Date Code 20001006 Trip and Target Logic 5-9
SEL-351 Instruction Manual
Determining Three-Pole Open Condition Without Circuit Breaker Auxiliary Contact
If a circuit breaker auxiliary contact is not connected to the SEL-351 Relay, SELOGIC Control
Equation setting 52A is set:
52A = 0 (numeral 0)
With SELOGIC Control Equation setting 52A continually at logical 0, 3PO logic is controlled
solely by load detection element 50L. Phase pickup 50LP is set below load current levels.
When the circuit breaker is open, Relay Word bit 50L drops out (= logical 0) and the 3PO
condition asserts:
3PO = logical 1 (circuit breaker open)
When the circuit breaker is closed, Relay Word bit 50L picks up (= logical 0; current above phase
pickup 50LP) and the 3PO condition deasserts after the 3POD dropout time:
3PO = logical 0 (circuit breaker closed)
Note that the 3PO condition is also routed to the permissive overreaching transfer trip (POTT)
logic (see Figure 5.6) and the loss-of-potential (LOP) logic (see Figure 4.1).
Circuit Breaker Operated Switch-Onto-Fault Logic
Circuit breaker operated switch-onto-fault logic is enabled by making time setting 52AEND
(52AEND ¹ OFF). Time setting 52AEND qualifies the three-pole open (3PO) condition and then
asserts Relay Word bit SOTFE:
SOTFE = logical 1
Note that SOTFE is asserted when the circuit breaker is open. This allows elements set in the
SELOGIC Control Equation trip setting TRSOTF to operate if a fault occurs when the circuit
breaker is open (see Figure 5.1). In such a scenario (e.g., flashover inside the circuit breaker tank),
the tripping via setting TRSOTF cannot help in tripping the circuit breaker (the circuit breaker is
already open), but can initiate breaker failure protection, if a breaker failure scheme is
implemented in the SEL-351 Relay (see output contact OUT3 example in Output Contacts in
Section 7: Inputs, Outputs, Timers, and Other Control Logic) or externally.
When the circuit breaker is closed, the 3PO condition deasserts (3PO = logical 0) after the 3POD
dropout time (setting 3POD is usually set for no more than a cycle). The SOTF logic output,
SOTFE, continues to remain asserted at logical 1 for dropout time SOTFD time.
Close Bus Operated Switch-Onto-Fault Logic
Close bus operated switch-onto-fault logic is enabled by making time setting CLOEND (CLOEND
¹ OFF). Time setting CLOEND qualifies the deassertion of the load detection element 50L
(indicating that the circuit breaker is open).
Circuit breaker closure is detected by monitoring the dc close bus. This is accomplished by wiring
an optoisolated input on the SEL-351 Relay (e.g., IN7) to the dc close bus. When a manual close
Courtesy of NationalSwitchgear.com
To Next Page
