Schweitzer Engineering Laboratories SEL-352-1 - Fault Clearing Pickup Time (Fcpu); Fault Logic Enable (FTLOG)

3-18 Breaker Logic Date Code 20010731
SEL-352-1, -2 Instruction Manual
In the example, minimum Zone 1 three-phase fault duty after accounting for fault study errors,
I
3ph
, equals:
I
3ph
secondaryA9.09=
600
primaryA5456
=
The 50FT element accuracy specification for a nominal 5 A relay is ±0.025 A secondary ±5% of
setting (±0.005 A ± 5% of setting for 1 A I
nom
.) Thus, the worst-case error for a setting of 9.09 A
secondary is:
50FT error
(
)
(
)
A9.090.05A0.025 ¼+=
secondary A0.479=
To ensure that 50FT asserts for all Zone 1, three-phase faults, we set the 50FT element:
50FT
A479.0A09.9 -=
secondary A61.8=
Fault Logic Enable (FTLOG)
Select the best scheme for your application based on the bus configuration of the protected
breaker, operating considerations listed, and timing requirements.
Because the example protected breaker is in a single-breaker installation, any of the five
available schemes may be used. We selected Scheme 2 based on the simplicity of the scheme
and the fact that the trip input may be deasserted once the breaker failure scheme initiates.
Fault Clearing Pickup Time (FCpu)
For the example application, all Zone 1, three-phase faults must clear within 8.5 cycles to
maintain system stability. Thus, total fault clearing time (Tt) is 8.5 cycles. Nominal tripping
time of local breakers (Tbl) and remote breakers (Tbr) is 2 cycles. Transfer trip channel time to
the remote breaker (Tc) is 0.5 cycle. Operating time of the line protective relay (Tpr) for a 3-
phase fault near the Zone 1 boundary is 2 cycles. Specified reset time of the 50FT overcurrent
element T50 is less than 0.75 cycle. The FCpu time and safety margin (Ts) are calculated as
follows:
Ts = Tt – (Tpr + Tbl + T50 + Tbr + Tc) cycles
Ts = 8.5 – (2.0 + 2.0 + 0.75 + 2.0 + 0.5) cycles
Ts = 1.25 cycles
You can now use the following equation to calculate FCpu:
FCpu = Tbl + T50 + Ts = 4.0 cycles
Note that auxiliary relay operate times are not separated from breaker operate times. If the high
current interrupting output contacts are used to trip breakers directly, then you do not need to
consider auxiliary relay operate times.