Voltage, Current, and Resistance in DC Systems
What V, I, and R actually are — and how each is measured in the field.
You’re at a test station, kneeling next to the post. The pipe header lead is in your left hand. In your right, a copper rod sits inside a small clear cylinder filled with bright blue saturated copper sulfate solution — a Cu/CuSO4 half-cell, the standard portable reference for cathodic protection work. You push the half-cell’s porous tip down into the moist soil right next to the pipe.
Multimeter set to V DC. Black lead from the meter to the half-cell, red lead to the pipe header. The display settles at −0.94 V.
That number is what every CP system on every pipeline is ultimately judged against. It’s how we answer the only question that matters: is the pipe protected? — and in this case, the answer is yes. But the reading itself isn’t current, isn’t resistance, isn’t a calculation. It’s a voltage. A voltage between the pipe and the soil, referenced through the half-cell. To read it correctly — to know what the meter is actually telling you — you have to know what voltage is in this context, where it lives in the CP circuit, and what makes it different from current and resistance.
That’s what this module is for. EC-001 named the shapes of the circuit. EC-002 walked the math that runs inside those shapes. EC-003 is about the three quantities themselves — what each one is, where each one shows up, and how each one is measured in a real CP system.
Why three quantities deserve their own module
The math earns its keep when the readings make sense.
EC-002 showed you that V = I × R works in three forms — solve for any one when the other two are known. That’s the calculator. This module is the lab.
You can plug numbers into Ohm’s Law all day, but if you don’t know what kind of voltage you’re looking at, what current actually does in a CP system, or what resistance is doing the work — the math gives you a number without telling you what to do with it. The mental model that makes the math worth doing is the one we build here: three quantities, three different jobs, three different ways they show up in the field.
Voltage — pressure, with a reference
Voltage is electrical pressure. It pushes current through a circuit the way water pressure pushes water through a pipe. The unit is the volt (V). The variable is V (sometimes E in older texts and CP-specific manuals).
Here’s the thing about voltage that makes it different from current or resistance: voltage is always measured between two points. There’s no such thing as “the voltage at this spot.” A voltmeter has two leads for a reason — it reads the difference in electrical potential between wherever you put the red lead and wherever you put the black lead. Move either lead and the reading changes.
For most CP voltages, both points are obvious. Rectifier output: between the (+) and (−) terminals at the cabinet. Cable IR drop: between the two ends of the cable. Junction box voltage: between the (+) bus and the soil-side return. Easy enough — two physical points you can clip leads to.
The one CP voltage that needs more explanation is structure-to-soil potential — the canonical reading the field uses to judge whether a pipe is protected. The two points being measured are: (1) the structure (the pipe) and (2) the soil right next to the pipe. The pipe end is easy: clip a lead to the test station header. But “the soil” isn’t a wire you can clip to — it’s a volume of dirt with electrical potential that varies from place to place. So we use a half-cell to give us a defined, repeatable contact with the soil at one specific point.
The Cu/CuSO4 half-cell — your reference for soil voltage
A copper-sulfate half-cell (Cu/CuSO4, sometimes written CSE) is a small portable cell built around three things: a copper rod, a saturated solution of copper sulfate, and a porous plug at the bottom that lets the solution make ionic contact with whatever the cell is touching. Press the plug into moist soil and the half-cell becomes a defined electrical contact — its own potential is fixed (within a few millivolts) by the chemistry inside the cell.
Structure-to-soil potential reading. The half-cell gives a defined contact with the soil at one specific point.
The half-cell is a portable, repeatable, well-understood reference. Two CP techs walking the same line on the same day with the same kind of half-cell will read the same potential at the same spot — within a few millivolts. That repeatability is why it’s the standard.
The −850 mV criterion
The classic CP criterion for a buried steel structure: a structure-to-soil potential of −0.85 V (or more negative) measured against a Cu/CuSO4 reference is considered evidence of cathodic protection. That’s the number on the wall. A reading of −0.94 V — the one in the hook — is more negative than −0.85 V, so it meets the criterion.
The negative sign isn’t optional. CP makes the structure the cathode of a deliberate electrochemical circuit, which means the structure ends up at a more negative potential than the surrounding soil. We measure that negativity by putting the half-cell at a defined potential and reading the difference. A pipe sitting in moist soil with no CP applied typically reads somewhere between −0.40 V and −0.65 V. With CP working, that reading drives more negative — past −0.85 V — and stays there.
Current — the flow that does the actual job
Current is the rate of electrical charge flow through a circuit. The unit is the ampere (or amp, A). The variable is I.
If voltage is the pressure, current is the water actually moving through the pipe. In CP work, current is what does the protective job — current entering the structure surface from the surrounding soil drives the electrochemical reaction that prevents corrosion. No current, no protection. The voltage criterion (−0.85 V) is how we confirm that enough current is flowing.
Where current shows up in a CP system
Three places, three different stories about the same circuit:
- Rectifier output current. What the impressed-current power supply is putting into the system — measured at the rectifier with the cabinet’s internal shunt (or a portable shunt across the output terminals). Reads in amps, often 5 A to 50 A on typical impressed-current systems.
- Bond shunt current at a test station. What’s flowing through a galvanic anode bond, a foreign-line bond, or a test connection — measured in millivolts across a calibrated shunt and converted to amps via Ohm’s Law (the move you walked through in EC-002). Reads from a few milliamps up to several amps.
- Current entering the structure surface. What’s happening at the metal-soil interface itself — the current density that determines whether enough current is reaching the pipe to maintain the −0.85 V criterion. Measured indirectly via current density coupons or polarization techniques (a later module).
The three are connected. The rectifier sources the current. The cable + groundbed + soil delivers it to the structure. The interface is where it does the work. A failure at any one of those points shows up as a different reading at a different place, which is why a CP tech walks all three.
Current density — a quick name
The reason current density (current per unit area of structure surface) matters more than total current alone: a 30-amp system on a 50-mile pipeline is delivering one current density, the same 30 amps on a 5-mile pipeline is delivering ten times more. The CP criterion isn’t really about total amps — it’s about whether enough current per unit of bare-steel surface is reaching the structure to drive the cathodic reaction. Coating quality, structure geometry, and groundbed placement all show up as differences in the current density delivered, even if the amperage stays the same.
Resistance — opposition, sets the budget
Resistance is opposition to current flow. The unit is the ohm (Ω). The variable is R.
Resistance in a CP circuit isn’t one thing. It’s a stack of different resistances in different places, each measured differently, each contributing to how much current the rectifier has to push to deliver enough protective current at the pipe. The “budget” framing matters here: the source can only push so much voltage, and Ohm’s Law says I = V ÷ R. Total resistance sets the ceiling on current at any given source voltage.
The four resistances every CP tech meets
In rough order of how big each one tends to be in a typical impressed-current installation:
- Anode-to-soil (or anode-to-earth) resistance — the contact resistance between each anode and the soil it’s buried in. Usually the largest single component. Climbs as anodes consume and as soil dries. Calculated from the anode dimensions and the surrounding soil resistivity; measured indirectly by reading rectifier V and I and applying Ohm’s Law.
- Soil resistivity — a property of the soil itself, expressed in ohm-centimeters (Ω-cm). Not a circuit resistance you can measure with a regular ohmmeter — it’s an intrinsic property measured with a 4-pin Wenner test. Drives anode-to-soil resistance directly.
- Cable resistance — the resistance of the conductor between the rectifier and the groundbed. Usually fractions of an ohm, but at high current it eats meaningful voltage as IR drop (EC-002 worked this).
- Coating resistance — the resistance of the pipe coating itself, expressed per unit area. The whole reason coating exists from a CP perspective: high coating resistance means most of the current goes around the coating (through coating defects called holidays), which is exactly where the protection needs to land.
Each one is measured differently and reported in different units. None of them are read with a generic “ohms” setting on a multimeter applied at the rectifier output — the multimeter’s internal ohmmeter circuit isn’t built for the scales involved or the soil-electrochemistry effects. The R you “read” in the field is almost always backed out from V and I using Ohm’s Law (R = V ÷ I — the diagnostic Ohm’s-Law rearrangement from EC-002), or measured via a purpose-built instrument like a soil resistivity meter with the 4-pin Wenner setup.
The four readings every CP tech makes
Different setup, different instrument mode, different question being answered — but each one is a measurement of one of the three quantities. Here’s what each looks like in the field.
Scenario 1 — Structure-to-soil potential (a voltage)
The hook scenario, restated
Setup: Cu/CuSO4 half-cell pressed into moist soil near the pipe. Pipe lead clipped from the test station header. Multimeter on V DC. Black lead to the half-cell, red to the pipe.
What’s being measured: the difference in electrical potential between the structure and the soil at that location, referenced through the half-cell’s known potential.
Typical reading: −0.85 V to −2.5 V on a working impressed-current system. −0.40 V to −0.65 V on unprotected pipe.
What it tells you: whether the pipe meets the −0.85 V criterion at that point. A series of such readings along the pipeline, taken at multiple test stations, is what builds the protective-status picture for the whole system.
Scenario 2 — Rectifier output (a voltage and a current)
At the cabinet
Setup: rectifier-mounted V meter and A meter (or shunt + portable meter on mV).
What’s being measured: the source voltage the rectifier is putting onto the cable, and the current it’s pushing into the system. Two separate readings, two different stories.
Typical reading: 12 V to 50 V on V; 5 A to 50 A on I. Both depend on the system size and the auto-tap setting.
What it tells you: the source-side state of the system. V and I together also let you back out total circuit resistance via R = V ÷ I (the same diagnostic rearrangement EC-002 covered) and compare today’s R against the design.
Scenario 3 — Bond current at a test station (a current via a shunt)
The hook revisited from a different angle
Setup: bond shunt in series with a galvanic anode group (or foreign-line bond, or test connection). Multimeter on mV DC across the shunt.
What’s being measured: a voltage drop across a known resistance — converted via Ohm’s Law to a current.
Typical reading: a few milliamps to a few amps for a galvanic anode bond; up to tens of amps on heavier bonds.
What it tells you: how much current is actually flowing through that specific connection. Compared against design current and prior readings, it tells you whether the anode group is performing.
Scenario 4 — Soil resistivity (a resistance, named only)
Wenner 4-pin — the right tool for the right job
Setup: a soil resistivity meter with four pins driven in a straight line at equal spacing, connected to the meter’s four terminals.
What’s being measured: the bulk electrical resistivity of the soil, in ohm-centimeters (Ω-cm).
Typical reading: 500 Ω-cm for very wet, mineral-rich soil up to 100,000+ Ω-cm for dry, sandy soil. Most pipeline corridors fall between 1,000 and 30,000 Ω-cm.
What it tells you: the soil’s intrinsic resistance per unit volume — the input that drives anode-to-soil resistance, groundbed sizing, and CP system design. The procedure itself is in a later module; the point here is that this is a resistance reading, and it requires a purpose-built tool.
Side by side — the three quantities at a glance
| Quantity | What it is | Where you measure it | What it tells you |
|---|---|---|---|
| V | Electrical pressure between two points | Cabinet output, between half-cell and pipe at a test station, across a shunt, between two ends of a cable | Source state, protective status (vs. −0.85 V criterion), IR drop, calibrated shunt readings |
| I | Rate of charge flow — what does the actual protective work | Rectifier output (via shunt), bond shunts at test stations, indirectly at the structure surface (current density) | System output, anode performance, whether enough current is reaching the structure |
| R | Opposition to current flow — sets the budget | Backed out from V and I (Ohm’s Law); soil resistivity via Wenner 4-pin; anode-to-soil and coating R via design + measurement | System resistance vs. design, soil conditions, groundbed health, coating integrity |
Voltage requires a reference. Current does the work. Resistance sets the budget. Three quantities, three jobs, three different ways to read them.
Three readings, one diagnosis
Walk back to the test station from the hook. You took the structure-to-soil potential — −0.94 V. Before leaving the site, you stop at the rectifier cabinet and read the source: 22 V output, 12 A current. Three readings in one visit. Each one tells you a piece of the story; together they tell you how the system is sitting.
- −0.94 V structure-to-soil at the test station. That’s a voltage, referenced against Cu/CuSO4. More negative than the −0.85 V criterion. The pipe meets criterion at that location. Voltage answers: is the pipe protected here?
- 22 V rectifier output. Also a voltage, but a different one — it’s the source pressure the rectifier is delivering. Compared to the rectifier’s design max (often 24 V or 50 V), this tells you how hard the system is having to push to maintain the protective state. 22 V on a 24 V max rectifier is close to the ceiling — if the system needs more, there’s nowhere to go.
- 12 A rectifier current. That’s the current — what’s actually flowing into the soil to do the work. Compared to design current (say, 10 A nominal), 12 A is a little over — could be coating defects letting more current escape, could be wetter soil today than at design.
Now back out the total circuit resistance from V and I using R = V ÷ I:
If commissioning recorded Rtotal at 1.5 Ω, today’s 1.83 Ω is a quarter of an ohm higher than design. That’s a piece of information you don’t get from any single reading — it comes from combining V and I, knowing what each one represents, and applying the math.
That’s the point of distinguishing the three quantities. Each reading individually answers one question. Together, they tell you the system’s health. And the only way to do that synthesis honestly is to know what each one is.
Key takeaways
- Voltage is electrical pressure, measured between two points. In CP, the canonical voltage is structure-to-soil potential — pipe vs. soil, referenced through a Cu/CuSO4 half-cell. The −0.85 V Cu/CuSO4 criterion is the standard floor for protected steel.
- The Cu/CuSO4 half-cell is the practical default reference for buried-pipeline CP work — stable, repeatable, well-correlated with decades of CP criteria.
- Current is the rate of charge flow — what does the protective electrochemical work at the structure surface. It shows up in three places: rectifier output, bond shunts at test stations, and current density at the structure-soil interface.
- Resistance is opposition to current flow and isn’t a single thing in CP. The four R’s: anode-to-soil resistance, soil resistivity, cable resistance, coating resistance. Each is measured differently.
- R is almost always backed out from V and I via Ohm’s Law (R = V ÷ I — the diagnostic rearrangement from EC-002), or measured with a purpose-built instrument like a 4-pin Wenner soil resistivity meter — not via the ohms scale on a regular multimeter at the rectifier output.
- Three readings at one site, one diagnosis. Structure-to-soil V tells you protection state. Rectifier V and I tell you source state. The trio together tells you system health.
References & further reading
- AMPP CP 1 Cathodic Protection Tester Course Manual — the half-cell, structure-to-soil potential measurement, and the −0.85 V Cu/CuSO4 criterion.
- AUCSC Basic Course — V/I/R foundations and field-measurement coverage of reference electrodes and CP criteria.
- Peabody’s Control of Pipeline Corrosion — A.W. Peabody. CP criteria framework and survey/measurement practice.
- AMPP CP 2 Cathodic Protection Technician Course Manual — extended CP-1 treatment with technician-level depth on field measurements, current density, and survey practice.
- CP Survey Procedures — practical procedural depth on close-interval surveys, depolarization, and reference-electrode practice.
- Corrosion Basics: An Introduction — broader textbook framing on the role of V, I, and R in CP design.
Listen — narrated walkthrough
Voltage, Current, and Resistance in DC Systems
Same scope as the read — what voltage, current, and resistance each are, where each one shows up in a CP system, and how each one is measured at a real test point — walked through visually with the structure-to-soil reading as the anchor.
Narrated by Mike Roberts · ~21 min
Listen on the drive in or while waiting for the coating to cure. Come back for the deck or the worked problems whenever you want.
Once you’ve worked through the audio or the deck, head to the Apply lesson for three measurement scenarios — and then the quiz to lock it in.
Apply — three problems
Read the meter, name the quantity, make the call.
Three problems. The point isn’t arithmetic — it’s recognizing what each instrument setup is actually measuring, what reading means what, and how to put two or three readings together into a system diagnosis. Work through each one before you click. Match your reasoning to ours.
How to use this lesson. Read the setup. Think it through. Each step reveals our reasoning. If your answer doesn’t match, the steps are there to compare against.
Three setups, three readings — name what’s being measured.
−1.05 V.180 mV.4,800 Ω-cm.For each setup: which of the three quantities (V, I, R) is being measured, what does the reading represent, and what does it tell you about the system at that point?
Setup A — what’s being measured
Quantity: Voltage (V).
What it represents: the structure-to-soil potential at this test station — the difference in electrical potential between the pipe and the soil, referenced through the Cu/CuSO4 half-cell.
What it tells you: the pipe is at −1.05 V Cu/CuSO4 — more negative than the −0.85 V criterion. The pipe meets criterion at this location.
Setup B — what’s being measured
Quantity: Current (I) — measured indirectly via a voltage drop across a known resistance.
What it represents: the rectifier output current. The voltmeter reads the IR drop the shunt makes visible. Apply Ohm’s Law (EC-002):
Convert to base units first: 180 mV → 0.18 V.
I = V ÷ R = 0.18 V ÷ 0.01 Ω = 18 A
What it tells you: the rectifier is putting 18 A into the system right now. Compared to design current and prior readings, that says whether the source side is performing.
Setup C — what’s being measured
Quantity: Resistance (R) — specifically soil resistivity, an intrinsic property of the soil itself.
What it represents: the bulk electrical resistivity of the soil, expressed in ohm-centimeters (Ω-cm), at the location and depth corresponding to the 5-ft pin spacing. (Wenner-method depth roughly equals pin spacing.)
What it tells you: the soil at this location is moderately conductive. Around 4,800 Ω-cm sits in the middle of the typical pipeline-corridor range (1,000–30,000 Ω-cm). This is a design input — it drives anode-to-soil resistance calculations and groundbed sizing — not a direct measure of system health.
Three setups, three different quantities, three different stories. The instrument and the test setup tell you what’s being measured before the number does. V requires a reference. I needs a known resistance to be inferred from a voltage drop. R demands a purpose-built tool — you don’t measure soil resistivity with a regular ohmmeter.
A structure-to-soil potential reading on a section that “should be fine.”
−0.78 V Cu/CuSO4. The system criterion is −0.85 V or more negative.
What does that reading mean, and what should you do next?
Step 1 — read the number against the criterion
The criterion is −0.85 V or more negative (i.e., −0.85 V, −0.90 V, −1.00 V, etc., all meet criterion). The reading is −0.78 V — less negative than −0.85 V. So this location does not meet criterion as read.
The structure is under-protected at this point relative to the standard. That doesn’t automatically mean corrosion is occurring — the criterion includes safety margin — but it does mean the system is not delivering the protective state the design intended at this location, and that’s worth resolving.
Step 2 — what could be causing it
Several possibilities, none of them mutually exclusive:
- No CP installed (or disconnected) at this section. A −0.78 V reading sits close to the natural-state range for unprotected steel in moist soil. Worth verifying the system here is actually energized and that the bond to this section is intact before chasing the other causes.
- Galvanic anode consumption. The anodes serving this section are doing the work of staying negative; as they consume, their output drops and the structure drifts toward less-negative readings. This is the most common cause on long-served galvanic systems.
- Drier soil than design assumed. Higher soil resistivity means more anode-to-soil resistance, which means less current onto the structure, which means less negative readings.
- Coating damage near this test station. A localized coating defect would mean more current is needed to protect that exposed area than the original design budgeted for. Other test stations might still meet criterion while this one drifts.
- Foreign-line interference or stray current. Less common but possible — another structure or DC source nearby could be pulling protective current away.
Step 3 — what to do next
- Read more locations. Is this isolated to one test station or is the whole section drifting? If isolated, the cause is local. If widespread, anode consumption or design assumptions changing are more likely.
- Check the anode bond currents. If a galvanic anode group is putting out less current than design (say, 50 mA today vs. 250 mA in the original commissioning report), that’s a strong signal toward anode consumption.
- Check the trend. The history matters. A single −0.78 V reading is one data point; six months of progressively less-negative readings at this station is a story.
- Document and elevate. A reading that misses criterion gets logged, gets a planned action, and gets re-read on the next visit. Not protected today doesn’t mean abandoned today — it means we know something about the system we didn’t know yesterday, and we have a plan.
The reading is just a number until you put it next to the criterion. The criterion turns the number into a status. The status turns the status into the next action. That chain — number → criterion → status → action — is the work of CP.
Three readings at one site, one health picture.
−0.86 V26 V. Across the rectifier’s “50 mV = 10 A” shunt, your multimeter on millivolts DC reads 36 mV.22 V, Idesign = 8 A, Rdesign,total = 2.75 Ω. Structure-to-soil at TS-04 at commissioning: −0.95 V.What’s the system telling you, and what would you flag for follow-up?
Step 1 — convert the shunt reading to current
The shunt rating is “50 mV = 10 A” — a 0.005 Ω shunt (50 mV ÷ 10 A → 0.05 V ÷ 10 A = 0.005 Ω in base units).
Convert the live reading to base units first: 36 mV → 0.036 V.
I = V ÷ R = 0.036 V ÷ 0.005 Ω = 7.2 A
Today’s rectifier output current: 7.2 A.
Step 2 — back out today’s total circuit resistance
Apply Ohm’s Law solve-for-R (the move you walked through in EC-002):
Rtotal = V ÷ I = 26 V ÷ 7.2 A ≈ 3.61 Ω
Compare to design: design Rtotal was 2.75 Ω. Today’s Rtotal is 3.61 Ω — about 0.86 Ω higher than design. That’s a meaningful climb (≈31% higher).
Step 3 — read the structure-to-soil against criterion
−0.86 V Cu/CuSO4 at TS-04 today. The −0.85 V criterion is met — barely. Compare against commissioning’s −0.95 V at the same test station: today’s reading is 0.09 V less negative than at commissioning. The system is meeting criterion but trending in the wrong direction.
Step 4 — what the three readings together say
The story the readings tell when read together:
- Source side: Rectifier voltage has climbed from 22 V (design) to 26 V — the system is auto-tapping up to compensate for something. Current has dropped from 8 A (design) to 7.2 A. That combination is the signature of rising total circuit resistance.
- Resistance side: Rtotal has climbed from 2.75 Ω to 3.61 Ω. Most likely candidates: anode consumption (lifting parallel-array R), drier soil (increasing anode-to-soil resistance), or a connection drift on the series side.
- Protected side: The structure-to-soil at TS-04 has drifted from −0.95 V at commissioning to −0.86 V today. Still meeting criterion (barely), but the headroom against criterion is shrinking. The rectifier has been compensating — but it’s nearly out of voltage headroom (most rectifiers max at 24 V, 36 V, or 50 V; 26 V says we’re well into the rectifier’s output range).
This is a system that’s still working, but is slowly losing margin. Not an emergency. Definitely a flag for follow-up.
Step 5 — what would you flag
- Trend the source readings. Pull the last 6–12 months of rectifier V/I logs. If V has been climbing month over month while I drops, we’re watching a slow groundbed degradation play out.
- Walk more test stations. Read structure-to-soil at TS-01, TS-02, TS-03, TS-05, TS-06. If TS-04 is the lowest reading on the line, the issue is local. If readings are uniformly tight, it’s the whole groundbed.
- Inspect the groundbed. If anode consumption is suspected, schedule a groundbed inspection: visual, electrical, and ideally individual anode current readings if the array has accessible per-anode shunts.
- Plan ahead, not in panic. 26 V on a system designed at 22 V means we have voltage left. But not unlimited. Knowing the trajectory now lets you plan a groundbed supplement or replacement before criterion is missed — instead of after.
One reading tells you status at a point. Three readings tell you the source state, the resistance state, and the protective state — and Ohm’s Law lets you derive a fourth piece (total R) from two of them. None of that synthesis happens unless you can name what each reading is. That’s why we distinguish V, I, and R.
Three problems, three different jobs the same trio of quantities does — identify the measurement, read the value against criterion, synthesize across multiple readings into a system picture.
You’ll do this same kind of work every time you walk a CP system. The instruments are different at every test station. The numbers change every visit. What stays the same is the discipline: name the quantity, know what it represents, compare it to the right reference, and read it alongside what else the system is telling you.
Up next: a quiz to confirm V, I, and R are wired in.
Voltage, Current, and Resistance in DC Systems
Foundation tier · EC TRACK · ELECTRICAL BASICS FOR CP
One module done. Keep going — you'll earn the certificate when you finish this section, and the Foundation medal when you complete every section in the tier.
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