Protection Characteristics; Time / Current Characteristics; Iec Characteristic - ABB REF601 Applications Manual

Section 4
Protection and Control Function
4.4
4.4.1
4.4.2
16

Protection characteristics

Time / Current characteristics

Relay offers three-stage overcurrent and two stage earth-fault protection functions.
The low-set stage of overcurrent protection and earth-fault protection are equipped
with standard Inverse Definite Minimum Time (IDMT) characteristics – (Normal
Inverse (NI), Extreme Inverse (EI), Long Inverse (LI), and Very Inverse (VI))
along with definite time (DT) characteristics for better co-ordination with rest of
the network. Additionally special characteristic curve RI is also provided. The
high stage and instantaneous stage for over current protection and high stage earth
fault protection come with DT characteristics.
When IDMT characteristic has been selected, the operating time of the stage will
be a function of the current; the higher the current, the shorter the operating time.
The stage includes six different time/current curve sets – four according to the BS
142 and IEC 60255 standards namely normal inverse, very inverse, extremely
inverse, longtime inverse and one special curve, named RI type curve along with
DT characteristics.

IEC characteristic

The relationship between current and time for standard normal inverse, very
inverse, extremely inverse and long-time inverse complies with the BS 142.1966
and IEC 60255-3 standards and can be expressed as follows:
)
=
( ) 1
Where,
t = operate time in seconds
K = time multiplier
I = measured current value
Iset = set start current value
The slope of the time/current characteristics shall be determined by the
constants and ß as indicated below:
Table 10: Values of constant
Slope of the time/current curve set
Normal inverse
Very inverse
Extremely inverse
Long time inverse
and
0.02
1.0
2.0
1.0
1MDU07212-YN Rev. A
0.14
13.5
80
120
REF601 / REJ601
Application Manual