Switchgear vs switchboard

The difference in one table

What makes switchgear switchgear

Switchgear is designed so that one fault, one breaker failure or one maintenance job affects one circuit and nothing else. Four things follow from that.

Power circuit breakers

Low-voltage power circuit breakers are bigger, slower and sturdier than molded-case breakers: a steel frame with an operating mechanism you can see, stored-energy closing, and a trip unit with adjustable long-time, short-time, instantaneous and ground-fault settings. They’re built to be maintained for decades rather than replaced.

Draw-out mounting

Each breaker racks into a cell and connects through primary stabs; racking it out disconnects it and closes shutters over the live bus. A breaker can be inspected, tested or swapped for a spare without an outage on the bus.

Compartments

The breaker cells, the main bus and the cable terminations are each in their own grounded steel compartment, so an arc in one doesn’t spread, and a technician in the cable compartment is separated from the bus.

The short-time rating

The bus, the connections and the breakers are rated to carry a short-circuit current for 30 cycles without damage, so the breakers can be set to wait.

Switchgear is rear-connected, so it needs clearance behind it (the NEC’s working space in 110.26 applies on both sides), and a lineup of a main, a tie and ten feeders might be 25 feet long and 5 feet deep.

What a switchboard is

A switchboard does the same job with less. Its breakers are molded-case (sealed units, replaced rather than serviced) or insulated-case (larger molded-case breakers with electronic trip units and sometimes a draw-out cradle), mounted on a common interior and connected to the bus with bolts. The cabinet is a single enclosure with sections; the breakers in a section share its air. It can be front-accessible only, which lets it sit against a wall and saves a yard of floor.

Because the breakers clear a fault in a few cycles and the bus isn’t rated to wait, a switchboard’s main and feeders trip on overlapping curves: a large fault on a feeder may open the main as well. For most buildings that’s accepted; the whole floor goes dark for the few minutes it takes to reset. A switchboard with insulated-case breakers and a short-time setting narrows the gap, but the bus still isn’t rated for it.

Short-time withstand and selective coordination

The engineering difference comes down to one number. A low-voltage power circuit breaker and the switchgear around it are rated to carry their full short-circuit current for 30 cycles, half a second, without tripping or being damaged. That lets a main breaker be set with a short-time delay: when a fault appears, it waits a few cycles to see whether a feeder breaker below it clears the fault first. If one does, the main stays closed and every other feeder keeps running. That’s selective coordination, and the NEC requires it for emergency, legally required standby and critical operations power systems (700.32, 701.32, 708.54), which is why hospitals and data centers are built with switchgear.

A molded-case breaker has no short-time rating; it trips instantaneously on a large fault, because its contacts and case can’t hold the current longer. Two molded-case breakers in series both see the fault and both may open. Coordination studies can narrow the overlap by choosing breakers with different instantaneous settings and letting the feeder’s current-limiting action work in its favor, but the main can’t be made to wait the way a power breaker can.

Medium voltage: metal-clad and metal-enclosed

Above 1,000 V the same split appears under different names. Metal-clad switchgear (IEEE C37.20.2) is the medium-voltage counterpart of low-voltage switchgear: draw-out vacuum circuit breakers, every major part in its own grounded compartment, insulated bus, shutters over the stabs, and mechanical interlocks on racking. Lineups run 5 kV to 38 kV and 1,200 to 4,000 A, and feed plants, campuses and utility substations. Metal-enclosed switchgear (C37.20.3) is the simpler kind: fixed-mounted load-interrupter switches with power fuses, or fixed breakers, in a common enclosure without the compartmenting. Pad-mounted switchgear, such as S&C’s PMH units, is metal-enclosed switchgear in a tamper-resistant outdoor cabinet, used to loop and fuse a campus’s medium-voltage feeders.

The buying questions are the same as at low voltage: the bus rating, the short-circuit rating, whether the breakers draw out, and what the interlocks are. Our Kirk key guide covers the last.

Cost, footprint and maintenance

New, a switchgear lineup costs on the order of two to three times a switchboard of the same ampacity, takes about twice the floor area once rear access is counted, and weighs more. Used, the gap narrows, because switchgear from the major makers (Square D and Schneider Masterpact, Eaton Magnum and Cutler-Hammer DS, GE AKR and WavePro, Siemens WL and ABB Emax) is built to be reconditioned: the breakers are serviced, the trip units updated or replaced, the bus inspected and insulation-tested, and the lineup carries on. Surplus switchgear in good condition sells for a fraction of new, and often ships in weeks where new is a year out.

Maintenance runs the other way. Switchgear expects it: breaker lubrication and exercise, contact resistance, insulation resistance, trip-unit testing with primary or secondary injection, on a three-to-five-year cycle (NETA MTS). A switchboard gets an infrared scan and a check of connections, and its breakers are replaced when they fail or when the trip units become obsolete.

Buying used switchgear or a switchboard

Five things settle whether a used lineup fits, before anything about its condition:

• Voltage and bus rating. 480Y/277 V (or 208Y/120, 600 V) and a main bus at or above the transformer’s full-load amps; a 2,500 kVA transformer at 480 V needs 3,000 A or more (kVA to amps chart).
• Short-circuit rating. The gear’s rating must exceed the available fault current at its terminals, which your utility and transformer impedance set; 65 kA covers most 480 V services, 100 kA the big ones.
• Breaker frames and trip units. Which frames fill the cells (800, 1,600, 3,200, 4,000 A) and whether the trip units are electronic and current (Masterpact Micrologic, Magnum Digitrip, AKR MicroVersaTrip) or 1980s solid-state units that are no longer supported.
• Sections and spare cells. A main, a tie, the feeders you need and a couple of spare cells, in a lineup that fits the room, with the incoming cable or bus duct entry where yours arrives.
• Age and makers. Breakers and parts are still available for the lines above; gear from a maker that has left the market is harder to keep.

On condition, what matters is what we test before shipping: insulation resistance of the bus and each breaker, contact resistance, breaker timing and operation, protective relay or trip-unit testing, and a dielectric withstand test on the bus. The switchgear listings show each unit’s test status, and a listing that has been through the tests says so.

Which one you need

Switchgear

A hospital, a data center, a process plant, a utility, a building with a legally required standby system that must coordinate, any service over about 3,000 A, or any site where an hour of lost production costs more than the price difference.

Switchboard

An office, a store, a school, a warehouse, light industrial: services up to 3,000 or 4,000 A where a trip is a nuisance rather than a loss, and the budget and the electrical room are tight.

Either, with care

Mid-size manufacturing. Often switchgear for the main and the motor control center feeds, and switchboards or panelboards beyond them.

Tell Magic Search the voltage, main amps, fault rating and the breakers you want, and it finds the used lineups that fit, switchgear and switchboard, and says which is which.