Relay protection threshold start time

A protective relay starts operating when the current through it exceeds its set pickup value, with the operating time determined by its time characteristic and settings.Pickup Current and Start ConditionThe start of a relay is defined by the pickup current, which is the minimum current required to overcome the relay's controlling force and initiate operation. This current is adjustable through the current setting of the relay, often expressed as a percentage of the current transformer (CT) rated secondary current. For example, according to standards, relays with normal, very, or extremely inverse time characteristics start when the energizing current exceeds 1.3 times the set start current, while long-time inverse relays start at 1.1 times the set start current .Time CharacteristicsThe operating time of a relay depends on its time characteristic:Definite time relays operate after a fixed time delay, independent of fault current magnitude.Inverse time relays operate faster as the fault current increases, making them suitable for radial networks where fault currents vary significantly . The Time Setting Multiplier (TSM) adjusts how quickly the relay contacts close, while the Plug Setting Multiplier (PSM) relates the fault current to the relay's pickup current. The combination of PSM and TSM determines the total operating time, which can be visualized on a time vs. PSM curve .Coordination and GradingRelay start time is also influenced by coordination with other relays in the network. Time-graded protection ensures that the relay closest to the fault operates first, while upstream relays operate with a delay to maintain selectivity. The time interval between relays must be sufficient to prevent upstream relays from tripping before the fault is cleared by the nearest relay .SummaryIn practice, a relay starts when:The current exceeds the pickup threshold (adjusted by current settings and PSM).The time delay (definite or inverse) elapses according to the relay's characteristic and TSM.Coordination with other relays ensures selective operation and system protection. This combination of current threshold and time delay defines the relay protection start time, ensuring both fast fault clearance and selective protection of the network .

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Relay protection threshold start time