The operation of 50BF can be described using a module diagram. All the modules in the
diagram are explained in the next sections. Also further information on the retrip and
backup trip logics is given in sub-module diagrams.
A070445-ANSI V1 EN
Figure 155:
Functional module diagram
Level detector 1
The measured phase currents are compared phasewise to the set
Current value
. If the
measured value exceeds the set
Current value
, the level detector reports the exceeding of
the value to the start, retrip and backup trip logics. The parameter should be set low enough
so that breaker failure situations with small fault current or high load current can be
detected. The setting can be chosen in accordance with the most sensitive protection
function to start the breaker failure protection.
Level detector 2
The measured residual current is compared to the set
Current value Res
. If the measured
value exceeds the set
Current value Res
, the level detector reports the exceeding of the
value to the pickup and backup trip logics. In high-impedance grounded systems, the
residual current at phase-to-ground faults is normally much smaller than the short circuit
currents. To detect a breaker failure at single-phase ground faults in these systems, it is
necessary to measure the residual current separately. In effectively grounded systems,
also the setting of the ground-fault current protection can be chosen at a relatively low
current level. The current setting should be chosen in accordance with the setting of the
sensitive ground-fault protection.
Start logic
The start logic is used to manage the starting of the timer 1 and timer 2. It also resets the
function after the circuit breaker failure is handled. On the rising edge of the
PICKUP
input, the enabling signal is send to the timer 1 and timer 2.
1MRS240050-IB C
Section 5
Protection-related functions
REF615R
319
Technical Manual
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