Arc-Resistant Switchgear: Types, Testing and Limits

What arc-resistant means

An internal arcing fault inside switchgear releases a pressure wave within the first several milliseconds, followed by superheated gas, vaporized copper and burning debris. Arc-resistant switchgear is built with reinforced structures, latched doors, pressure relief flaps and a controlled exhaust path so the energy is vented where people are not.

The performance is established by type testing under IEEE C37.20.7, which applies to metal-enclosed switchgear, both medium-voltage and low-voltage. The term arc-resistant is precise: the equipment is not arc-proof, it does not reduce the energy of the arc, and it does not make the compartments safe to open during a fault. It changes the exposure of a person standing outside the closed enclosure, and only that.

Accessibility types

IEEE C37.20.7 defines where around the assembly the protection is provided. Type 1 provides arc-resistant performance at the front only, which suits gear installed against a wall with restricted access to the sides and rear. Type 2 provides it at the front, sides and rear, so personnel anywhere around the lineup at floor level are covered.

Suffix letters extend the rating. Type 2B adds protection with the low-voltage instrument or control compartment door open, so relay work and metering checks can be performed with the primary compartments still protected. Type 2C adds protection between adjacent compartments within the assembly, meaning an arc in one compartment does not propagate to a neighboring compartment whose door is open.

• Type 1: front only
• Type 2: front, sides and rear
• Type 2B: Type 2 plus protection with the low-voltage control compartment open
• Type 2C: Type 2 plus protection between adjacent compartments

The internal arc test

The test initiates an arc with a fusible wire across the phases in each type of primary compartment, breaker, bus and cable, at a rated arcing current and for a rated duration. The arcing current is typically equal to the short-circuit rating of the switchgear, for example 40 kA or 63 kA, and the duration is commonly 0.5 seconds, although shorter durations may be specified where fast protection is credited. Cotton indicators are mounted at defined distances around the accessible surfaces.

To pass, doors and covers must stay closed and latched, no parts may be ejected, no openings may appear in the accessible surfaces, the indicators must not ignite, and the grounding connections must remain effective. The rating applies to the tested configuration. Changes to cable entry, front-mounted devices, plenum arrangement or the mounting surface can invalidate it.

Pressure relief, plenums and ducts

Each compartment has a pressure relief flap or panel, usually in the roof, that opens under the initial pressure rise and directs gas upward into a plenum running the length of the lineup. From the plenum an exhaust duct carries the gas to a discharge point, ideally outdoors through a wall or roof with a hood that keeps weather out and prevents anyone from standing at the outlet.

Where an outdoor discharge is not practical, some designs are tested to vent into the electrical room itself, which requires a minimum room volume and ceiling height so the pressure and temperature rise stay within limits. Low-voltage designs are also available with arc-quenching devices that detect the arc optically and divert it into a controlled short circuit in a few milliseconds, an approach that limits pressure rather than venting it.

What arc-resistant switchgear does not do

The rating holds only with every door and cover closed and latched, except where the B or C suffix specifically covers an open compartment. It does not lower the incident energy at the arc, so the arc-flash labels that apply to work with doors open are unchanged. Racking a breaker with the door open removes the protection unless the design is tested for closed-door racking or remote racking is used, and that is the operation most likely to start an arc.

Arcs in the cable vault, at external terminations or in the exhaust path are outside the tested boundary. NFPA 70E treats normal operation of properly installed and maintained Type 2 arc-resistant switchgear with doors closed and secured differently from conventional equipment in its likelihood tables, but the arc-flash risk assessment still governs every task, and no arc-resistant rating permits skipping lockout, testing for absence of voltage or PPE when compartments are opened.

Room design and specification

Arc-resistant switchgear imposes requirements on the building. The plenum and duct need clear space above the lineup, a penetration through the wall or roof with appropriate fire stopping, and a discharge location away from walkways, air intakes and other equipment. Floor openings, conduit entries and trench covers must be sealed as tested, because an unsealed opening is a path for hot gas.

The specification should state the accessibility type, arcing current and duration, exhaust method and required proof of testing for the supplied configuration, along with clearances for breaker removal and lifting. Arc-resistant construction pairs naturally with maintenance-mode relay settings, arc-flash detection and remote racking; each addresses a different part of the exposure, and the combination is what an arc-flash mitigation plan should describe.