4 Protection Functions and Specifications
4-20
L-PRO 4000 User Manual
D02706R02.30
A secondary. If this current has been that low for the 1 second pickup time de-
lay this input to the AND gate would go high. For a de-energized line this input
would normally be high to allow the blocking scheme to operate if the other
two inputs are high. This input along with input two are present to help prevent
any distance relay blocking for a line that is in service and is now experiencing
a line fault.
Switch On To
Fault (SOTF)
When energizing a line that has been out of service, the line may have a fault
on it. If the line potential is derived from bus PTs, the distance relay function
acts normal and operates for any fault that exists when the line is energized. If
the line uses line PTs, the output of these line PTs is zero before energizing.
Because there is no memory voltage, operation of the line distance functions
may be undefined. To provide protection to detect faults when a line is first en-
ergized, the Switch On To Fault function (SOTF) is involved.
Figure 4.14: Switch On To Fault (SOTF)
The logic diagram, Figure 4.14: Switch On To Fault (SOTF) on page 4-20,
shows how the circuit actually works. The SOTF function has options as Close
Command or Status Monitoring.
Summary of Contents for L-PRO 4000
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