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DT characteristic
(i)
The DT operates time characteristic is a constant one and will not be affected by the level of
energizing current. The reset time is set to be zero so that resetting is performed immediately.
The user can have time settings discussed in section (iv).
IDMT characteristic
(ii)
The IDMT operate-time characteristic is inversely proportional to the level of the energizing
current. The user can choose a operate time characteristic: ten characteristic curves are
available; i.e. IEC, UK, IEEE, US and an original (user programmable characteristic curve;
ORG that the user can set the equation parameters). The curve equation and parameters will
be explained in section (iii).
In IEEE, US, and ORG time characteristics, the users can select a reset characteristic
time: independent (definite)-time-reset (DT) or inverse-time-reset (DEP) or definite-time-reset
(DEF). The inverse-time-reset (DEP) equation and parameters will be explained in section (v).
Operate time of IDMT
(iii)
The dependent time in the IDMT are defined by Equation (2.4-4). (ref IEC 60255-151):
where:
t = operating time for constant current I (seconds),
I = energizing current (amperes),
Is = earth fault current setting (amperes),
TMS = time multiplier setting,
k, α, c = constants defining curve.
The nine pre-programmed standard characteristic curve are defined in Table 2.4-1. In
addition, one original (user-programmable) curve can be applied. One curve can be chosen for
each SEF stage. Figure 2.4-3 IDMT Characteristics curves illustrates these characteristic
curves. For example, the user should set one for the scheme switch [SEF1-Timer]. (That is,
"IEC-NI" for IEC Normal Inverse, "IEC-VI" for IEC Very Inverse, "IEC-EI" for IEC Extremely
Inverse, "UK-LTI" for UK Long Time Inverse, "IEEE-MI " for IEEE Moderately Inverse, "IEEE-
VI" for IEEE Very Inverse, "IEEE-EI" for IEEE Extremely Inverse, "US-CO2" for US CO2 short
time inverse, "US-CO8" for US CO8 inverse, and "Original" for user-programmable
characteristic.)
In the IDMT, the user can also program the characteristic using time-multiplier-setting
k
�� ( �� ) = ������ × {[
α
( I I
⁄ )
s
- 72 -
] + c}
− 1
6F2T0207 (0.01)
(2.4-4)
GRE200 (5,6)
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