A definite time over-current (DTOC) relay is a relay that operates after a definite period of time once the current exceeds the pickup value. Hence, this relay has current setting range as well as time setting range.OverviewIn, a protective relay is a device designed to trip a when a is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving par. Electromechanical protective relays operate by either, or. Unlike switching type electromechanical with fixed and usually ill-defined operating voltage thresholds.