Zone 3 relays monitor what percent of a line?

Zone 3 relays are designed to provide backup protection for transmission lines and are typically set to monitor a reach that extends significantly beyond the end of the protected line. The common practice for Zone 3 settings is a pickup distance that is approximately 150% of the line length. This setting allows the relay to detect fault conditions effectively while ensuring that it doesn't inappropriately trip for faults far from the line.

The percentage reflects the capability of the relay to account for potential faults that could occur on adjacent lines or in the case of long transmission lines where the length of the line may require a broader monitoring scope. This extended coverage is vital for ensuring system reliability during unexpected outages or faults, thereby contributing to the overall safety and operational integrity of the power grid.