Reference · 9 min read · Updated June 2026

IEEE/ANSI Device Numbers Cheat Sheet (C37.2)

If you have ever looked at a substation or switchgear one-line and wondered why there are two-digit numbers scattered around the breakers and little relay circles — a 52 here, a 50/51 there, an 87T on the transformer — those are device function numbers from ANSI/IEEE C37.2. They are the shorthand North-American protection engineers use to say what a device does without writing it out in words.

The standard assigns each protective, control, and auxiliary function a number from 1 to 99. A "51" is always an AC time-overcurrent relay, anywhere in the world a C37.2 drawing is used. That consistency is the whole point: a relay tech in Texas and a consultant in Toronto read the same one-line the same way. This page is a working cheat sheet of the numbers you will actually meet on single-line diagrams, plus the suffix letters and combinations that modify them.

Draw a protected feeder — breaker, CT and relay — on smartsld.com — free

How the numbering works

A device number is a function identifier, not a part number. It tells you the job the device performs; the manufacturer, model, and settings are called out separately. A few things are worth knowing before you read the table:

52 is the circuit breaker itself. It is the one number that is not a relay — it is the power switching device. You will see it more than any other symbol on a one-line.
Protective relays cluster in the 20s through 80s. Overcurrent (50/51), voltage (27/59), directional (67), differential (87) and frequency (81) are the bread-and-butter feeder and transformer functions.
The number stays the same whether the function lives in an electromechanical relay or a modern microprocessor relay. One digital relay may perform 50, 51, 67, 27, 59, 81, and 87 all at once — each is still cited by its C37.2 number.

On a diagram, the function is usually drawn as a circle (the relay) with its number inside, tapped from a current transformer (CT) or voltage/potential transformer (VT/PT) and wired to trip the breaker it protects.

A phase-overcurrent relay (50/51) senses feeder current through a CT and trips breaker 52. Dashed lines are secondary (sensing) and control wiring, not power.

Master device number reference

These are the functions you will encounter most often on distribution, generation, and industrial one-lines. The table is condensed from ANSI/IEEE C37.2; confirm exact wording against the current edition of the standard when it matters for a submittal.

No.	Function	Typical use
21	Distance relay	Impedance-based fault detection. Primary and backup protection on transmission lines and generator step-up connections.
25	Synchronism-check / synchronizing	Confirms two systems match in voltage, frequency, and phase angle before a breaker closes — tie breakers and generator paralleling.
27	Undervoltage relay	Trips or alarms on sagging voltage. Motor protection, undervoltage load shedding, and part of anti-islanding schemes.
32	Directional power relay	Detects real-power flow in a defined direction. Reverse-power protection for generators and utility interties.
37	Undercurrent / underpower relay	Detects loss of load — pump loss of prime, conveyor break, or an unloaded motor.
40	Loss-of-field (field) relay	Detects loss of excitation on synchronous generators and motors.
46	Negative-sequence / phase-balance current	Detects current unbalance, open phase, or phase reversal. Motor and generator protection.
47	Phase-sequence / phase-balance voltage	Detects phase reversal or voltage unbalance — supply-quality and rotation checks.
49	Thermal relay	Overload protection modeling machine or transformer heating, often driven by RTDs or a thermal replica.
50	Instantaneous overcurrent relay	Trips with no intentional delay above a set current — fast clearing of high-magnitude faults.
51	AC time-overcurrent relay	Inverse time-current curve. The backbone of coordinated overcurrent protection and backup.
52	AC circuit breaker	The power breaker itself — not a relay. The most common number on any one-line.
55	Power-factor relay	Monitors or controls power factor. Capacitor-bank switching and synchronous-machine control.
59	Overvoltage relay	Trips or alarms on sustained overvoltage. Anti-islanding, capacitor, and generator protection.
63	Pressure switch	Sudden-pressure, Buchholz, or gas-accumulation protection on liquid-filled transformers.
67	AC directional overcurrent relay	Overcurrent that only operates for current in one direction. Loops, parallel feeders, and ties.
79	AC reclosing relay	Automatically recloses a breaker after a trip. Common on overhead distribution feeders.
81	Frequency relay	Over/under-frequency (81O/81U) and rate-of-change (81R). Load shedding and anti-islanding.
85	Pilot / carrier / communications relay	The communications-channel element in pilot and line-differential schemes (paired with 87L).
86	Lockout relay	Hand-reset auxiliary that trips and locks out multiple devices until manually reset after a major fault.
87	Differential relay	Compares current entering vs leaving a zone. Primary protection for transformers, buses, machines, and lines.
94	Tripping / trip-free relay	Fast auxiliary relay that trips the breaker and provides the trip-free (anti-pump) function.

Suffix letters

A bare number tells you the function; a suffix letter tells you what it is protecting or which quantity it measures. These are the ones you will see constantly:

Suffix	Meaning	Example
N	Neutral / residual	`51N` — time overcurrent operating on the residual (the vector sum of the three phase CTs).
G	Ground	`51G` — time overcurrent fed from a dedicated ground or core-balance (zero-sequence) CT.
T	Transformer	`87T` — differential protection whose zone is a power transformer.
B	Bus	`87B` — differential protection whose zone is a bus.

The N vs G distinction trips people up. Both catch ground faults, but N derives ground current from the residual connection of the phase CTs, while G measures it directly with a separate ground CT. Conventions vary slightly between manufacturers, so check the relay's own documentation. Other suffixes you may meet include L (line, as in 87L), A (alarm), and 1/2 to number multiple identical devices.

Common combinations

Functions are routinely stacked when a single device performs several jobs. A few you should be able to read on sight:

50/51 — instantaneous plus time overcurrent in one phase-overcurrent element. The single most common relay callout on a feeder.
50N/51N — the same instantaneous-plus-time pair applied to the neutral/residual for ground-fault protection.
27/59 — combined under- and overvoltage, often a single voltage-protection block.
67N — directional ground overcurrent, used where fault current can flow either way through the point of measurement.
87T — transformer differential; 87B bus differential; 87L line-current differential (paired with a 85 communications channel).

How relays and breakers get labelled on an SLD

On the diagram the mapping is straightforward once you know the convention:

The breaker carries the number 52, usually with an equipment tag alongside it (for example 52-M1 for main breaker 1). The symbol is the breaker itself — a small square in the conductor (ANSI) or a switch with crossbars (IEC).
The relay is drawn as a circle containing its function number or numbers (50/51, 87T, 27). A dashed line ties it to the CT or VT that feeds it, and another dashed line runs to the trip coil of the breaker it operates. Dashed = secondary and control wiring; solid = primary power.
Instrument transformers — the CT and VT/PT that the relays read — are shown in the primary circuit and labelled with their ratios (for example 600:5).
Auxiliaries like the 86 lockout and 94 trip relay usually sit near the breaker they command; the 86 often shows the several breakers it locks out.

Put together, a typical protected feeder reads: bus → CT → breaker 52, with a 50/51 (and often 50N/51N) relay sensing the CT and tripping the 52 through an 86 lockout. Read enough of them and the pattern becomes automatic.

Lay out a switchgear one-line with breakers and relays — free in the browser

North-American vs IEC practice

Device numbers are a North-American convention. C37.2 numbering dominates in the United States, Canada, and markets that follow ANSI practice. Elsewhere — Europe, most of Asia, Australia, the Middle East — drawings follow IEC conventions and generally do not use these two-digit numbers.

Under IEC, protection functions are more often written out or referenced through IEC 61850 logical nodes (for example PTOC for a time-overcurrent function, PDIF for differential, PDIS for distance). Many relay vendors and multinational projects now print both — the familiar ANSI number and the 61850 logical node — so the two systems increasingly appear side by side rather than in competition. If you are reading a drawing and the protection is labelled by function name or a four-letter node instead of a number, you are almost certainly on an IEC-style document. When a project spans both worlds, agree the convention up front and keep one drawing internally consistent.

C37.2 is periodically revised and relay manufacturers occasionally use their own extensions and sub-letters. Treat this page as a reading aid, not a specification — verify function assignments against the current edition of ANSI/IEEE C37.2 and the specific relay's instruction manual before you rely on them.

Spot an error or want a device number added? Email [email protected] and we'll fix it.