1MDU07205-YN rev. F
REF601 CEI
User’s manual
32
4.7 Protection characteristics
4.7.1 Time / Current characteristics
REF601 offers three-stage overcurrent and two stage earth-fault protection functions.
The low-set stage of overcurrent protection is equipped with standard Inverse-defi nite
Minimum Time (IDMT) characteristics - Very Inverse (VI), while the low set stage of
earth-fault protection has a DMT characteristics for better co-ordination with rest of
the network. The high-set and very high-set stages for over current protection and
high stage earth fault protection come with DMT 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 seven time/current curve sets – four according to the BS 142 and IEC
60255 standards namely normal inverse, very inverse, extremely inverse and long-
time inverse and one special curve, named RI type curve.
4.7.2 IEC characteristics
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:
where,
t = operate time in seconds
K = time multiplier
I = measured current value
I set = set start current value
The slope of the time/current characteristics shall be determined by the
constants à and ß as indicated below:
1)(
*
−
=
a
b
Iset
I
K
t
ß
Slope of the time/current curve set à ß
Very inverse 1.0 13.5
Protection Functions
In case of REF601, accuracy class 5 is applicable. The details of accuracy points as
per BS: 142:1966 are given below:
I/I> Very Inverse
2 2.34E
5 1.26E
7 ---
10 1.01E
20 1.00E
Note: E stands for accuracy class i.e. in this case it is 5 for phase and external earth fault whereas for
internal earth fault the accuracy class is 7.5
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