IR Drop
IR drop is voltage drop caused by current flowing through resistance. In cathodic protection, IR drop can affect potential measurements and make ON readings misleading.
Quick Definition
IR drop is the voltage drop equal to current multiplied by resistance.
Why This Term Matters
IR drop is one of the main reasons ON potentials can be misleading. When CP current flows through soil, water, concrete, cables, bonds, or other resistance, voltage drop becomes part of the measured circuit.
If IR drop is not considered, a structure may appear more protected than it actually is.
Core Concept
Ohm’s law
IR drop follows Ohm’s law: voltage drop equals current multiplied by resistance.
ON potential error
ON potentials are measured while current is flowing, so they can include voltage drop through the electrolyte.
Instant-off reduction
Instant-off testing interrupts CP current to reduce current-flow voltage drop while the structure remains polarized.
Not only soil
IR drop can occur in electrolyte paths, cables, shunts, bonds, splices, and other resistive parts of the circuit.
Common Mistakes
-
Assuming ON potentials are free of IR drop.
Why it is wrong: ON readings are collected while current is flowing. -
Assuming instant-off removes all possible errors.
Why it is wrong: Instant-off reduces current-flow voltage drop but does not eliminate every measurement limitation. -
Ignoring resistance in bonds or cables.
Why it is wrong: Resistive conductors and connections can create voltage drops and limit current.
Field Example
A pipeline has an ON potential of −1,100 mVCSE and an instant-off potential of −820 mVCSE. The difference suggests that the ON reading included a significant IR drop component.