Functions
2.17 Auxiliary Functions
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
221
A double-logarithmic diagram provided by the CB manufacturer illustrates the relationship of operating cycles
and tripping current (see example in Figure 2-74). This diagram allows the number of yet possible trips to be
determined (for tripping with equal tripping current). According to the example, approximately 1000 trips can
yet be carried out at a tripping current of 10 kA. The characteristic is determined by two vertices and their con-
necting line. Point P1 is determined by the number of permitted operating cycles at rated operating current Ir,
point P2 by the maximum number of operating cycles at rated fault tripping current Isc. The associated four
values can be configured.
Figure 2-74 Diagram of operating cycles for the 2P procedure
As Figure 2-74 illustrates a double-log diagram, the straight line between P1 and P2 can be expressed by the
following exponential function:
n = b·I
b
m
where n is the number of operating cycles, b the operating cycles at I
b
= 1A, I
b
the tripping current, and m the
directional coefficient.
The general line equation for the double-logarithmic representation can be derived from the exponential func-
tion and leads to the coefficients b and m.
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