The RD inverse curve gives a logarithmic delay, as used in the Combiflex protection
RXIDG. The curve enables a high degree of selectivity required for sensitive residual
earth-fault current protection, with ability to detect high-resistive earth faults. The
curve is described by equation
122:
[ ] 5.8 1.35 ln= - ×
× >
æ ö
ç ÷
è ø
i
t s
k in
EQUATION1195 V1 EN (Equation 122)
where:
in> is the set start current for step n,
k is set time multiplier for step n and
i is the measured current
The timer will be reset directly when the current drops below the set start current level
minus the hysteresis.
21.3 Inverse time characteristics
When inverse time overcurrent characteristic is selected, the operate
time of the stage will be the sum of the inverse time delay and the set
definite time delay. Thus, if only the inverse time delay is required, it
is of utmost importance to set the definite time delay for that stage to
zero.
Table 574: ANSI Inverse time characteristics
Function
Range or value Accuracy
Operating characteristic:
( )
1
P
A
t B k tDef
I
æ ö
ç ÷
= + × +
ç ÷
ç - ÷
è ø
EQUATION1249-SMALL V2 EN
I = I
measured
/I
set
k = (0.05-999) in steps of 0.01 -
ANSI Extremely Inverse A=28.2, B=0.1217, P=2.0
ANSI Very inverse
A=19.61, B=0.491, P=2.0
ANSI Normal Inverse A=0.0086, B=0.0185,
P=0.02, tr=0.46
ANSI Moderately Inverse A=0.0515, B=0.1140, P=0.02
ANSI Long Time Extremely Inverse A=64.07, B=0.250, P=2.0
ANSI Long Time Very Inverse A=28.55, B=0.712, P=2.0
ANSI Long Time Inverse A=0.086, B=0.185, P=0.02
1MRK 502 048-UEN A Section 21
Time inverse characteristics
689
Technical manual
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