FANOX SIA-C, User Manual

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4.13. Application examples
It is important to know that if both overcurrent protection functions (50 and 50/51), phase or neutral,
are enable, definite time function (function 50) must be more restrictive. So, if overcurrent fault
values are low, inverse time overcurrent function (function 50/51) must work, and if overcurrent fault
reaches a certain value, definite time overcurrent function will always work. This is because, when
overcurrent fault reach high values (I>>), it is necessary to be sure that trip is going to be
instantaneous to get that the element we are protecting, does not be damaged.
It is shown somo examples below:
APPLICATION EXAMPLE 1
Starting from the following information:
Line details:

Transformation ratio of CT =100/1

Primary current: I
p
=100 A
50/51 function settings

Curve type: IEC Inverse

Dial: 0.05

Tap: 1xI
n
50 function settings

Tap: 11xI
n

Operating time: 0.05 s
Figure 1. 50 and 50/51 IEC Inverse
If overcurrent fault is 11xI
n
=1100 Ap, IEC inverse curve defines a tripping value of 0.1425s (Figure1)
for 50/51 function. It is considered that this time is too high, so when current fault reaches 11xI
n
,
definite time overcurrent function will be work.