Section 12/Installation and Maintenance of Equipment in Supply Stations
NESC Section 12 (Rules 120 through 129) covers equipment enclosure, guarding, grounding of frames, and installation requirements for electric supply stations.
NESC Section 12 applies to the installation and maintenance of equipment in electric supply stations including substations, generating stations, and switching stations. Rule 120 requires current-carrying parts to be enclosed or guarded to prevent accidental contact by authorized personnel. Rule 121 establishes minimum heights and clearances for unguarded live parts based on voltage. Rule 122 requires all non-current-carrying metal parts of equipment, including frames, tanks, enclosures, fences, and supporting structures that may become energized, to be grounded in accordance with Section 9. Rule 123 covers conductor terminations and connections, requiring secure mechanical and electrical connections that withstand thermal cycling and fault currents. Rule 124 addresses identification and marking of equipment for safe operation.
Why this section exists
Electric supply stations contain equipment operating at voltages from distribution levels (4 kV to 35 kV) through transmission levels (69 kV to 765 kV). The energy levels at these voltages make accidental contact or equipment failure potentially fatal. The enclosure and guarding requirements protect workers performing routine maintenance and switching operations. The grounding requirements, implemented through IEEE 80 grounding grid design, ensure that fault currents are safely conducted to earth and that step and touch voltages remain within tolerable limits for personnel inside and outside the station.
What plan reviewers look for
Plan reviewers check substation general arrangement drawings for adequate clearances between energized parts and accessible surfaces per Rule 121. They verify that all equipment frames, tanks, and enclosures are shown bonded to the grounding grid. They check fence grounding connections and verify adequate clearance from the fence to energized conductors. For indoor stations, they check that metal-enclosed switchgear and transformers have proper grounding bus connections. They verify that the grounding plan shows connections from every piece of equipment to the station ground grid per IEEE 80 requirements.