8-6 Breaker Monitor, Metering, and Load Profile Functions Date Code 20001006
SEL-351 Instruction Manual
In Figure 8.2, note that the breaker maintenance curve levels off horizontally below set point
KASP1, COSP1. This is the close/open operation limit of the circuit breaker (COSP1 = 10000),
regardless of interrupted current value.
Also, note that the breaker maintenance curve falls vertically above set point KASP3, COSP3.
This is the maximum interrupted current limit of the circuit breaker (KASP3 = 20.0 kA). If the
interrupted current is greater than setting KASP3, the interrupted current is accumulated as a
current value equal to setting KASP3.
Operation of SELOGIC Control Equation Breaker Monitor Initiation Setting BKMON
The SELOGIC Control Equation breaker monitor initiation setting BKMON in Table 8.2
determines when the breaker monitor reads in current values (Phases A, B, and C) for the breaker
maintenance curve (see Figure 8.2) and the breaker monitor accumulated currents/trips [see BRE
Command (Breaker Monitor Data) in Section 10: Serial Port Communications and
Commands].
The BKMON setting looks for a rising edge (logical 0 to logical 1 transition) as the indication to
read in current values. The acquired current values are then applied to the breaker maintenance
curve and the breaker monitor accumulated currents/trips (see references in previous paragraph).
In the factory default settings, the SELOGIC Control Equation breaker monitor initiation setting is
set:
BKMON = TRIP (TRIP is the logic output of Figure 5.1)
Refer to Figure 8.3. When BKMON asserts (Relay Word bit TRIP goes from logical 0 to
logical 1), the breaker monitor reads in the current values and applies them to the breaker monitor
maintenance curve and the breaker monitor accumulated currents/trips.
As detailed in Figure 8.3, the breaker monitor actually reads in the current values 1.5 cycles after
the assertion of BKMON. This helps especially if an instantaneous trip occurs. The instantaneous
element trips when the fault current reaches its pickup setting level. The fault current may still be
“climbing” to its full value and then levels off. The 1.5-cycle delay on reading in the current values
allows time for the fault current to level off.
Figure 8.3: Operation of SELOGIC Control Equation Breaker Monitor Initiation Setting
See Figure 8.8 and accompanying text for more information on setting BKMON. The operation of
the breaker monitor maintenance curve, when new current values are read in, is explained in the
following example.
Courtesy of NationalSwitchgear.com
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