Overview
IR drop is the voltage loss that occurs when electrical current flows through a resistance. In cathodic protection work, that usually means current flowing through soil, water, or another electrolyte. This matters because the meter can see that voltage loss along with the structure’s actual electrical condition.
In plain English, IR drop can make a cathodic protection reading look more negative or otherwise different than the true polarized condition of the structure. That is why technicians and engineers must understand IR drop before interpreting ON potentials, instant OFF readings, or cathodic protection criteria.
Many measurement misunderstandings in cathodic protection come from confusing a meter reading with the actual electrical state of the structure. IR drop is one of the main reasons that happens.
Standard Summary
What the Standard Is Referring To
In cathodic protection, IR drop refers to the voltage loss associated with current flowing through resistance in the measurement path. The letters come from I for current and R for resistance. When current flows through soil or another electrolyte, a voltage gradient is created.
If a reference electrode is placed in an area affected by that voltage gradient, the meter may capture both the structure’s electrical condition and the voltage drop in the electrolyte. That means the measured value may not represent the true polarized condition of the structure alone.
Plain-English Explanation
Think of IR drop as electrical pressure lost while current is moving through something that resists that movement. In cathodic protection systems, the electrolyte is not a perfect conductor. Soil, for example, resists current flow.
When cathodic protection current travels through that soil, some voltage is dropped along the way. If the reference electrode is located where that voltage gradient exists, the meter may display a value that looks more negative than the structure itself actually is.
This does not mean the structure suddenly changed. It means the measurement includes an additional electrical effect caused by current flow through the environment.
Why This Concept Matters
IR drop matters because many cathodic protection decisions are based on measured potentials. If a reading is strongly affected by IR drop, a technician may think the structure meets a criterion when the true polarized potential does not.
This is especially important when evaluating criteria such as the −850 mV criterion. An ON potential may appear strongly protective, but part of that apparent protection may come from voltage loss in the electrolyte rather than from actual polarization of the structure.
Understanding IR drop helps explain why instant OFF measurements, proper electrode placement, and careful interpretation are so important in field work.
How It Is Observed in the Field
IR drop is not usually measured as a separate number by itself during routine field testing. Instead, it is recognized by how it affects structure-to-electrolyte potential readings.
A common field clue is a large difference between an ON potential and an instant OFF potential. If the ON reading is much more negative than the instant OFF reading, part of that difference may be caused by IR drop in the electrolyte.
Technicians also become suspicious of IR drop when readings change significantly with electrode placement, when current output is high, when coatings are poor, or when soil resistivity is low enough to support substantial current flow patterns around the structure.
What Can Affect the Amount of IR Drop
The amount of IR drop present in a measurement can vary widely depending on field conditions and system behavior.
Amount of Current Flow: More cathodic protection current generally creates larger voltage gradients.
Electrolyte Resistance: Soil or water resistivity affects how current moves and how much voltage is lost.
Reference Electrode Placement: Readings can change depending on whether the electrode is positioned in a stronger or weaker part of the voltage gradient.
Coating Condition: Poor coatings or bare areas can concentrate current flow and increase local gradients.
System Complexity: Multiple rectifiers, bonds, foreign structures, and interference sources can complicate the measurement field.
Current Interruption Quality: If all relevant current sources are not interrupted together, residual current flow can continue to produce error.
Field Interpretation
In the field, IR drop is best understood as a measurement effect that can interfere with accurate interpretation of cathodic protection data. The key question is whether the meter reading represents the structure’s true polarized condition or whether the value is being significantly influenced by voltage gradients in the electrolyte.
This is why technicians often compare ON and instant OFF readings, pay close attention to reference electrode placement, and use accepted test methods when evaluating cathodic protection criteria.
Good interpretation does not mean assuming every negative reading is correct. It means understanding how current flow, resistance, and measurement geometry can change what the meter displays.
Common Misunderstandings
Mistake 1: Assuming the ON potential always represents the true condition of the structure.
Mistake 2: Thinking a very negative reading automatically proves adequate cathodic protection.
Mistake 3: Treating IR drop as a problem only on impressed current systems. It can affect other systems too, depending on current flow and measurement setup.
Mistake 4: Ignoring reference electrode placement when evaluating whether IR drop may be present.
Mistake 5: Assuming that any difference between ON and OFF readings is caused only by IR drop without considering other system behavior.
Example Scenario
A technician measures a buried pipeline and records an ON potential of −1.15 V vs CSE. At first glance, the reading appears comfortably more negative than the −850 mV criterion.
The technician then interrupts the cathodic protection current and captures an instant OFF potential of −0.81 V vs CSE. The large difference between the two readings suggests that a significant portion of the ON reading was influenced by IR drop.
In this case, the ON value alone would be a poor basis for interpretation. The technician must evaluate the structure using the more appropriate measurement and the applicable criterion.
Standards Context
The concepts discussed on this page originate from AMPP SP0169 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems.
These explanations are simplified educational summaries intended to help readers understand the concepts used in cathodic protection standards. They are not a substitute for the complete standard or for professional engineering training and judgment.
The official standard can be obtained from the AMPP Knowledge Hub.