Protective Relay Reference

IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays

Updated 2026-10-24

Edition timelineIEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays: 1996, 20182000200520102015IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent RelaysIEEE C37.112-2018IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays1996IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays2018
Edition timeline. Editions recorded for this standard from the publisher's catalog. The filled dot marks the edition listed as current.
Edition timeline: edition years from the publishers' catalogs; editions without a four-digit year are listed but not plotted.
StandardEditionKindCurrentRecord
IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays1996Revision ofstd.ieee-c37-112.revision-of
IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays2018Approvalstd.ieee-c37-112.approved

Data records

  • The IEEE-SA Standards Board approved the 2018 edition of IEEE C37.112 on 2018-12-05. Value: 2018. Source: IEEE C37.112-2018, clause Front matter. Verified 2026-10-24. Record std.ieee-c37-112.approved
  • IEEE C37.112-2018 states in clause 3.4 that its conformance bands are templates for classifying the shape of a curve and are not tolerance bands for timing accuracy or repeatability; the standard gives no timing-accuracy figure, which remains a manufacturer claim for each relay. Source: IEEE C37.112-2018, clause 3.4. Verified 2026-10-24. Record std.ieee-c37-112.bands-not-tolerance
  • Clause 3.4 of IEEE C37.112-2018 requires the operate times of a conforming relay to fall inside the band drawn around each standard characteristic in Figures 2 through 4 over a stated range of multiples of pickup; each band lies a fixed proportion above and below the Table 1 time. Source: IEEE C37.112-2018, clause 3.4. Verified 2026-10-24. Record std.ieee-c37-112.conformance-bands
  • The constants in Table 1 of IEEE C37.112-2018 represent the mean of the induction-relay characteristics analyzed in Annex A, and their digits are chosen so that a change in the last digit moves the relative operate time by only a small fraction over the stated range of multiples of pickup. Source: IEEE C37.112-2018, clause Table 1, Annex A. Verified 2026-10-24. Record std.ieee-c37-112.constants-basis
  • Table 1 of IEEE C37.112-2018 lists the trip constants and the optional reset constant of three standard characteristics named moderately inverse, very inverse and extremely inverse; the standard defines no other curve families, so short-time, definite-time and IEC shapes lie outside it. Source: IEEE C37.112-2018, clause 3.4, Table 1. Verified 2026-10-24. Record std.ieee-c37-112.curve-families
  • In IEEE C37.112-2018 the time dial is the relay setting that fixes the integral quantity at which the relay operates, and so sets the time scale of the whole operate curve. Source: IEEE C37.112-2018, clause 2. Verified 2026-10-24. Record std.ieee-c37-112.definition.time-dial
  • IEEE C37.112-2018 is the designation of the document titled IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays. Value: IEEE C37.112-2018. Source: IEEE C37.112-2018, clause Front matter. Verified 2026-10-24. Record std.ieee-c37-112.designation
  • Clause 3.4 of IEEE C37.112-2018 makes dynamic behavior part of conformance: a microprocessor relay conforms only when it implements the integral equation with the trip-equation time rather than a static curve fit, so that coordination with induction relays holds while the current changes. Source: IEEE C37.112-2018, clause 3.4. Verified 2026-10-24. Record std.ieee-c37-112.dynamic-conformance
  • The dynamic equation of clause 3.2 of IEEE C37.112-2018 integrates the reciprocal of the trip-equation time over the current history and declares operation when the integral reaches unity, which lets the characteristic follow a changing fault current. Source: IEEE C37.112-2018, clause 3.2. Verified 2026-10-24. Record std.ieee-c37-112.equation.dynamic-form
  • The trip equation of clause 3.2 of IEEE C37.112-2018 expresses operate time as the time dial multiplied by the sum of two terms: an inverse term, in which a curve constant is divided by the multiple of pickup raised to a curve exponent less unity, and a definite-time constant that represents the core saturation component of an induction relay. Source: IEEE C37.112-2018, clause 3.2. Verified 2026-10-24. Record std.ieee-c37-112.equation.operate-form
  • The optional reset equation of clause 3.2 of IEEE C37.112-2018 applies while the current is below pickup and gives a negative time, representing the travel of an induction disk back toward its reset position, derived from a reset constant defined as the reset time at zero current. Source: IEEE C37.112-2018, clause 3.2. Verified 2026-10-24. Record std.ieee-c37-112.equation.reset-form
  • The trip equation of IEEE C37.112-2018 has the same form as the inverse-time characteristic of IEC 60255-151:2009 with a definite-time constant added, the added term modeling the saturation of an induction relay core. Source: IEEE C37.112-2018, clause 3.2. Verified 2026-10-24. Record std.ieee-c37-112.iec-relationship
  • Clause 3.1 of IEEE C37.112-2018, with Figure 1, presents the close coordination between the extremely inverse characteristic and a high-voltage fuse as the rationale for inverse characteristics in the grounded four-wire distribution practice of North America. Source: IEEE C37.112-2018, clause 3.1. Verified 2026-10-24. Record std.ieee-c37-112.rationale.fuse-coordination
  • IEEE C37.112-2018 is a full revision of IEEE C37.112-1996. Value: 1996. Source: IEEE C37.112-2018, clause Front matter. Verified 2026-10-24. Record std.ieee-c37-112.revision-of
  • IEEE C37.112-2018 supplies analytic equations for the typical inverse-time overcurrent characteristic shapes so that microprocessor and computer based relays can reproduce the curves of induction-disk relays and coordinate with them. The document covers the equation forms, the constants of three standard characteristics and the conformance criteria for a relay that claims one of those characteristics. Source: IEEE C37.112-2018, clause 1.1, 1.2. Verified 2026-10-24. Record std.ieee-c37-112.scope
  • The sponsor of IEEE C37.112-2018 is the Power System Relaying Committee of the IEEE Power and Energy Society. Source: IEEE C37.112-2018, clause Front matter. Verified 2026-10-24. Record std.ieee-c37-112.sponsor
  • Annex A of IEEE C37.112-2018 is informative; it derives the trip and reset equations from the dynamics of an induction disk and tabulates measured constants for two induction relay models in Tables A.1 and A.2. Source: IEEE C37.112-2018, clause Annex A. Verified 2026-10-24. Record std.ieee-c37-112.structure.annex-a
  • Clause 2 of IEEE C37.112-2018 defines the terms inverse-time overcurrent relay, reset characteristic, reset and time dial. Source: IEEE C37.112-2018, clause 2. Verified 2026-10-24. Record std.ieee-c37-112.structure.definitions
  • Clause 3.2 of IEEE C37.112-2018 states three equations: a trip equation for current above pickup, an optional reset equation for current below pickup, and an integral form that governs operation when the input current varies with time. Source: IEEE C37.112-2018, clause 3.2. Verified 2026-10-24. Record std.ieee-c37-112.structure.equations
  • Clause 3.3 of IEEE C37.112-2018 states that the time dial scales the two trip constants and the reset constant together, each in direct proportion to the dial setting, and that a microprocessor relay holds the ratio of the two trip constants fixed whereas an induction relay may let it drift somewhat. Source: IEEE C37.112-2018, clause 3.3. Verified 2026-10-24. Record std.ieee-c37-112.time-dial-scaling
  • The licensed copy of IEEE C37.112-2018 carries the full title IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays. Value: IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays. Source: IEEE C37.112-2018, clause Front matter. Verified 2026-10-24. Record std.ieee-c37-112.title

Sources

  • IEEE C37.112-2018, IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays, 2018 edition, IEEE. Cited: clause Front matter; clause 3.4; clause Table 1, Annex A; clause 3.4, Table 1; clause 2; clause 3.2; clause 3.1; clause 1.1, 1.2; clause Annex A; clause 3.3. Held as: Purchased single-user copy.

Obtaining the documents

Changelog (45 entries)
DateRevisionChange
2026-10-24created (std.ieee-c37-112.approved)
2026-10-24revised (std.ieee-c37-112.approved)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.approved)
2026-10-24created (std.ieee-c37-112.bands-not-tolerance)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.bands-not-tolerance)
2026-10-24created (std.ieee-c37-112.conformance-bands)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.conformance-bands)
2026-10-24created (std.ieee-c37-112.constants-basis)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.constants-basis)
2026-10-24created (std.ieee-c37-112.curve-families)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.curve-families)
2026-10-24created (std.ieee-c37-112.definition.time-dial)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.definition.time-dial)
2026-10-24created (std.ieee-c37-112.designation)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.designation)
2026-10-24created (std.ieee-c37-112.dynamic-conformance)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.dynamic-conformance)
2026-10-24created (std.ieee-c37-112.equation.dynamic-form)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.equation.dynamic-form)
2026-10-24created (std.ieee-c37-112.equation.operate-form)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.equation.operate-form)
2026-10-24created (std.ieee-c37-112.equation.reset-form)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.equation.reset-form)
2026-10-24created (std.ieee-c37-112.iec-relationship)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.iec-relationship)
2026-10-24created (std.ieee-c37-112.rationale.fuse-coordination)
2026-10-24revised (std.ieee-c37-112.rationale.fuse-coordination)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.rationale.fuse-coordination)
2026-10-24created (std.ieee-c37-112.revision-of)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.revision-of)
2026-10-24created (std.ieee-c37-112.scope)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.scope)
2026-10-24created (std.ieee-c37-112.sponsor)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.sponsor)
2026-10-24created (std.ieee-c37-112.structure.annex-a)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.structure.annex-a)
2026-10-24created (std.ieee-c37-112.structure.definitions)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.structure.definitions)
2026-10-24created (std.ieee-c37-112.structure.equations)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.structure.equations)
2026-10-24created (std.ieee-c37-112.time-dial-scaling)
2026-10-24revised (std.ieee-c37-112.time-dial-scaling)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.time-dial-scaling)
2026-10-24created (std.ieee-c37-112.title)
2026-10-24review_status unreviewed -> software_reviewed (std.ieee-c37-112.title)

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Cite the governing standard for the fact itself; cite this page as the finding aid.

"IEEE Standard for Inverse-Time Characteristics Equations for Overcurrent Relays." Protective Relay Reference. 2026-10-24. https://www.protectiverelayreference.com/standards/ieee-c37-112/.