IEC 60255 1xx: Protection relay functional standards for all

Electrical Tester – 15 October 2021

Authors: Thierry Bardou, Andrea Bonetti, Volker Leitloff, and Murty Yalla

The International Electrotechnical Commission (IEC) is currently working on a new series of standards that covers the functional requirements of measuring relays and related equipment used to protect electrical transmission and distribution systems. The new protection relay functional standards are designated as the IEC 60255-1xx series. The standardisation of various test methodologies and measurement metrics promises benefits for the entire protection relay community. A total of fifty international experts from seventeen national committees of the IEC are working together to develop these standards. Andrea Bonetti of Megger actively represents Sweden on the committee and together with three other delegates graciously agreed to provide information about this important work.

Electrical systems are becoming more and more complex and sophisticated. As a consequence, protection systems are also becoming more complex and, at the same time, the consequences of protection failures are becoming costlier and more disruptive. Additionally, the widespread implementation of smart grid concepts is also creating new challenges for protection systems. It is clear, therefore, that there is a real need for internationally recognised standards to define the functionality of the key components – the protection relays and their protection functions – that form the basis of these systems. To meet this need, the IEC is currently working on the IEC 60255-1xx series of functional standards dedicated to protection relays and protection functions. Before looking at the benefits these standards can provide, let us review some background information.

The scope of TC 95

The standards are being developed and published by the Maintenance Team 4 (MT4) of IEC Technical Committee 95 (TC 95): “measuring relays and protection equipment”. The scope of TC 95 is the standardisation of measuring relays, protection equipment, and protection functions embedded in any equipment or systems used in various fields of electrical engineering covered by the IEC, including combinations of devices and functions that form schemes for power systems protection. The TC 95 scope includes control, monitoring, and process interface related functions and equipment used with protection systems (such as automatic reclosing, fault location, teleprotection or process data interfaces, and fault recording); as well as protection and protectionrelated functions of distributed energy resources (DER) or inverter-based resources (IBR).

It can be seen that the scope is very broad and, as a result, there will be many standards developed in this series. Some of these have already been published while others are in various stages of development. All these standards have a common objective: they specify minimum functional requirements, testing methodologies and methods of performance evaluation, as well as the format for publishing the test results for each function. The aim is to help users in evaluating protection functions on a standardised basis with respect to relay selection, setting, commissioning, application, and operation.

Relay standards available and planned

At the time of writing, the IEC 60255-1xx standards that have already been published are:

  • IEC 60255-151:2009, Functional requirements for over/under current protection
  • IEC 60255-127:2010, Functional requirements for over/under voltage protection
  • IEC 60255-149:2013, Functional requirements for thermal electrical relays
  • IEC 60255-121:2014, Functional requirements for distance protection
  • IEC/IEEE 60255-118-1:2018, Synchrophasor for power systems - measurements
  • IEC 60255-181:2019, Functional requirements for frequency protection

Work on the following standards is at various stages of development and they are expected to be completed and published in the following order:

  • IEC 60255-187-1, Functional requirements for restrained and unrestrained differential protection of motors, generators, and transformers
  • IEC 60255-187-3, Functional requirements for biased (percentage) differential relays for transmission lines
  • IEC 60255-187-2, Functional requirements for busbar differential protection

Protecting the smart grid: IEC 60255-181:2019

In 2012, an ad hoc working group – AHG2 “New protection requirements for the smart grid” – was formed and it submitted its report to TC 95 in 2014. This identified a need for revising some of the existing standards and for developing new standards taking into account the high penetration of distributed generation, which is posing new protection challenges. Revisions of IEC 60255-1, IEC 60255-26, and the new functional standard IEC 60255-181 were identified as high priorities.

While IEC 60255-1 and IEC 60255-26 are still under revision, the new IEC 60255-181 Standard for Frequency and ROCOF protection functions was published in February 2019. Moreover, the IEC 60255-181 standard has been adopted relatively recently (autumn 2019) as the European standard EN IEC 60255-181, to be used without modifications in 34 countries in Europe.

Figure 1: Importance of standardisation (recreated from original slide courtesy of

New technical report for IEC 61850 applications in relaying

The TC 95 committee is also looking at the application of the IEC 61850 standard in relay protection systems. IEC 61850 relates to communication networks and systems for power utility automation. To facilitate this work, the committee has created a dedicated working group (TC 95/WG 2) to address “the use of digital sampled values instead of analog inputs and IEC 61850 GOOSE messages instead of binary inputs and outputs”. WG 2 is preparing a technical report (TR) which, at the time of writing, has the provisional title “Part 216-1: Guidelines for Requirements and Tests for Protection Functions with Digital Inputs and Outputs”. This report is expected to be published by the end of 2021.

It is planned that the work being carried out by this group will be coordinated with the existing IEC 60255- 1xx series functional standards that consider voltage, current, temperature, etc. inputs in analog form, plus binary inputs and outputs. Future editions of the existing functional standard, as well as the new one, will take into account the voltage, current, temperature, etc. inputs in analog form and digital form in accordance with IEC 61850, as well as binary inputs and outputs and IEC 61850 GOOSE messages.

Standards are not just for relay manufacturers!

Having explained the background for these standards and discussed their status, let us move on to consider their applications and benefits. Many engineers and technicians who work with relay protection systems believe that the standards apply only to relay manufacturers and that as users they do not need to be aware of their contents. This is unfortunate because, although it is certainly true that the standards apply to relay manufacturers, it is equally true that users will benefit from knowing about and understanding the issues that the standards address. Relay users also need to take the content of the standards into account in their work.

As an example, the IEC 60255-121:2014 standard for distance protection function has two test methods from which a relay manufacturer can choose for measuring the accuracy (so-called basic accuracy) of a distance protection relay [1]. One method is called ‘pseudocontinuous ramp’ and the second method is called ‘ramp of shots’. For technical reasons, the two methods may give different results. Consequently, the manufacturer is requested to provide not only the value of the basic accuracy but also the method used to assess it. Further, during commissioning, it may be necessary to check the accuracy of the protection relay to verify that it is healthy. For these tests, it is desirable to use the same test method as that used by the manufacturer to be sure of comparing ‘apples with apples’, and to avoid unnecessary misunderstandings if the test results are questioned.

Another example relates to the IEC 60255-181:2019 standard for frequency and frequency-related protection functions [2]. This standard specifies a particular frequency ramp, the formula for which can be found in normative annex A of IEC 60255-181. Testing frequency relays with a different frequency ramp may result in unexpected results, depending on the relay algorithms and, once again, this can lead to unnecessary discussions if the test result is declared as failed. Since no test engineer nor any end-user wants to have doubts about the protection relay’s behavior during commissioning, it is best to always use the same standardised frequency ramp.

Not only should the test engineer be aware of these requirements, but test equipment manufacturers are also expected to implement standardised test methods in their test sets. As these two examples have shown, the standardisation chain extends far beyond the relay manufacturers.

Benefits for the users

What about the end-users? Is it enough to specify “the relay protection devices shall be conformant to the applicable parts of the IEC 60255-1xx series of functional standards”? Well, that sentence is better than nothing. It ensures that the protection relays used in the system will at least have performed all the tests required in the standard and will also be covered by the essential declarations that relay manufacturers must provide. Unfortunately, however, including that sentence will not ensure that the protection relays match the requirements of the user’s application. But how can this be?

The reason is that the IEC 60255-1xx series of standards does not include pass/fail criteria for the tests, except in a few very particular cases. It provides a large set of standardised tests to verify relay performance (operate time, start time, accuracy, security, etc.), but it does not state which is the best relay because this depends on the application.

Another example, which relates to IEC 60255-121: 2014, may make this clearer. Distance protection Relay A exhibits a standardised typical ‘operate time’ of 18 ms, but in other tests, the relay shows some inaccuracy in selecting the faulty phase(s). Relay B exhibits a typical ‘operate time’ of 28 ms, but all tests have shown that the relay correctly reports the faulty phases. If my application has only a three-phase trip, I may decide to use relay A. But if my application has single or multi-phase tripping, with blocking of auto-reclose operations when multiphase faults are detected, my choice will probably be relay B.

As a result, it is strongly recommended that relay manufacturers, relay test equipment manufacturers, and relay commissioning and maintenance engineers should be aware of functional standards and that endusers should be equally aware! End-users are expected to create user profiles which include acceptance criteria and requirements relating to their specific applications. However, when creating these profiles, the users will be guided by the information and definitions contained in the standards [1] and [2]. Additionally, users could adopt a subset of the standardised tests described in the IEC 60255-1xx series as the basis for formulating their acceptance tests. In fact, it is not uncommon to see factory acceptance tests directly referring to, or at least inspired by, IEC 60255-1xx series standards.

As previously mentioned, the standards also contain several declarations that relay manufacturers are required to make to aid the engineering processes involved in the protection of electrical substations. Knowing that the manufacturers must provide this information will make it easier for engineers to design reliable protection systems. In this case, as before, there are no pass/fail criteria for these declarations. How well they have been addressed is a judgement for the user. It is clear, however, that if some declarations are missing, the vendor cannot claim compliance with the standard.

As an example, particular emphasis is given by the standards to the current transformer dimensioning formulas that manufacturers are obliged to provide, which now have to be in a standardised mathematical format. In fact, gaining a consensus in the protection relay community about the importance of these formulas for relay users has been one of the most important goals achieved by the TC 95/MT 4 committee. Relay manufacturers are required to carry out extensive testing to comply with this requirement, which will result in benefits that include easier, safer, and more reliable design and engineering of protection systems.

It is worth mentioning that the standards do not specifically detail commissioning or routine tests for protection functions. Nevertheless, several clauses can be applied to these tests and doing so will help to minimise the risk of misunderstandings about validation in the field compared with the performance declared by the relay manufacturer or requested by the user, as previously explained. Also, manufacturers of relay test equipment are expected to implement the definitions and test methodologies detailed in the standards, in relation to both commissioning and maintenance tests.

Many are waiting for the technical report on IEC 61850

The entire relay community is eagerly awaiting the technical report that TC 95/WG 2 is preparing about IEC 61850 applications for GOOSE and Sampled Values in relay protection applications. The report aims to provide significant guidelines for the use of IEC 61850 in protection applications, with many mandatory requirements in the report that are not mandatory in the IEC 61850 standard itself. It can also be said that the report provides a relay protection profile for the IEC 61850 standard. The report gives in-depth details about process bus, merging units, time synchronisation, and testing. It is planned that the work being carried out by this group will be introduced into the existing IEC 60255-1xx series standards that consider the voltage, current, temperature, and other inputs in analog form only, plus binary inputs and outputs. As already mentioned, the next editions of the existing functional standard, as well as the new one will take into account the voltage, current, temperature, etc. inputs in analog form and digital form in accordance with IEC 61850, as well as binary inputs and outputs and IEC 61850 GOOSE messages. Once the report is published, the most significant parts of it will be integrated into the IEC 60255-1xx series becoming full IEC standards [3].


In conclusion, we have seen that for relay manufacturers, the standards provide detailed requirements to which they must adhere. For end-users, they provide clear and unambiguous guidance on the performance relays should provide, and the documentation they can expect from the manufacturers. For manufacturers of relay test equipment, the standards dictate the testing methods they are expected to provide. Testing engineers have an easier task. The use of the standardised concepts and methods by all the sections of the protection community minimises misunderstandings at procurement, order, commissioning/maintenance, and FAT/SAT phases. This common technical understanding facilitates a smoother, faster, and cheaper project path. In short, standards in the new IEC 60255-1xx series are relevant and important for everyone whose work involves relay protection systems, whatever their role in this wonderful technical community.

Note from the editor: This article is written by the authors on behalf of the IEC committees/working groups mentioned, and not on behalf of their affiliated companies. The editor of Electrical Tester expresses gratitude for the considerable efforts in terms of time and competence that have been involved in writing this article and gives special thanks to the authors’ affiliates for actively participating and allocating important resources to standardisation work, which is an activity beyond pure everyday business.


[1] Bonetti, Andrea, Murty V.V.S. Yalla, and Stig Holst. “The IEC 60255-121:2014 Standard and Its Impact on Performance Specification, Testing and Evaluation of Distance Protection Relays”. In 2016 IEEE/PES Transmission and Distribution Conference and Exposition (T D), 1–6, 2016.

[2] Bonetti, Andrea, Janez Zakonjsek, and Urban Rudez. “Bringing ROCOF into Spotlight in Smart Grids: New Standardization and UFLS Method”, 2020.

[3] Leitloff, Volker, Hao Chen, Dehui Chen, Andrea Bonetti, Lei Xu, and Ahmed Mohamed. “Standardisation Challenges for Digital Inputs and Outputs of Protection Functions in IEC 60255 Series”. In ResearchGate. Glasgow, UK, 2019.…

About the authors

Andrea Bonetti - Andrea is a senior specialist in relay protection and IEC 61850 applications at Megger Sweden AB. He is an active member of the IEC:

  • Member of IEC TC 95/MT 4. Measuring relays and protection equipment - Functional standards.
  • Member of IEC TC 95/WG 2. Protection functions with Digital Input/Output.
  • Member of IEC TC JWG 17. Documentation of communication in power utility automation.
  • Andrea received the IEC 1906 award in 2013. He is a corresponding member of CIGRÉ WG B5.53.

Murty Yalla - Murty is president of Beckwith Electric Co, Florida USA. Murty is an active member of the IEC:

  • Chair of IEC TC 95.
  • Convenor of TC 95/MT 4.
  • Murty received the IEC 1906 Award in 2010.

He is an active member of the IEEE, NERC, and CIGRÉ and was elected in 2006 to Fellowship by the IEEE Fellow Committee for his contributions in computer relays for power systems.

Thierry Bardou - Thierry is a senior engineer in protection relay at Schneider Electric. Thierry is an active member of the IEC:

  • Secretary of IEC TC 95 “Measuring relays and protection equipment”.
  • Member of IEC TC95/MT4 “Measuring relays and protection equipment – Functional standards”.
  • Member of IEC ACTAD (Advisory Committee on Electricity Transmission and Distribution).

Volker Leitloff - Volker is an expert in domain protection and automation with RTE (French TSO). Volker is an active member of the IEC:

  • Convenor of IEC TC 95/WG 2 “Protection functions with Digital Input/Output”.
  • Chair of IEC TC 38 and CENELEC TC 38 “Instrument transformers” and member of IECTC 38 WG 37 and WG 47.
  • Member of TC 95/MT 4 “Measuring relays and protection equipment - Functional standards”.

Volker is an active member of CIGRÉ:

  • Convenor of CIGRE B5 TG51 and member of WG B5.69 and B5.64.