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Automatic switch onto fault logic (SOTF)
The external activation is achieved by an input (SOTF-BC), which should be set high
for activation, and low when the breaker has closed. This is carried out by an NC aux-
iliary contact of the circuit breaker or by the closing order to the breaker.
The internal automatic activation is controlled by the DLD function and its functional
output DLD--START. The DLD--START functional output is activated when all phase
voltages and phase currents have been below their set operate values. The DLD--
START functional output is usually configured to the SOTF-DLCND functional input.
It activates the operation of the SOTF function, if present for more than 200 ms without
the presence of a non-directional impedance starting signal SOTF-NDACC.
Operation of a SOTF function can be blocked by the activation of a SOTF-BLOCK
functional input.
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The parameters for the switch-onto-fault function are set via the local HMI or PST (Pa-
rameter Setting Tool). Refer to the Technical reference manual for setting parameters
and path in local HMI.
The low voltage and low current criteria for automatic activation is settable under the
DLD-- function. Refer to the Technical reference manual for setting parameters and
path in local HMI.
This setting is not critical as long as it is lower then the lowest operation voltage during
normal and emergency conditions.
The distance protection zone used for a switch-onto-fault criterion (SOTF zone) have
to be set to cover the entire protected line with a safety margin of minimum 20%.
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