ANSI/IEEE C37.2 Device Function Numbers. A Working Reference.
What the numeric codes on protection relays and switchgear single-line diagrams mean, where to look them up, and how they fit alongside ISA 5.1 instrument letters on a P&ID.
If you have ever stood in front of a single-line diagram with the numbers 52, 87, 50/51, and 27/59 printed next to a relay symbol and wondered where to look them up, this is for you. ANSI/IEEE C37.2 is the standard that defines those numbers, and once you know how to read it, an SLD opens up the same way a P&ID does after you learn ISA 5.1.
The standard itself
ANSI/IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations. The numbering convention dates from early 20th-century switchgear practice and has been maintained by the IEEE Power and Energy Society for several decades. The current published version is available from the IEEE for purchase.
The standard does three things. It assigns a number to every common power-system protection or control function, the device function numbers, it defines short acronyms for the same functions, so they can appear in software where a numeric code is awkward, and it specifies suffix letters for refining a base function, 52A vs 52B, 50N vs 50G.
The most common device numbers
A working SLD reader can recognise about 25 device numbers on sight. The rest can be looked up. These are the ones worth memorising.
Breakers and switchgear
| Number | Function | Where you see it |
|---|---|---|
| 52 | AC circuit breaker | Every SLD. The most common single number. |
| 89 | Line switch, disconnect | Outdoor switchyards, isolating disconnects. |
| 86 | Lockout relay | Trip-and-lock function on serious faults. |
| 94 | Tripping relay | Auxiliary tripping from protection scheme. |
| 5 | Stopping device | Manual stop switches. |
Overcurrent and short-circuit protection
| Number | Function | Where you see it |
|---|---|---|
| 50 | Instantaneous overcurrent | Fast trip on fault current, no time delay. |
| 51 | AC time overcurrent | Time-curve trip. Protects against sustained overload. |
| 50, 51 | Both functions in one relay | Modern numerical relays bundle both. |
| 50N | Instantaneous neutral overcurrent | Earth-fault on neutral conductor. |
| 50G | Instantaneous ground overcurrent | Earth-fault on dedicated CT. |
| 67 | AC directional overcurrent | Trips only for fault current flowing in one direction. |
Voltage and frequency
| Number | Function | Where you see it |
|---|---|---|
| 27 | Undervoltage | Loss-of-voltage trip. |
| 59 | Overvoltage | Over-voltage trip. |
| 27, 59 | Both in one relay | Modern voltage protection. |
| 81 | Frequency relay | Underfrequency or overfrequency tripping. |
| 81U | Underfrequency | Common on grid-connected generators. |
| 81O | Overfrequency | Generator runaway protection. |
| 32 | Reverse power | Generator motoring protection. |
Differential and distance
| Number | Function | Where you see it |
|---|---|---|
| 87 | Differential relay | Transformer, busbar, generator differential. The premium protection scheme. |
| 87T | Transformer differential | Specifically transformer protection. |
| 87B | Busbar differential | Specifically bus-protection scheme. |
| 87L | Line differential | Transmission-line protection. |
| 21 | Distance relay | Transmission-line distance protection. |
Generator-specific
| Number | Function |
|---|---|
| 24 | Volts-per-hertz, overexcitation |
| 40 | Loss of field |
| 46 | Negative-sequence current, phase imbalance |
| 49 | Thermal overload |
| 64 | Ground fault |
Transformer-specific
| Number | Function |
|---|---|
| 26 | Apparatus thermal device |
| 49 | Machine or transformer thermal |
| 63 | Pressure switch, Buchholz on oil-filled units |
Auxiliary and control
| Number | Function |
|---|---|
| 86 | Lockout relay |
| 94 | Tripping relay |
| 4 | Master contactor |
| 6 | Starting circuit breaker |
| 30 | Annunciator relay |
| 74 | Alarm relay |
Suffix letters
The suffix on a device number refines what specific contact or function the symbol represents. The most common.
| Suffix | Meaning | Common context |
|---|---|---|
| A | Closed when the device is closed | 52A is the breaker auxiliary closed when the breaker is closed. |
| B | Closed when the device is open | 52B is closed when the breaker is open. The complement of A. |
| X, Y, Z | Auxiliary relay copies | When a function uses multiple auxiliary contacts. |
| N | Neutral or zero-sequence | 50N, 51N, 27N for neutral-related variants. |
| G | Ground | 50G, 51G for ground-fault variants. |
| T | Time-delayed variant | 52T for a time-delayed trip contact. |
| U | Under, frequency or voltage | 81U is underfrequency. Complement is 81O. |
| O | Over, frequency or voltage | 59O, 81O. |
Suffix usage drifts slightly between published C37.2 and what operators put on drawings. A project legend page is always the source of truth for that specific drawing set.
How C37.2 numbers fit alongside ISA 5.1
A unified plant has an electrical scope and an instrumentation scope, and the two use different identification systems. Most projects keep them on separate drawings. SLD pages with C37.2 numbers and protective-relay tags, P&ID pages with ISA 5.1 letter codes and instrument bubbles.
There is a narrow overlap where the two domains meet. Motor controls, electrical-driven valves, and shutdown-related interlocks where a process condition trips a breaker or a breaker status feeds the BPCS. On those interfaces you typically see an ISA tag for the process side, e.g. XV-101 for an emergency-isolation valve and a C37.2 device number for the breaker, 52 that drives its actuator. Both appear on a unified electrical-and-instrumentation drawing or on a cause-and-effect matrix that connects them.
Reading an SLD with C37.2 numbers
Walk through any SLD with the same loop you would use on a P&ID.
- Find the source. Bus bar at the top, with its voltage and busbar tag.
- Trace down through breakers and disconnects. Each
52is a breaker. A feeder breaker has the bus on one side and the load on the other. - Read the protection devices on each breaker. The numbers next to the breaker symbol,
52,87,50/51,27/59tell you the protection scheme. - Resolve the load. Motor, transformer, sub-bus, or another switchgear lineup.
- Catch the auxiliaries.
86lockouts,94tripping relays,30annunciator relays. These tie the protection logic to the operator interface.
Every modern SLD is an organized stack of these patterns. Once you know 52 is a breaker and 87 is differential, the drawing reads as compactly as a P&ID reads to an instrumentation engineer.
For keeping the instrument side of an integrated drawing set organised across the project lifecycle, the instrument index master class guide covers column structure and cross-document reconciliation conventions.