Protection Basics
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Relay protection time calculation
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Current setting, actual current & curve type constants determine relay operation time according to the International Electrotechnical Commission's mathematical formula. The typical IEC equation is: t = TMS x [k / ( (I/I p)^α – 1)] Where: t – Operating time in secondsSelective short-circuit protection can be achieved in different ways, such as: Time-graded protection Time- and current-graded protection A straightforward way of obtaining selective protection is to use time grading. For successful protection coordination, relay working times must be accurately calculated since overcurrent relays activate when circuit current exceeds a predetermined threshold limit. The free online Time Overcurrent Relay Calculator lets electrical engineers immediately calculate relay operate. Use this Protection Relay Setting Calculator to calculate pickup current, time multiplier settings (TMS), operating time, coordination time interval (CTI), and plug setting multiplier (PSM) using fault current, CT ratio, and IEC 60255 curve parameters. Zone1 is consid-ered to be the main protection for the line to be protected, hence no intentional time delay is allowed. Direction: Forward Typically required zone 2 reach impedances = 100% line impedances.
