The Wrong Shunt Reads Zero

Around Five miles of new 12-inch line, well coated, protected by magnesium anode banks connected in test stations along the right-of-way. High-potential anodes, 10 to a bank, just as the design required. You are there for the first annual readings.

The potentials look good. Using portable GPS-sync’d interrupters at every connection point and your reference cell you’re looking at −1.658 V ON and −1.164 V OFF. Polarized, with some margin. The anodes are doing what they were installed to do.

Then you move the leads to the shunt to measure a voltage drop and calculate current output. The meter reads 0.0 mV. You check the leads, clean the posts, try again. 0.0 mV, with a flicker to 0.1.

Both measurements cannot be true. The 494 mV between ON and OFF is IR drop, and you know you’ve added polarization - which needs current. 0.0 mV on the shunt says there is none. One of those two readings is telling you about the instrument, not the pipe.

Look at the shunt. It is orange.

A shunt is a resistor with a known resistance, installed in series so you can measure current without opening the circuit. Put the voltmeter across it, read the millivolt drop, and Ohm's law gives you the current: I = V ÷ R, with the millivolts converted to volts.

Two habits go with every shunt reading.

Read across the small potential posts, not the bolts. The posts sit at the ends of the calibrated element. The bolts and lugs are outside it, and a reading taken there includes whatever resistance is in the connection.

Read the ohm value off the shunt every time. Most shunts for test stations are color-coded and labeled with the ohms and the amp capacity.

Why not put the multimeter on the milliamp range and break the circuit? Because the milliamp range on many digital multimeters puts several ohms in series with a circuit that only has a few ohms in it to begin with, and the current you read is not the current that was flowing before you got there. AMPP CP 2 discusses this and that a permanently installed shunt as the more accurate method. The shunt is part of the circuit from the day the test station is installed, so every reading you take is of the system as it actually operates.

When a galvanic test station gets a shunt, it is usually one of three board-style shunts. Same footprint, holes in the right spot, three colors.

The colors are not one company's scheme. Cott, M.C. Miller, and Tinker & Rasor all sell board shunts in the same three colors at the same three values. Other values exist for specific jobs, and bar shunts for rectifiers and bonds are rated in amps and millivolts rather than by color. The color is a convention. The value marked on the shunt is the fact.

The last column is the one this article is about.

AMPP CP 2: "When selecting a shunt, its current rating must exceed the anticipated circuit current, and the millivolt drop at the anticipated operating current should be easily measurable on a standard digital multimeter." Two conditions. Most of us learned the first one and never heard the second.

The ceiling is the amp rating. A red shunt is rated 2 A. At 2 A it drops 200 mV and dissipates 0.4 W across a short length of resistance wire. Run it past its rating and it heats, and once the magic smoke gets out it will burn open, and now the anodes or the bond is disconnected and nobody knows it until the next survey. On a magnesium bank the ceiling is usually not a concern. On a bond it can be.

The floor is your meter. The common field multimeter resolves 0.1 mV on its millivolt range. Whatever the shunt drops below that does not exist as far as the display is concerned. So one digit (0.1) is worth 1 mA on a red shunt, 10 mA on a yellow, and 100 mA on an orange.

Peabody's puts it plainly in the instrumentation chapter: a 0.001 ohm shunt read with a 0.1 mV meter gives you current to plus or minus 0.1 A. Another example: Put 54 mA through a 0.01 ohm shunt and the drop is 0.54 mV. A meter that resolves 0.1 mV shows 0.5 mV, and you write down 50 mA. Nothing is broken. The meter just cannot see the 4.

Peabody's answer is a meter that resolves 0.01 mV: if you get a zero reading, use a more sensitive meter. That works, if every tech and contractor for the next twenty years carries one. Swapping the shunt is a fix that stays in the test station.

The rule only works if you have a number in mind for the current before you pick the shunt. For galvanic anodes: "Anode currents can vary between a few milliamperes up to several hundred milliamperes, depending upon the number of anodes and the soil resistivity."

A single 17-lb packaged magnesium anode on well-coated pipe in 5,000 ohm-cm soil can do around 25 to 30 mA depending on the alloy. In 10,000 ohm-cm soil, about half that. After the line polarizes, less again. Ten anodes in a bank do not give ten times one anode; the multiple-anode factors put a ten-anode bank at roughly six to nine times a single anode, depending on spacing.

A new, well-coated line is the low end of all of this. It needs very little current, and a galvanic bank's output is set by the driving voltage across the whole circuit (the pipe's resistance to earth included) not just by how much magnesium is in the ground. Anode weight is typically more to do with the design life and once the pipe polarizes, the current output drops. The bank on a new line is oversized for the first survey on purpose. It is sized for year 20 or more.

So the realistic range for a magnesium anode bank at a test station is tens of milliamps. It may be a little higher in some situations. Take a look at the table. On an orange shunt you essentially see zero or a flickering digit. On a yellow you get one or two digits. On a red you get two or three digits, and a 2 amp ceiling you won’t need.

Take the bank from the intro and say it is putting out 38 mA.

Same current. Three readings. One of them is a measurement.

Does the higher-resistance shunt change the current? Rarely enough to matter in a galvanic circuit. In Peabody's single-anode example the anode-to-earth resistance plus the pipe comes to 6.81 ohms, and a 0.1 ohm shunt is 1.5% of that. In the AUCSC Advanced Course's seven-anode example in 1,000 ohm-cm soil the whole circuit is 1.67 ohms, and a red shunt would be 6%. Small either way, and more to the point, constant. Peabody notes that the common 0.01 ohm shunt has no substantial effect on anode output in most cases, and CP 2 is comfortable recommending a full ohm when the current is down at 5 mA. The shunt stays in the circuit, so whatever resistance it adds has been part of the system since day one, and every reading you take is of the system as it actually runs.

Magnitude is half the reading. Put the positive lead on the pipe-side post and the negative lead on the anode-side post.

Conventional current leaves the anode, crosses the soil, collects on the pipe, comes up the pipe lead, passes through the shunt, and goes back down the anode lead to the anode. In the metallic path, current flows from pipe to anode. With your leads set as above, current enters the meter on the positive terminal and the display reads positive.

Positive means the anode is discharging. That is what a galvanic anode is for. Record the sign along with the number.

Negative means the anode is picking up current. A magnesium anode is bare metal sitting in low-resistivity backfill and connected to your pipe. Where a foreign rectifier's current is flowing in the soil, the anode can behave like a very large holiday, collecting stray current onto the structure instead of discharging its own. The anode shunt is where you catch it. Once the leads are confirmed on the right posts and the terminals are confirmed as labeled, a negative reading at an anode station is not a sign to flip on the data sheet. It is an interference lead to be followed.

The same two limits apply to a bond between structures. The numbers move.

A resistance bond draining interference, a continuity bond across a coupling, or a bond across an isolation joint can carry amps rather than milliamps. Bonds should have a shunt between them for measurement of magnitude and direction of current flow. Here the ceiling matters more. A red shunt on a 5 A bond isn’t going to work. A yellow at 8 A, an orange at 25 A, or a bar shunt rated for the design current is the right part, and the sizing math for the bond itself is in "Drainage Bond Sizing: The Calculation Behind the Mitigation."

Direction does the same job at a bond that it does at an anode. The bond was designed to carry current one way. The shunt tells you whether it is, and how much, every time you visit.

Which leads to the one rule that covers anodes and bonds both. Any time two structures, or an anode and a structure, are connected through a test station, connect them through a shunt. A direct connection or a shorting bar proves the connection exists. It cannot tell you what is flowing across it, and it cannot tell you which way.

The orange shunt in the intro was not defective. Its smallest digit is 100 mA and the bank is putting out 38. Swap the orange for a red. The same bank now reads 3.8 mV. Divide by 0.1 ohm, 38 mA, positive, discharging.

Then write the shunt value on the record. A year from now somebody else is going to read this station, and 3.8 mV means one thing on a red shunt and another thing entirely on another.

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.

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