Arc-Resistant Switchgear: IEEE C37.20.7 Types and Suffixes

An internal arcing fault produces hot gases and pressure inside a switchgear enclosure. Arc-resistant construction is tested to direct the effects away from specified accessible locations while the equipment is installed and operated in its rated configuration. IEEE C37.20.7-2024 covers test and evaluation procedures for switchgear rated up to 52 kV and discusses service conditions, installation, and application. The designation is a test result for an assembly, not a general promise about every fault or every task.

The useful question is where a person may be relative to the installed gear during normal operation. A lineup with a freely accessible rear aisle has a different exposure than one installed against a wall. An operator opening a low-voltage control compartment is doing a different task from someone opening an MV cable compartment. The tested type and suffix describe some of these cases, but the instruction manual still has to define required door positions and exhaust routing.

A product label should state the test standard, accessibility designation, fault-current level, and arc duration. Eaton’s arc-resistant switchgear FAQ describes a separate label for the arc rating and another for the operating conditions that preserve it. A label saying only “arc resistant” omits the conditions needed to compare it with the project’s available fault current and layout.

Construction differs from clearing. IEEE’s metal-clad assembly standard describes a switchgear construction category, while C37.20.7 addresses internal-arc testing. The enclosure can redirect fault products without making the fault disappear. The protective devices must still trip, and their clearing time influences the duration of the event. OSHA’s arc-flash publication explains that protective-device operation and maintenance affect incident-energy estimates. A building design should consider both the assembly test and the site’s fault-clearing study.

Type 1 addresses the front, where a person typically operates the switchgear. Type 2 addresses the freely accessible exterior around the front, rear, and sides. Eaton’s medium-voltage fundamentals and its Power Xpert IGX design guide list those accessibility meanings. Type 2 includes Type 1’s front case, but the exact installation and accessible surfaces must match the tested configuration.

Imagine a lineup with a rear cable aisle. A worker may be at the back when an internal fault starts in another compartment. A front-only rating would not establish the same tested exterior protection there. If the room gives access to the rear and sides, the specification should name the needed perimeter performance. If the lineup is against a wall, the designer still needs to account for exhaust and the inaccessible face; it should not assume a front-only label solves all room hazards.

Type 2 does not make a door-open task equivalent to a door-closed one. The base exterior rating is evaluated for the stated normal operating condition. When an instrument or control door will be opened during work, ask whether suffix B applies. When adjacent compartments must be protected from an internal fault’s effects, ask about suffix C. These additions describe different test outcomes rather than “more voltage.”

For comparison, request the manufacturer’s test report or clear test summary rather than simply “Type 2.” Ask which compartments were fault-initiated, which surfaces were accessible, what covers and doors were secured, and how an exhaust duct was fitted. A test claim copied from one lineup configuration may not transfer to a different bus transition or end section.

B extends the accessibility claim for the specified low-voltage instrument or control compartment door open in normal operating conditions. C addresses protection between adjacent compartments within the assembly. BC combines the B and C claims where the tested product carries both. Eaton describes Type 2B and Type 2C in its arc-resistant guide; ABB’s SafeGear FAQ describes Type 2BC as its Type 2C configuration with the low-voltage isolation feature. The suffixes should be checked against the actual product test and standard edition.

D is different. Eaton explains D as a special-application designation for equipment installed where some exterior surfaces are inaccessible and a Type 2 design is not needed. Eaton’s definition of suffix D does not describe a door between compartments. A project that asks for Type 2D because it wants a compartment barrier has confused two separate issues. If the concern is adjacent compartments, investigate C and the product test report.

A Type 2B line is a useful example. Under the conditions in Eaton’s IGX guide, the Type 2 perimeter behavior remains with the identified control compartment door open. It does not mean an MV cable or breaker door may be open. A worker who opens the wrong compartment has changed the configuration beyond the stated B case.

Record the entire designation in the equipment schedule. “Type 2BC” conveys a different set of evaluated locations than “Type 2B,” while neither suffix alone states the arc current or duration. If the design depends on a particular operating task, write that task and door state in the specification and require the submittal to show the test covers it.

Do not assume every arc-resistant label means a half-second test. The rating is the duration on the offered product’s test record at its stated current. Eaton’s low-voltage arc-resistant FAQ gives a particular product’s 0.5-second label example; an ABB medium-voltage application table lists some transition configurations at 0.5 second and others at 10 cycles. The products and editions are not interchangeable, so a project must compare its calculated clearing time to the offered assembly’s tested time.

Current also matters. A lineup tested at one internal-arc current cannot simply be assumed to withstand a higher available fault current. Request the test value and compare it with the site’s available short-circuit current at the assembly. The study should account for the source configuration in which the fault is greatest, such as a closed tie or a generator contribution, if those operating modes are possible.

A shorter protective-device clearing time can reduce the energy released before the fault is isolated. But a relaying promise needs an end-to-end check: sensing, logic, trip circuit, breaker opening, and backup protection. OSHA’s arc-flash guidance notes that if a protective device fails to operate as intended, the event can last longer than an estimate assumed. Preventive maintenance and relay testing are therefore part of applying the rating.

Do not turn the tested duration into a “safe work time.” The test describes how the enclosure behaved during an internal fault under laboratory conditions. It does not permit a worker to remain near an exposed energized conductor for that interval. Electrical work practices and PPE depend on the task and site hazard assessment.

Arc-resistant switchgear often redirects hot gas upward or into a duct or plenum. Eaton’s medium-voltage fundamentals describes venting through a plenum to a location where it can be released away from personnel. The receiving building must provide a compatible route. If the ceiling is too low or the duct turns into an occupied area, an electrically correct lineup may not have a suitable installation.

Treat the exhaust path as a room-interface drawing. Ask for minimum overhead clearance, plenum dimensions, duct length and turns permitted by the manufacturer, termination location, and whether an external wall or roof penetration is required. Also check the lineup end sections and any bus transition: a transition that changes the pressure path can invalidate an assumed layout. The arc-test designation belongs on the installed assembly, not just on freestanding cubicles in an OEM photograph.

Consider a data-hall electrical room with an overhead cable tray. If the gear exhausts upward, the tray may obstruct or occupy the discharge path. Moving the tray can be easier before procurement than after the equipment arrives. If the manufacturer requires a duct outdoors, allocate that route before structural and fire-stopping details are frozen. The exact solution depends on the tested OEM configuration.

Room access matters too. A Type 2 lineup may be reachable from its rear and sides, but those aisles still need maintenance space and an exit route. Do not trade away service access to make a drawing appear to fit. The OEM installation manual and site safety review should settle the final room arrangement.

It changes a tested enclosure behavior; it does not issue a blanket PPE exemption. OSHA’s arc-flash guide says the hazard assessment considers the task, likelihood, severity, and controls. OSHA’s interpretation on breaker operation also explains that arc risk can exist even when a breaker is inside an approved enclosure and that the appropriate PPE depends on site-specific factors. The equipment rating and the workplace decision are related but separate.

A closed, secured lineup during ordinary operation is the case many arc-resistant designs target. Opening an MV cable compartment, removing a breaker, or changing a cover can alter the boundary. A B suffix applies only to the specified control or instrumentation door, not to every door. The owner should put the actual task beside the OEM’s tested condition before changing procedures or labels.

The short-circuit study and arc-flash assessment must reflect the operating modes. A tie closed during maintenance may increase available fault current. A protection maintenance mode may shorten clearing time if it is designed and enabled correctly. Neither condition can be inferred from the arc-resistant nameplate. OSHA warns that changed systems and poorly maintained protective devices can make earlier incident-energy calculations inaccurate.

Separate prevention, clearing, and containment. Good installation and maintenance reduce the likelihood of a fault; relays and breakers clear it; an arc-resistant enclosure redirects the effects during its tested interval. A single feature cannot substitute for the other two. The metal-clad versus metal-enclosed guide addresses assembly construction, which is a different selection axis.

State the nominal system voltage, equipment class and BIL, available fault current at the lineup, required arc current and duration, accessibility type and suffix, and the operating tasks the designation must cover. Include the one-line and a room drawing showing accessible sides, rear aisle, ceiling, and proposed exhaust route. Ask the vendor to identify the tested configuration and any conditions for maintaining the rating.

Include every physical variation that might affect an arc path: bus transitions, end sections, future extension sections, transformer throat, outdoor housing, and cable-entry arrangement. Ask for installation instructions and the label language that will appear on the actual equipment. IEEE C37.20.7-2024 includes installation and application in its scope; the product submittal supplies the design-specific conditions.

Pantheon’s electrical catalog can start a quote request for an arc-rated lineup. Send the ratings, accessibility type and suffix, and room plan so we can check which tested configurations can be sourced. Have the engineer compare an offered test report and installation instructions with the site’s fault and hazard studies.

Reading D as a compartment-door suffix. D addresses a special installation with inaccessible exterior surfaces. C concerns adjacent compartments; B concerns the stated low-voltage door-open case.

Assuming 0.5 second without checking the label. Product configurations can carry different test durations. Compare the offered current and time with the site study.

Specifying a type but no exhaust route. The tested pressure path must have a compatible room or outdoor termination.

Using an arc designation as a work permit. Opening a door outside the tested condition changes the case. Task-specific electrical safety rules and PPE decisions still apply.