8.6
SEL-400 Series Relays Instruction Manual Date Code 20171006
Monitoring
Circuit Breaker Monitor
Another part of the circuit breaker maintenance curve falls vertically at the right
set point (B1KASP3, B1COSP3). This is the maximum interrupted current limit
(for the Example 8.1 circuit breaker, this is at B1KASP3 := 20.0). If the inter-
rupted current exceeds setting B1KASP3, the relay sets contact wear at 105 percent.
Preloading Contact Wear Data
Upon the first commissioning of the relay, the associated circuit breakers can
already have some wear. You can preload a separate amount of wear for each pole
of each circuit to preload existing contact wear data. The relay accepts integer
values of percentage wear as great as 100 percent. The relay adds the incremental
contact wear at the next circuit breaker monitor initiation (and at all subsequent
initiations) to the preloaded value to obtain a total wear value. The limit for
reporting circuit breaker contact wear is 150 percent for each pole.
Example 8.2 I
2
t Criteria Application
Some circuit breaker manufacturers do not provide a circuit breaker mainte-
nance curve, but specify the accumulated fault current arcing time (I
2
t) for
circuit breaker maintenance. For example, manufacturer’s data specify I
2
t
per phase at 750 kA
2
seconds for a particular circuit breaker, at a rated arcing
duration for each trip of 1 cycle. The circuit breaker maximum interrupting
current rating is 40 kA, and the continuous load current rating is 2 kA.
You can construct the contact wear curve for this circuit breaker from the
specified I
2
t. Choose B1KASP1 := 2.0 (the continuous current rating) and
B1KASP3 := 40.0 (the maximum interrupting current rating). Choose the
middle of the contact wear curve based on experience and system fault stud-
ies. The majority of faults a typical circuit breaker interrupts are single-line-
to-ground faults. Therefore, set BnKASP2 at or slightly greater than the
expected single-line-to-ground fault current (B1KASP2 := 10.0 kA in this
example). Using the following equations, calculate these settings points to
obtain the number of close/open operations:
Equation 8.1
Equation 8.2
Equation 8.3
In these equations, t
arc
is the arcing time in seconds; t
arc
= (1/f
NOM
)• (arc
duration in cycles); f
NOM
is the nominal power system frequency (50 Hz or
60 Hz). These calculations show the number of close/open operations
rounded to the nearest unit.
B1COSP1 =
I
2
t
(B1KASP1)
2
• t
arc
----------------------------------------------
750
2
2
• (0.01667 • 1)
----------------------------------------- := 11250=
B1COSP2 =
I
2
t
(B1KASP2)
2
• t
arc
--------------------------------------------
750
10
2
• (0.01667 • 1)
-------------------------------------------- := 450=
B1COSP3 =
I
2
t
(B1KASP3)
2
• t
arc
--------------------------------------------
750
40
2
• (0.01667 • 1)
-------------------------------------------- := 28=
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