• Protective Relaying Terms and Definitions

    Quick reference of over 200 protective relay terms and definitions, compiled from the Institute of Electrical and Electronics Engineers and the National Association of Relay Manufacturers.

    Acceptance test — A calibration and functional test of a new or replacement relay to ensure it is in proper working order prior to installation.

    Air gap — Sometimes used for contact separation or for magnetic air gap.

    Ampere-turns — The product of the number of turns in a magnetic coil and the root means square (rms) current in amperes passing through the coil.

    Armature — Hinged or pivoted moving part of the magnetic circuit of an electromagnetic relay. It is used in a general sense to mean any moving part that actuates contacts in response to a change in coil current.

    Armature contact — Sometimes used for a movable contact.

    Armature relay — A relay operated by an electromagnet that, when energized, causes an armature to be attracted to a fixed pole (or poles).

    Auxiliary relay — A relay that operates in response to opening and closing of its operating circuit to assist another relay or device in performing a function.

    Back contacts — Sometimes used for the stationary contact of single-pole, normally closed contacts. Same as normally closed contacts.

    Backstop — The part of a relay that limits movement of the armature away from the pole piece or core.

    Backup relaying — Supplementary relaying designed to operate if a primary relay should malfunction or a circuit breaker fails to operate. Backup relaying usually disconnects more of the power system than just the part with the faulty element, since this is necessary to remove the abnormal condition and to minimize the effect on the remainder of the system.

    Bar relay — A relay so designed that a bar actuates several contacts simultaneously.

    Break-before-make contacts — Contacts that interrupt one circuit before establishing another.

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    Break contact — Same as back contact.

    Break delay — Sometimes used for release time.

    Bridging — Describes a contact transfer in which the movable contact touches the normally open contact before leaving the normally closed contact during the transfer action, thus never completely opening the circuit of the movable contact.

    Brush — Sometimes used for wiper.

    Burden — The impedance of the circuit connected to the secondary winding. For potential transformers the burden is often expressed in terms of equivalent volt-amperes (VA) and power factor at a given voltage and frequency.

    Calibration — See relay calibration.

    Chatter — A sustained rapid opening and closing of contacts caused by variations in the coil current, mechanical vibration, shock, problems with laminations in the magnetic core, or incorrect travel of the armature.

    Clapper relay — Sometimes used for armature relay.

    Close-differential relay — Sometimes used for marginal relay.

    Coil — A magnetic or thermal winding to which energy is supplied to activate the relay.

    Commissioning test — A test of the total relay system after installation or modification or for troubleshooting purposes. It includes tests of the instrument transformers and all wiring and relay outputs with actual trip testing of the circuit breaker.

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    Contact arrangement — The combination of different basic contact forms to make up the entire relay switching structure.

    Contact bounce — Uncontrolled making and breaking of contact when relay contacts are moved to the closed position.

    Contact follow — The distance two contacts travel together after just touching.

    Contact gap — Same as contact separation.

    Contact over travel — Sometimes used for contact follow.

    Contact separation — Maximum distance between mating relay contacts when the contacts are in the open position.

    Contact spring — A current-carrying spring to which contacts are fastened.

    Contacts — Current-carrying parts of a relay that engage or disengage to make or break electrical circuits.

    Contactor — Sometimes used for a relay with heavy-duty contacts.

    Continuity-transfer contacts — Same as make-before-break contacts.

    Continuous-duty relay — A relay that may be energized with rated coil voltage or current at rated contact load for a period of 3 hours or more without failure and without exceeding specified temperature requirements.

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    Current balance relay — A relay that allows tripping whenever there is an abnormal change in the division of current between two circuits.

    Current rating — See rated coil current and rated contact current.

    Current relay — A relay designed to operate at a particular rated coil current rather than at a given rated coil voltage.

    Current transformer — A transformer used to reduce a primary current to a secondary value for use with protective relaying, control device, and metering circuits, typically 5 amps.

    Cycle timer — A controlling mechanism that opens or closes contacts according to a preset cycle.

    De-energize — To de-energize a relay is to disconnect the relay coil from its power source.

    Definite-purpose relay — A relay with a feature distinguishing it from a general-purpose relay. Types of definite purpose relays are interlock, selector, stepping, sequence, latch-in, and time-delay.

    Delay relay — A relay that is intentionally designed for a time delay between the energizing or de-energizing instant and the time that the relay contacts open or close.

    Diagnostic tests — Tests to find and correct relay settings, design or wiring errors, or malfunctions. These tests usually are conducted after a protective system problem is identified or suspected, such as failure to trip.

    Differential relay — A relay having multiple elements that function when voltage, current, or power difference between elements reach a predetermined value.

    Directional relay — A relay that trips when current flow is in one direction only.

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    Directional test — A test of directional relay elements to verify that the relay will operate or block properly when the relay input quantities are in the proper direction. A directional test is sometimes performed with the equipment carrying normal load. It always is performed as part of a relay system commissioning and as part of routine maintenance testing or troubleshooting tests.

    Double-break contacts — A combination in which a movable contact simultaneously makes and simultaneously breaks connection between two stationary contacts.

    Double-make contacts — For normally open contacts, the Double-break combination may be called double-make contacts.

    Double-throw contacts — A combination of two stationary contacts and one movable contact, which engages one of them when the coil is energized and the other when the coil is de-energized.

    Double-wound coil — A winding consisting of two parts wound on the same core.

    Drop-out values — Drop-out current, voltage, or power is the maximum value for which contacts of a previously energized relay will always assume their de-energized positions.

    Duty cycle — Rated working time of a device compared to its idle time.

    Electric reset — A term applied to a relay indicating that, following an operation, its contacts must be reset electrically to their original positions.

    Electromagnetic relay — A relay whose operation involves using a magnetic field, produced by an electromagnet.

    Electrostatic spring shields — Metallic shields between two relay springs to minimize capacitance between them.

    Enclosed relay — A relay with both coil and contacts protected from the surrounding medium by a cover not normally airtight.

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    Energize — To energize a relay is to apply rated voltage to its coil.

    Extension spring — Same as restoring spring.

    Fast-operate relay — A high-speed relay specifically designed for short-operate time but not short-release time.

    Fast-operate fast-release relay — A high-speed relay specifically designed for both short-operate time and short-release time.

    Fast-operate slow-release relay — A relay specifically designed for short-release time but not short-operate time.

    Fast-release relay — A high-speed relay specifically designed for short-release time but not short-operate time.

    Fixed contacts — Stationary contacts of a relay that are engaged and disengaged by moving contacts to make or break circuits.

    Flight time — Sometimes used for transfer time.

    Follow-through contacts — Contacts with contact follow.

    Frame — The structure on which the coil and contact assembly are mounted.

    Front contacts — Sometimes used for the stationary contact of single-pole; normally open contacts.

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    Functional test — (relay) A calibration, if required, and functional test of a relay to verify the relay functions according to its settings and specifications. Typical tests include characteristics tests, timing tests, pickup tests, and instantaneous trip (fault trip) tests. These tests typically are performed as part of the acceptance test, part of routine maintenance, or part of troubleshooting testing.

    Gasket-sealed relay — An airtight relay, sealed with a gasket that is not bonded to the other sealing material.

    General-purpose relay — A relay with ratings, design, construction, and operational characteristics, which make it adaptable to a wide variety of uses.

    Hand-reset — A qualifying term applied to a relay indicating that, following an operation, the contacts must be reset manually.

    Harsh conditions — Defined as excessive vibration, dusty atmospheres, and extreme temperature changes.

    Header — The part of a hermetically sealed relay through which electrical terminals pass.

    Hermetically sealed relay — An airtight relay, the sealing of which involves fusing or soldering but does not use a gasket.

    High-speed relay — A relay specifically designed for short-operate time, short-release time, or both.

    Hold values — The hold current, voltage, or power is the minimum value for which contacts of a previously energized relay always will maintain their energized positions.

    Homing — A qualifying term applied to a stepping relay indicating that wipers, upon completion of an operational cycle, are stepped around or back to the start position.

    Hum — As applied to relays, the sound caused by mechanical vibration resulting from alternating current flowing in the coil.

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    Impregnated coils — Coils that have been permeated with phenolic or similar varnish to protect them from mechanical vibration, handling, fungus, and moisture.

    Inductive winding — As contrasted with a noninductive winding, an inductive winding is a coil having a specifically designed inductance.

    Input — A physical quantity or quantities to which the relay is designed to respond. For microprocessor-based relays, the input consists of an analog to digital convertor that must be tested to ensure the input values match the predetermined input values.

    Instrument relay — A relay, the operation of which depends upon principles employed in electrical measuring instruments, such as the electrodynamometer, iron-vane, and D'Arsonval.

    Instrument transformer — Instrument transformers comprise current transformers, potential transformers, and coupling capacitor voltage transformers (CCVTs), which reduce current and voltage to levels useable by protective relays, control devices, and metering circuits.

    Interlock relay — A relay composed of two or more coils with their armatures and associated contacts so arranged that freedom of one armature to move or its coil to be energized is dependent upon the position of the armature.

    Intermittent-duty relay — A relay that must be de-energized at occasional or periodic intervals to avoid excessive temperature.

    Latch-in relay — A relay having contacts that lock in either the energized or de-energized position until reset, either manually or electrically.

    Level — As applied to a stepping relay, level is used to denote one bank or series of contacts.

    Level contact — Sometimes used for movable contact.

    Load test — This test involves measuring the alternating currents and/or voltages applied to the relay when the equipment is under normal load. The relative phase angles of the currents and/or voltages also are measured during the load test. This test may be performed while troubleshooting a suspected problem to ensure the relay is receiving the proper quantities.

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    Locking relay — Sometimes used for latch-in relay.

    Lock-Out Relay — A device that trips and maintains associated equipment or devices as inoperative until it is reset by an operator, either locally or remotely.

    Low-capacitance contacts — A type of contact construction providing low intercontact capacitance.

    Make contact — Same as front contact.

    Magnetic air gap — A nonmagnetic portion of a magnetic circuit.

    Magnetic freezing — The sticking of a relay armature to the core, after de-energization, due to residual magnetism of the core.

    Magnetic switch — Sometimes used for relay.

    Make-before-break contacts — Double-throw contacts so arranged that moving contacts establish a new circuit before disrupting the old one.

    Make delay — Sometimes used for operate time.

    Mercury-contact relay — A relay in which the contacting medium is mercury.

    Microprocessor relay — Relay in which the input current or voltage waveform is sampled three or more times per power cycle and conversion of samples to numeric values for measurement calculation by microprocessor electronics that also are performing self-diagnostics.

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    Motor-driven relay — A relay actuated by the rotation of the shaft of some type of motor (i.e., a shaded-pole, induction-disk, or hysteresis motor).

    Movable contact — A contact that moves, when the relay is energized or de-energized, to engage or disengage one or more stationary contacts.

    Multiple-break contacts — Contacts so arranged that, when they open, the circuit is interrupted in two or more places.

    Multiple pileup — An arrangement of contact springs composed of two or more separate pileups.

    Multiple stack — Same as multiple pileup.

    Neutral relay — In contrast to a polarized relay, a relay where the movement of the armature is independent of direction of flow of current through the relay coil.

    Nonbridging — Describes a contact transfer where the movable contact leaves one contact before touching the next.

    Nonhoming — A qualifying term applied to a stepping relay indicating that wipers, upon completion of an operational cycle, do not return to the home position but are at rest on the last used set of contacts.

    Noninductive windings — A type of winding where the magnetic fields produced by two parts of the winding cancel each other and provide a noninductive resistance.

    Nonmagnetic shim — A nonmagnetic material attached to the armature or core of a relay to prevent iron-to-iron contact in an energized relay.

    Nonoperate value — The nonoperate voltage, current, or power is the maximum value for which contacts of a previously de-energized relay will always maintain their de-energized positions.

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    Normal position — De-energized position, open or closed, of contacts due to spring tension or gravity.

    Normal sequence of operation — The sequence in which all normally closed contacts open before closure of normally open contacts of the assembly.

    Normal-speed relay — A relay in which no attempt has been made either to increase or decrease the operate time or the release time.

    Normally open contacts — A combination of a stationary contact and a movable contact that are not engaged when the coil is de-energized.

    Off-limit contacts — Contacts on a stepping relay used to indicate when the wiper has reached the limiting position on its arc and must be returned to normal before the circuit can function again.

    Off-normal contacts — Stationary contacts on a homing stepping relay used to indicate when the wiper is not in the starting position.

    Operate time NC — Operate time for normally closed contacts: Total elapsed time from the instant the coil is energized until contacts have opened (i.e., contact current is zero).

    Operate time NO — Operate time for normally open contacts: Total elapsed time from the instant the coil is energized until contacts are closed and all contact bounce has ceased.

    Operate values — Same as pickup values.

    Operating frequency — The rated alternating current frequency of the supply voltage at which the relay coil is designed to operate.

    Output — The relay output consists of the terminals connected to internal tripping contacts that open or close based upon the input signal to the relay.

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    Overload relay — A relay specifically designed to operate when its coil current reaches a predetermined value above normal.

    Partially enclosed relay — A relay with either contacts or coil (but not both) protected from the surrounding medium by a cover that is not airtight.

    Partially sealed relay — A relay with either contacts or coil (but not both) sealed.

    Periodic testing — Time-based routine maintenance testing and calibration of the relays and overall protection system on a schedule.

    Pickup values — Pickup voltage, current, or power is the minimum value for which contacts of a previously de-energized relay will always assume their energized position.

    Pileup — A set of contact arms, assemblies, or springs placed one on top of the other with insulation between them.

    Plant protection system testing — A total end-to-end test of the relay system. It includes primary injection (if possible) of test currents or voltages in the primaries of the instrument transformers and an actual trip of the circuit breaker. This proves all of the elements of the protection system, including wiring and tripping, functions.

    Plunger relay — A relay operated by energizing an electromagnetic coil that, in turn, operates a movable core or plunger by solenoid action.

    Polarized relay — A relay that depends on the polarity of the energizing current to operate.

    Pole — Each movable contact of a relay is a pole.

    Pole face — The part of the magnetic structure on the end of the core nearest the armature.

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    Potential Transformer — A transformer used to reduce a primary voltage to a secondary value for use with protective relaying, control device, and metering circuits, typically 69 or 120 volts.

    Protective relay system — The entire protective system, including the relays themselves, all associated wiring and terminals, all relay inputs and their current and voltage sources, control relays, and associated sensors and transducers. This includes all relay outputs and all devices in breaker trip circuits such as lockout relays, limit switches, etc. This also includes breaker trip coils and circuit breakers. Applies to both the alternating current and direct current supply systems, including the battery, battery chargers, and all associated wiring and circuits.

    Pull-in values — Same as pickup values.

    Pull-on values — Sometimes used for pickup values.

    Qualified Person — A qualified person must be properly trained, knowledgeable, and experienced in relay and protection system maintenance and safety, as well as testing techniques for specific protection equipment located at their facility.

    Ratchet relay — A stepping relay actuated by an armature-driven ratchet.

    Rated coil current — Steady-state coil current at which the relay is designed to operate.

    Rated coil voltage — Coil voltage at which the relay is designed to operate.

    Rated contact current — Current that the contacts are designed to carry for their rated life.

    Relay — A device that is operated by variation in conditions of one electric circuit to affect operation of other devices in the same or other electric circuits by either opening circuits or closing circuits or both.

    Relay calibration — To adjust relay operation to ensure that the relay operates within factory specifications. Electromechanical relays may require the adjustment of spring tension, time dial, magnets, contacts, etc. Solid-state relays may require the adjustment of potentiometers, variable capacitors, or variable inductors to ensure correct operation.

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    Release factor — Ratio, expressed in percent, of dropout current to rated current or the analogous voltage ratio.

    Release time NO — Release time for normally open contacts: Total elapsed time from the instant the coil current starts to drop from its rated value until contacts have opened (i.e., contact current is zero).

    Release time NC — Release time for normally closed contacts: Total time from the instant the coil current starts to drop from its rated value until contacts are closed and all contact bounce has ceased.

    Release values — Same as dropout values.

    Repeating timer — A timing device that, upon completion of one operating cycle, continues to repeat automatically until excitation is removed.

    Residual gap — Length of magnetic air gap between the pole-face center and nearest point on the armature, when the armature is in the energized position.

    Residual pins — Nonmagnetic pins or screws attached to either the armature or core of a relay to prevent the armature from directly contacting the magnetic core.

    Residual screws — See Residual pins.

    Residual setting — Value of the residual gap obtained by using an adjustable residual screw.

    Residual shim — Same as nonmagnetic shim.

    Restoring spring — A spring that moves the armature to, and holds it in, the normal position when the relay is de-energized.

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    Retractile spring — Sometimes used for restoring spring.

    Rotary relay — Sometimes used for motor-driven relay.

    Rotary stepping relay — Same as stepping relay.

    Rotary stepping switch — Same as stepping relay.

    Sealed relay — A relay that has both coil and contacts enclosed in an airtight cover.

    Self-cleaning contacts — Sometimes used for wiping contacts.

    Selector relay — A relay capable of automatically selecting one or more circuits from a number of circuits.

    Sequence control — Automatic control of a series of operations in a predetermined order.

    Sequence relay — A relay that controls two or more sets of contacts in a definite predetermined sequence.

    Shading coil — Sometimes used for shading ring.

    Shading ring — A shorted turn surrounding a portion of the core of an alternating current magnet, causing a delay in change of magnetic flux in that part, thereby preventing contact chatter.

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    Slave relay — Sometimes used for auxiliary relay.

    Slow-operate fast-release relay — A relay specifically designed for long-operate time and short-release time.

    Slow-operate relay — A slow-speed relay designed for long-operate time but not for long-release time.

    Slow-operate slow-release relay — A slow-speed relay specifically designed for both long-operate time and long-release time.

    Slow-release relay — A slow-speed relay specifically designed for long-release time but not for long-operate time.

    Slow-speed relay — A relay designed for long-operate time, long-release time, or both.

    Slug — A highly conductive sleeve placed over the core to aid in retarding the establishing or decay of flux within the magnetic path.

    Solenoid relay — Sometimes used for a plunger relay.

    Solid-state relays — Relays that use various low-power diodes, transistors, and thyristors and associated resistor and capacitors. These components are designed into logic units used in many ways.

    Special protection system — Designed to detect abnormal system conditions and take automatic, preplanned action (other than the isolation of faulted elements) to provide acceptable system performance. Special protection system actions may result in reduction in load or generation or changes in system configuration to maintain system stability, acceptable voltages, or acceptable facility loading.

    Special-purpose relay — A relay with an application requiring special features that are not characteristic of conventional general-purpose or definite-purpose relays.

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    Specified duty relay — A relay designed to function with a specified duty cycle but might not be suitable for other duty cycles.

    Spring buffer — A bearing member made of insulating material that transmits motion of the armature to the movable contact and from one movable contact to another in the same pileup.

    Spring pileup — Same as pileup.

    Spring stud — Same as spring buffer.

    Stack — Same as pileup.

    Stationary contact — A contact member that is rigidly fastened to the relay frame and is not moved as a direct result of energizing or de-energizing the relay.

    Stepping relay — A relay whose contacts are stepped to successive positions as the coil is energized in pulses. Some stepping relays may be stepped in either direction. (The stepping relay is also called a rotary stepping switch or a rotary stepping relay.)

    Target — A supplementary device operated either mechanically or electrically, to indicate visibly that the relay has operated or completed its function.

    Telephone-type relay — Sometimes used for an armature relay with an end-mounted coil and spring pileup contacts mounted parallel to the long axis of the relay coil.

    Tension spring — Sometimes used for restoring spring.

    Thermal relay — A relay operated by the heating effect of electric current flow.

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    Throw — For a relay on which the moving contact engages more than two stationary contacts during its cycle of operation, the contact arrangement is described as MPNT, where M is the number of poles and N is the number of throws (e.g., 8P 20T).

    Time-delay relay — A relay in which a delayed action purposely is introduced.

    Timing relay — A motor-driven time-delay relay.

    Transfer time — Total elapsed time between breaking one set of contacts and the making of another set of contacts.

    Transfer time on operate — Total elapsed time from the instant the normally closed contacts start to open until the normally open contacts are closed and all contact bounce has ceased.

    Transfer time on release — Total elapsed time from the instant the normally open contacts start to open until the normally closed contacts are closed and all contact bounce has ceased.

    Transit time — Same as transfer time.

    Trip values — Trip voltage, current, or power is a rated value where a bistable polarized relay will transfer from one contact to another.

    Troubleshooting — See Diagnostic tests.

    Undercurrent relay — A relay specifically designed to function when its coil current falls below a predetermined value.

    Undervoltage relay — A relay specifically designed to function when its coil voltage falls below a predetermined value.

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    Unenclosed relay — A relay that does not have its contacts or coil protected from the surrounding medium by a cover.

    Winding — Same as coil.

    Wiper — A moving contact on a stepping relay.

    Wiping contacts — Contacts designed to have some relative motion during the interval from the instant of touching until completion of closing motion.


    • Operation, Maintenance, and Field Test Procedures for Protective Relays and Associated Circuits

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