What does a relay compliance claim actually tell you?

A protection relay needs to make the right decision within the required time. IEC 60255 provides a family of requirements and performance evaluations for measuring relays and power system protection equipment. It covers several distinct questions: how the protection function behaves, how the equipment handles electromagnetic disturbances, and whether the product presents electrical or physical hazards. Evidence for one part does not establish compliance with every other part.

The relay normally issues a trip command; the circuit breaker interrupts the fault current. Relay operating time is therefore only one contribution to fault clearance. The trip circuit, any intervening logic and the breaker's opening time belong in the overall assessment.

Start with the requirement, then choose the part

IEC 60255-1:2022 common requirements provides the foundation for measuring relays and protection equipment. Its scope also includes relevant control, monitoring, communication and process interface equipment used in protection schemes. Function-specific parts then address the behaviour and performance of particular protection functions.

IEC 60255-26:2023 EMC requirements addresses emissions and immunity. Equipment must limit the disturbances it creates and satisfy the applicable criteria when exposed to external disturbances. Conducted and radiated effects, transients and electrostatic discharge are among the concerns. IEC 60255-27:2023 product safety addresses hazards such as electric shock, fire and injury. A product safety result does not validate the protection algorithm's response to every power system fault.

Match the protection function to its document

The protection functions and standard parts guide is a useful entry point. Identify the functions your scheme actually uses, then check the scope of the supporting evidence for each one.

Overcurrent and undercurrent: IEC 60255-151:2009 addresses current protection functions. Assess pickup, timing characteristics and operating accuracy together. A suitable characteristic still needs settings that coordinate with the devices elsewhere in the network.

Overvoltage and undervoltage: IEC 60255-127:2010 covers voltage protection. The measured quantity, threshold, delay and reset behaviour matter in practice. A nominal voltage rating alone does not define a working protection scheme.

Thermal protection: IEC 60255-149:2013 addresses thermal electrical relays. Examine how the thermal model accounts for previous loading. A reduction in present current does not mean the protected equipment has already cooled.

Distance protection: IEC 60255-121:2014 addresses operating characteristics in the impedance plane and their performance evaluation. Zone reach, direction, phase selection and timing have to be considered alongside line data and the instrument transformers.

Frequency protection: IEC 60255-181:2019 deals with frequency and rate-of-change-of-frequency protection. Measurement windows and delays influence the balance between dependable response and resistance to unwanted operation.

Differential protection: IEC 60255-187-1:2021 covers longitudinal differential protection for AC motors, generators and transformers. Its scope should not be extended to every differential application. Transformer energisation and current transformer saturation are examples of conditions where stability matters.

The supporting parts have different jobs

The IEC 60255-21 mechanical test family separates vibration in Part 21-1, shock and bump in Part 21-2, and seismic testing in Part 21-3. Specify the required severity class, mounting arrangement and acceptance criteria. An older publication date, by itself, does not mean that a document has been withdrawn.

IEC 60255-24:2013 COMTRADE defines a file exchange format for transient waveform and event data. It supports disturbance analysis and replay rather than defining a live communications bus. IEC/IEEE 60255-118-1:2018 addresses time-referenced synchrophasor, frequency and ROCOF measurements, without prescribing a single calculation algorithm.

IEC TS 60255-216-1:2025 is a Technical Specification for testing protection functions with digital interfaces. Relevant interfaces include Sampled Values inputs, GOOSE status and trip messages, and time synchronisation. Successful message delivery and correct end-to-end protection behaviour require different evidence.

Ask for evidence you can use

A purchasing specification should identify the part number and edition, followed by the tested model, hardware, firmware, enabled functions and settings. Ask for the method, input conditions, performance limits and acceptance criteria. For EMC, check the tested ports and disturbance levels; for mechanical testing, check the installation arrangement. The phrase “IEC 60255 compliant” cannot supply these missing details.

Type testing evaluates a product design under defined conditions. Site commissioning checks the installed measurement and trip chain. CT ratios and polarity, wiring, relay settings, the trip circuit and breaker response still need verification on the actual installation. A laboratory result cannot compensate for an incorrectly wired measurement circuit.

Editor's note

Build the evidence around the protection scheme: identify the asset and credible faults, select the required functions, and connect each function to its applicable document and commissioning checks. The useful report is the one that states what was tested, under which conditions, and against which limits. A long list of standard numbers on a brochure provides much less guidance.

Continue with a related topic

Inverse-time and IDMT protection curves · EMC for protection relays · Protection relay product safety · IEC 61850 power system communications · How current transformers work

References and sources

Official IEC publication scopes; checked 11 October 2026.
IEC 60255-1:2022 · IEC 60255-121:2014 · IEC 60255-127:2010 · IEC 60255-149:2013 · IEC 60255-151:2009 · IEC 60255-181:2019 · IEC 60255-187-1:2021 · IEC 60255-21-1:1988 · IEC 60255-21-2:1988 · IEC 60255-21-3:1993 · IEC 60255-24:2013 · IEC 60255-26:2023 · IEC 60255-27:2023 · IEC/IEEE 60255-118-1:2018 · IEC TS 60255-216-1:2025