CHAPTER 5: SETTINGS MONITORING ELEMENTSF60 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 5-3055Figure 5-167: IEC 255-8 sample operate and reset curvesThe thermal overload protection element estimates accumulated thermal energy E using the following equationscalculated each power cycle. When current is greater than the pickup level, In > k × I B , element starts increasing thethermal energy:Eq. 5-35When current is less than the dropout level, In > 0.97 × k × I B , the element starts decreasing the thermal energy:Eq. 5-36where∆t is the power cycle durationn is the power cycle indext op(In) is the trip time calculated at index n as per the IEC255-8 cold curve or hot curve equationstrst(In) is the reset time calculated at index n as per the reset time equation
