Toshiba GR200 Series Instruction Manual page 248

The OC element will reset later in a period determined by the characteristics of dependent
time resetting. In other words, the user can choose the behavior of the OC element for either
faults that are repetitive intermittent or faults that occur in rapid succession. The
characteristic for dependent time resetting is selectable from five dependent time
characteristics in conformance with IEEE and US standards.
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
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6F2S1914 (0.49)
GRL200 (Soft 033 & 037)