Protection coordination is the proof that the correct device clears each fault within equipment limits while healthy sections remain energised. It requires more than visually separated time-current curves.

Learning objectives

Build coordination cases, include measurement and breaker delays and document margins across every permitted network topology.

Core engineering principles

Sensitivity and selectivity pull in opposite directions

Low pickup detects weak faults, while higher pickup or delay avoids upstream operation. Settings must satisfy both using staged or directional functions where necessary.

Clearing time includes the full chain

Relay algorithm, intentional delay, output contact, trip coil, breaker opening and arcing all contribute. Coordination based only on relay time is incomplete.

Curves are valid on a stated current base

CT ratio, relay pickup, IEC curve family and time multiplier determine the plotted characteristic. Fuse tolerances and breaker clearing bands also matter.

Topology changes grading

Closed couplers, parallel transformers, generators and minimum-source states alter current distribution and direction. One curve set may not cover all states.

Equipment withstand sets the upper time limit

Cable damage, transformer through-fault and arc energy curves constrain how slowly backup protection may operate.

Engineering application method

  1. Step 1: Model normal and contingency network configurations.
  2. Step 2: Calculate maximum and minimum faults at relevant locations.
  3. Step 3: Plot protection, fuse, recloser and equipment-damage characteristics.
  4. Step 4: Add CT, relay and breaker tolerances and grading margin.
  5. Step 5: Issue approved setting files with calculation revision traceability.

Practical example

Two curves can appear separated at high current yet cross near the minimum feeder fault, causing the incomer to trip first. Coordination must be checked over the complete current range.

Common mistakes

  • Using relay time without breaker time.
  • Checking only three-phase maximum faults.
  • Ignoring fuse tolerance bands.
  • Applying one setting group to incompatible topologies.
  • Changing CT ratio without recalculating curves.

Design and review checklist

  • Are maximum and minimum faults included?
  • Is complete clearing time plotted?
  • Are damage curves respected?
  • Are all topologies covered?
  • Are setting files revision-controlled?

Standards basis and official sources

  • IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
  • IEC 61869-1:2023 — General requirements for instrument transformers and low-power instrument transformers.
  • IEEE C37.2-2022 — Device function numbers, acronyms and contact designations.

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

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