When you measure the potential of a cathodically protected pipe with the current flowing, the number you get is flattered by an error that has nothing to do with how well the steel is actually protected. That error is the IR drop, and removing it is what instant-off potential is all about. By interrupting the protection current and reading the potential in the instant before the pipe starts to relax, the true polarized potential is captured without the IR error baked in. This guide explains why the raw on potential overstates protection, how synchronized interrupters capture the instant-off value, why the negative 850 millivolt instant-off level is the compliance standard, and how remote monitoring logs it automatically.
Instant-Off Potential in one line: Instant-off potential is the pipe-to-soil potential measured in the moment immediately after the cathodic-protection current is switched off, before the pipe begins to depolarize. Reading it at that instant removes the IR drop, the voltage error caused by protection current flowing through the resistance of the soil, which inflates the ordinary on potential. Because the instant-off value reflects the true polarized potential of the steel, it is the basis of the negative 850 millivolt polarized criterion used to judge whether a buried pipeline is adequately protected.
When cathodic protection is working, current is continuously flowing through the soil to the pipe, and that current has to pass through the electrical resistance of the soil and the coating to get there. Any current flowing through a resistance produces a voltage across it, and this voltage is the IR drop, named for current times resistance. When you measure the pipe-to-soil potential with the current on, your voltmeter reads the genuine potential of the steel plus this IR drop, so the number is more negative, and therefore looks more protected, than the steel really is.
This matters because protection is judged by how negative the potential is, and the IR drop pushes the on reading in the more-negative direction for reasons unrelated to the actual condition of the steel. A pipe could show a comfortably negative on potential and yet, once the IR error is stripped out, fall short of the protection level. Relying on the raw on potential can therefore give false confidence, and the size of the error varies with soil resistivity and how much current is flowing, so it is not even a constant offset that could be simply ignored.
What integrity engineers actually care about is the polarized potential, the true potential the steel has reached because the protection current has polarized it, with the IR drop excluded. That polarized value reflects the real electrochemical state of the metal and whether corrosion is being suppressed. The whole purpose of the instant-off technique is to get at this polarized potential by eliminating the IR drop from the measurement, so the judgement about protection rests on the steel's true condition rather than on an inflated reading.
The trick that removes the IR drop is timing. The IR drop exists only while current is flowing, so it vanishes the instant the current stops, whereas the polarization of the steel decays only gradually over seconds and longer. If you could read the potential in the brief moment right after the current switches off but before the steel begins to relax, you would see the polarized potential without the IR error. That reading is the instant-off potential, and capturing it depends on switching the current off and measuring at exactly the right moment.
To do this across a protected system, current interrupters are fitted to the protection current sources and set to switch on and off in a repeating cycle, and crucially they are synchronized so that all sources turn off together. Synchronization matters because a pipe is often protected by more than one current source, and if they did not all interrupt at the same instant, current from an unswitched source would still be flowing and its IR drop would still contaminate the reading. With every source interrupted together, there is a clean instant with no protection current flowing anywhere, and the potential read in that instant is the true instant-off value.
The cycle is kept short, with the off period lasting only long enough to take the reading before the pipe depolarizes appreciably, then the current returns so protection is barely interrupted overall. A surveyor, or an automated meter, captures the potential in the off window of each cycle. Getting this right is a matter of clean, well-synchronized switching and reading at the correct instant, because a reading taken too late catches the pipe already depolarizing, and a reading taken with any source still on still contains IR drop.
Because the instant-off potential is the polarized potential free of IR drop, it is the value the polarized protection criterion is written against. The widely used benchmark is that a buried steel pipeline is considered adequately protected when its polarized, instant-off potential is at least negative 850 millivolts relative to a copper-copper sulfate reference. Framing the criterion on the instant-off value rather than the on value is deliberate, precisely because the on value carries the IR error that could make an under-protected pipe appear compliant. Demonstrating compliance therefore means showing the instant-off potential meets the level, not just the on potential.
Historically, capturing instant-off data meant sending technicians into the field to install temporary interrupters and walk the line reading potentials during the off cycle, which is labour-intensive and only gives a snapshot on the day of the survey. Automated interrupters built into the protection current sources, together with permanently installed reference electrodes, change this by letting the on and instant-off potentials be captured on a schedule without anyone on site. The interrupter cycles the current, and a logger reads the potential in the off window, producing instant-off records continuously rather than once a year.
Feeding that data into remote cathodic-protection monitoring is what turns it into an ongoing compliance and diagnostic record. On a cloud SCADA platform such as Merobix the on potential, the instant-off potential, and the current-source output can be logged and trended together, so an operator sees whether the polarized potential is holding above the criterion over time and receives a notification if an instant-off value drifts toward or below the level. For pipelines spread across remote territory, this means compliance is monitored continuously and problems are caught between formal surveys, with the interrupter and remote logger doing the work that once required a technician standing over each test point.
The on potential is measured while the cathodic-protection current is flowing, so it includes the IR drop, a voltage error caused by that current passing through the resistance of the soil, which makes the reading look more negative and more protected than the steel really is. The instant-off potential is read in the moment right after the current is switched off, before the pipe depolarizes, so the IR drop is gone and the reading reflects the true polarized potential of the steel. Protection is judged on the instant-off value for that reason.
A pipeline is often protected by more than one current source, and the IR drop disappears only when no protection current is flowing at all. If one source stayed on while another switched off, current from the unswitched source would still be flowing and its IR drop would still contaminate the reading. Synchronizing all the interrupters so every source turns off at the same instant creates a clean moment with no current flowing anywhere, which is when the true instant-off potential can be captured.
Automated current interrupters can be built into the protection current sources to cycle the current on and off on a schedule, and permanently installed reference electrodes provide a fixed measuring point. A remote logger reads the potential during the off window of each cycle and records both the on and instant-off values. Feeding these into a cloud CP monitoring platform lets the polarized potential be trended continuously and alarmed if it drifts below the criterion, so compliance is watched between formal surveys without anyone on site.
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