Areva MiCom P120, Technical Manual, Page: 232 / 668

P12x/EN AP/A96 Application Guide
Page 10/80
MiCOM P120/P121/P122/P123
t
I
I>> I>
t_I>
I> >> >>>Interlock NO
P0358ENa
I> t
I>> t
I>>> t
I> t
I>> t
I>>> t
t
I
I>> I>
t_I>>
I> >> >>>Interlock YES
t_I>>>
I>>>
2.3 DMT thresholds
The three phase (earth) overcurrent thresholds can be selected with a time constant delay.
The time to operate is equal to the time delay set, plus the time for the output contact to
operate (typically about 20 to 30 ms ; 20ms for a current exceeding or equal to 2 times the
threshold) and the time required to detect the overcurrent state (maximum 20ms at 50Hz).
For DMT curves, a reset timer "t Reset" is associated with the first and second thresholds
(phase and earth elements).
2.4 IDMT thresholds
2.4.1 Inverse time curves
The first and second phases (earth) overcurrent threshold can be selected with a dependent
time characteristic. The time delay is calculated with a mathematical formula.
There are eleven inverse time characteristics available.
The mathematical formula applicable to the first ten curves is:
t=T x (
K
(I/I
S
)
α
–1
+L )
Where:
t = Tripping time
K = Coefficient (see table)
I = Value of measured current
I
S
= Value of the programmed threshold (Pick-up value)
α
= Coefficient (see table)
L = ANSI/IEEE coefficient (zero for IEC curves)
T = Time multiplier between 0.025 and 1.5