User manual 09/2009 - Schneider Electric

schneider.electric.dk

User manual 09/2009 - Schneider Electric

The Protection FunctionConductor Heat Rise and Tripping CurvesAnalysis of the equation of heat rise in a conductor, through which a current I runs, can be used todetermine the nature of physical phenomena:• For low or medium intensity currents (I < Ir), the conductor equilibrium temperature (for an infinite time)only depends on the current quadratic demand value (see Quadratic Demand Value (Thermal Image),page 79). The limit temperature corresponds to a limit current (Ir pick-up for trip unit long timeprotection).• For low overcurrents (Ir < I < Isd), the conductor temperature only depends on the I 2 t energy providedby the current. The limit temperature is an I 2 t IDMT curve.• For high overcurrents (I > Isd), the phenomenon is identical if the I 2 t ON function of the short timeprotection has been configured (see I2t ON/OFF Function, page 44).The figure below (in double log scales) represents a heat rise curve A (for an equilibrium temperature θ)and a trip curve B (for the limit temperature θL):1 Low intensity current zone2 Low overcurrent zoneThermal MemoryMicrologic 5 and 6 trip units incorporate the thermal memory function which ensures that the conductorsare cooled even after tripping: cooling lasts for 20 minutes before or after tripping.42 LV434104 09/2009

More magazines by this user
Similar magazines