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3.1.2
CTs on 'Circuit' Side of the Circuit Breakers
Where CTs are located on only one side of the circuit breaker some protection zones do not extend fully to the CB
and protection 'blind spots' are formed. The areas of primary plant between the CTs and CB may not be protected
by the relevant protection zone e.g. F3 and F4 in Figure 3-2.
Faults F1 and F2 should be correctly cleared as before,
Fault F3 will only cause operation of busbar 1 protection because the fault is outside the zone of busbar 2
protection. Thus the busbar 2 fault will remain uncleared. Arrangements must therefore be made to cause the
busbar 2 CBs to be tripped to clear the fault.
F4: This is a circuit fault, the circuit CB is tripped by the busbar protection, however the busbar zone 1 is
unnecessarily tripped, this is usually considered as acceptable in view of the rare occurrences of such faults. On
interconnected systems the circuit fault will be cleared by the remote end protection.
Faults F1 and F2 should be correctly cleared as before, but F3 will only cause operation of the busbar protection
because the fault is outside the circuit protection zone. Thus the fault may remain fed from the remote end of the
circuit. Arrangements must therefore be made to cause the CB at the remote end of the line to be tripped under
these circumstances.
©2013 Siemens Protection Devices Limited
Figure 3-2
CTs on Circuit Side of CBs
Figure 3-3
CTs on Circuit Side of CBs
Chapter 7) 7SR23 DAD Applications Guide
Chapter 7 Page 21 of 42
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