Protection Characteristics; Time / Current Characteristics; Iec 60255-3 Idmt Characteristic - ABB REF601 Applications Manual

1MDU07212-YN Rev. C
4.10
4.10.1
4.10.2
REF601 / REJ601
Application Manual

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 ten different time/current curve sets – four according to the BS
142 and IEC 60255 standards namely normal inverse, very inverse, extremely
inverse, longtime inverse, four according ANSI C37.xxx standard namely moderate
inverse, normal inverse, very inverse, extremely inverse and one special curve,
named RI type curve along with DT characteristics.

IEC 60255-3 IDMT 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 α and ß as indicated below:
Table 19: Values of constant
Slope of the time/current curve set
IEC – Normal inverse
IEC – Very inverse
IEC – Extremely inverse
IEC – Long time inverse
Protection and Control Function
α β
and
0.02
1.0
2.0
1.0
Section 4
a b
0.14
13.5
80
120
31