GuideSwitchgear and motor control8 min read
Metal-clad vs metal-enclosed switchgear
Metal-clad switchgear and metal-enclosed switchgear are both medium-voltage lineups in steel enclosures, and the difference is in the construction the standards require. Metal-clad (IEEE C37.20.2) has draw-out circuit breakers, with the breaker, the bus, the cable terminations and the controls each in its own grounded metal compartment, automatic shutters over the live stabs and insulated bus. Metal-enclosed (IEEE C37.20.3) puts fixed-mounted load-interrupter switches and power fuses, or fixed breakers, in a common enclosure without the compartmenting. Metal-clad is for a substation that must be worked on live and coordinated; metal-enclosed is for switching and fusing a feeder or a transformer at a fraction of the cost.
The short version
- Every metal-clad lineup is metal-enclosed; the reverse isn’t true. Metal-clad is the stricter construction.
- Metal-clad (C37.20.2): draw-out breakers, compartments with grounded barriers, shutters, insulated bus, interlocks; 5 to 38 kV, 1,200 to 4,000 A.
- Metal-enclosed interrupter switchgear (C37.20.3): fixed load-interrupter switches with fuses, viewing windows, key interlocks; bare or insulated bus; for feeders, transformer primaries and campus loops.
- Pad-mounted switchgear (S&C PMH and PME, Eaton VFI units) is metal-enclosed gear in a tamper-resistant outdoor cabinet; vault and submersible gear likewise.
- Low-voltage switchgear is “metal-enclosed” by definition (C37.20.1); the term metal-clad is only used above 1,000 V.
- Used: metal-clad breakers from the major makers are serviceable for decades; metal-enclosed gear is checked for its switch, fuses and bus condition and is often the better value for a simple job.
The difference in one table
| Metal-clad | Metal-enclosed (interrupter switchgear) | |
|---|---|---|
| Standard | IEEE C37.20.2 | IEEE C37.20.3; pad-mounted C37.74 |
| Switching device | Draw-out circuit breakers (vacuum; SF6 or air-magnetic on older gear) | Fixed load-interrupter switches with power fuses; or fixed breakers; vacuum fault interrupters in pad-mounted gear |
| Protection | Protective relays tripping the breakers; coordinated | Fuses (expulsion or current-limiting); sometimes a relay on a fixed breaker |
| Compartments | Breaker, bus, cable and control each in a grounded metal compartment | A common enclosure, with barriers where the maker adds them |
| Shutters | Automatic, closing over the stabs when a breaker is withdrawn | None; the switch is fixed |
| Bus | Insulated | Often bare; insulated as an option |
| Interlocks | Racking, door and breaker interlocks required; key interlocks added | Key interlocks on the switch and fuse door; viewing window to confirm blade position |
| Voltage and current | 5, 15, 27 and 38 kV; 1,200 to 4,000 A | 5 to 38 kV; 600 and 1,200 A switches |
| Serviceability | Breaker withdrawn and serviced with the bus live | An outage to change fuses safely; the switch is maintained in place |
| Cost and size | High; deep sections, rear access | A third to a half of metal-clad; shallow; front access |
| Where | Main substations, utilities, plants with motors and generators | Transformer primaries, loops, feeders, small substations |
What makes switchgear metal-clad
C37.20.2 lists the features, and each exists to let someone work on the gear with the rest of it live.
- Draw-out breakers
- The breaker sits on a carriage and connects through primary and secondary disconnects; racking it out isolates it and lets it be removed for service or swapped for a spare without touching the bus (the vacuum breaker guide).
- Compartments
- The breaker cell, the main bus, the cable termination and the low-voltage control compartment are separated by grounded steel, so a fault in one is contained and a technician in the cable compartment is separated from the bus.
- Shutters
- When the breaker is withdrawn, metal shutters close over the stationary stabs automatically, covering the live bus.
- Insulated bus
- The main bus and connections are covered with insulation, so that a dropped tool or an animal can’t bridge phases and a fault doesn’t propagate along the bus.
- Interlocks
- The breaker can’t be racked in or out while closed; the cell door and the breaker position are interlocked; the control circuit disconnects before the primaries. Key interlocks are added on top for sequences between cells and other equipment.
- Relaying
- Protective relays on the cell door (overcurrent, differential, ground fault), with instrument transformers in the gear, trip the breakers. Coordination between breakers is what lets a fault take out one feeder and not the substation.
Lineups from Eaton (VacClad-W), ABB (SafeGear, ADVAC), Schneider (Masterclad), Siemens (GM-SG), GE (PowerVac) and Powell share this construction and differ in breaker design, dimensions and the relays fitted. A vintage lineup with air-magnetic breakers (GE Magneblast, Westinghouse DHP, Allis-Chalmers) is still metal-clad, and its breakers are commonly replaced by roll-in vacuum retrofits.
What metal-enclosed switchgear is
Metal-enclosed interrupter switchgear does the two things a feeder or a transformer primary needs, switching and fault protection, with the two simplest devices that do them. The load-interrupter switch is a three-pole air switch with arc chutes, rated to open and close its full load current (600 or 1,200 A) and to close onto a fault but not to interrupt one; a viewing window lets the operator see the blades open. The power fuses below it, expulsion or current-limiting, clear faults. Key interlocks make the fuse door open only with the switch open, and the switch close only with the door shut.
Everything sits in one enclosure, front-accessible, often with bare bus, in sections of one switch and fuse set each, or a switch, a bus and several fused ways. Makers: S&C (Metal-Enclosed Switchgear), Eaton (MVS and MEF), Powell, Federal Pacific, Siemens. The gear is also built with a fixed-mounted vacuum breaker or fault interrupter in place of switch and fuses, which adds relaying without adding the compartments, and the standard still calls that metal-enclosed.
Pad-mounted, vault and arc-resistant gear
- Pad-mounted switchgear
- Metal-enclosed gear in a padlockable outdoor cabinet with the cables entering from below through elbows, built to IEEE C37.74. S&C’s PMH (fused ways) and PME (with fault interrupters), and Eaton’s VFI and Cooper VFI units, loop and fuse underground campus and subdivision feeders. The site’s switchgear listings include them.
- Vault and submersible gear
- The same functions in a stainless, submersible tank for manholes and vaults, with elbow-connected ways; S&C Vista is the common one.
- Arc-resistant switchgear
- Either class, built and type-tested to IEEE C37.20.7 so that an internal arc is contained and vented away from the operator; Type 2 protects on all sides, Type 2B with the low-voltage compartment door open. Specified for data centers and plants where people stand in front of the gear.
- Low-voltage switchgear
- Below 1,000 V the compartmented, draw-out construction is called metal-enclosed low-voltage power circuit breaker switchgear (C37.20.1); the switchgear vs switchboard guide covers that side.
Which one a job needs
- Metal-clad for a main substation with several feeders that must coordinate, for any bus with generators or large motors, for anything operated often, and for anywhere a breaker has to be serviced without taking the plant down.
- Metal-enclosed for the primary of one transformer, for sectionalizing and tapping a loop, for a feeder to a building that can wait for a fuse change, and for a budget that won’t stretch to breakers and relays.
- Pad-mounted where the gear lives outdoors on a campus or a subdivision, on a pad with underground cable.
- Both in most plants: metal-clad at the substation, metal-enclosed at each outlying transformer, which is why the Kirk key guide’s first example is a feeder breaker and a transformer’s switch compartment.
Buying either used
The questions are the same as for any switchgear (voltage class, bus rating, short-circuit rating, sections and cable entry), plus what the construction class adds.
- Metal-clad
- The breakers are the value: their make, type, interrupting rating and condition, whether they’re vacuum or air-magnetic, and whether parts and roll-in replacements exist (the vacuum breaker guide). Then the relays (electromechanical relays are replaced; microprocessor relays may be kept), the shutters and interlocks working, the bus insulation uncracked, and a dielectric test on the bus.
- Metal-enclosed
- The switch: that it opens and closes cleanly, the arc chutes are intact, the blades aren’t pitted; the fuse type, since some fuses are obsolete and a lineup built for them is worth less; the bus for tracking and water; the key interlocks and viewing windows; the cabinet for rust on outdoor gear.
- Both
- Nameplate ratings matched to the supply’s available fault current; the lineup’s dimensions against the room; a test before shipping (insulation resistance, contact resistance, breaker timing, relay or fuse check, hi-pot), which the units we sell get.
The switchgear listings say which class each lineup is, and Magic Search reads the construction, breakers and ratings off sellers’ pages when you describe the lineup you need.
Questions people ask
What is the difference between metal-clad and metal-enclosed switchgear?
Construction. Metal-clad switchgear, built to IEEE C37.20.2, has draw-out circuit breakers with every major part in a separate grounded metal compartment, shutters that cover the live stabs when a breaker is withdrawn, insulated bus, and mechanical interlocks. Metal-enclosed switchgear, built to IEEE C37.20.3, has fixed-mounted interrupter switches with fuses, or fixed breakers, in a single enclosure without those compartments and shutters. Metal-clad is a subset of metal-enclosed: all metal-clad gear is metal-enclosed, but most metal-enclosed gear is not metal-clad.
What is metal-clad switchgear used for?
Distributing medium-voltage power (5 to 38 kV) where circuits must be protected by breakers that can be coordinated, serviced without a bus outage, and operated often: plant main substations, utility distribution substations, large campuses, data centers, mines and generating plants. A typical lineup is a main breaker, a tie and several feeders, each a draw-out vacuum breaker in its own cell with protective relays on the door.
What is metal-enclosed switchgear used for?
Switching and fusing medium-voltage circuits where a breaker and relays would be more than the job needs: the primary of a transformer, the taps of a campus loop, a feeder to a remote building, a small substation. A load-interrupter switch opens and closes the circuit under load; power fuses clear faults. It costs a fraction of metal-clad, takes up less room and needs little maintenance.
Is pad-mounted switchgear metal-clad or metal-enclosed?
Metal-enclosed. Pad-mounted switchgear such as S&C’s PMH and PME series and Eaton’s VFI units is built to IEEE C37.74 as metal-enclosed gear in a locked, tamper-resistant outdoor cabinet that sits on a pad like a transformer, with elbow connections underneath. It holds load-interrupter switches and fuses, or vacuum fault interrupters, for looping and fusing underground feeders. No pad-mounted gear is metal-clad.
Is low-voltage switchgear metal-clad?
No. Low-voltage switchgear is built to IEEE C37.20.1 and is called metal-enclosed low-voltage power circuit breaker switchgear; the term metal-clad is reserved for medium voltage. In practice low-voltage switchgear has the same features as metal-clad (draw-out breakers, compartments, shutters), which is why people say it, but the standards don’t.
What does arc-resistant switchgear mean?
Switchgear built and tested to IEEE C37.20.7 to contain an internal arcing fault and vent its gases away from where people stand, through flaps and a plenum, so that an operator at the front (Type 1) or on any side (Type 2) is protected for the rated fault current and duration. It can be metal-clad or metal-enclosed; the arc rating is an addition to the construction class, not a replacement for it.
Sources: IEEE C37.20.2 (metal-clad switchgear); IEEE C37.20.3 (metal-enclosed interrupter switchgear); IEEE C37.20.7 (arc-resistant switchgear); IEEE C37.74 (pad-mounted switchgear); IEEE C37.20.1 (low-voltage switchgear).