Inverse Definite Time Reset Curves ( Idmt ) - DEMA 400 Series User Manual

4.3.1.4.2

Inverse Definite Time Reset Curves ( IDMT )

RIDMT curves are used to obtain the inverse D.M.T. resetting characteristics for IEEE/ANSI
and custom curves that are needed to provide selectivity schemes when electromechanical
relays are protecting a primary line in series with the one 400 Series protects. The RIDMT
parameters differ with the tripping curve they are based on. The table 400 Series Protection
and Reset Curves Parameters on page 16 shows these parameters. The sample image below
shows the CO2 short time inverse trip characteristic and its RIDMT curve. The formula given
below explains the calculation method of the reset delay for CO2 RIDMT curve.
6.9 s
t= ×RTMS
2
I
1-( )
I
[ ]
s
Sample IDMT formula: CO2 IDMT Reset Curve Formula
Please check the table on page 16 for other IDMT formulas
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