Mixing Amps and Milliamps
Convert milliamps to amps before using Ohm's law with volts and ohms. For example, 500 mA is 0.5 amps.
Ohm's Law for Cathodic Protection
Ohm's law is one of the most common calculations used in cathodic protection. It relates voltage, current, and resistance, and it is used when evaluating rectifier output, shunt current, voltage drop, circuit resistance, and expected system response.
Formula
V = I x R
| Symbol | Meaning | Common Unit |
|---|---|---|
| V | Voltage | volts |
| I | Current | amps |
| R | Resistance | ohms |
Rearranged Forms
| Solving For | Formula | Use Case |
|---|---|---|
| Voltage | V = I x R | Estimate voltage required to drive a known current through a known resistance. |
| Current | I = V / R | Estimate current output when voltage and circuit resistance are known. |
| Resistance | R = V / I | Estimate effective circuit resistance from rectifier voltage and current. |
Worked Example 1: Solve for Current
A rectifier is operating at 24 volts and the effective circuit resistance is 6 ohms. Estimate the current.
I = V / R
I = 24 / 6
I = 4 amps
The estimated current is 4 amps. In the field, this value should be compared with the rectifier ammeter and historical output to determine whether the circuit behavior is reasonable.
Worked Example 2: Solve for Resistance
A rectifier is operating at 48 volts and 4 amps. Estimate the effective CP circuit resistance.
R = V / I
R = 48 / 4
R = 12 ohms
The effective circuit resistance is 12 ohms. A sudden increase in effective resistance may indicate an open lead, failed splice, depleted anode bed, poor electrolyte contact, or other circuit problem.
Worked Example 3: Solve for Voltage
A CP circuit requires 5 amps and has an effective resistance of 8 ohms. Estimate the voltage required.
V = I x R
V = 5 x 8
V = 40 volts
The estimated voltage required is 40 volts. If the rectifier cannot provide the required voltage, the system may not be able to drive the intended current through the circuit.
CP Interpretation Notes
- Ohm's law helps explain circuit behavior, but it does not prove adequate cathodic protection by itself.
- Rectifier voltage and current should be compared with structure-to-electrolyte potentials, current distribution, and historical data.
- High voltage with low current often suggests high resistance or an open circuit condition.
- Low voltage with unusually high current may suggest a short, added continuity, coating deterioration, or increased current demand.
- Always keep amps, milliamps, volts, millivolts, and ohms separated correctly.
Common Mistakes
Using Nameplate Values
Rectifier nameplate capacity is not the same as actual operating voltage and current. Use measured output for circuit calculations.
Ignoring Field Conditions
A calculated resistance is only an effective circuit value. It does not identify the exact location of a problem without additional testing.
Practice Problems
- A rectifier operates at 36 volts and 6 amps. What is the effective resistance?
- A CP circuit has 3 amps flowing through 10 ohms. What voltage is required?
- A shunt has 25 mV across 0.01 ohm. What current is flowing?