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purpose, the binary input „>BF Start w/o I" No. 1439 is provided (Figure 2-148
left). This input initiates the breaker failure protection even if no current flow is detect-
ed.
Common Phase
Common phase initiation is used, for example, for lines without automatic reclosure,
Initiation
for lines with only three-pole automatic reclosure, for transformer feeders, or if the bus-
bar protection trips. This is the only available initiation mode if the actual 7SD5 model
is able to trip three-pole only.
If the breaker failure protection is intended to be initiated by further external protection
devices, it is recommended, for security reasons, to connect two starting criteria to the
device. Besides the trip command of the external relay to the binary input „>BF
Start 3pole" No. 1415 it is recommended to connect also the general device
pickup to binary input „>BF release" No. 1432. For Buchholz protection it is rec-
ommended that the trip command is connected to the device by two separate wire
pairs.
Nevertheless, it is possible to initiate the breaker failure protection in single-channel
mode should a separate release criterion not be available. The binary input „>BF
release" (No. 1432) must then not be assigned to any physical input of the device
during configuration.
Figure 2-148 shows the operating principle. When the trip signal appears from any in-
ternal or external feeder protection and at least one current flow criterion (according
to Figure 2-145) is present, the breaker failure protection is initiated and the corre-
sponding delay time(s) is (are) started.
If the current criterion is not fulfilled for any of the phases, the position of the circuit
breaker auxiliary contact(s) is queried provided that this is available according to
Figure 2-147. If the circuit breaker poles have individual auxiliary contacts, the series
connection of the three normally closed (NC) auxiliary contacts is used. The circuit
breaker has operated correctly after a three-pole trip command only when none of the
phases carries current or when all three NC auxiliary contacts have closed.
Figure 2-147
If an internal protection function or an external protection device trips without current
flow, the internal input „Start internal w/o I" or the external input „>BF Start w/o I"
7SD5 Manual
C53000-G1176-C169-1
Creation of signal "CB ≥ any pole closed"
2.20 Circuit Breaker Failure Protection
361
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