Toshiba GR-200 Series Instruction Manual page 41

When definite time resetting is selected with the operation of the inverse time OC
selection and the reset time set to zero (instantaneous), no intentional delay is added for
the resetting: that is, when the energizing current falls below the reset threshold of the
OC element, the OC element returns to its reset condition. If a value is set to the reset
time in seconds, the resetting of the OC element is delayed by the reset time.
Definite time delay (independent time characteristic)
In a system in which the fault current does not vary a great deal relative to the location
of the fault, definite time overcurrent protection is usually applied. Because the fault
current does not change greatly with respect to fault position the previous advantages
described with regard to the inverse time function are not applicable. Therefore, the
definite time OC element is used instead of the inverse time OC element. The definite
time OC element has a fixed operating time regardless of the magnitude of the fault
current.
The operating time of the definite time OC element is set depending on the
distance from the power source and should be set shorter for relays furthest from the
power source. Definite time OC protection provides selectivity dependent on the
difference between relay operating times. It is limited to use in networks with a power
source at one side only i.e. in radial networks. The threshold values can be set identically
at each terminal, but preferably, the threshold value is reduced, little by little to provide
a margin in terms of the current sensitivity of the OC element. Moreover, a higher
sensitivity of threshold current is required for a protective relay that is located remote
from the power source, that is, the threshold of the definite time OC element is required
to be lower than the ordinary threshold.
The operate time of the definite time OC element is constant irrespective of the
magnitude of the fault current and the threshold level of operation of the definite time
OC element with an on-delay timer provides a selective protection. As a result, the
circuit breaker (CB) at the remote terminal far from the power source can be tripped in
the shortest time. The on-delay timer, which generates a delay in starting the operation
of the OC element, provides a time grading margin (Tc); Tc is discussed later.
Definite time resetting as well as the on-delay starting setting of the definite time
OC element is provided. The operate time for the on-delay timer for starting and for the
resetting time is set using the OC element settings.
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6F2T0200 (0.15)
GRE200 (1,2)