Toshiba GRD140 Instruction Manual

⎯ 14 ⎯6 F 2 S 0 7 5 8IEEE Reset Curves(Time Multiplier = 1)1.0010.00100.001000.000.1 1Current (Multiple of Setting)Time (s)MIVIEICO2CO8Figure 2.1.4 Dependent Time Reset Characteristics2.1.1.2 Definite Time Overcurrent ProtectionIn a system in which the fault current does not vary a great deal in relation to the position of thefault, that is, the impedance between the relay and the power source is large, the advantages of theIDMT characteristics are not fully utilised. In this case, definite time overcurrent protection isapplied. The operating time can be constant irrespective of the magnitude of the fault current.The definite time overcurrent protection consists of instantaneous overcurrent measuring elementsand delayed pick-up timers started by the elements, and provides selective protection with gradedsetting of the delayed pick-up timers. Thus, the constant time coordination with the downstreamsection can be maintained as shown in Figure 2.1.5. As is clear in the figure, the nearer to thepower source a section is, the greater the delay in the tripping time of the section. This isundesirable particularly where there are many sections in the series.Operate timeTCTCA B CFigure 2.1.5 Definite Time Overcurrent Protection