Over-Voltage Protectors
An over-voltage protector, or OVP, is used at isolation points when the system needs normal isolation but also needs a controlled bypass path during over-voltage events.
Quick Definition
An OVP blocks current below a selected voltage threshold and conducts when the voltage across the device attempts to exceed that threshold. In CP isolation applications, it is commonly used to protect isolated joints from lightning, AC fault current, static buildup, or other over-voltage conditions.
OVP below threshold
isolation maintained
device clamps
bypass path closes
device returns
to blocking state
Why OVPs Matter to CP Technicians
An OVP is not just a safety accessory. It can affect the CP system when it conducts. If one side of an isolation joint is cathodically protected and the other side is grounded, a conducting OVP can allow CP current to flow into the grounded side.
This is why continuous clamping matters. If the CP system drives the protected side beyond the device threshold for an extended period, the OVP may remain conductive and effectively bypass the isolation point while the condition persists.
Operational Example
Suppose an OVP is selected to clamp at approximately 1 volt in one polarity and a rectifier drives the isolated pipeline side to a condition that places approximately 1.25 volts across the OVP. The device may conduct as long as that voltage condition remains. In that condition, part of the CP current can be diverted through the OVP into the grounded facility side instead of staying on the intended buried pipeline.
What to Remember
- The OVP protects the isolation point from over-voltage stress.
- The selected blocking voltage matters.
- Conduction is not automatically a failure; it may be the device doing its job.
- Continuous conduction can become a CP current-loss problem.
- Lead length matters for lightning performance because conductor inductance contributes to clamping voltage.
OVP vs. SSD
OVPs are a good introductory decoupling concept because they teach normal isolation plus emergency bypass. Solid-state decouplers are more advanced because they can block DC cathodic protection current while allowing AC current to pass continuously.