Phase Arrangement Standards

According to IEEE C37.20.1, the phase arrangement of assembled busbars and primary connections in low-voltage switchgear must follow a standardized sequence. Viewed from the front, the order should be 1, 2, 3 (or A, B, C), organized front-to-back, top-to-bottom, or left-to-right. Some equipment configurations require neutral conductors or alternative phasing, in which case the orientation must be clearly identified to ensure operational consistency and safety.

Cable Termination and Connections

Switchgear must provide adequate space for both mechanical and electrical cable terminations. Each connection point must conform to NEMA CC1-1993 bolt-hole specifications, guaranteeing uniformity and structural reliability. Bus joints, when joined using bolts, nuts, and washers, must be designed to protect insulation integrity. Grounding connections must allow circuits to be safely disconnected without interrupting the ground bus. All removable elements require grounding continuity until the device is fully isolated and safely withdrawn.

Grounding of Equipment and Doors

All enclosures—whether for meters, transformers, relays, or control instruments—must be securely grounded through metallic mounting hardware capable of penetrating paint coatings. Doors mounted with electrical devices must be bonded to the main structure using at least No. 14 AWG wire or equivalent bonding conductors. Wiring passing through hinges must be flexible, strain-relieved, and adequately protected against abrasion or sharp edges. Conductors larger than 14 AWG must use Class C or D stranding for durability.

Wiring Protection and Standards

Conductors routed through barriers or enclosures require bushings, grommets, or sleeves to prevent insulation damage. Wires must be adequately supported, with ratings of 600 V and 90°C, flame-retardant, and compliant with NEMA WC70-1999/ICEA S-95-658. SIS-type wire listed in NFPA 70 (NEC) 2002 is preferred, though equivalent compliant types are acceptable.

Control Power Transformers and Safety Devices

Switches, fuse pullouts, and molded-case circuit breakers used to connect equipment to main circuits must employ “dead-front” designs, preventing direct exposure to energized components. For outdoor switchgear, lighting fixtures should allow safe replacement without de-energizing the main system. Convenience outlets must be grounded, two-pole, three-wire types, and protected by ground-fault circuit interrupters (GFCIs).

Ventilation, Exhaust, and Service Disconnects

Pressure relief and ventilation must be positioned to ensure gases released during normal or fault conditions do not endanger personnel. Components designated as service disconnecting means must comply with NEC Article 230, ensuring safe integration into building power systems.

Materials and Structural Requirements

Low-voltage switchgear must use adequately supported sheet metal. Each breaker must be housed in a dedicated ventilated compartment, with barriers preventing gases from one compartment affecting another. Where bus tie breakers are present, barriers must isolate each bus segment. Sheet thicknesses must meet or exceed MSG No. 11 (3.0 mm) for barriers and MSG No. 14 (1.9 mm) for covers and doors. Alternative metals, such as aluminum, require increased thickness to maintain equivalent mechanical strength.

Accessibility, Barriers, and Insulation

Hinged doors cover breaker compartments, while removable plates provide access to bus and connection areas. Covers for inspection must be manageable—no larger than 1.12 m² or heavier than 27 kg—unless assisted by lifting devices. Interlocks must prevent exposure to energized fuses and ensure circuit breakers cannot be inserted or withdrawn under unsafe conditions. Operators must be protected from stored energy discharges by mechanical safety mechanisms.

Breaker Interlocks and Locking Systems

Mechanical interlocks must prevent moving a breaker while closed, prevent closing unless fully engaged, and control the release of stored energy. Locking mechanisms must secure breakers in disconnected positions and allow fuses to be accessed only when devices are safely withdrawn. All removable elements of identical type and rating must be interchangeable, but safeguards must prevent mismatched installations that could compromise ratings.

Secondary Disconnecting Devices

Control wiring between removable and fixed structures must use automatic contacts or manual plug-and-socket connectors, with interlocks preventing unsafe disconnections. Manual connectors must not allow the insertion of removable elements until fully engaged.

Cable Space and Clearances

Net wiring space must be no less than 250% of the aggregate cross-sectional area of all installed cables, excluding obstructions or moving parts. This ensures sufficient clearance for heat dissipation and safe cable management.

Labels, Lifting, and Safety Measures

Each switchgear assembly must bear ANSI Z535.4-1998-compliant warning labels, alerting personnel to residual hazards. Lifting devices, whether wheeled or overhead, must be available for safely inserting or withdrawing drawout breakers from their compartments.