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6 F 2 S 0 7 5 8
IEEE Reset Curves
(Time Multiplier = 1)
1000.00
100.00
EI
VI
10.00
CO8
MI
CO2
1.00
0.1
1
Current (Multiple of Setting)
Figure 2.1.4
Dependent Time Reset Characteristics
2.1.1.2 Definite Time Overcurrent Protection
In a system in which the fault current does not vary a great deal in relation to the position of the
fault, that is, the impedance between the relay and the power source is large, the advantages of the
IDMT characteristics are not fully utilised. In this case, definite time overcurrent protection is
applied. The operating time can be constant irrespective of the magnitude of the fault current.
The definite time overcurrent protection consists of instantaneous overcurrent measuring elements
and delayed pick-up timers started by the elements, and provides selective protection with graded
setting of the delayed pick-up timers. Thus, the constant time coordination with the downstream
section can be maintained as shown in Figure 2.1.5. As is clear in the figure, the nearer to the
power source a section is, the greater the delay in the tripping time of the section. This is
undesirable particularly where there are many sections in the series.
Operate time
T
C
T
C
A
B
C
Figure 2.1.5 Definite Time Overcurrent Protection
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