Toshiba GRL150-100 Series Instruction Manual page 28

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Definite Time Protection
In a system in which a fault current does not vary significantly with the position of the fault, the
advantages of the IDMT characteristics are less apparent. In this case, definite time overcurrent
protection is applied. The operating time can be set irrespective of the magnitude of the fault
current.
Definite time overcurrent protection consists of instantaneous overcurrent elements with on-delay
timers.
Identical current values can be set for all terminals, but graded settings are better than identical
settings in order to provide a margin for current sensitivity. The further 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 the inverse time protection setting.
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