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2.5.2
Definite Time Overcurrent Protection
In a system in which fault current does not change greatly with the position of the fault, the
advantages of the IDMT characteristics are not fully realised. In this case, the definite time
overcurrent protection is applied. The operating time can be set irrespective of the magnitude of
the fault current.
The definite time overcurrent protection consists of instantaneous overcurrent elements and
on-delay timers started by them.
Identical current values can be set for terminals, but graded settings are better than identical
settings in order to provide a margin for current sensitivity. The farther from the power source
the terminal is located, the higher sensitivity (i.e. the lower setting) is required.
The operating time of the overcurrent element of each terminal is constant irrespective of the
magnitude of the fault current and selective protection is implemented by graded settings of the
on-delay timer. As a result, the circuit breaker of the terminal most remote from the power source
is tripped in the shortest time.
When setting the on-delay timers, time grading margin Tc is obtained in the same way as
explained in Section 2.5.1.
Setting
The setting elements necessary for the definite time overcurrent protection and their setting
ranges are shown below.
Element
Range
OC
0.5 - 100.0 A
( 0.1 - 20.0 A
TOC
0.00 - 10.00 s
OCBT
ON/OFF
EF
0.5 - 5.0 A
( 0.10 - 1.00 A
TEF
0.00 - 10.00 s
[EFBT]
ON/OFF
[EFBTAL]
ON/OFF
(*) Current values shown in the parentheses are in the case of 1 A rating. Other current values are in
the case of 5 A rating.
Step
Default
0.1 A
10.0 A
0.1 A
2.0 A) (*)
0.01 s
3.00 s
ON
0.1 A
5.0 A
0.01 A
1.00 A) (*)
0.01 s
3.00 s
ON
ON
86
6 F 2 S 0 8 5 0
Remarks
Phase overcurrent
OC delayed tripping
OC backup protection
Residual overcurrent
EF delayed tripping
EF backup protection
EF backup trip alarm
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