Quick Hits

EPISODE 9 — Shielding The Corona Ozone Problem — Part III Finale (315.44, 315.45)

Slide 1 — Hook "Closing out Part III — WHY shielding exists at all, and the narrow situations where you're allowed to skip it."

Slide 2 — 315.44 The core shielding rule and why it matters

• General rule: solid dielectric insulated conductors operating above 2000V in PERMANENT installations must have ozone-resistant insulation AND be shielded. All metallic insulation shields connect to a grounding electrode conductor, grounding busbar, EGC, or grounding electrode. EGCs installed with medium-voltage circuits are sized per 270.122.

• WHY: at high voltage, "corona" (a faint discharge glow) can form at the conductor surface, generating ozone that attacks and breaks down the jacket/insulation over time. The shield sits at GROUND potential, which keeps voltage off the jacket outside the shield and prevents that discharge/ozone formation from happening in the first place.

Slide 3 — The narrow exceptions to skip shielding

• Nonshielded, ozone-resistant conductors up to 5000V phase-to-phase ARE allowed in Type MC cable at INDUSTRIAL establishments with qualified-persons-only maintenance/supervision.

• Exception No. 1: nonshielded conductors up to 2400V, listed by a qualified testing lab, IF insulation resists electric discharge/surface tracking (or is covered with ozone/discharge/tracking-resistant material), wet-location use has a nonmetallic jacket or continuous metallic sheath, and thickness follows Table 315.10(B).

• Exception No. 2: nonshielded conductors up to 5000V, listed by a qualified testing lab, but ONLY to replace EXISTING nonshielded conductors on EXISTING equipment in industrial establishments — same qualified-persons-only, insulation-resistance, and thickness conditions apply.

• Exception No. 3: covers the same airfield lighting allowance from 315.36 Exception No. 2.

• 315.45: cable joints/terminations must be provided with a means to connect the metallic insulation shield to ground, where required.

Slide 4 — Quiz + CTA Q: "Why is a metallic shield on medium-voltage cable connected to ground?" A) To boost the cable's ampacity B) To keep the jacket at ground potential and prevent corona/ozone-driven insulation breakdown C) It's purely a labeling requirement with no electrical function D) To reduce the cable's physical weight

✅ Answer: B — grounding the shield prevents the voltage stress, corona discharge, and resulting ozone damage that would otherwise degrade the jacket/insulation.

CTA: Follow for more daily quick-hit study notes 📚⚡ — Part III complete! Part IV (Ampacities) is next — the biggest part of this article.

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