Siemens SIPROTEC 7SS523 V3.2 Manual page 350

Appendix
A.3 Connection Diagrams
Legend:
BI: Binary Input
CZ: Check zone
BU: Bay unit
I-DIFF: Differential current
I-SUM: Summation current
CB: Circuit breaker
M: Alarm reley
BZ: Bus zone
BF: Breaker failure
ISO: Isolator
CU: Central unit
Figure A-3 Connection diagram central unit (continued in Figure A-4, page 335)
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7SS52 V4 Manual
C53000-G1176-C182-1