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72
Automatic switch onto fault logic (SOTF)
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The switch-onto-fault function is a complementary function to impedance measuring
functions.
With the switch-onto-fault (SOTF-) function, a fast trip is achieved for a fault on the
whole line, when the line is being energized. The SOTF tripping is generally non-direc-
tional in order to secure a trip at fault situations where directional information can not
be established, for example, due to lack of polarizing voltage when a line potential
transformer is used.
Automatic activation can be used only when the potential transformer is situated on the
line side of a circuit breaker.
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The switch-onto-fault function can be activated either externally or automatically, in-
ternally, by using the information from a dead-line-detection (DLD) function (see fig-
ure 37).
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After activation, a distance protection zone (usually its non-directional starting signal)
is allowed to give an instantaneous trip. The functional output signal from the distance
protection zone to be used should be connected to the SOTF-NDACC functional input
of the SOTF function, see figure 37. The distance protection zone used together with
the switch-onto-fault function shall be set to cover the entire protected line. The non-
directional instantaneous condition is maintained for 1 s after closing the line circuit
breaker.
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