ANSI Device Numbers — IEEE C37.2 Protective Relay Function Codes
ANSI device numbers — codified in IEEE C37.2 — are the two-digit codes that label every protective and control function in an electrical-power system. This page lists the full table of numbers 1–99, the standard suffix letters (N, G, T, L, B), worked usage on a substation single-line diagram, and the mapping to IEC 61850 logical nodes used by modern microprocessor relays. Reviewed by a licensed PE.
ANSI device-number tools
For settings calculation of the most-used number — ANSI 51 time overcurrent — the IDMT relay calculator computes trip time from pickup multiple, time-dial, and IEC / IEEE curve constants. Pair the result with the per-device ANSI number on the protection one-line for a complete settings package.
→ IDMT relay calculator (ANSI 51) · → Short-circuit calculator · → Fault current calculator
The ANSI device number system — formula and structure
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- Optional prefix N before the base number identifies neutral or specific phase
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- Base number 1–99 identifies the device function (IEEE C37.2 master list)
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- Suffix letters identify the protected element (T, G, L, B, M, N, P)
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- Common composites: 50N, 51N, 51G, 87T, 87B, 87L, 67N, 27/59
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- 50/51 = combined instantaneous + time overcurrent in one device
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- 21/21N = phase distance + neutral distance
Standards governing device numbers
| Document | Scope |
|---|---|
| IEEE C37.2-2008 | Standard Electrical Power System Device Function Numbers, Acronyms, and Contact Designations — the master list |
| ANSI C37.90 / C37.90.1 / .2 | Surge withstand, dielectric, and EMI tests for protective relays carrying ANSI device numbers |
| IEEE C37.110 | Guide for the application of current transformers used for protective relaying |
| IEEE C37.91 | Guide for protecting power transformers (use of 87T, 49T, 50/51, 64) |
| IEEE C37.96 | Guide for AC motor protection (use of 49, 50, 51, 87M, 14) |
| IEC 61850-7-4 | Logical-node naming used by modern IEC 61850-compliant IEDs (PIOC, PTOC, PDIF, etc.) |
Reference: ANSI device numbers — most-used (1–99)
| No. | Function | Common use |
|---|---|---|
| 2 | Time-delay starting / closing relay | Sequence control |
| 21 | Distance relay | Transmission line protection (mho, quadrilateral) |
| 24 | Volts-per-hertz | Generator / transformer overexcitation |
| 25 | Synchronism check | Closing onto a live system |
| 27 | Undervoltage | Bus, motor, generator UV |
| 32 | Directional power / reverse power | Generator anti-motoring |
| 40 | Loss of excitation | Generator field loss |
| 46 | Phase balance current | Motor unbalance |
| 47 | Phase sequence voltage | Reverse-phase detection |
| 49 | Thermal — machine or transformer | RTD-based or replica thermal |
| 50 | Instantaneous overcurrent | Close-in fault, no intentional delay |
| 50N | Instantaneous neutral overcurrent | Ground-fault detection |
| 51 | Time overcurrent (IDMT) | Backup feeder protection |
| 51G | Time overcurrent — ground | Earth-fault feeder protection |
| 52 | AC circuit breaker | The breaker itself |
| 59 | Overvoltage | Bus, generator OV |
| 62 | Time-delay stopping / opening | Reclose timer |
| 63 | Pressure switch (transformer) | Sudden-pressure relay (Buchholz family) |
| 64 | Ground detector | Ungrounded system fault detector |
| 67 | Directional overcurrent | Parallel feeders, ring buses |
| 67N | Directional neutral overcurrent | Directional ground-fault |
| 78 | Out-of-step / loss of synchronism | Generator pole slip |
| 79 | Reclosing relay | Automatic line reclose |
| 81 | Frequency (over- or under-) | Load shedding, islanding |
| 81U / 81O | Underfrequency / overfrequency | Frequency-element variants |
| 85 | Communications / pilot relay | Permissive / blocking schemes |
| 86 | Lockout relay | Hand-reset trip after major fault |
| 87 | Differential | Transformer / bus / generator / line / motor diff |
| 87T | Transformer differential | Primary protection for power transformers |
| 87G | Generator differential | Stator winding protection |
| 87B | Bus differential | High-impedance / low-impedance bus protection |
| 87L | Line differential | Pilot-wire / fibre-channel line protection |
| 94 | Trip relay | Auxiliary trip output |
Suffix-letter reference
| Suffix | Meaning | Example |
|---|---|---|
| N | Neutral / residual | 50N — instantaneous neutral overcurrent |
| G | Ground | 51G — time ground-overcurrent |
| T | Transformer | 87T — transformer differential |
| L | Line | 87L — line differential |
| B | Bus | 87B — bus differential |
| M | Motor | 87M — motor differential |
| P | Phase | 50P — instantaneous phase overcurrent |
| U / O | Under / over | 81U — underfrequency, 81O — overfrequency |
- Open IEEE C37.2 IEEE C37.2-2008 is the source standard for ANSI device numbers. The current revision lists numbers 1 through 99 (some retired), suffix letters (A–Z), prefixes (e.g. "50N" = neutral instantaneous), and the relationship to IEC 61850 logical nodes.
- Identify the protection function Examples: instantaneous overcurrent → 50; time overcurrent → 51; differential → 87; transformer differential → 87T; bus differential → 87B; line distance → 21; loss of excitation → 40; reverse power → 32; over/under voltage → 59 / 27; over/under frequency → 81O / 81U.
- Add suffix letters as needed "N" denotes neutral (50N is neutral instantaneous OC); "G" denotes ground (51G); "T" identifies the protected element (87T = transformer differential); "L" line; "B" bus. Multiple suffixes stack, e.g. 51NG = neutral ground time overcurrent.
- Use the function code in the one-line and protection narrative On a single-line diagram, every relay box is labelled with its ANSI device number(s). In the protection narrative or relay setting calculation document, every setting paragraph leads with the device number for traceability between drawings, settings, and the test report.
- Map to IEC 61850 logical nodes for substation automation Modern microprocessor relays implement IEC 61850 logical nodes (e.g. PIOC for instantaneous OC = ANSI 50, PTOC for time OC = ANSI 51). The ANSI number stays in the protection drawings; the LN reference is used in the SCD/CID files for inter-IED communication.
Worked example — labelling a transformer protection package
A 25 MVA 138/13.8 kV substation transformer with primary breaker, secondary breaker, ground bank, and aux PT. List the ANSI device numbers on the one-line.
- 87T — transformer differential (primary protection — high-set restraint with 2nd-harmonic inrush block)
- 50/51, 50N/51N — backup overcurrent (phase + neutral) on both primary and secondary breakers
- 49T — top-oil and winding-temperature thermal relay (RTD or replica)
- 63 — sudden-pressure (Buchholz / SPR) relay on transformer tank
- 50G/51G — ground-bank ground overcurrent (zero-sequence detection)
- 27/59 — bus undervoltage / overvoltage at LV side
- 86T — lockout relay after major internal fault — requires hand reset before re-energisation
- 52 — primary and secondary circuit breakers themselves
Comparison — ANSI device numbers vs. IEC 61850 logical nodes
| ANSI # | Function | IEC 61850 LN |
|---|---|---|
| 50 | Instantaneous overcurrent | PIOC |
| 51 | Time overcurrent | PTOC |
| 87 | Differential | PDIF |
| 21 | Distance | PDIS |
| 27 | Undervoltage | PTUV |
| 59 | Overvoltage | PTOV |
| 81 | Frequency | PTOF / PTUF |
| 40 | Loss of excitation | RFLO (or LOEX in vendor extension) |
| 52 | Circuit breaker | XCBR |
| 79 | Recloser | RREC |
Variants and related queries
Numbers — the device-number family
"Numbers" in protection-engineer talk almost always means the ANSI device numbers per IEEE C37.2. The same numbers also appear as the function-block IDs in microprocessor-relay documentation (SEL, GE Multilin, ABB REL, Siemens 7SA, Areva MiCOM). Vendors typically list the ANSI number alongside the proprietary function code for legacy compatibility.
ANSI numbers for relays — generation, transmission, distribution
Generation typically uses 87G (generator differential), 40 (loss of excitation), 32 (reverse power / anti-motoring), 24 (V/Hz), 46 (negative-sequence), 49 (thermal). Transmission lines use 21 (distance), 67 (directional OC), 87L (line differential), 79 (auto-recloser), 25 (sync check). Distribution feeders rely on 50/51/50N/51N, 67/67N, 79.
ANSI standard device numbers PDF and list
The authoritative list lives in IEEE C37.2-2008 (paid standard). For most engineers, vendor protection manuals (SEL Application Guide, Schweitzer Engineering Labs Library) reproduce the table free in their relay documentation. Public references include the IEEE Power & Energy Society wiki entry and university lecture notes from power-system protection courses.
Relay code numbers and IEEE numbers
"Relay code numbers" and "IEEE numbers" are colloquial names for ANSI / IEEE C37.2 device numbers. The codes are managed by the IEEE Power System Relaying Committee (PSRC). In international IEC-aligned documents, the same protection functions are often labelled by IEC 61850 logical-node abbreviations — see the comparison table above.
Frequently asked questions
- What are ANSI device numbers?
- ANSI device numbers — formally defined in IEEE C37.2 — are two-digit codes (1 to 99) that identify the function of each device in an electrical-power protective and control system. Number 50 is instantaneous overcurrent; 51 is time overcurrent; 87 is differential; 27 is undervoltage; 81 is frequency relay. Suffix letters refine the function (N = neutral, G = ground, T = transformer, L = line, B = bus). The numbering system originated in the 1928 ANSI standard and has been maintained by IEEE since 1957.
- Why are protective relays numbered?
- Standardised numbering provides unambiguous communication on single-line diagrams, in protection-setting documents, and in operator-training material. Saying "the 87T tripped" is faster and less error-prone than "the transformer differential relay tripped". The numbers are language-independent and have been adopted internationally (with IEEE C37.2 widely cited even outside ANSI/IEEE jurisdictions).
- What is ANSI 50 vs. 51?
- ANSI 50 is instantaneous overcurrent — trips with no intentional time delay when current exceeds the pickup threshold. ANSI 51 is time overcurrent — trip time decreases as current increases per the relay's inverse curve (IEC very-inverse, US extremely-inverse, etc.). Used together: 51 provides backup overcurrent grading while 50 catches close-in faults faster than the 51 curve allows. The IDMT relay calculator handles 51 trip-time computation.
- What is ANSI 87?
- ANSI 87 is differential protection — compares the current entering and leaving a protected zone (transformer, generator, motor, bus, or line) and trips if the difference exceeds a set value. Suffix letters identify the element: 87T (transformer differential), 87G (generator differential), 87M (motor differential), 87B (bus differential), 87L (line differential). Differential is the gold-standard primary protection for the protected element because it isolates faults inside the zone with no time delay.
Historic source — symmetrical components
Fortescue\'s 1918 paper introduced symmetrical components — the mathematical basis for negative-sequence (46), zero-sequence ground (51N / 51G) and directional ground (67N) protection schemes that occupy a large portion of the modern ANSI device-number table.
Related calculators and references
Sources and further reading
- IEEE C37.2-2008 — Standard Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
- IEEE C37.90 / C37.91 / C37.96 — relay testing and application guides.
- IEC 61850-7-4 — Basic communication structure: Compatible logical node classes.
- Blackburn, J. L.; Domin, T. J. Protective Relaying — Principles and Applications, 4th edition. CRC Press, 2014.
- SEL — Protective Relay Application Guide, current edition (free PDF).
- Fortescue, C. L. Method of Symmetrical Co-ordinates Applied to the Solution of Polyphase Networks. AIEE, 1918.