Schweitzer Engineering SEL-387-0, Instruction Manual

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5.11
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Metering and Monitoring
Breaker Monitor
and (B n KAP3, B n COP3). As shown in Figure 5.5 , the point (B n KAP1,
B n COP1) must represent the lowest current value, point (B n KAP2, B n COP2)
an intermediate current value, and point (B n KAP3, B n COP3) the maximum
current value. The relay will not accept the settings unless B n KAP1 <
B n KAP2 < B n KAP3.
For values of current in kA (I) below B n KAP1, the number of operations is
assumed to be the same as for B n KAP1. In this part of the curve, the number
of operations may be governed more by the cumulative mechanical wear-and-
tear on the breaker operating mechanism, rather than actual contact
degradation. For values of I above B
n KAP3, there is assumed to be no breaker
capability to interrupt, and 100 percent contact wear is assumed. B n KAP3,
then, is typically set at the maximum rated interrupting current for the
particular breaker. B
n KAP1 is set at a value approximating the continuous
load current rating of the breaker. B n KAP2 is set at some intermediate value
of current, chosen to provide the closest visual “fit” to the manufacturer’s
curve.
The two straight line segments of the curve between the three defined points
define number of operations as a function of current in kiloamperes by an
equation of the form:
Equation 5.3
To determine the constants K and alpha for a given segment, any two current-
operations pairs in that segment must be known. For any given pairs (I1, O1)
and (I2, O2), the alpha constant is determined by the equation:
Equation 5.4
The K constant can be found by back-substitution:
Equation 5.5
Here, we can use the endpoint pairs (B n KAP1, B n COP1) and (B n KAP2,
B n COP2) to determine the equation that applies between these two input
points and pairs (B n KAP2, B n COP2) and (B n KAP3, B n COP3) for the
equation between the latter two points.
In Figure 5.5 , for example, the two segments have the following equations:
Equation 5.6
For a particular value of I in kA, the calculated value O(I) represents
100 percent wear of the breaker contacts. Thus, the incremental percent wear
for one trip operation at the defined current level is 100 / O(I) percent.
For I < B
n KAP1, O(I) = B n COP1. For B n KAP1 < I < B n KAP2, O(I) is
O I
  
K I

• =

log
10
O1
O2
-------
 
 
log
10
I1
I2
-----
 
 
-------------------------- =
K
O1
I1

-------- =
or
K
O2
I2

-------- =
O(I) 14 972

I
2.214
–
• =
and
O(I) 46 284

I
2.756
–
• =