The IEC 60255 series provides common and function-specific requirements for measuring relays and protection equipment. Engineers must distinguish evidence for the relay as a product from evidence that CT circuits, logic, tripping paths, settings and communications form a dependable protection scheme.

Why this topic matters

A numerical relay can pass product tests yet be applied incorrectly through unsuitable CT selection, weak DC supply, incomplete logic testing or an uncoordinated setting file. Standards compliance therefore needs a boundary: what the manufacturer proves and what the system integrator or project FAT must prove.

Scope and engineering boundary

IEC 60255-1 establishes common requirements. Other parts address EMC, product safety and particular protection functions or digital interfaces. The applicable set depends on the relay features used and the project architecture.

Core engineering principles

Common requirements define the product baseline

Rated auxiliary quantities, environmental conditions, accuracy concepts, inputs, outputs and documentation establish how the relay is declared and tested.

EMC performance is not only a laboratory concern

IEC 60255-26 addresses immunity and emissions for protection equipment, but installation segregation, shielding, bonding and surge paths determine whether the tested capability is preserved in the LVC.

Function-specific evidence has a defined model

Overcurrent, distance, differential or other functions are assessed against stated characteristics and test conditions. A product result must be connected to the enabled algorithm and firmware used in the project.

Scheme dependability requires end-to-end proof

CT polarity, VT circuits, binary inputs, trip contacts, breaker coils, interlocks, communications and settings are outside a narrow relay function test unless deliberately included.

Application workflow

  1. Step 1: List every enabled protection, supervision, communication and control function.
  2. Step 2: Map each function to applicable IEC 60255 parts and manufacturer evidence.
  3. Step 3: Record firmware, hardware variant, input type, output duty and auxiliary supply.
  4. Step 4: Check EMC installation measures against the relay manual and LVC design.
  5. Step 5: Create tests for settings, logic, binary I/O, analog inputs and complete trip paths.
  6. Step 6: Archive as-left files, event records and evidence linking the installed relay to the approved study.

Practical engineering example

A feeder relay’s 50/51 element may satisfy its declared characteristic, but a FAT still needs to verify the approved curve and pickup values, CT ratio, blocking logic, assigned output contact, trip-circuit wiring and actual breaker operation.

Common mistakes

  • Writing only ‘IEC 60255 compliant’ without applicable parts.
  • Treating relay certification as protection-scheme validation.
  • Ignoring output-contact duty for trip or close circuits.
  • Testing a setting file that is not the approved as-left revision.

Design and review checklist

  • Applicable IEC 60255 parts are listed by function.
  • Firmware and hardware variants are controlled.
  • EMC installation requirements are implemented.
  • Complete measurement-to-trip path is tested.
  • As-left configuration and evidence are archived.

Standards basis and official sources

  • IEC 60255-1:2022 — Official IEC common requirements for measuring relays and protection equipment.
  • IEC 60255-26:2023 — Official IEC EMC requirements for protection equipment.
  • IEC TS 60255-216-1:2025 — Digital-input and output protection functions using IEC 61850 and IEC 61869 interfaces.

Engineering note: Confirm the contracted edition, amendments, corrigenda, national adoption, project specification and manufacturer instructions before applying a requirement. This article explains engineering use and does not reproduce or replace the standard.

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