AC Interference — Induction & Mitigation (Diagram #007)
Explains AC induction on pipelines near power lines, how AC concentrates at coating holidays, and how mitigation provides a controlled grounding path.
What this visual explains
This diagram shows AC interference by induction on a pipeline paralleling an overhead power line and how mitigation reduces risk. The key idea is that induced AC can concentrate at coating holidays and create high AC current density at small exposed areas.
Diagram
How to read it
- Induction region: parallel exposure to AC transmission line creates induced voltage on the pipeline.
- Holiday focus: small exposed areas can see high AC current density.
- Mitigation path: controlled grounding components bleed AC while maintaining CP DC effectiveness.
Field interpretation
- Risk is driven by induced AC voltage and AC current density at defects, not just “being near power lines.”
- Mitigation is typically designed to provide a low-impedance AC path without shorting CP DC (decoupling devices).
- Evaluate exposure length, separation distance, soil resistivity, coating quality, and mitigation continuity.
Common mistakes
- Using DC-only thinking: AC interference behaves differently from stray DC interference.
- Assuming “good CP potentials” means AC risk is low (they are different hazards).
- Ignoring holiday concentration: the most severe effects can be at very small exposed areas.
CP 3 relevance
CP 3 troubleshooting includes identifying AC interference exposure, interpreting measurements, and understanding mitigation concepts. This visual supports separating AC induction from DC stray current and explaining mitigation intent.