what let you open a feeder, rack that breaker out behind closed shutters, and work on it while the main bus and every other feeder stay energized.

Metal-enclosed interrupter switchgear asks for far less. The standard is a performance requirement: the switch must safely interrupt load current inside a grounded enclosure. That is essentially the whole demand.

It does not require draw-out elements, full compartmentalization, automatic shutters, insulated bus, or the metal-clad interlock scheme. A typical metal-enclosed line-up uses fixed-mounted, fused load-interrupter switches: compact, mechanically simple, and cheaper. The trade is operational. With fixed-mounted or fused gear, servicing one device often means de-energizing the whole assembly, because there is no draw-out mechanism and no shutter to isolate a single cell while the rest stays live.

Neither approach is "wrong." A metal-enclosed interrupter line-up is proportionate for smaller, non-critical, or seldom-touched loads. It is only wrong when it is specified for a job that assumed metal-clad capability.

Here is the failure mode the standards exist to prevent. "Metal-enclosed" is the superset — every metal-clad line-up is metal-enclosed, but not the reverse. So a specification that simply says "metal-enclosed switchgear" is technically satisfied by the cheaper C37.20.3 gear, even if the one-line drawing assumes draw-out breakers, relay coordination, and the ability to maintain a feeder live.

That mismatch is a real and common defect: the design intends metal-clad capability, the words permit metal-enclosed, and the low bidder supplies exactly what the words allowed. The fix is one sentence. Write "metal-clad per IEEE C37.20.2" whenever the design needs draw-out breakers, compartmentalization, or live maintainability — and reserve "metal-enclosed per IEEE C37.20.3" for line-ups where a fixed or fused switch is genuinely enough.

The construction choice also interacts with the voltage class you need and with whether the line-up is arc-resistant — but the metal-clad-versus-metal-enclosed decision is the one that most often ships wrong. Metal-clad line-ups are available through Pantheon in the electrical catalog.