Protection relays detect faults by comparing the quantity (and angles in some cases) of the primary circuit current or voltage to a pre-determined setting. This comparison is done electromechanically for induction-type relays and digitally or electronically for digital or static. The relays are in round glass cases. : 4 The first. The main relay protection functions (overcurrent, directional, differential, distance, etc. ) are briefly explained in this technical article. Reduced Damage: Isolating faulty sections.