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in section (v).
(iii) Operate time of IDMT
The dependent time in the IDMT is defined by Equation (2.16-4). (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 2.16-2. In
addition, one original (user-programmable) curve can be applied. One curve can be chosen for
each EF stage. Figure 2.16-3 illustrates these characteristic curves. For example, the user
should set one for the scheme switches [EF1-Timer]. (That is, “IEC-NI” is 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 operation, the user can also program the characteristic using time-
multiplier-setting (TMS) for the required operate time. For example, the IEC characteristic is
selected, the user can set the TMS value using the setting [EF1-TMS-IEC] (incidentally, the
default value of the TMS is 1.00). The other TMSs are also available by the [EF1-TMS-UK],
[EF1-TMS-IEEE], [EF1-TMS-US] and [EF1-TMS-ORG].
Table 2.16-2 Constants for IDMT
Curve type
(IEC 60255-151)
IEC Normal Inverse (IEC-NI)
IEC Very Inverse (IEC-VI)
IEC Extremely Inverse (IEC-EI)
UK Long Time Inverse (UK-LTI)
IEEE Moderately Inverse (IEEE-MI)
IEEE Very Inverse (IEEE-VI)
IEEE Extremely Inverse (IEEE-EI)
US CO2 short time inverse (US-CO2)
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