What conditions must a protective relay device 67 meet to cause tripping?

A protective relay device, such as a type 67 relay, is designed to provide protection by detecting specific conditions in electrical systems that could indicate a fault, particularly during faults involving current flow. The correct choice aligns with the functionality of these relays which must respond to certain electrical characteristics—specifically, they monitor over-current levels.

In this case, the relay must function based on a desired value of alternating current (a-c) that is flowing in a predetermined direction. This means that the relay is set to activate when the current exceeds a set threshold while also ensuring that it is detecting faults from specific directions, which is especially important in applications like differential protection. This capability allows for the relay to function in a targeted manner, providing responsive trips when abnormal conditions occur, thus preventing damage to the equipment and ensuring the safety of the entire electrical system.

The focus on current in a defined direction is crucial for selective coordination of protection devices, ensuring that only the necessary devices trip when fault conditions arise, rather than interrupting service unnecessarily or too broadly.