order of 30% of this value, i.e. equal to the per phase charging current of the remaining system. Practically though,
the r
equired setting may well be determined on site, where suitable settings can be adopted based on practically
obtained results.
When using a core-balanced transformer, care must be taken in the positioning of the CT with respect to the
earthing of the cable sheath:
SEF
E00614
Cable box
Cable gland
Cable gland/shealth
earth connection
“Incorrect”
No operation
“Correct”
Operation
SEF
Figure 63: Positioning of core balance current transformers
If the cable sheath is terminated at the cable gland and directly earthed at that point, a cable fault (from phase to
sheath) will not result in any unbalanced current in the core balance CT. Therefore, prior to earthing, the
connection must be brought back through the CBCT and earthed on the feeder side. This then ensures correct
relay operation during earth fault conditions.
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 137
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