Why does an internal arcing fault put operators at risk?
When a phase-to-phase or phase-to-ground fault develops inside a medium-voltage enclosure, the air in the gap ionizes and a sustained arc forms. Within milliseconds it heats the surrounding air to tens of thousands of degrees, vaporizes copper, and drives a pressure wave and a jet of molten metal and plasma out of the weakest opening — often a door, a viewing window, or a cable-gland plate facing a worker. Standard metal-clad and metal-enclosed switchgear is built to interrupt fault current and to segregate live parts, but its compartment barriers are not, on their own, designed to steer that blast away from a person standing at the front of the lineup.
The hazard is quantified as incident energy, the thermal energy a worker at a given distance would absorb, expressed in calories per square centimetre. NFPA 70E defines the arc flash boundary as the distance at which incident energy reaches 1.2 cal/cm², the approximate threshold for the onset of a second-degree burn on unprotected skin. On a large medium-voltage bus the incident energy at working distance can be many times that value, which is why the way an enclosure fails matters as much as whether it clears the fault.
How does arc-resistant switchgear change the outcome?
Arc-resistant switchgear is engineered so that, during an internal arcing fault, the pressure and hot gas are contained and then vented along a controlled path — typically upward through a plenum or duct — instead of blowing out toward the operator. Reinforced doors, latches, and barriers hold the compartment closed while relief flaps and the exhaust channel release the energy above head height or outside the room. The design goal is not to prevent the fault but to protect anyone standing beside a closed, latched lineup while it runs.
Accessibility types tell you which sides are protected
The performance is graded by accessibility type. Under the North American test guide, Type 1 construction is arc resistant at the front of the equipment only, while Type 2 is arc resistant around the entire perimeter — front, sides, and rear. A suffix such as 2B extends the rating to the condition where a control or instrument compartment door is open. The distinction is practical: a unit set against a wall with only front access may be served by Type 1, whereas a free-standing lineup that crews can walk around generally calls for Type 2.
What does an IEEE C37.20.7 rating actually mean?
The governing reference is IEEE C37.20.7, the Guide for Testing Switchgear Rated Up to 52 kV for Internal Arcing Faults. It defines how a representative enclosure is subjected to a prospective fault current for a set duration, and how indicators mounted around the unit are used to judge whether escaping gas or flame would have injured a person. A rating is meaningful only for the tested current, duration, and accessibility type, and only when doors and covers are secured exactly as they were during the test.
Two limits are worth understanding. First, the guide applies to arcing faults occurring in air inside the enclosure with the unit properly closed; it does not cover a door left open or a fault inside a sealed component such as an instrument transformer. Second, an IEEE C37.20.7 rating is a test result for internal arcing performance — it is separate from the product safety listing and from the site NFPA 70E arc-flash risk assessment, which is still performed using the IEEE 1584 incident-energy model to set boundaries and select PPE. Arc-resistant construction reduces the consequence of a fault; it does not remove the need for a study, or for rated PPE when equipment is opened or worked energized.
Where does arc-resistant switchgear make sense?
The value is highest where people work close to high-energy medium-voltage gear and where an incident would also take critical load offline. Data centers, where operators switch live buses beside continuously loaded equipment, are a common case, as are utility substations and heavy-industry plants. Where calculated incident energy is low, or the gear is operated remotely from behind a barrier, standard construction with disciplined work practices may be sufficient. The decision should follow the arc-flash study rather than habit, because arc-resistant construction adds cost, footprint, and an exhaust-routing constraint that only pays off where the exposure is real.
What should a buyer specify?
State the accessibility type (1 or 2, plus any suffix) matched to how crews will access the lineup, and the prospective fault current and clearing time the rating must cover — those figures must align with the upstream protection and the incident-energy study, since a longer clearing time raises both the arc rating needed and the calculated incident energy. Confirm the exhaust path: an internally vented lineup needs room height or a plenum route, and an indoor install needs a room that can absorb the pressure. Specify the governing documents as context — designed and built to IEEE C37.20.7, coordinated with NFPA 70E and IEEE 1584 for the site study, and installed to the applicable NEC/CEC rules. For the product listing, require the mark your authority having jurisdiction expects.
Entogo builds low-voltage switchgear and motor control centers and gas-insulated switchgear on its own vertically integrated line, designed and built to IEEE C37.20.7; UL (cULus)/CSA certifiable on request. In-house engineering can match accessibility type, fault rating, and venting to a specific room and study, and align the switchgear with the transformers and substations feeding it — useful when a project cannot absorb the market’s long equipment waits. For a specification review, contact the engineering team.
