The First Tap is Misleading

The rectifier is hung, the cables are terminated, and the ground bed has had time to settle. You put the coarse tap on A and the fine tap on 1, flip the breaker, and read the output with your meter.

3.0 volts. 0.5 amps.

That's 6 ohms. The design planned for 1 - 2.

It's tempting to write it down as a high resistance bed and start making some phone calls. Hold off. The lowest tap is the worst place to quickly judge a ground bed, and the reason is a voltage that was sitting in the circuit before you ever turned the unit on.

Kicking on a new rectifier is a commissioning job, not a switch you flip. Review the system, pick the places you should watch, take a baseline, and step the taps up while monitoring, until the worst location is protected and the most negative one isn't overdone. Here's how that works in the field, and where the references back it up.

Walk through the installation against the plan before anything gets power. The positive cable goes to the anodes and the negative cable to the structure. Check any test stations at the ends of the bed where the positive cable comes up, and any bonds or junction boxes the design calls for.

Many operators ask for a loop cable on a new bed. The positive cable runs past each anode to the end of the bed and then loops back toward the rectifier, where it's left unconnected at a test station, junction box, or in the cabinet. A positive cable with damaged insulation discharges current and eventually breaks in two, cutting off every anode past the break. With a loop, you connect the far end and keep the bed running from the other side while the repair is planned and budgeted, instead of an unscheduled dig to stay in compliance. If the system has one, find the loop end and make sure it's set up the way the owner wants.

On a deep anode well, give the carbon backfill at least 24 hours after final pumping to settle before you energize.

At the rectifier, the safety habits from the rectifier troubleshooting issue apply:

Then check three readings before you put current to the line:

You can't read the whole line after every tap change, so pick the places that tell you what the whole line is doing.

Make them easy to get to. You'll visit each one several times before the day is over. On a 10-mile line that might be every road crossing, or at least five or six test stations spread end to end.

A few locations go on the list every time:

Mark each spot with a stake or paint so the reference electrode goes back in the same place every time.

Having some help to split the list is nice. One tech reading every location after every tap change makes for a long day. Divide the locations among the crew and call the results in. If one location matters more than the rest, leave someone there to watch it while you adjust. Data loggers can stand in for people, but you'd need several.

If additional CP sources exist, interrupt everything you can that influences your locations: other rectifiers, bonds and galvanic anodes, and foreign rectifiers where the owner agrees. Use GPS-synchronized interrupters with an easy to read cycle. 3 seconds ON and 1 second OFF is common.

Then, with the other sources cycling and the new unit still off, read ON and instant-off at every location. On a line with other systems already running, that's a baseline, not a native potential, and that's fine. The point is to see how much this unit moves the line and what it has to run at to do its job. If it was installed to help a trouble area, that area's baseline is the number to beat.

If your new rectifier is the only system you’re working with, this step is a lot easier.

One caution. A baseline taken with other systems running can't support a 100 mV polarization claim. That criterion counts from a true native, or from a decay with every influencing source turned off. This article focuses towards −850 mV CSE instant-off.

Start at the lowest tap: coarse on its lowest letter or number, fine on 1. It's a safety habit, and it's how the procedure is written in the books. Read volts and amps with your own meter, across the output terminals and across the panel shunt. Panel meters can and do fail.

The rectifier doesn't push any current until its output climbs past the back EMF. If the first tap shows a couple of volts and no amps, that may be all it is, so step up. If a higher tap still shows only about the back EMF and no amps, suspect the rectifier or its AC supply before you go looking for a cable break. The load-test resistor from the troubleshooting issue settles that in a minute.

Once current flows, volts divided by amps gives you the effective circuit resistance. On the first tap, though, it reads high, because the back EMF is a big share of a small voltage. "It is incorrect to simply divide the output voltage by the output current without accounting for the back EMF." (W. B. Holtsbaum, Cathodic Protection Survey Procedures, 3rd Edition).

Take the back EMF out first:

R = (V − back EMF) ÷ I

Here's one bed at four of the taps you'll step through, with 2.0 V of back EMF.

Illustrative example. The back EMF is held at 2.0 V to keep the math simple.

The bed didn't get better as the taps went up. That's how a good bed ends up labeled high resistance off a reading or two at the lower outputs.

If you expected 2 and you're looking at 50, stop and troubleshoot before you balance anything. The problem could be in the circuit: a cable, a connection or the bed itself. Locating and Repairing Cable Breaks in CP Systems covers the cable side. If it turns out to be a broken positive, the loop cable may let you keep the bed running while the repair gets scheduled.

The default is one step at a time. Raise the fine tap one position, energize, and read. When the fine tap tops out, put it back on 1 and move the coarse tap up one. A-6 goes to B-1, not B-6. If a step barely moves anything, going up two or three is reasonable.

Give each step time to settle before you read it. On a well-coated line, the drive between locations often covers it.

At every step, read the output volts and amps, then ON and instant-off at every location. Watch how much each location moves per amp. By the second or third step you can see where it's headed.

Here's the same illustrative line, with six locations and the bed near mile 2. The table shows the two locations that drive the decision.

Illustrative instant-off potentials, CSE.

After A-5, the far end is moving about 35 mV per amp. To get it past −850 mV with some margin it needs about 50 mV more, which is another 1.5 A, or 5 A total. The location next to the bed moves about 70 mV per amp, so 5 A puts it near −1.07 V.

Early instant-off readings understate where the line will settle, because polarization keeps building after each step. That's one more reason to come at the final setting from below.

The target is simple to say. Get the worst location more negative than −850 mV CSE instant-off without the most negative location going past −1.2 V instant-off, or your company's limit if it's tighter.

Peabody's Control of Pipeline Corrosion describes the same two limits. The point farthest from the bed has to reach the minimum potential for protection. The pipe near the bed has to stay less negative than about −1.1 V to avoid coating damage and hydrogen effects in susceptible steels. The Over-Protection issue used −1.2 V, which the AUCSC course material gives as the industry recommendation. NACE SP0169-2024 doesn't set an overprotection potential limit for steel, so if your company has a number, use it.

Good CP designs for large structures typically involve multiple smaller units rather than one large system.

If one location won't come up without overdoing the rest, find out why before you turn anything up. Turning up won't protect a section cut off by a broken bond, and it can increase interference there. Low spots also come from poor coating, poor continuity, a shorted insulating joint, contact with a foreign structure, or a shorted casing.

A well-coated new line can polarize quickly. The time it takes to drive between locations is often enough. Bare or poorly coated pipe can take days or weeks.

So the setting you leave with is your best read, not the final word. Go back in a few days to a week and read the locations again. In the example, a week later the far end had settled at −0.880 V and the location next to the bed at −1.095 V, still inside the window.

A congested corridor full of CP systems is a lot like an apartment building where everybody has a loud stereo. Each one gets turned up to be heard over the others, and pretty soon the whole building is blaring. A little cooperation and everybody can turn it down.

Peabody's says the same thing in engineering terms. Cooperating with the owners of other pipelines and buried structures is "an essential part of the pipeline corrosion engineer's job," so that everyone can run their systems "with the least practical effect on structures belonging to others." It also says to notify the owners of the structures that cross your line before the CP installation goes in, so the conversation can start early.

During commissioning, read the foreign lines at your crossings and near the bed, with the owner's permission. Keep the DC Interference vs. DC Influence issue in mind when you look at those numbers. A foreign line that swings more negative near your bed may be picking up current, or your bed's voltage gradient may just be reaching their reference cell.

Record the nameplate data, the as-found and as-left taps, and the output volts and amps at each step. Record the back EMF and both resistance numbers with the tap each came from, ON and instant-off at every location at every step, the interrupter cycle, and the foreign-line readings.

The commissioning numbers are the baseline that every rectifier inspection after this one gets compared to.

This article covers tap-type units. Auto-potential and constant-current rectifiers are set mainly with an adjustment on the control board, though many have tap bars too. The goal is the same, enough protection without too much, but the procedure isn't. Follow the manufacturer's manual for those.

Established in 2011, Roberts Corrosion Services, LLC delivers comprehensive, turn-key cathodic protection and corrosion control solutions nationwide. Our end-to-end expertise encompasses design and inspection, installation and repair, surveys and remedial work. We provide drilling services for deep anode installations and a full laboratory for analysis of samples and corrosion coupons, as well as custom CP Rectifier manufacturing.

While our initial focus was on the Appalachian Basin area, we complete field work all over the US. We are a licensed contractor in many states and can complete a wide range of services.

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