Quick Hits

PART II — SYSTEM GROUNDING

EPISODE 4 — Which AC Systems Must (and Needn't) Be Grounded (270.20, 270.21)

Slide 1 — Hook "New Part! System Grounding — starting with the basic decision: does this system even need to be grounded?"

Slide 2 — 270.20 Systems to be grounded

• Solidly grounded systems: follow 270.24 or 270.30.

• Impedance grounded systems: follow 270.36.

• Systems supplying portable/mobile equipment: follow 270.115.

Slide 3 — 270.21 Systems NOT required to be grounded

• Permitted (not required) to stay ungrounded: systems exclusively feeding industrial electric furnaces (melting/refining/tempering), separately derived systems exclusively feeding rectifiers for adjustable-speed industrial drives, and delta-connected systems.

• If ungrounded, a ground detector is REQUIRED, connected as close as practical to the system's supply point.

• Ungrounded systems must be legibly, durably marked at the source or first disconnect: "Caution: Ungrounded System Operating."

Slide 4 — Quiz + CTA Q: "An ungrounded system permitted under 270.21 must have what installed on the system?" A) A GFCI breaker B) A ground detector C) An AFCI breaker D) Nothing extra required

✅ Answer: B — 270.21(B): ground detectors are required on these permitted-ungrounded systems.

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