40
TB MRI3-ITE(R) 12.00 E
7.4.3
Inverse time overcurrent protection relay
According to IEC 255-4 or BS 142
Normal Inverse (type A)
[ ]
t
I
Is
t
I
s
=
−
>
014
0 02
1
,
,
Very Inverse (type B)
[ ]
t
I
Is
t
I
s
=
−
>
135
1
,
Extremely Inverse (type C)
[ ]
t
I
Is
t
I
s
=
−
>
80
2
1
Long time inverse
[ ]
s
t
1
Is
I
120
t
I
>
=
-
RI-inverse
[ ]
s
t
I
I
236
.
0
339
.
0
1
t
I
S
>
−
=
*RXIDG – characteristic
[ ]
s
t
I
I
I
*
35
.
1
8
.
5
t
I
S
n
>
⋅
⋅
−
=
Where:
t = tripping
time
t
I
> = time multiplier
I = fault
current
Is = Starting current
I
N
= natural logarithm
*only for earth current
