Date Code 20010731
Breaker Logic
3-17
SEL-352-1, -2 Instruction Manual
Trip Timer Dropout Time (TTdo)–Scheme 1
Range:
0–8191 cycles in 1/8-cycle steps
Use
SET <ENTER>
to access this setting. The TTdo setting controls the assertion time of the
62TT timer output (TTAD). During this time, the breaker failure logic cannot be reset.
Set TTdo longer than the maximum breaker operate time but at least one cycle less than the
FCpu setting. The TTdo setting should be similar to your SlowTr setting. A recommended
TTdo setting is 125 percent to 150 percent of the nominal breaker operating time if manufacturer
data are not available.
Setting TTdo longer than the maximum breaker operate time ensures that 62FC does not reset
before assertion of the fault current detector after an adjacent breaker clears in a multiple breaker
application. When two breakers serve one piece of equipment, the fault current detector (50FT)
may only pick up on one breaker. Once that breaker trips, the other breaker fault current detector
will pick up.
Setting TTdo at least one cycle behind the FCpu setting ensures that 62FC has an opportunity to
reset if there is no breaker failure.
Trip Timer Dropout Time (TTdo)–Scheme 4
Range:
0–8191 cycles in 1/8-cycle steps
Use
SET <ENTER>
to access this setting. It determines the duration of FBF assertion and,
therefore, the duration of the trip output to your lockout relay.
Set the TTdo timer to one cycle or longer.
Setting Calculations
This section contains setting calculations for an example SEL-352 Relay breaker failure
application for fault current conditions. Only settings necessary for this specific application are
referenced. Note that you will need different settings for different scheme selections. Please
select relay settings appropriate to your application. Refer to
Section 7: Setting the Relay
for a
complete list of settings.
The output of the fault current breaker fail protection is the assertion of the FBF Relay Word bit.
This bit must be used in control logic for energizing an external lockout relay. Refer to
Lockout
Relay Control
in
Section 4: Close Logic
.
Fault Detector (50FT)
For our example, system conditions necessitate clearing of Zone 1, three-phase faults by the fault
current breaker protection scheme if the protected breaker fails.
When you select the 50FT element setting, account for fault study errors and then account for
possible error in the current element measurement. By calculating the worst-case measurement
error and subtracting that value from the relay setting, you ensure that the element operates for
the current level your protection scheme specifies.
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