Treating Output as Compliance
A rectifier can be operating and still fail to provide adequate protection at all required locations.
Rectifier Output Formula for Cathodic Protection
Rectifier output readings are used to evaluate impressed current CP system operation. The most common values are DC voltage and DC amperage, which can also be used to estimate effective circuit resistance.
Rectifier output is operating data. It helps explain system behavior, but it does not prove adequate CP by itself.
Formula
R = V / I
| Symbol | Meaning | Common Unit |
|---|---|---|
| R | Effective circuit resistance | ohms |
| V | Rectifier DC voltage output | volts |
| I | Rectifier DC current output | amps |
Related Ohm's Law Forms
| Solving For | Formula | Typical CP Use |
|---|---|---|
| Resistance | R = V / I | Estimate effective CP circuit resistance from rectifier output. |
| Current | I = V / R | Estimate expected current if voltage and resistance are known. |
| Voltage | V = I x R | Estimate voltage required to drive a target current. |
Worked Example 1: Effective Circuit Resistance
A rectifier is operating at 48 volts and 4 amps. Estimate the effective circuit resistance.
R = V / I
R = 48 / 4
R = 12 ohms
The effective circuit resistance is 12 ohms. This value should be compared with historical readings to determine whether the circuit is behaving normally.
Worked Example 2: High Voltage and Low Current
A rectifier is operating at 60 volts and 2 amps. Estimate the effective resistance and interpret the result.
R = V / I
R = 60 / 2
R = 30 ohms
A relatively high effective resistance may indicate an open lead, failed splice, depleted or high-resistance groundbed, dry electrolyte conditions, or another circuit problem. Field troubleshooting is required before assigning a cause.
Worked Example 3: Required Voltage
A CP system needs 6 amps of current and has an effective circuit resistance of 8 ohms. Estimate the voltage required.
V = I x R
V = 6 x 8
V = 48 volts
The estimated voltage required is 48 volts. If the rectifier cannot supply that voltage, the system may not be able to deliver the target current through the existing circuit resistance.
CP Interpretation Notes
- Rectifier output confirms operating voltage and current, not full structure protection.
- High voltage with low current can indicate high resistance or an open circuit.
- Low voltage with unusually high current can indicate a short, unintended continuity, or increased current demand.
- As-found and as-left readings should both be documented when adjustments are made.
- Structure-to-electrolyte potentials are still required to evaluate CP criteria.
Common Mistakes
Ignoring Historical Trends
A single output reading is less useful without comparison to prior voltage, amperage, tap settings, and field potential response.
Missing Polarity Problems
Reversed polarity can make the intended protected structure discharge current and corrode rapidly.
Practice Problems
- A rectifier operates at 36 volts and 3 amps. What is the effective resistance?
- A CP circuit requires 5 amps through 10 ohms. What voltage is required?
- A rectifier changes from 8 amps to 2 amps while voltage increases. What type of condition should be investigated?