Definitions and Terminology

   1. All-or-nothing relay
A relay which is not designed to have any specified accuracy as to its operating value.

  2.     Auxiliary relay.
An all-or-nothing relay used to supplement the performance of another relay, by modifying contact performance for example, or by introducing time delays.

 3. Back-up protection.
A protective system intended to supplement the main protection in case the latter should be in-effective, or to deal with faults in those parts of the power system that are not readily included in the operating zones of the main protection.

 4. Biased relay.
A relay in which the characteristics are modified by the introduction of some quantity other than the actuating quantity, and which is usually in opposition to the actuating quantity.

 5. Burden.
The loading imposed by the circuits of the relay on the energizing power source or sources, expressed as the product of voltage and current (volt-amperes, or watts if D.C) for a given condition, which may be either at 'setting' or at rated current or voltage.

The rated output of measuring transformers, expressed in VA, is always at rated current or voltage and it is important, in assessing the burden imposed by a relay, to ensure that the value of burden at rated current is used.

 6. Characteristic angle.
The phase angle at which the performance of the relay is declared. It is usually the angle at which maximum sensitivity occurs.

7. Characteristic curve.
The curve showing the operating value of the characteristic quantity corresponding to various values or combinations of the energizing quantities.

 8. Characteristic quantity.
A quantity, the value of which characterizes the operation of the relay, e.g. current for an over current relay, voltage for a voltage relay, phase angle for a directional relay, time for an independent time delay relay, impedance for an impedance relay.

 9. Characteristic impedance ratio (C.I. R.)
The maximum value of the System Impedance Ratio up to which the relay performance remains within the prescribed limits of accuracy.
 10. Check protective system.
An auxiliary protective system intended to prevent tripping due to inadvertent operation of the main protective system.

 11. Conjunctive test.
A test on a protective system including all relevant components and ancillary equipment appropriately interconnected. The test may be parametric or specific.

    a. Parametric conjunctive test.
A test to ascertain the range of values that may be assigned to each parameter when considered in combination with other parameters, while still complying with the relevant performance requirements.

   b. Specific conjunctive test.
A test to prove the performance for a particular application, for which definite values are assigned to each of the parameters.

 12. Dependent time delay relay.
A time delay relay in which the time delay varies with the value of the energizing quantity.

 13. Discrimination.
The quality whereby a protective system distinguishes between those conditions for which it is intended to operate and those for which it shall not operate.

14. Drop-out.
A relay drops out when it moves from the energized position to the un-energized position.

15. Drop-out / pick ratio.
The ratio of the limiting values of the characteristic quantity at which the relay resets and operates. This value is sometimes called the differential of the relay.

16. Earth fault protective system.
A protective system which is designed to respond only to faults to earth.

17. Earthing transformer.
A three-phase transformer intended essentially to provide a neutral point to a power system for the purpose of Earthing.

 18. Effective range

The range of values of the characteristic quantity or quantities, or of the energizing quantities to which the relay will respond and satisfy the requirements concerning it, in particular those concerning precision.

19. Effective setting

The 'setting' of a protective system including the effects of current transformers. The effective setting can be expressed in terms of primary current or secondary current from the current transformers and is so designated as appropriate.

20. Electrical relay

A device designed to produce sudden predetermined changes in one or more electrical circuits after the appearance of certain conditions in the electrical circuit or circuits controlling it.

NOTE: The term 'relay' includes all the ancillary equipment calibrated with the device.

21. Energizing quantity.

The electrical quantity, either current or voltage, which alone or in combination with other energizing quantities, must be applied to the relay to cause it to function.

 22. Independent time delay relay.

A time delay relay in which the time delay is independent of the energizing quantity.

 21. Instantaneous relay.

A relay which operates and resets with no intentional time delay.

NOTE: All relays require some time to operate; it is possible, within the above definition, to discuss the operating time characteristics of an instantaneous relay.

 22. Inverse time delay relay.

A dependent time delay relay having an operating time which is an inverse function of the electrical characteristic quantity.

23. Inverse time delay relay with definite minimum
(I.D. M

A relay in which the time delay varies inversely with the characteristic quantity up to a certain value, after which the time delay becomes substantially independent.

24. Knee-point e.m.f.

That sinusoidal e.m.f. applied to the secondary terminals of a current transformer, which, when increased by 10 %, causes the exciting current to increase by 50%.

25. Main protection.

The protective system which is normally expected to operate in response to a fault in the protected zone.


26. Measuring relay.

A relay intended to operate with a specified accuracy at one or more values of its characteristic quantity.

27. Notching relay.

A relay which switches in response to a specific number of applied impulses.

28. Operating time.

With a relay de-energized and in its initial condition, the time which elapses between the application of a characteristic quantity and the instant when the relay operates.

29. Operating time characteristic.

The curve depicting the relationship between different values of the characteristic quantity applied to a relay and the corresponding values of operating time.

30. Operating value.

The limiting value of the characteristic quantity at which the relay actually operates.

31. Overshoot time.

The extent to which the condition that leads to final operation is advanced after the removal of the energizing quantity, expressed as time at the rate of progress of the said condition appropriate to the value of the energizing quantity that was initially applied.

32. Pick-up.

A relay is said to 'pick-up' when it changes from the un-energized position to the energized position.

33. Pilot channel.

A means of interconnection between relaying points for the purpose of protection.

34. Protected zone.

The portion of a power system protected by a given protective system or a part of that protective system.

35. Protective gear.

The apparatus, including protective relays, trans-formers and ancillary equipment, for use in a protective system.

36. Protective relay.

A relay designed to initiate disconnection of a part of an electrical installation or to operate a warning signal, in the case of a fault or other abnormal condition in the installation. A protective relay may include more than one unit electrical relay and accessories.

37. Protective scheme.

The coordinated arrangements for the protection of one or more elements of a power system.

A protective scheme may comprise several protective systems.

38. Protective system.

A combination of protective gear designed to secure, under predetermined conditions, usually abnormal, the disconnection of an element of a power system, or to give an alarm signal, or both.

39. Rating.

The nominal value of an energizing quantity which appears in the designation of a relay. The nominal value usually corresponds to the CT and VT secondary ratings.

40. Resetting value.

The limiting value of the characteristic quantity at which the relay returns to its initial position.

41. Residua/ current.

The algebraic sum, in a multi-phase system, of all the line currents.

42. Residua/ voltage.

The algebraic sum, in a multi-phase system, of all the line-to-earth voltages.

43. Setting.

The limiting value of a 'characteristic' or 'energizing' quantity at which the relay is designed to operate under specified conditions.

Such values are usually marked on the relay and may be expressed as direct values, percentages of rated values, or multiples.

44. Stability.

The quality whereby a protective system remains inoperative under all conditions other than those for which it is specifically designed to operate.

45. Stability limits.

The R.M.S. value of the symmetrical component of the through fault current up to which the protective system remains stable.

46. Starting relay.

A unit relay which responds to abnormal conditions and initiates the operation of other elements of the protective system.

47. System impedance ratio (S./.R.).

The ratio of the power system source impedance to the impedance of the protected zone.

48. Through fault current.

The current flowing through a protected zone to a fault beyond that zone.


 49. Time delay.

A delay intentionally introduced into the operation of a relay system.

 50. Time delay relay.

A relay having an intentional delaying device.

 51. Unit electrical relay.

A single relay which can be used alone or in combinations with others.

52. Unit protection.

A protection system which is designed to operate only for abnormal conditions within a clearly defined zone of the power system.

53. Unrestricted protection.

A protection system which has no clearly defined zone of operation and which achieves selective operation only by time grading.

       Fault Definitions and:

For the purpose of this International Standard, the following definitions, some of them based on IEC 60050(191), IEC 60050(212) and

IEC 60050(604) apply:


1- Fault

An unplanned occurrence or defect in an item which may result in one or more failures of the item itself or of other associated equipment

[IEC 604-02-011

NOTE - In electrical equipment, a fault may or may not result in damage to the insulation and failure of the equipment.


2- Non-damage fault

A fault which does not involve repair or replacement action at the point of the fault

NOTE - Typical examples are self-extinguishing arcs in switching equipment or general overheating without paper carbonization.

[IEC 604-02-091


3- Damage fault

A fault which involves repair or replacement action at the point of the fault

[IEC 604-02-08, modified]


4- Incident

An event related to an internal fault which temporarily or permanently disturbs the normal operation of an equipment [IEV 604-02-03, modified]

NOTE - Typical examples are gas alarms, equipment tripping or equipment leakage.


5- Failure

The termination of the ability of an item to perform a required function [IEC 191-04-01]


NOTE - In the electrical equipment, failure will result from a damage fault or incident necessitating outage, repair or replacement of the equipment, such as internal breakdown, rupture of tank, fire or explosion.


6- Electrical fault

a partial or disruptive discharge through the insulation.


7- Partial discharge

A discharge which only partially bridges the insulation between conductors. It may occur inside the insulation or adjacent to a conductor

[IEC 212-01-34, modified]


NOTE 1 - Corona is a form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation. This term is not to be used as a general term for all forms of partial discharges.


NOTE 2 - X-wax is a solid material which is formed from mineral insulating oil as a result of electrical discharges and which consists of polymerized fragments of the molecules of the original liquid

[IEV 212-07-24, modified].

Comparable products may be formed from other liquids under similar conditions.


NOTE 3 - Sparking of low energy, for example because of metals or floating potentials, is sometimes described as

Partial discharge but should rather be considered as a discharge of low energy.


8- Discharge (disruptive) .

The passage of an arc following the breakdown of the insulation

 [IEC 604-03-38, modified]


NOTE 1 - Discharges are often described as arcing, breakdown or short circuits.

The more specific following terms are also used:

- spark over (discharge through the oil);

- puncture (discharge through the solid insulation);

- Flashover (discharge at the surface of the solid insulation);

- tracking (the progressive degradation of the surface of solid insulation

   by local

  Discharges to form conducting or partially conducting paths);

- sparking discharges which, in the conventions of physics, are local

  Dielectric breakdowns of high ionization density or small arcs.


NOTE 2 - Depending on the amount of energy contained in the discharge, it will be described as a discharge of low or high energy, based on the extent of damage observed on the equipment .


9- Thermal fault

Excessive temperature rise in the insulation


NOTE - Typical causes are

- Insufficient cooling,

- Excessive currents circulating in adjacent metal parts (as a result of bad

  Contacts, eddy currents, stray losses or leakage flux),

- Excessive currents circulating through the insulation (as a result of high

  Dielectric losses), leading to a thermal runaway,

- overheating of internal winding or bushing connection lead.


10- Typical values of gas concentrations.

gas concentrations normally found in the equipment in service which have no symptoms of failure, and which are over passed by only an arbitrary percentage of higher gas contents, for example 10 % .


NOTE 1 - Typical values will differ in different types of equipment and in different networks, depending on operating practices (load levels, climate, etc.).


NOTE 2 - Typical values, in many countries and by many users, are quoted as "normal values", but this term has not been used here to avoid possible misinterpretations.



        2 Time delay starting or closing relay.

        3 Checking or interlocking relay

        21 Distance relay

        25 Synchronizing or synchronism check relay

        27 Under voltage relay

        30 Annunciator relay

        32 Directional power relay

        37 Undercurrent or under power relay

        40 Field failure relay

        46 Reverse phase or phase balance current relay

        49 Machine or transformer thermal relay

        50 Instantaneous over current or rate-of-rise relay

        51 A.c. time over current relay

        52 A.c. circuit breaker

        52a Circuit breaker auxiliary switchnormally open

        52b Circuit breaker auxiliary switchnormally closed

        55 Power factor relay

        56 Field_application relay

        59 Over voltage relay

        60 Voltage or current balance relay

        64 Earth fault protective relay

        67 A.c. directional over current relay

        68 Blocking relay

        74 Alarm relay

        76 D.c. over current relay

        78 Phase angle measuring or out-of-step protective relay

        79 A.c. reclosing relay

        81 Frequency relay

        83 Automatic selective control or transfer relay

        85 Carrier or pilot wire receive relay

        86 Locking-out relay

        87 Differential protective relay

        94 auxiliary tripping relay

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