AC Induced Voltage and AC Corrosion Density
Computes the AC current density at a coating holiday beside the personnel touch limit.
Example
You enter
- Induced AC voltage on the pipe (V) 15
- Local soil resistivity (ohm-m) 25
- Assumed holiday area (sq cm) 1
- Personnel touch limit (V) 15
- AC corrosion threshold (A/m^2) 30
- Alternative soil resistivity (ohm-m) 100
You get
- Equivalent holiday diameter 11.3 mm
- AC current density at the holiday 135.406
- Threshold multiple 4.51352
- Voltage the steel actually needs 3.32335
- Stringency ratio 4.51352
- In the alternative soil 33.9 A/m^2 (-75%) at 100 ohm-m
- Alternative change (%) -75
Checked against published worked examples from: R. Allen (Appalachian Underground Corrosion Short Course).
Details, formula, and sources
ScopeThe worked line sits exactly on the 15 V touch limit while running 4.5 times the AC corrosion threshold, so mitigating to 15 V protects the crew and does nothing for the pipe.
Formula or tableJ_AC = 8 V_AC / (rho pi d) at a circular holiday of equivalent diameter d, against the AC corrosion threshold and the personnel touch limit.
Source and editionISO 18086, NACE SP21424 (now AMPP), and a corridor AC interference study govern.
Free accessISO 18086 is a purchased standard; the spreading-resistance relation is public.
What governsEstimate. AHJ and licensed professional govern.
API field namesinduced_ac_v, soil_resistivity_ohm_m, holiday_area_cm2, touch_limit_v, risk_threshold_a_per_m2, alternative_resistivity_ohm_m, holiday_diameter_mm, ac_current_density_a_per_m2, threshold_multiple, voltage_for_threshold_v, stringency_ratio, alternative_current_density_a_per_m2, alternative_change_pct
- Holiday size an assumption rather than a measurementcorridor study
- Thresholds commonly 30 A/m^2 for AC corrosion and 15 V for touchISO 18086 / safety practice