IEC 60255 Framework for Protection Relays and Protection Functions

IEC 60255 is a layered product-and-function framework for measuring relays and protection equipment; it is not a complete protection-coordination study or a guarantee that a relay is correctly applied. A defensible specification combines common requirements, function-specific performance, EMC, safety and the separate communication/system standards required by the project.

This guide maps IEC 60255-1:2022, IEC 60255-26:2023, IEC 60255-27:2023 and representative 1xx functional parts into a practical relay specification, type-test evidence review, FAT/SAT plan and lifecycle baseline for MV switchgear protection.

Executive conclusions

IEC 60255-1:2022 supplies common rules for measuring relays/protection equipment, including distributed schemes, merging units and communication/process interfaces within its scope.
IEC 60255-26:2023 addresses electromagnetic compatibility; IEC 60255-27:2023 addresses product safety. Neither proves protection-function accuracy.
Use the applicable IEC 60255-1xx functional part for overcurrent, voltage, distance, differential or other functions where one exists.
A multifunction IED can require several functional standards plus common, EMC and safety evidence.
IEC 60255 defines minimum product/function performance and test methods; it does not calculate settings, grade protection, select CTs or prove the complete trip scheme.
IEC 61850 conformance is separate from IEC 60255 functional performance, and neither alone proves the engineered GOOSE/SV application.
Review the exact hardware, firmware, function package, analogue-input type and configuration used in type tests.
Define accuracy and timing across current/voltage/frequency, temperature, auxiliary supply, waveform distortion, harmonics and CT saturation relevant to the application.
FAT should test settings, characteristics, logic, I/O, communications and trip paths against an approved cause-and-effect matrix; SAT proves installed primary/secondary interfaces.
Control firmware, settings, SCL files, logic, cybersecurity and regression evidence throughout the relay lifecycle.

1. What IEC 60255 covers

The IEC 60255 family applies to measuring relays and protection equipment. The 2022 second edition of IEC 60255-1 explicitly includes combinations of equipment forming distributed protection schemes and recognizes merging units, communications and process interfaces as integral elements. It establishes common terminology, ratings, environmental/reference conditions, input/output behaviour and test principles.

It does not replace the power-system study. Correct protection still requires fault/load studies, CT/VT application, breaker capability, DC-system design, settings calculations, selectivity, arc-flash/clearing-time objectives, logic design and commissioning.

2. The layered compliance model

Layer	Representative standard	Evidence purpose
Common relay requirements	IEC 60255-1:2022	General ratings, conditions, I/O, documentation and tests
Functional performance	Applicable IEC 60255-1xx part	Pickup, reset, characteristic, timing, dynamic behaviour and published results
EMC	IEC 60255-26:2023	Immunity/emissions at defined equipment ports
Product safety	IEC 60255-27:2023	Electric shock, fire, mechanical/thermal and related product hazards
Communication engineering	IEC 61850 parts	Data model, services, SCL, MMS/GOOSE/SV and conformance
Cybersecurity	IEC 62351/project controls	Authentication, roles, keys/certificates, security management
Instrument transformers	IEC 61869 parts	CT/VT/LPIT performance and interface
Complete scheme	Project specification/studies/tests	Selectivity, dependability, security and end-to-end tripping

3. Representative functional standards

Function family	Representative IEC 60255 part	Main focus
Over/under voltage	IEC 60255-127:2010	Voltage-function measurement and time characteristics
Over/under current	IEC 60255-151:2009	Current pickup/reset, definite/inverse time and dynamic performance
Distance protection	IEC 60255-121	Distance-function performance and test methods
Frequency/ROCOF protection	Applicable IEC 60255 functional part	Frequency-based measurement/timing behaviour
Differential—motor/generator/transformer	IEC 60255-187-1:2021	Restrained/unrestrained longitudinal differential performance
Other differential applications	Applicable IEC 60255-187 part	Application-specific differential behaviour and documentation

Some functional standards are older than the current common/EMC/safety documents but remain current according to the IEC catalogue. Publication year alone does not prove obsolescence; check lifecycle status, amendments, corrigenda and the contract baseline.

4. Build the relay compliance matrix by installed function

Do not assign standards only by the IED’s marketing name. List each enabled function and interface:

50/51 phase overcurrent and 50N/51N earth fault;
67/67N directional overcurrent/earth fault;
27/59 voltage and 81 frequency/ROCOF;
87 transformer, motor, generator or bus differential;
64REF/87N restricted earth fault;
21 distance, 46 negative sequence, 49 thermal and motor functions;
breaker failure, auto-reclose, synchrocheck and trip-circuit supervision;
analogue CT/VT, LPIT, Sampled Values and binary inputs/outputs;
GOOSE/MMS/reporting/time synchronization;
disturbance records, SOE, measurements and cybersecurity interfaces.

For each row, state applicable product/function standard, manufacturer guarantee, project requirement, type-test evidence, FAT method and site end-to-end test.

5. Rated inputs and application interfaces

nominal current input (for example 1 A/5 A) or LPIT/SV interface;
nominal voltage input and open-delta/residual connection;
frequency range and off-nominal performance;
thermal withstand of current/voltage inputs;
input burden and impact on CT/VT circuit;
binary-input wetting voltage, threshold, debounce and current;
output contact making/carrying/breaking duty for DC inductive trip circuits;
auxiliary supply nominal range, ripple, dips/interruptions and inrush;
communication ports, fibre/copper type and isolation;
earthing, screen and panel-mounting requirements.

Match the actual relay ordering code. A low-level sensor input, conventional 1 A input and IEC 61850-9-2 SV subscription are different designs and require different evidence and commissioning methods.

6. Accuracy is multi-dimensional

A single “±x%” data-sheet value is insufficient unless its reference, range and influencing quantities are known. Review:

operating-value accuracy versus setting and measured quantity;
reset ratio and hysteresis;
time accuracy for definite and inverse characteristics;
minimum operating time and overshoot/overtravel behaviour;
temperature and auxiliary-supply influence;
frequency and waveform/harmonic influence;
DC offset and CT saturation/dynamic performance;
memory, polarization and directional-element behaviour;
measurement filtering/window and transient response;
repeatability and measurement uncertainty of test equipment.

Set FAT tolerances from the applicable functional standard, manufacturer’s published performance and test-set uncertainty. Do not simply add arbitrary percentages or ignore uncertainty near a boundary.

7. Time characteristics and curve verification

identify curve family and equation/version;
test pickup/reset around the threshold;
test multiple current/voltage points across operating range;
include definite minimum time or high-set stage transitions;
define timing start/stop and binary output used;
verify cold/warm memory or reset model where relevant;
test transient overreach/overshoot for fast stages;
check timer interactions with blocking, breaker failure or reclose;
separate element operate time from output contact and breaker time.

The complete clearing time is the sum of measurement/decision, logic, output, trip path, breaker opening/arcing and system margins. A relay type-test time does not equal plant clearing time.

8. Directional, distance and differential dynamic performance

Dynamic functions require more than steady-state magnitude injection:

Directional: polarizing quantity, characteristic angle, forward/reverse boundary, low-voltage memory and zero-/negative-sequence connection.
Distance: zone reach/time, resistive/reactive boundary, load encroachment, power swing, source impedance, CVT transient and fault resistance.
Differential: ratio/vector compensation, restraint slope, breakpoint, through-fault CT saturation, inrush/overexcitation blocking and zero-sequence removal.
REF: CT ratio/polarity, star-point connection, stabilizing/high-impedance circuit or low-impedance bias, sensitive pickup and external-fault stability.

The relevant functional standard sets minimum performance/test reporting, while the project study determines settings and stability requirements for the actual CTs and protected zone.

9. IEC 60255-26 EMC evidence

IEC 60255-26:2023 specifies emissions limits and immunity tests for measuring relays/protection equipment, including continuous/transient conducted and radiated disturbances and electrostatic discharge. Review evidence by equipment port and acceptance criterion:

enclosure, auxiliary-power, CT/VT, binary I/O and communication ports;
electrostatic discharge;
radiated/conducted RF immunity and emissions;
electrical fast transient/burst;
surge and oscillatory disturbances;
power-frequency magnetic field and conducted disturbances;
AC/DC supply dips, interruptions, ripple and gradual shutdown/startup where applicable;
functional acceptance during/after test—not only survival;
exact wiring, cable length, grounding and auxiliary devices used.

A relay can pass laboratory EMC tests yet fail in a poorly bonded panel with bad screen termination or mixed cable routing. Product evidence and installation engineering are complementary.

10. IEC 60255-27 product safety evidence

IEC 60255-27:2023 addresses safety of measuring relays/protection equipment up to the stated AC/DC voltage limits. It minimizes electric-shock, fire and injury/property risks and covers mounting in cabinets/racks/panels and auxiliary devices tested with the equipment.

accessible parts, protective bonding and insulation;
clearance/creepage and dielectric strength;
temperature, fire enclosure and material flammability;
terminal/cable security and energy limits;
mechanical strength and mounting;
marking, warnings and installation instructions;
batteries, fuses and replaceable parts where applicable;
pollution/overvoltage environment and intended use;
ordering-code differences affecting safety.

IEC 60255-27 explicitly does not cover functional performance. Safety certification cannot replace IEC 60255 functional evidence.

11. Environmental and mechanical service conditions

operating/storage temperature and derating;
humidity and condensation;
altitude and cooling/insulation impacts;
vibration, shock and seismic requirements;
dust/corrosion and enclosure protection;
heat generated in the low-voltage compartment;
fan/filter dependency and hot spots;
service life of display, capacitors, power supply and output relays.

IEC 60255-1:2022 added environmental operating conditions and manufacturer derating concepts. Apply the manufacturer’s declared limits to the actual switchgear compartment, not the substation room average alone.

12. Binary outputs and the physical trip circuit

contact make/carry/break capability at actual DC voltage and inductive L/R;
trip-coil current, inrush and minimum voltage;
contact wetting, bounce and operate/release time;
seal-in, lockout relay and master-trip duty;
trip-circuit supervision current and false-pickup risk;
dual trip coils and DC segregation;
output contact wear and replacement policy;
arc suppression that does not delay release or defeat supervision;
test switch and isolation facilities.

A relay output’s AC resistive rating is not evidence that it can interrupt a high-inductance DC trip coil. Use the manufacturer’s DC duty data or an interposing/lockout relay designed for the duty.

13. IEC 61850 is a separate system layer

IEC 61850-6 for SCL engineering exchange;
IEC 61850-7 series for data/service models;
IEC 61850-8-1 for MMS/GOOSE station-bus mapping;
IEC 61850-9-2/profile documents for Sampled Values where used;
IEC 61850-10 for conformance testing;
IEC 62439-3 for PRP/HSR;
IEC/IEEE 61850-9-3 for utility PTP profile;
IEC 62351/project requirements for security.

An IEC 61850 conformance certificate demonstrates specified protocol/model behaviour for a tested implementation. It does not prove the project’s dataset, subscription, logic, network performance or physical trip result. Conversely, IEC 60255 function tests do not prove SCL or GOOSE engineering.

14. Type-test evidence review

manufacturer, model, hardware revision and ordering code;
firmware/platform and enabled function package;
conventional, LPIT or SV input type;
auxiliary supply and I/O board variants;
standard part, edition, amendment and test cases;
reference conditions and influencing quantities;
raw performance tables/curves and acceptance criteria;
EMC port/configuration and safety construction;
laboratory identity/accreditation scope;
failures, deviations, modifications and repeats;
manufacturer assessment covering the offered revision.

Firmware can change algorithms, timing and communications without changing the front label. Require a controlled product-change process and regression evidence for safety/protection-critical changes.

15. From type test to project FAT

Type evidence	Project FAT/SAT evidence
Product/function performance under standardized tests	Actual settings, CT/VT scaling, logic and wiring
EMC immunity/emissions	Correct panel bonding, routing and switching behaviour
Product safety construction	Safe installation, terminal covers, earthing and access
Protocol conformance	Actual SCL, GOOSE/SV/MMS application and network
Output contact rating	Actual trip coil/interposing circuit and end-to-end operation

16. FAT test architecture

Freeze approved settings, logic, firmware, SCL and checksums.
Verify model/order code, CT/VT/SV scaling and wiring diagrams.
Test pickup/reset and timing at selected boundary/curve points.
Test directional/restraint/differential/distance behaviour as applicable.
Test blocking, interlocks, timers, latches, breaker failure and reclose logic.
Verify binary inputs/outputs, trip matrix, LEDs, alarms and SOE.
Test GOOSE/SV/MMS, quality, test/simulation and communication loss where used.
Operate the real final trip path safely at least once where the ITP requires it.
Verify disturbance records, event time and setting/report exports.
Remove forces/blocks/test flags and archive the as-left baseline.

Secondary injection proves the relay and much of the scheme. Primary injection/end-to-end testing additionally proves CT wiring, polarity, phase identity, ratio and physical path within its test limits.

17. Negative and failure-mode tests

loss of one/two/three voltage or current channels;
CT saturation/open-circuit supervision logic as safely simulated;
bad/invalid/questionable SV or MMS quality;
GOOSE publisher timeout and subscription mismatch;
loss of auxiliary supply, reboot and recovery;
time-source loss and SOE quality;
trip-circuit open, breaker fails to clear and breaker-failure initiation;
local/remote authority conflict and unauthorized command;
setting-group change during abnormal conditions;
redundant network/supply path failure;
stale/forced/test data and restoration.

Security and dependability are both protection objectives. Proving only that the relay trips for an internal fault does not prove stability for load, external faults, inrush, CT saturation or communication failure.

18. Settings are outside the certificate

system topology and fault levels;
maximum load/start/inrush and allowable overload;
CT/VT ratios, classes, saturation and residual connection;
downstream/upstream curves and breaker clearing times;
earthing method and minimum earth-fault current;
transformer vector group/inrush/overexcitation;
motor starting/stall/thermal model;
distributed generation and reverse power flow;
arc-flash/energy objectives and equipment withstand;
credible contingency and maintenance configurations.

A relay can conform perfectly to IEC 60255 and still be dangerously mis-set. Require independent settings review, controlled approval and comparison of the loaded file to the approved calculation.

19. Lifecycle and change control

asset/model/serial/hardware/firmware inventory;
approved settings and logic with checksum/version;
SCL/CID/SCD and network configuration baseline;
role/account/certificate/key governance;
manufacturer notices, vulnerabilities and firmware assessment;
backup/restore verification;
impact analysis for every settings/logic/firmware change;
targeted regression and end-to-end test;
event/disturbance record retention and time quality;
periodic trip-path, battery and breaker tests;
obsolescence and replacement equivalence plan.

20. Common mistakes

writing “IEC 60255 compliant” without naming applicable parts;
using EMC/safety evidence as functional proof;
assuming a multifunction IED certificate covers every enabled element;
ignoring hardware/firmware/input-board differences;
testing one point on an inverse curve;
testing magnitude but not direction, restraint or failure states;
using IEC 61850 conformance as proof of project GOOSE/SV logic;
rating output contacts from AC resistive data for a DC trip coil;
ignoring test-set uncertainty and timing boundaries;
accepting correct relay operation without proving the breaker trip path;
loading settings without checksum/readback and independent review;
leaving test flags, forced values or temporary accounts active.

Primary references

Engineering note: This guide explains the standards architecture; it does not provide project settings. Use licensed standards, manufacturer manuals, approved power-system studies and controlled test procedures.