What percent of a line do Zone 2 relays look at?

Zone 2 relays are designed to provide protection for transmission lines and associated equipment. They are typically set to operate based on a specified reach of the line, which is generally beyond the immediate line segment. Zone 2 relays usually cover a portion of the line length that extends outward from the end of the protected section to provide backup protection in case of a fault that is not detected by Zone 1 relays.

The correct answer indicates that Zone 2 relays look at 120% of a line, meaning they can detect faults not only on the specific section that is protected but also extend that coverage to reach slightly beyond the line segment. This additional reach enhances reliability by ensuring that any faults occurring at the endpoints or beyond are still effectively coordinated and managed by the protection system.

In practice, having a Zone 2 reach of 120% is useful for minimizing blind spots that could pose operational risks. It allows for coverage of potential faults that may not directly affect the line but could still impact system reliability or stability.