AC vs. DC Electricity: Characteristics and Differences

You’re at a test station on a stretch of pipeline that runs parallel to a 138 kV transmission corridor. The towers march along the right-of-way a couple hundred feet to the east, exactly the way they have for the last forty miles of pipe. You open the lid on the test post and clip your meter leads to the pipe header and to a Cu/CuSO₄ half-cell pressed into moist soil at the base of the post.

Meter on V DC. The display settles at −1.05 V. The pipe meets the −0.85 V criterion at this location with comfortable margin. CP is doing its job here.

Same lead pair, same half-cell, same pipe. You flip the meter to V AC. The display jumps to 6.5 V AC. Steady. Repeatable.

Two readings. Same conductor pair. Two completely different stories. The DC reading is a CP question and the answer is good. The AC reading is a different question entirely — induced from the power line above, having nothing to do with the rectifier feeding this section, and not something more CP current is going to make go away. This module is about why those are two different regimes, why CP only addresses the first one, and how the meter mode is the discipline that keeps them straight.

Why this module sits here in the rung

The hinge between the math and the hardware

EC-001 named the shapes of CP circuits — series, parallel, and the rules that govern them. EC-002 walked the math that runs inside those shapes. EC-003 distinguished voltage, current, and resistance and showed where each one is read at a real CP test point. Every one of those modules assumed something we never said out loud: the current is DC. The shapes are DC shapes. The Ohm’s Law math is DC math. The structure-to-soil readings are DC voltages.

That assumption is correct, and it’s load-bearing. Cathodic protection is a DC technique. But CP systems live in a world where AC is everywhere — feeding the rectifier, riding on cables that pass nearby, induced onto the pipeline itself by parallel power lines. The meter you take into the field reads both regimes, and a fair share of CP fieldwork is knowing which one you are reading at any given moment, and what that reading does and does not tell you.

Three jobs sit on this module:

• Compare AC and DC as electrical regimes — waveform, frequency, peak vs. RMS, where each shows up in a CP system, what mode the meter is in to read each.
• Explain why CP requires DC — and why no amount of AC can substitute, regardless of how strong it is.
• Name how AC ends up on a pipeline from sources unrelated to the CP system, and why it is a separate problem on its own work track, with its own techniques and its own reading.

The math here is light — one small piece of arithmetic on peak vs. RMS — because the work of this module is conceptual. EC-005 will pick up power and energy calculations in CP circuits. The deeper treatment of AC interference — coupling models, induced-current thresholds, decoupling devices, mitigation design — lives in a later EC module. EC-004’s lane is to make sure the difference between the two regimes is wired in cleanly before any of that downstream work has to land.

DC — constant in one direction

Direct current is electrical current that flows steadily in one direction. The polarity is fixed: there’s a positive terminal and a negative terminal, and current flows from one to the other without reversing. Plotted on a graph of current versus time, pure DC is a flat horizontal line — the value doesn’t change as time passes.

Voltage in a DC circuit behaves the same way. Source voltage is a fixed level. Drops across resistors are fixed levels. Read it on a meter and the display sits at one number until something in the circuit changes.

Where DC lives in CP work

Every part of a cathodic protection system that does protective work is on the DC side:

• Rectifier output. The DC bus inside an impressed-current rectifier cabinet — the (+) terminal feeding the anode header cable, the (−) terminal returning from the structure. Typical values: 12 V to 50 V on V; a few amps to fifty amps on I.
• Galvanic anode systems. A magnesium or zinc anode wired to a buried structure produces DC by simple electrochemistry — no rectifier needed. The anode sits at one potential, the structure at another, and the difference drives a small DC current through the soil and back through the bond cable. Typical bond currents: tens of milliamps to a few amps.
• Structure-to-soil potential readings. The −0.85 V Cu/CuSO₄ criterion is a DC voltage between the structure and the soil. The whole reason a Cu/CuSO₄ half-cell works as a reference is that its own electrochemistry sits at a stable, known DC potential.
• Bond shunt readings. The millivolts you read across a calibrated shunt to find a bond current — those are millivolts DC. The current through the shunt is DC.
• Batteries. Where DC is needed for a small load with no utility power available — coupon cells, monitoring equipment, cathodic-disbondment test rigs in the lab.

The meter mode for every one of those readings is DC: V DC for voltages, mA DC or A DC for currents, mV DC for the small voltage drops across shunts. A meter on AC mode looking at the same conductors will read close to zero — because pure DC has no alternation for the AC mode to register.

AC — alternating, sinusoidal

Alternating current is electrical current that reverses direction periodically. The “wave” of an AC system is a sine wave — current rises smoothly from zero up to a positive peak, falls back through zero down to a negative peak, then rises back to zero again. One full rise-and-fall is one cycle. The number of cycles per second is the frequency.

DC sits at a steady level. AC swings sinusoidally above and below zero. Same axes, two regimes.

The standard utility frequency in the United States is 60 Hz — sixty full cycles per second. (Most of the rest of the world runs at 50 Hz; the principles are identical, only the number is different.) Each cycle has two reversals — once on the way down through zero, once on the way back up — so US-grid AC reverses direction 120 times per second. Faster than a human eye perceives, but well-defined and the dominant signal everywhere on the utility grid.

Peak vs. RMS

An AC waveform is described by two voltage numbers, and they are not the same. The peak voltage is the maximum instantaneous value the waveform reaches at the top of its swing. The RMS voltage (root-mean-square) is the equivalent steady DC voltage that would deliver the same average power into a resistive load. The two are linked by a fixed factor for a clean sine wave:

So a 120 V RMS outlet has a peak voltage of about 120 × 1.414 ≈ 170 V. A 240 V RMS service has a peak of about 240 × 1.414 ≈ 339 V. A 480 V three-phase line has a line-to-line peak of about 480 × 1.414 ≈ 679 V. Catalog ratings, breaker labels, and nameplates use RMS by convention — when somebody says “120 volt outlet,” they mean 120 V RMS, not 120 V peak.

Where AC lives in CP work

Three places, three different stories:

• Utility input to the rectifier. Every impressed-current CP system starts with utility AC at the cabinet — typically 120 V or 240 V single-phase for small units, 240 V or 480 V three-phase for larger ones. That AC enters the cabinet, crosses through the transformer, and only then becomes the DC the system actually uses. This AC is on purpose, on the utility side of the cabinet, and the rectifier wouldn’t work without it.
• Induced AC on the pipeline. When a buried pipeline runs parallel to a high-voltage transmission line for any meaningful distance, the pipe acts like the secondary of an enormous transformer — the AC current in the power line induces a smaller AC current and voltage on the pipe itself. This AC is incidental, not on purpose, and it has nothing to do with the rectifier. We unpack it in the next section.
• Ripple on rectified DC. The DC coming out of a rectifier isn’t perfectly smooth. There’s a residual AC component — ripple — riding on top of the DC level, at twice the input frequency (120 Hz for full-wave rectification of 60 Hz AC). Modern rectifiers reduce this with filtering, but a small amount remains. We treat it briefly in Scenario C below.

The meter mode for any of those readings is AC: V AC for voltages. A meter on DC mode looking at clean AC reads close to zero — because the average of a symmetric sine wave over time is zero, and the DC mode reports the average.

Why CP is fundamentally a DC technique

This is the load-bearing section of the module. Cathodic protection cannot run on AC. Not at low voltage, not at high voltage, not at any frequency. Three reasons explain why, and they are all rooted in the electrochemistry that CP exists to drive.

1. Polarity has to be fixed.

The whole point of CP is to make the structure the cathode of a deliberate electrochemical cell — to push it to a more negative potential than it would naturally sit at, so that the metal-loss reactions that normally happen on the structure surface stop. That requires the structure to be the cathode continuously, not on average and not most of the time.

AC reverses polarity 120 times a second on a 60 Hz system. For a 60th of a second the structure would be cathodic; for the next 60th it would be anodic. Anodic means metal loss — exactly what CP exists to prevent. You don’t get net protection from polarity that flips on you that fast; you get a structure that is corroding for half of every cycle and protected for the other half, which from the structure’s point of view is no protection at all.

2. The reference electrode only reads steady DC.

The Cu/CuSO₄ half-cell, the silver/silver-chloride cell used in seawater work, the zinc reference embedded in some test stations — all of them depend on their own electrochemistry sitting at a stable potential. We measure structure-to-soil potential by reading the difference between the structure’s potential and the half-cell’s known fixed potential. A 0.94 V difference means something. A reading that swings from +6 V to −6 V at 60 Hz means nothing — the average is zero, and the only number the meter can extract from it (the AC RMS) doesn’t tell us anything about whether the structure is sitting at a protective potential.

The −0.85 V criterion is a DC criterion. Every CP criterion in every governing standard, every survey procedure, every coupon-based polarization measurement — all DC. There is no AC version because there can’t be one.

3. The electrolyte responds to direction, not magnitude.

Current flowing through soil from an anode to a structure isn’t a flow of electrons — it’s a flow of ions. Positively charged ions move one way, negatively charged ions move the other, and the electrochemical reactions at each electrode (oxidation at the anode, reduction at the cathode) only proceed if the ions keep moving in their respective directions. Reverse the polarity and the ions reverse too — and the chemistry that the CP system is trying to drive at the cathode unwinds in the half-cycle the polarity is wrong, then has to start over again.

Put the three together and the conclusion is unavoidable: CP requires unidirectional, polarity-stable current. The only way to deliver that on a system fed by utility power is to convert the utility AC to DC inside the rectifier. The transformer steps the voltage up or down. The rectifier diodes (or SCRs) flip every other half-cycle so the output is unidirectional. Filter capacitors smooth the result. What comes out of the cabinet onto the cable is DC — sometimes with a little ripple still on it, but DC at heart. That’s the system the rest of CP is built around.

AC on the pipeline — the second story

The first story — CP runs on DC — is the whole reason rectifiers exist. The second story is what happens when AC ends up on a pipeline that has nothing to do with the rectifier feeding that section.

A buried pipeline running parallel to a high-voltage transmission line for any significant distance is electromagnetically coupled to that line. The transmission line carries an AC current — typically tens to hundreds of amps at 60 Hz, depending on the line’s loading. The magnetic field around the line oscillates at the same frequency. That oscillating field cuts through the pipeline’s path, and the result is the same as the secondary winding of a transformer: an induced AC voltage and current on the pipeline itself.

Inductive coupling. The AC in the transmission line induces an AC voltage and current on the parallel pipeline below.

How big the induced AC ends up being depends on the geometry — how close the pipe runs to the line, how long the parallel run is, what the line’s current is, what the soil resistivity is along the route, and where the pipeline is grounded. On a long parallel run with a heavily-loaded line, induced pipe-to-soil AC voltages of several volts to tens of volts are not unusual. We saw 6.5 V AC at the test station in the hook; on heavier corridors, readings can exceed 50 V AC at unprotected points.

Why this matters — AC on the pipe is its own corrosion mechanism

What you read on the meter is V AC pipe-to-soil. What actually drives AC corrosion is the AC current density at coating defects (holidays). The two are linked — higher V AC means more current can leak through a holiday — but the corrosion mechanism lives in the current density, not the voltage reading itself. At around 4 V AC and above, conditions on a typical pipeline can support enough current density at holidays to drive AC-induced corrosion — a regime that proceeds independently of, and unmitigated by, the cathodic protection system. The mechanism involves alternating polarization at the holiday, partial film disruption during the anodic half-cycle, and pitting that continues even while the structure-to-soil DC potential meets the −0.85 V criterion. The exact V AC threshold varies with soil resistivity, coating quality, and holiday geometry; field practice uses the V AC reading as the trigger to investigate further.

AC interference is also a personnel safety issue. Industry standards generally treat 10 to 15 V AC and higher pipe-to-soil as a worker shock hazard — including for corrosion techs reading test stations, opening valve boxes, or working at flanges and other above-ground appurtenances. Even a few volts AC can be enough to deliver a startling shock under wet hands and a good return path; double-digit V AC pipe-to-soil reliably is. Mitigation systems address both the corrosion side and the safety side.

Three readings, three meter modes

Three measurement vignettes from real CP work. Each one shows the same lesson from a different angle: the meter mode is part of the measurement. Switching modes on the same lead pair gives different numbers because they are different questions.

Scenario A — At the rectifier cabinet

Scenario B — At the test station next to the power line

Scenario C — Ripple on the rectified output

Meter-mode discipline

Every CP measurement starts with a question we mostly leave implicit: which regime is this reading actually in? The answer decides the meter mode (V DC vs. V AC, mA DC vs. mA AC), the unit interpretation (RMS vs. peak vs. steady-state), and what the reading actually means for the system.

The discipline that keeps the readings honest is small but non-negotiable: name the mode in the field notes. Write “−1.05 V DC,” not “−1.05 V.” Write “6.5 V AC,” not “6.5 V.” On a routine visit you will read both regimes on the same conductors within a few minutes of each other. The notebook entry that distinguishes them is what makes it possible — months later, after the visit is forgotten — to know which reading was which, and what each one was doing.

The CP-tester habit of always recording the meter mode comes from exactly this lesson. Two different regimes. Two different questions. Two different stories. The mode is part of the answer.

Back at the test station

Close the lid on the math for a minute and stand back at the test station you opened in the hook. Two readings on the same lead pair: −1.05 V DC and 6.5 V AC. Two different regimes, two different questions, two different parts of the field-notes page.

The DC reading was the CP question. The −1.05 V Cu/CuSO₄ potential meets the −0.85 V criterion with a comfortable margin, and that is the only thing the CP system itself is responsible for. The rectifier feeding this section is doing its job. The pipe is at a protective potential. From the rectifier’s point of view, the visit could end here.

The AC reading is something else. The 6.5 V AC is induced from the transmission line above and has nothing to do with the CP rectifier — turning the rectifier output up to 28 V or down to 16 V will not change that 6.5 V. The 6.5 V is on its own work track: AC interference assessment, soil and coating evaluation, mitigation design with decoupling devices and gradient control where warranted, and ongoing monitoring. That track does not run through this module — it runs through later work in this rung and beyond.

What the field-notes page captures, then, is two readings on two regimes, distinguished by mode, both real, neither one swept up by the other. The math gives you the numbers; the meter mode tells you which question the numbers are answering; the job tells you what to do with each.

Side by side — DC and AC at a glance