IR Drop and Voltage-Drop Error
IR drop is voltage drop across a resistance caused by current flow. In plain language, when current moves through soil, water, concrete, coating defects, bonds, or metallic paths, part of the measured voltage can be from that current path instead of only from the structure surface condition.
The practical problem is simple: a measured potential can look more protective than the actual polarized structure surface condition if voltage drop is not properly considered.
The term comes from Ohm's law, where voltage drop is related to current and resistance. This page uses that idea as a field interpretation concept, not as a detailed correction formula.
Where Voltage-Drop Error Comes From
- Current flowing through soil or water between the structure and reference electrode.
- Current flowing along the pipeline or other metallic structure.
- Nearby anode beds, distributed anodes, bonds, foreign structures, and interference sources.
- Poor reference electrode placement, poor soil contact, or high contact resistance.
- Dry soil, frozen soil, pavement, rock, standing water, or other field conditions that make the electrolyte contact unrepresentative.
- Dynamic stray current, telluric current, or induced AC effects that distort DC readings.
Ways to Reduce or Consider IR Drop
| Method | Use | Caution |
|---|---|---|
| Current interruption | Reduces voltage drop from interrupted CP sources | Does not remove error from sources that remain active and does not guarantee every influence is gone. |
| Reference electrode close placement | Reduces electrolyte path between reference and structure | Access, shielding, poor contact, and local electrolyte conditions can limit effectiveness. |
| Coupons | Allows local interruption of a representative exposed surface | Coupon must represent the structure condition and environment. |
| Stepwise current reduction | Estimates voltage-drop component by controlled output changes | Requires stable conditions and careful interpretation. |
| Engineering review | Uses history, corrosion evidence, system characteristics, and survey data | Must be documented and technically defensible. |
Common Mistake
The common mistake is assuming that every ON potential more negative than a criterion proves adequate protection. ON readings can be useful, but voltage drops other than those across the structure-to-electrolyte interface must be considered before drawing conclusions.
Field notes should identify whether the reading was ON, instant-off, depolarized, or native; where the reference electrode was placed; what current sources were operating or interrupted; and any condition that could change the electrolyte path or contact quality.