A071142 V1 EN
Figure 114: Example of three-stage overcurrent protection
The maximum three-phase fault current is 41.7 kA and the minimum three-phase
short circuit current is 22.8 kA. The actual accuracy limit factor of the CT is calculated
to be 59.
The start current setting for low-set stage (3I>) is selected to be about twice the
nominal current of the cable. The operate time is selected so that it is selective with
the next relay (not visible in the figure above). The settings for the high-set stage and
instantaneous stage are defined also so that grading is ensured with the downstream
protection. In addition, the start current settings have to be defined so that the relay
operates with the minimum fault current and it does not operate with the maximum
load current. The settings for all three stages are as in the figure above.
For the application point of view, the suitable setting for instantaneous stage (I>>>)
in this example is 3 500 A (5.83 x I
2n
). For the CT characteristics point of view, the
criteria given by the current transformer selection formula is fulfilled and also the
relay setting is considerably below the F
a
. In this application, the CT rated burden
could have been selected much lower than 10 VA for economical reasons.
Section 11 1MRS756378 D
Requirements for measurement transformers
228 REF615
Application Manual
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