Introduction

In North America, medium-voltage metal-clad switchgear is governed primarily by the ANSI/IEEE C37.20.2 standard. Under this standard, internal arc fault capability is considered an optional requirement — statistical data shows that approximately 60% of procurement specifications call for non-arc-rated enclosures with NEMA 1 enclosure ratings. However, in certain industrial sectors — including petrochemical, data center, and critical infrastructure applications — buyers increasingly require switchgear to also meet the internal arc fault classification requirements of IEEE C37.20.7.

This article examines the design principles, classification system, pressure relief technologies, and key engineering challenges associated with arc-resistant metal-clad switchgear built to North American standards.

1. IEEE C37.20.7: Arc Fault Classification Overview

The ANSI/IEEE C37.20.7 standard is the primary North American test standard for internal arc fault withstand capability in metal-enclosed switchgear. When specified by the end user, it applies to a broad range of equipment including:

• Low-voltage power distribution switchgear

• Motor control centers (MCCs) at low voltage

• Medium-voltage metal-clad switchgear

• Medium-voltage motor control centers

Arc Fault Type Classification

The standard defines two fundamental arc-resistance types:

IEEE C37.20.7 additional requirements:

• Indicator distance from switchgear: 100 mm

• Arc duration: 0.5 seconds

Comparison with IEC: IEC-based standards (e.g., IEC 62271-200) typically require IAC A FLR rating for authorized personnel, with indicators placed at 300 mm from the equipment — a significantly more conservative test setup than the ANSI requirement.

Type 2b — The Most Demanding Classification

The "b" suffix in Type 2b represents a more stringent test condition: during the internal arc fault test, the low-voltage compartment (instrument/relay) door is left open. The equipment must still satisfy all arc-resistant performance criteria under this condition.

This simulates a realistic operational scenario where maintenance personnel may have the instrument door open while working on relay settings or metering — one of the highest-risk activities in a substation environment. Satisfying Type 2b requirements is considerably more difficult to achieve than standard Type 2, particularly for double-tier circuit breaker lineups.

2. Pressure Relief Design Strategies

The fundamental design challenge in arc-resistant switchgear is managing the rapid pressure rise caused by arc-generated plasma and hot gas during a fault event. Different manufacturers have developed distinct approaches to this problem.

2.1 ABB SafeGear Three-Way Valve Flap System

ABB holds multiple patents in the arc fault protection space. One of its most notable innovations is the three-way pressure relief valve flap system deployed in its SafeGear lineup. The system operates as follows:

• The instrument compartment rear channel serves as a shared pressure relief corridor

• Pressure from the lower circuit breaker compartment and bus compartment is routed through this shared channel

• Gas and pressure are then directed through the rear of the upper instrument-only compartment

• Finally, pressure exits through a horizontal top-of-cabinet duct that vents outside the substation building

This architecture efficiently handles multi-compartment pressure relief while minimizing cross-contamination between compartments. The valve flap geometry is optimized to enable rapid depressurization while limiting the blast effect on adjacent compartments.

Key advantage: By routing all pressure through a single shared rear corridor, ABB avoids the need for individual side-mounted pressure relief channels that would compromise structural integrity and thermal ratings in high-fault-current applications.

Patent note: The three-way valve flap system and associated pressure routing architecture are covered by ABB patents. Any alternative design that mimics this routing topology must ensure freedom-to-operate through independent design differentiation.

2.2 Independent Compartment Pressure Relief

For equipment to comply with Type 2b requirements, each compartment in the switchgear lineup must vent independently. Internal arc faults in one compartment must not propagate into or damage adjacent compartments.

ABB's switchgear — including the circuit breaker compartment, bus compartment, and cable termination compartment — features dedicated pressure venting paths that direct gas to the cabinet top or designated exit points. This design satisfies the IEEE C37.20.7 requirements for internal arc fault containment and is critical for the Type 2b classification.

3. Engineering Challenges for 15/27 kV Double-Tier Lineups

3.1 Why Double-Tier Switchgear is Particularly Difficult

For 15 kV and 27 kV double-tier circuit breaker switchgear, achieving Type 2b certification is one of the most demanding tasks in MV switchgear engineering. A double-tier lineup contains six distinct compartments, and the following constraints make the design extremely challenging:

• No ventilation holes permitted between compartments

• No cross-compartment pressure pathways are allowed under the Type 2b test

• Each of the six compartments must demonstrate independent venting capability

• The lower-tier breaker compartment, bus compartment, and lower-tier cable compartment must all vent without relying on adjacent spaces

3.2 Patent Avoidance Strategies

Because the most elegant solution — using the rear instrument compartment corridor as a shared plenum — is patented by ABB, alternative designs must find architecturally differentiated approaches. Some documented strategies include:

Option A: Forward-shifted auxiliary compartment

Move the upper auxiliary/instrument compartment forward in the cabinet footprint, freeing rear space behind the lower breaker and bus compartments for dedicated pressure relief ducting. Limitation: Severely constrains primary equipment layout and is incompatible with true double-tier breaker arrangements.

Option B: Side-mounted pressure relief channels

Add bilateral pressure relief channels on each side of the cabinet structure to handle gas venting from lower compartments. Limitation: Channel cross-sectional area is typically insufficient for high fault currents (e.g., 63 kA), and lateral space is rarely available in dense substation layouts.

Option C: Arc absorption chamber with multi-compartment connectivity

A pressure absorption chamber connected to multiple or individual compartments, with a mechanism that simultaneously activates dedicated flaps for specific compartments while closing flaps connected to other spaces. This Type 2b-compliant design pattern mirrors the functional intent of the ABB system without duplicating the patented valve geometry.

4. Instrument Protection in Arc-Rated Lineups

When the dedicated instrument compartment is too small to accommodate all required secondary devices (protection relays, meters, transducers), the instruments must be relocated to the circuit breaker draw-out compartment door. In this configuration, protective cover panels must be designed and installed over the instrument mounting area on the breaker door. These panels serve two critical functions:

• Blast protection: Prevent arc-generated plasma and pressure waves from damaging sensitive electronic components during a fault event

• Thermal shielding: Protect relay logic and wiring from radiated heat

5. Standards Comparison: IEEE C37.20.7 vs IEC 62271-200

6. Conclusion

Arc-resistant metal-clad switchgear designed to ANSI/IEEE C37.20.7 represents a significant engineering challenge — particularly when Type 2b performance is required for 15/27 kV double-tier configurations. Key design considerations include:

• Selecting an appropriate arc type classification based on application requirements

• Implementing a pressure relief architecture that achieves independent compartment venting without infringing on existing patents

• Ensuring proper blast protection for instrument and control components

• Carefully managing the structural geometry constraints of double-tier configurations

As industrial facilities continue to prioritize personnel safety and equipment availability, arc-resistant switchgear will remain a growing segment of the North American MV market — driving continued innovation in pressure management and enclosure engineering.

Related topics: Medium-voltage switchgear specifications | IEEE C37.20.2 | NEMA enclosure ratings | arc flash hazard analysis (IEEE 1584) | motor control center design