# IEC 60255 — Measuring relays and protection equipment > IEC 60255 is the series governing measuring relays and protection equipment — the protective relays and intelligent electronic devices (IEDs) that detect faults and trip breakers in power systems. It defines common requirements, electromagnetic compatibility, product safety and the functional behaviour of protection functions. It is the device-level companion to IEC 61850 substation communication. - Source : https://industryhub.site/en/standards/iec-60255 - Body : IEC - Family : Protection / Grid - Status : current - First published : 1970s - Latest revision : 60255-26:2023 (EMC); part-specific - Edition : Multi-part - Next revision : Active maintenance - Access : Paid (IEC webstore) - Language : English ## Document structure - **IEC 60255-1** — Common requirements : The baseline for all measuring relays and protection equipment — definitions, ratings, environmental and documentation requirements shared across the series. - **IEC 60255-26** — EMC requirements : Electromagnetic compatibility (emission and immunity) for protection equipment operating in the harsh environment of a substation. Current edition 2023. - **IEC 60255-27** — Product safety requirements : Safety against electric shock, fire and injury for equipment rated up to 1000 V AC / 1500 V DC. - **IEC 60255-1xx** — Functional requirements : Function-specific behaviour — e.g. -151 over/under-current protection, -127 over/under-voltage — defining how a protection function must operate and be tested. ## Key concepts - **Protective relay / IED** : A device that monitors currents and voltages, detects a fault (overcurrent, distance, differential), and commands a breaker to trip. Modern relays are microprocessor-based intelligent electronic devices (IEDs). - **Type testing** : IEC 60255 defines the type tests a relay must pass — accuracy, timing, EMC, safety, environment — so a protection function behaves predictably across manufacturers and installations. - **EMC in the substation (60255-26)** : A substation is electromagnetically violent: switching surges, fault currents, radio. A relay that misoperates under disturbance can trip a healthy line or fail to trip a faulted one; 60255-26 sets the immunity and emission bar. - **Functional requirements (-1xx)** : The -1xx parts standardise specific protection functions — over/under-current (-151), over/under-voltage (-127) and others — defining characteristics, accuracy and test methods so settings mean the same thing everywhere. - **Companion to IEC 61850** : 60255 specifies the relay as a product; IEC 61850 specifies how relays (IEDs) communicate inside the digital substation — GOOSE tripping, sampled values. A modern IED conforms to both. ## The devices that decide when to trip When a fault hits a power system — a short circuit, an earth fault — something must detect it in milliseconds and open the right breaker, isolating the fault without taking down the healthy network. That something is a **protective relay**. **IEC 60255** is the series that defines these **measuring relays and protection equipment**: how they must perform, survive their environment, and be tested. ## One series, several concerns Protection devices are judged on more than logic. The series splits the requirements: **60255-1** carries the common baseline — definitions, ratings, documentation. **60255-26** sets **electromagnetic compatibility**, because a substation is electromagnetically violent and a relay that misoperates under a switching surge can trip a healthy line or, worse, fail to trip a faulted one. **60255-27** covers **product safety** against shock and fire. And the **-1xx** parts define **functional requirements** for specific protection functions — over/under-current (-151), over/under-voltage (-127) — so a setting means the same thing on any vendor's relay. ## From electromechanical to IED The relay has evolved from an electromechanical disc to a microprocessor-based **intelligent electronic device (IED)** that measures, protects, records and communicates. IEC 60255 type tests ensure that whatever the technology, the protection behaves predictably: correct pickup, correct timing, correct restraint. ## Companion to IEC 61850 60255 specifies the relay *as a product*; [IEC 61850](/en/standards/iec-61850) specifies how those relays *communicate* inside the digital substation — GOOSE for fast tripping, sampled values for measurements. A modern IED conforms to both at once, and like every connected substation device it falls under the control-system cybersecurity discipline of [IEC 62443](/en/standards/iec-62443), with its EMC framed by [IEC 61000](/en/standards/iec-61000). ## Applicable industries Power transmission and distribution, Substations and switchgear, Industrial and generation electrical protection, Relay manufacturers and test laboratories ## References & further reading - [IEC 60255-26:2023 — EMC requirements (official)](https://webstore.iec.ch/en/publication/62084)