2-38 Specifications Date Code 20011026
SEL-321/321-1 Instruction Manual
Selecting Single-Pole or Three-Pole Tripping Logic
The relay uses single-pole tripping logic if:
• Single-pole trip enable setting ESPT = Y, Y1, or
• Programmable input variable SPTE is asserted.
Otherwise, the relay uses three-pole tripping logic.
Tripping Output Signals
There are four tripping output variables, which can be programmed to drive output contacts for
tripping breakers: TPA, TPB, TPC, and 3PT.
Variables TPA, TPB, and TPC are phase-dependent tripping signals, designed to control the
individual poles of a circuit breaker in single-pole tripping schemes.
Variable 3PT indicates three-pole trips.
Single-Pole Tripping Conditions
When the single-pole tripping logic is active, the SEL-321 Relay trips single-pole for the
following trip conditions:
• Zone 1 SLG faults detected by the ground distance elements
• Zone 2 SLG faults detected by the ground distance elements and permitted to trip through
the communications scheme logic.
• DTA, DTB, DTC assigned and asserted.
• See the special single-pole tripping appendix, Appendix F, for other schemes.
All other tripping conditions (including switch-onto-fault, time-step trips, overcurrent trips, and
trips during the SPO condition) cause three-pole trips.
Selecting Elements to Trip
The user selects the desired relay elements for unconditional trip, for switch-onto-fault, and for
tripping with communications assistance.
The user programs three variables with the desired elements, using the SET L command: MTU,
MTO, and MTCS.
MTU (Unconditional Trip Variable)
Include Zone 1 and time-delayed protective elements in this equation, for example:
MTU = M1P + Z1G + M2PT + Z2GT + M3PT + Z3GT + 51NT
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