Forgetting mV to V Conversion
A shunt reading in millivolts must be converted to volts before using Ohm's law with ohms.
Voltage Drop Formula for Cathodic Protection
Voltage drop occurs when current flows through resistance. In cathodic protection, voltage drop is important when evaluating circuit behavior, shunt readings, bond currents, and structure-to-electrolyte potential measurements.
Voltage drop is useful for calculations, but unwanted IR drop can also distort CP potential measurements if the test condition is not controlled.
Formula
V = I x R
| Symbol | Meaning | Common Unit |
|---|---|---|
| V | Voltage drop | volts or millivolts |
| I | Current | amps |
| R | Resistance | ohms |
Common Rearranged Forms
| Solving For | Formula | Use Case |
|---|---|---|
| Voltage drop | V = I x R | Calculate voltage drop through a known resistance. |
| Current | I = V / R | Calculate current through a known shunt or bond resistance. |
| Resistance | R = V / I | Estimate resistance from measured voltage drop and current. |
Worked Example 1: Voltage Drop in a Bond
A bond carries 3 amps through a resistance of 0.2 ohms. Calculate the voltage drop.
V = I x R
V = 3 x 0.2
V = 0.6 volts
The voltage drop is 0.6 volts. Bond current direction and magnitude should be interpreted with the structure potentials and the purpose of the bond.
Worked Example 2: Shunt Current from Millivolts
A 0.01 ohm shunt has 25 mV measured across it. Calculate the current.
25 mV = 0.025 volts
I = V / R
I = 0.025 / 0.01
I = 2.5 amps
The current through the shunt is 2.5 amps. Always convert millivolts to volts before using volts, amps, and ohms together.
Worked Example 3: CP Circuit Voltage Requirement
A CP circuit must drive 4 amps through 12 ohms of effective resistance. Estimate the voltage required.
V = I x R
V = 4 x 12
V = 48 volts
The estimated voltage requirement is 48 volts. If the required voltage is higher than the rectifier can provide, the system may not achieve the intended current output.
CP Interpretation Notes
- Voltage drop is expected anywhere current flows through resistance.
- ON potentials can include voltage drop from current flowing in the electrolyte.
- Instant-off readings are used to reduce current-flow voltage drop effects when interruption is valid.
- Shunt calculations require correct resistance and correct unit conversion from millivolts to volts.
- Measured voltage drop does not identify the exact problem without additional field checks.
Common Mistakes
Confusing Useful and Unwanted Drop
Voltage drop can be useful for shunt current calculations, but unwanted IR drop can distort potential measurements.
Ignoring Current Direction
For bonds and interference testing, current direction can be as important as magnitude.
Practice Problems
- A 0.01 ohm shunt has 40 mV across it. What current is flowing?
- A bond carries 1.5 amps through 0.4 ohms. What is the voltage drop?
- A CP circuit has 6 amps flowing through 7 ohms. What voltage is required?
Related Pages
Field Use Reminder
Voltage drop calculations are most useful when they are tied to a specific field purpose, such as calculating shunt current, evaluating bond current, estimating circuit voltage requirement, or recognizing when current-flow voltage drop may be affecting a potential reading.