Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / 13 terms concerning relaying, measurements, and breakers used by protection engineers

Terminology in relay protection

It’s not unusual to see graduates and engineers from other disciplines experience difficulties in properly interpreting the terminology used in applying relays, analyzing their performance, and designing protection systems. Actually, this is normal, but during the project, this lack makes it difficult for relay engineers to communicate effectively with their colleagues and convey their interpretations of relaying issues and questions effectively.

13 terms concerning protective relays, measurements, and breakers used by protection engineers
13 terms concerning protective relays, measurements, and breakers used by protection engineers (on photo: SEL's 351S Relay Module)

This technical article is dedicated to graduates and engineers coming from other disciplines as well to experienced power system and protection engineers. It will shed some light on terms concerning the quality of measurements, philosophy of protection and circuit breakers often used by protection engineers.

The specific terms included in this article are:

  1. Accuracy,
  2. Accuracy class,
  3. Reliability,
    1. Dependability,
    2. Security,
  4. Sensitivity,
  5. Relay stability,
  6. Primary protection,
  7. Backup protection,
  8. Dual protection,
  9. Device number,
  10. Breaker failure,
  11. Phase disagreement,
  12. Pole flashover, and
  13. Single-phase tripping

1. Accuracy

This term is used for at least two different purposes, one to describe the accuracy of a device and the other to specify the accuracy of a measurement. In the first context, accuracy is the degree to which a device (relay, instrument or meter) conforms to an accepted standard.

The statement of an accuracy is only as good as the methods used to express it for individual components and the manner in which they affect the overall accuracy of the device.

In the second case, the accuracy of a measurement specifies the difference between the measured and true values of a quantity. The deviation from the true value is the indication of how accurately a reading has been taken or a setting has been made.


If a relay is specified to have ±5% accuracy, it means that the relay should operate when its exciting quantity (current or voltage) is between -5% and +5% of its setting. Let us consider the case of Figure 1 and assume that the CT provides secondary current which is an accurate representation of the primary current.

A line protected by a current relay
Figure 1 – A line protected by a current relay

When the fault current is 12,000 A, the current in the relay will be 100 A. If the relay accuracy is ±5%, it could interpret the current to be of any level from 95 A to 105 A. In case the relay is set to operate at 100 A, it mayor may not operate depending on its interpretation of the level of current in the circuit.

Go back to Contents Table ↑

2. Accuracy Class

This term is used to define the quality of the steady state performance of a current transformer. The accuracy class of a current transformer (CT) used for protection functions is described by a letter which indicates whether the accuracy can be calculated (class C) or it must be obtained from physical tests (class T).

This letter is followed by a number which is equal to the maximum secondary terminal voltage that the CT will produce at 20 times the rated secondary current with no more than 10% error.

Examples of accuracy classes for 10% error class C CTs are C1OO, C200, C400 and C800. At this time, there is no accuracy class higher than C800. Examples of accuracy classes for 10% error class T CTs are T105, 1250, T375 and T750.

IEEE C57.13IEC 60044-1
C10025 VA 5P 20
C20050 VA 5P 20
C400100 VA 5P 20
C800200 VA 5P 20

The IEC accuracy designation gives the burden VA at rated input, the accuracy rating (5P), and the limit of 20 times rating.

Go back to Contents Table ↑

3. Reliability

Reliability is an index that expresses the attribute of a protective relay or a system to operate correctly for situations in which it is designed to operate. This also includes the attribute of not operating (incorrectly) for all other situations.

Reliability is expressed in terms of two competing fundamental attributes, dependability and security.

The essentials of power systems: Relay protection and communication systems
Figure 2 – Protection engineer testing the secondary circuits

Go back to Contents Table ↑

3.1 Dependability

Dependability is the aspect of reliability that expresses the degree of certainty that a relay will operate correctly. For relay systems, dependability is assured by using redundant protection systems and backup relays.


The primary protection for a transmission line may be provided by using a phase comparison protection scheme. The degree of certainty that this scheme will operate for all faults on the transmission line is the dependability index of the scheme.

To increase this index for the transmission line protection system, distance relays can be included to act as backup relays.

Primary and backup relay protection in case of failures
Figure 3 – Primary and backup relay protection in case of failures (on photo: Relay protection panels in East Lake 132-11kV substation; credit: PSD Energy)

Go back to Contents Table ↑

3.2 Security

Security is the aspect of reliability that expresses the degree of certainty that a relay will not operate incorrectly irrespective of the nature of the operating state of the power system. Pretty simple.

Premium Membership Required

This technical article/guide requires a Premium Membership. You can choose an annually based Plus, Pro, or Enterprise membership plan. Subscribe and enjoy studying specialized technical articles, online video courses, electrical engineering guides, and papers. With EEP’s premium membership, you get additional essence that enhances your knowledge and experience in low- medium- and high-voltage engineering fields.

Check out each plan’s benefits and choose the membership plan that works best for you or your organization.

Good To Know!Save 50% on all video courses by purchasing Enterprise plan.

Log In »Purchase »

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

six  ×    =  twelve

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge