Internal arc faults are rare, but when one occurs inside a metal-enclosed switchgear cubicle the energy is released in milliseconds. We do not certify arc performance — the assembly is tested and classified as a complete unit. What we control is the geometry, joints, venting path and stiffness that decide whether a cubicle relieves pressure in a designed direction or fails at its weakest seam. Here is how we approach arc pressure, relief design and compartmentalisation when building to your drawings.
Key takeaways
- An internal arc fault develops in three stages — compression, expansion and thermal — each loading the enclosure differently.
- Most cubicle failures are structural: bulged side panels, cracked welds, doors released at peak pressure.
- Pressure relief must be directional: hot gas goes to a designated exhaust outlet, never to the operator face.
- Stiffness comes from folded profiles, continuous welds and compartment separation, not thicker flat plate.
- IAC classification belongs to the tested complete assembly; a sheet metal shop builds to drawings and cannot grant it.
The three stages of an internal arc fault
We treat an internal arc fault as a pressure event first, because pressure is what breaks the structure.
- Compression stage. The arc ionises and heats the air inside the compartment almost instantaneously. With the volume initially fixed, pressure rises sharply and loads every panel, seam and fastener at once.
- Expansion stage. Copper and aluminium parts melt and vaporise as the arc continues. The added vapour keeps increasing the hot gas volume, so the gas seeks the path of least resistance — a door gap, a vent, or the weakest weld.
- Thermal stage. Radiation and hot gas keep acting on the walls until protection clears the fault. The thermal load has already softened the material locally — which is why stiffness and relief must be designed together.
How cubicles actually fail
Failure rarely starts with plate thickness. It starts with a detail.
- Side panel bulging and weld cracking. Flat panels with unsupported spans deform plastically under the pressure peak; once the panel moves, its perimeter welds load in peel rather than shear and a seam that looked sound in the workshop opens.
- Door hinges and locks releasing at peak pressure. A door is only as strong as its latching system. If hinge leaves, pins and lock points are not rated for the separating force, the door becomes a projectile and gas discharges towards the operating position.
- Secondary damage in the substation. Gas escaping through an uncontrolled path can pressurise adjacent cubicles, damage cabling and instrumentation, and reach personnel areas.
Pressure relief: give the gas a designed way out
Our job in relief design is to make the designed path the easiest path.
- Directional exhaust ducts. Hot gas is guided through a defined channel to a designated outlet — usually the top or rear — instead of escaping at the front operating face.
- Pressure relief flaps. A flap stays closed in normal service against dust and vermin and opens when its set condition is reached, lowering the pressure load on walls, doors and frame joints.
- Fast response. The flap and its mounting must react within the compression stage; a sticking mechanism transfers the full pressure peak into the structure.
- Internal deflectors. Baffles steer gas away from sensitive compartments and busbar joints, so a fault in one compartment does not become a fault in the next.
Relief only works if the surrounding structure does not deform first, so stiffness and relief are never separate packages: a vented box without stiffness deforms before the vent acts, and a stiff box without a vent fails at a higher pressure.
Materials and hardware that hold up over time
- Galvanised and stainless steel parts. Corrosion protection preserves section thickness and joint integrity across the service life.
- Laser-cut relief parts and specified hinges. Laser cutting gives repeatable slot and flap geometry, so relief devices behave consistently from unit to unit; calibrated hinges and latches remove generic-hardware variability.
- Consistent joint preparation. Fit-up controls the gap before welding, and stable penetration keeps a seam from becoming the origin of a failure. Insulation dimensions also stay exactly as drawn — see our guide to clearance, creepage and insulation coordination.
What IEC 62271-200 and IEEE C37.20.7 mean for a sheet metal supplier
IEC 62271-200 covers metal-enclosed high-voltage switchgear and includes the internal arc classification (IAC) concept, with accessibility types for different sides of the assembly. IEEE C37.20.7 is the guide for testing switchgear rated for internal arcing faults. Neither is a rating a sheet metal supplier can issue for a part.
We state the boundary clearly: IAC classification is the result of a type test on the complete assembly. It belongs to the switchgear maker's tested design. We build enclosures to the customer's drawings — vent geometry, reinforcement and compartment arrangement as specified, held to tolerances that let a tested design be reproduced — but we cannot and do not assign an IAC rating. Arc-related claims come from testing, not from calculation or material choice.
Stiffness and compartmentalisation
Our stiffness strategy here is structural, not simply thicker.
- Multi-bend folded profiles instead of heavy flat plate. A folded member carries load in several planes and controls deflection over a large span better than adding millimetres to a sheet.
- Heavy-duty continuous welds. Continuous seams distribute the pressure load along the joint instead of concentrating it at the ends of intermittent welds.
- Compartment separation. Keeping the busbar, breaker and cable compartments as distinct volumes limits fault spread and keeps ionised gas away from adjacent parts.
These points are checked in our DFM review before any panel is cut. Our sheet metal welding guide explains how we specify structural welds, and our industrial control panel enclosure guide covers large-span reinforcement.
| Failure mode | Cause | Countermeasure |
|---|---|---|
| Side panel bulging | Unsupported flat span | Folded frame members, reinforcement |
| Weld cracking at panel edge | Peel load after deflection | Continuous welds, controlled fit-up |
| Door released at peak pressure | Hinges or latches under-rated | Heavy-duty hinges, multi-point latching |
| Gas discharged at operator face | No defined exhaust path | Directional duct to a designated outlet |
| Fault spreading between compartments | Shared volume | Segregated busbar, breaker, cable compartments |
| Relief device opening inconsistently | Slot geometry variance, friction | Laser-cut relief parts, free-moving flap |
Build the enclosure your tested design depends on
Send us the drawings and structural requirements for your switchgear enclosure. We manufacture to your design, review manufacturability before tooling, and produce frames, panels, doors and relief features on laser cutting and CNC bending equipment so every unit matches the one you tested. Free CAD support comes with the first article, and we flag any dimension we cannot hold rather than changing it silently.