Toshiba GR200 Series Instruction Manual page 148

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.
DT characteristic
(ii)
The operate time in the DT characteristic will be a constant time and is not affected by the
level of current. The reset time is set to zero so that resetting is performed immediately. The
user can set the operate time using a setting; it is discussed in in sections (vi) and (vii).
IDMT characteristic
(iii)
The operate time in the IDMT characteristic is inversely proportional to the level of the fault
current. The user can determine the operate time from a preset curve. Ten characteristic curves
are available; i.e. nine pre-installed curves covering IEC, IEEE, US and UK standard
characteristic curves and one original (user programmable) characteristic curve for which the
user can set the equation parameters. The IDMT curve equation and parameters will be
explained in section (iv).
The reset time in the IDMT is user-selectable; either inverse-time-reset or definite-time-
reset is selected. For the inverse-time-reset, the user can choose the reset time for the preset
characteristic from six characteristic curves; i.e. IEEE and US standard five pre-installed
characteristic curves and one user original characteristic curve where the user can set the
equation parameters. The inverse-time-reset curve equation and parameters will be discussed
in section (vi).
Operate time of IDMT
(iv)
The operate time in the IDMT is defined by Equation (2.6-1). (ref. IEC 60255-151):
where:
t = operate time with constant current I (seconds),
I = operate (energizing) current (amperes),
Is = overcurrent setting (amperes),
k
t ( I ) = TMS × {[
α
( �� ��
⁄ )
��
- 134 -
] + ��}
− 1
Centralized GRB200
(Soft: 030, 031, 032, 033, 034)
6F2S1931 (0.20)
(2.6-1)