ELECTROPEDIA – The IEC vocabulary

Electrical Tester – 17 December 2021

By Andrea Bonetti and Xicai Zhao


When dealing with a global technical community day after day, it is increasingly problematic to use a language where the meaning is ‘assumed’.

Is the voltage the phase-to-phase voltage or the phase-to-earth voltage? Not all schools in the world assume that in a three-phase system the voltage is the phase-to-phase voltage, so it is better to be clear from the beginning.

Is a current with an angle of 15 degrees representing a phasor in the first quadrant or in the fourth? The lead system is related to the math convention of having the phasor/vector in the first quadrant, with phasors rotating counter-clockwise, but the lag system would see it in the fourth quadrant. People using the lag reference system are not more or less correct than people using the lead reference system, but today they need to understand each other more than ever before as it’s likely they will need to work more closely than they did previously.

To communicate clearly from the beginning potentially involves using many words to explain each concept at the outset, essentially declaring “when I say that, this is what I actually mean”. This is difficult and time consuming, so a better way of facilitating communication is to get help from standardisation. This allows us to be concise but exact in our expressions, and avoid (or at least minimise) the need for explanations before communicating.

The Electropedia

Few of us, even if we work every day in the electrical field, know that there is a readily available and reliable reference for technical definitions: Electropedia. Electropedia is the World’s Online Electrotechnical Vocabulary; it is accessible to everybody and it is often known as ‘IEV Online’ (Figure 2).

Behind Electropedia there is a huge amount of standardisation work. In fact, what Electropedia presents for each subject area is the result of correlating IEC standards. Indeed, Figure 3 shows the subject areas that are currently available in Electropedia and the IEC 60050- 447 standard, for example, is the background for subject 447: measuring relays.

Figure 1: Example of different phasor representations of the same balanced three-phase positive-sequence voltage system. The left-hand picture is in the ‘ordinary’ reference system, counter-clockwise rotation of the phasors, positive angle in the ‘leading direction’ (counter clockwise). The right-hand picture has the same counter-clockwise reference of the phasor rotations, but the angle is positive in the ‘lagging’ direction, which is clockwise.

Figure 2: Screenshot from Electropedia, or IEV Online (http://www.electropedia.org/iev/iev.nsf/welcome?OpenForm&Seq=1)

In this article, we are going to look at some details of Electropedia for measuring relays. We will explain who takes care of this standardisation work, some important concepts for everyday use, what has been done, and what is planned.

The committees and working groups behind Electropedia for protection relays

The International Electrotechnical Commission (IEC) Technical Committee 95 (TC 95: Measuring relays and protection equipment) has set up Maintenance Team 1 (IEC TC 95/MT1) to update vocabulary and terminology for measuring relays and protection equipment. The task assigned to MT1 is to revise IEC 60255-447:2010, and to review Chapter 3 - terms and definitions - in each individual standard that's drafted within TC 95.

As the result of work done by MT 1, IEC 60050-447 Ed. 2 "International Electrotechnical Vocabulary (IEV) - Part 447: Measuring relays and protection equipment" was published in May 2020. Note that the title of this standard has been changed from 'measuring relays' to 'measuring relays and protection equipment' (Figure 4).

Also, the nomenclature for this application area is available online (Figure 5).

Some nomenclature that is good to know ...

Operate time or trip time, pick-up or start?

The correct IEC wording is 'operate time'. Operating time, operational time or other terms should not be used when talking about the 'trip time' of a protection relay. 'Trip time' is a definition associated with IEEE nomenclature.

The wording 'trip time' does not exist in the IEV for relay protection. Nevertheless, in many IEC standards for protection relays (IEC 60255-1 xx series) it is possible to find sentences like this: " ... start the time until the relay operates (trips) ... " to facilitate understanding for IEEE colleagues.

What must be noted is that 'operating time' for a relay does not exist. The words 'operating time' refer more to circuit breakers (IEC 60050-441, switchgear, controlgear and fuses).

Also, for 'start time' , the correct IEC wording is 'start time' or 'start threshold' . In IEEE it is defined as 'pick-up time' or 'pick-up threshold'. The protection relay starts (or picks up for IEEE).

There is an interesting but subtle difference between 'operate' and 'trip' that is worth mentioning: the 'start condition' is different from the 'operate condition' . For measuring relays and protection equipment, 'start' means a fault or abnormality occurs which has been detected by the measuring relay or protection equipment, something happens inside it (it leaves its initial state). An output signal (start signal) may be issued to indicate this, but such a signal is not essential. ‘Operate’ means that the relay has completed its intended function, such as tripping the target circuit breaker via an output signal (operate signal). In some special applications, e.g. generator protection, the ‘operate signal’ may trigger control logic first rather than just tripping the target circuit breaker, for example to start load shedding. So, the ‘operate signal’ may not be the same as the ‘trip signal’.

Figure 3: List of the subject areas available in Electropedia (http://www.electropedia.org/iev/iev.nsf/welcome?OpenForm&Seq=1)

Figure 4: IEC 60050-147:2020, from https://webstore.iec.ch/publication/60935

Figure 5: Nomenclature for measuring relays, IEC 60050-447 (http://www.electropedia.org/iev/iev.nsf/ index?openform&part=447)

Figure 6: Relay ‘operate time’ according to IEC 60050-447 (http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=447-05-…)

Reset or disengage?

Frankly speaking, not many relay protection engineers know about the word ‘disengage’ relating to relays. How many of you have heard someone say “the relay has disengaged”? Yet the ‘disengage condition’ is different from the ‘reset condition’.

In the ‘reset condition’, all internal status (e.g. integrators) and output signals shall return to their initial conditions after the fault or abnormality. The ‘reset condition’ shall appear after ‘start condition’, whether or not the ‘operate condition’ has occurred.

The ‘disengage condition’ is a transitional condition between the ‘operate condition’ and the ‘reset condition’, in which the ‘operate signal’ has returned to its initial state after being issued.

An electromechanical relay is a simple example of this. At the appearance of the fault, the relay starts, and the disc begins to rotate. At the end of the rotation, the operate signal is given. When the fault is cleared, the operate signal and start signal are not active, but the disc is still rotating back to its initial situation. The relay has disengaged but has not yet reset. When the disc is at its initial position, the relay is reset.

Why, in the year 2020, does the IEV define notions related to electromechanical relays? Many digital relays simulate the disc movement (integrator) because this is necessary to facilitate selectivity in networks where there is a prevalence of electromechanical relays - so this definition is important!


The International Electrotechnical Vocabulary (IEV) serves to promote global unification of terminology in the fields of electrotechnology, electronics and telecommunications. It is a common language among experts in these domains. Along with newly adopted technologies, new terms will be amended over time to avoid misunderstanding. IEC/TC95/MT1 will continue to contribute to that.

We have just shown two typical wordings that are sources of misunderstandings in the relay community. There are many other words and definitions, but to consider them all would take a lot of time and space!

The main message of this article is: “please be aware that there is a standard way to express yourself, and the IEC provides it”. Consider the Electropedia in your work, in your reports, and in your specification. This will be of great benefit for the entire relay community and indeed for the entire electrotechnical community.

About the authors

Andrea Bonetti

MSEE, La Sapienza University, Rome, Italy

Andrea is senior specialist in relay protection and IEC

61850 applications at Megger Sweden AB.

Andrea is an active member of IEC:

  • IEC TC 95/MT 4. Measuring relays and protection equipment - Functional standards
  • IEC TC 95/WG 2. Protection functions with digital input/output
  • IEC TC JWG 17. Documentation of communication in power utility automation


Xicai Zhao

MSEE, Shanghai Jiao Tong University, Shanghai, China

Xicai is senior engineer in relay protection and substation

automation at NR Electric China.

Xicai is an active member of IEC:

  • IEC TC 95/MT 1. Vocabulary and terminology
  • IEC TC 95/MT 4. Measuring relays and protection equipment - Functional standards