Medium-voltage switchgear explained

In North American practice, medium voltage means above 1000 V and up through roughly 38 kV. Equipment is not rated at the nominal system voltage — it is rated at a maximum voltage class with a matching basic impulse level (BIL). The preferred classes in IEEE C37.06 are 4.76, 8.25, 15, 27, and 38 kV; IEEE C37.20.2 extends metal-clad construction to 48.3 kV.

So a 13.8 kV system takes 15 kV class gear, and a 24.94 kV system takes 27 kV class. Asking a supplier for "13.8 kV switchgear" is ambiguous; asking for "15 kV class, 95 kV BIL" is not. The full mapping from system voltage to equipment class — which class 4.16, 13.8, and 34.5 kV each need — is worth pinning down before you write anything else.

Switchgear is an assembly, not a breaker. A line-up contains the breakers or switches, the main and riser bus, current and voltage transformers, protective relays, control power, and the enclosure that ties the safety story together. Most of the specification effort goes into the things that are not the breaker.

These are two different IEEE standards, and the words are not interchangeable.

Metal-clad switchgear (IEEE C37.20.2) has a defined structural recipe. It requires draw-out (removable) circuit breakers; grounded metal barriers separating every major compartment — breaker, main bus, cable, instrumentation; automatic shutters that cover the primary stabs whenever the breaker is racked out; insulated primary bus; and mechanical interlocks that prevent racking a closed breaker in or out.

Metal-enclosed interrupter switchgear (IEEE C37.20.3) is a performance standard. It requires that the switch safely interrupt load current inside a grounded enclosure. It does not require draw-out elements, full compartmentalization, or the same interlock scheme. Metal-enclosed low-voltage power circuit-breaker switchgear is a third standard, IEEE C37.20.1.

The operational consequence is the whole argument. With metal-clad gear, 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. With a fixed-mounted or fused metal-enclosed line-up, servicing one device often means de-energizing the assembly.

"Metal-enclosed" is the superset — every metal-clad line-up is metal-enclosed, but not the reverse. A specification that says "metal-enclosed switchgear" when the one-line assumes draw-out breakers and relay coordination is a real and common failure mode. Write "metal-clad per IEEE C37.20.2" if that is what the design needs. The full metal-clad vs metal-enclosed breakdown walks the requirement-by-requirement difference between the two standards.

Two ratings do most of the work, and they answer different questions.

Continuous current is how much load the bus and the breaker can carry indefinitely without exceeding their temperature rise. Preferred values are 1200, 2000, 3000, and 4000 A. Main bus and breaker frame are rated separately — a 4000 A main bus with 1200 A feeder breakers is normal.

Short-circuit (interrupting) rating is how much fault current the breaker can clear. Preferred values run 25, 31.5, 40, 50, and 63 kA symmetrical. Two companion numbers travel with it and belong in the specification:

• Close-and-latch, the peak asymmetrical current the breaker can close into without welding — conventionally 2.6 times the rated short-circuit current on an rms-asymmetrical basis.
• Short-time withstand, how long the assembly can carry fault current while a downstream device clears, typically 2 seconds. This is what lets you coordinate rather than trip everything at once.

One historical trap when matching older equipment to a modern study: breakers built to the pre-1999 rating structure were rated on an MVA class with a voltage-range factor K, so their interrupting capability changed with operating voltage. Post-1999 IEEE C37.04 breakers are rated on a constant-kA basis. If a nameplate shows a K factor, do the conversion before you compare it against an ETAP or SKM result.

Practically all new medium-voltage breakers use vacuum interrupters, and they have displaced oil, air-magnetic, and SF6 designs in this class.

The mechanism is simple. The contacts open inside a sealed ceramic bottle at very high vacuum. The arc is sustained only by metal vapor boiled off the contacts, and at the first natural current zero — within half a cycle — that vapor condenses onto the shield and dielectric strength recovers almost instantly. There is nothing to burn, no gas to handle, no oil to test.

What wears is the contact face. A vacuum breaker will typically survive tens of thousands of mechanical operations and on the order of 10,000 full-load switching operations, but only a few dozen full-rated short-circuit interruptions. That is why the acceptance checks on a used line-up are contact-erosion measurement against the manufacturer’s wear mark and a vacuum integrity test (a DC hi-pot across the open contacts, or a magnetron test), alongside the usual contact-resistance and timing checks.

Vacuum is also the forward-compatible choice. Regulatory pressure on SF6 as a greenhouse gas is pushing new medium-voltage designs toward vacuum and clean-air insulation, which is worth weighing before adopting an SF6 line-up for a twenty-year asset.

An internal arcing fault in a medium-voltage enclosure releases enormous energy as pressure, molten metal, and plasma. Arc-resistant switchgear per IEEE C37.20.7 is tested to contain and redirect that energy — typically for a full 0.5 second, 30 cycles — venting it upward through a plenum or duct instead of out the front of the cubicle.

The standard defines accessibility types. Type 1 protects only at the front; Type 2 protects at all freely accessible sides. Suffixes describe what may be open during the event: B adds protection with the low-voltage control or instrument compartment door open, C covers the barrier between adjacent compartments, D covers the door between compartments in the same cell. Type 2B is the common data-center and industrial specification. The arc-resistant switchgear page breaks down the types and suffixes in full.

Two things engineers routinely get wrong here:

• Arc-resistant construction does not reduce incident energy. It redirects it while the equipment is closed and latched to its tested configuration. Open a door outside that configuration and you are back to the calculated incident energy and the PPE that goes with it. Arc-resistant gear is a complement to an arc-flash study, not a substitute.
• Venting is a building problem. The plenum needs somewhere to discharge — overhead clearance, a roof penetration, or ducting to the outside. Discovering that after the room is designed is expensive.

The real incident-energy reduction comes from clearing the fault faster: arc-flash relays that sense light plus overcurrent and trip in a few milliseconds, and maintenance-mode settings that temporarily remove intentional time delay while someone is working in front of the gear.

A supplier cannot price or match a line-up from "15 kV switchgear." A quotable specification carries at least this much:

• System data — nominal voltage, frequency, and grounding method (solidly grounded, low- or high-resistance grounded, ungrounded). Grounding changes relay philosophy and sometimes the equipment.
• Ratings — rated maximum voltage and BIL, main bus continuous amps, breaker frame amps, short-circuit kA with the X/R basis, close-and-latch, short-time withstand duration.
• Construction — metal-clad per C37.20.2 or metal-enclosed per C37.20.3, stated explicitly. Arc-resistant type if required.
• Arrangement — number of sections and the one-line: main-tie-main, main-only, feeder count, bus-transition or auxiliary sections, future spaces.
• Protection and metering — relay make and model, CT and PT ratios and accuracy class, metering points, and the communications protocol you expect to poll over.
• Control power — 125 VDC, 48 VDC, or 120 VAC, plus battery and charger scope.
• Environment — indoor NEMA 1 or outdoor NEMA 3R / walk-in, altitude and ambient temperature derating, seismic or IBC requirements.
• Physical — cable entry top or bottom, shipping split limits, and the door swing and aisle depth the room can actually accommodate.

For used and surplus line-ups add the condition evidence you expect: operation counts, contact wear, insulation resistance and power-factor test records, relay firmware, and whether the assembly retains its original nameplate ratings. Metal-clad line-ups in 15 kV and 38 kV classes are available through Pantheon in the electrical catalog; the unit substation page covers how these sections combine with a transformer into one assembly.