ANSI Device Numbers List: Complete Guide to IEEE C37.2 Relay Functions

ANSI device numbers are short numeric codes that tell you what a protection, control or switching device does in a power system. They come from the IEEE C37.2 standard. For example, 50 is instantaneous overcurrent, 51 is time-delayed overcurrent, 52 is an AC circuit breaker, 86 is a lockout relay and 87 is differential protection. The full list, suffix letters and IEC 61850 equivalents are all below.

Table of contents

1. ANSI Device Numbers: Quick Answer Table
2. What Are ANSI Device Numbers?
3. IEEE C37.2: The Standard Behind the Numbers
4. Complete ANSI Device Numbers List (1–94)
5. ANSI Device Numbers 1–20: Control and Machine Functions
6. ANSI Device Numbers 21–40: Protection and Generator Functions
7. ANSI Device Numbers 41–60: Protection and Switching Functions
8. ANSI Device Numbers 61–80: Monitoring and Protection Functions
9. ANSI Device Numbers 81–94: Frequency, Differential and Auxiliary Functions
10. How the Device Numbers Are Grouped
11. ANSI Device Number Suffix Letters and Their Meanings
12. Difference Between 50N, 50G, 51N and 51G
13. Combined ANSI Device Numbers
14. ANSI Device Numbers by Protected Equipment
15. ANSI Device Numbers vs IEC 61850 Logical Nodes
16. ANSI Numbers vs IEC Graphical Symbols
17. How to Read ANSI Device Numbers on a Single-Line Diagram
18. Worked Examples of Relay Labels
19. Common Misconceptions About ANSI Device Numbers
20. Frequently Asked Questions About ANSI Device Numbers
21. What are ANSI device numbers?
22. What is the ANSI device number for an instantaneous overcurrent relay?
23. What is the difference between ANSI 50 and 51?
24. What does ANSI 87 mean?
25. What is an ANSI 86 lockout relay?
26. Why is a circuit breaker called device 52?
27. What does the N suffix mean?
28. What does G mean in ANSI relay numbers?
29. What is ANSI 79 used for?
30. What does ANSI 81 mean?
31. What is the IEC equivalent of ANSI device numbers?
32. Are ANSI device numbers used in India?
33. How many ANSI device numbers are there?
34. Conclusion

ANSI Device Numbers: Quick Answer Table

These are the numbers you will meet most often on single-line diagrams and relay settings sheets. Use this table first, then jump to the full list if you need a rarer number.

ANSI no.	Function	What it does
21	Distance relay	Measures impedance to find line faults
24	Volts-per-hertz relay	Detects over-fluxing
25	Synchronism check	Allows two supplies to be paralleled safely
27	Undervoltage relay	Operates when voltage is too low
32	Directional power relay	Detects reverse or directional power flow
40	Loss-of-excitation relay	Detects generator field failure
46	Phase-balance current relay	Detects current unbalance
47	Phase-sequence / voltage-balance relay	Detects wrong phase sequence or unbalanced voltage
49	Thermal relay	Protects against overheating
50	Instantaneous overcurrent	Trips at once on very high current
51	AC time overcurrent	Trips after a delay that depends on current
52	AC circuit breaker	Opens and closes the AC circuit
59	Overvoltage relay	Operates when voltage is too high
63	Pressure device	Detects abnormal pressure, such as gas surge in a transformer
64	Ground protective relay	Detects earth faults
67	Directional overcurrent	Detects overcurrent in one direction only
79	AC reclosing relay	Recloses a breaker automatically after a trip
81	Frequency relay	Detects over- or underfrequency
86	Lockout relay	Latches a trip until someone resets it
87	Differential relay	Compares current in and out of a protected zone

What Are ANSI Device Numbers?

An ANSI device number identifies a function, not a specific box. The number is printed on drawings, relay nameplates, settings files and alarm lists, so everyone reads the same thing. A “51” on a drawing means time-delayed overcurrent protection, whoever made the relay.

In older electromechanical schemes, one number usually meant one relay. Modern numerical relays, also called intelligent electronic devices (IEDs), can run dozens of numbered functions in a single unit. Device number 11 is used for such a multifunction device. If you are new to the hardware side, our guide to the types of protection relays explains how these devices have changed over time.

You will see the numbers on:

• Single-line diagrams (SLDs) and protection schematics
• Relay panels and nameplates
• Numerical relay configuration and settings documents
• Substation drawings, commissioning records and alarm schedules

IEEE C37.2: The Standard Behind the Numbers

The governing standard is IEEE C37.2, titled IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations. The current edition is C37.2-2022, published on 14 December 2022. It replaces C37.2-2008.

The 2022 edition assigns 95 device numbers and 22 acronyms. The 2008 edition assigned 95 numbers and 17 acronyms. The extra acronyms cover modern substation functions such as arc-flash detection, phasor measurement and trip-circuit monitoring. The numbering system itself dates back to 1928, when it began as AIEE Standard No. 26.

The numbers are used well beyond the United States, including in India, the UK, the Middle East and Latin America. Europe and many newer digital substations also use IEC symbols and IEC 61850 logical nodes, covered later in this guide.

Complete ANSI Device Numbers List (1–94)

Not every integer from 1 to 99 is an assigned function. The standard assigns 95 numbers, and numbers 95 to 99 are for specific applications where no other number fits. The table below covers 1 to 94 in plain language. Names are standard-style and the meanings are written in our own words. Always confirm exact wording against the purchased IEEE C37.2-2022 text before using a number in a contract or drawing.

ANSI Device Numbers 1–20: Control and Machine Functions

No.	Device name	Plain-language meaning	Category	Typical application
1	Master element	Starts or permits a control sequence	Control	Control switch
2	Time-delay starting or closing relay	Adds a deliberate delay before starting or closing	Control	Motor starting sequences
3	Checking or interlocking relay	Allows an action only when set conditions are met	Control	Interlock schemes
4	Master contactor	Main control contactor in a scheme (legacy use)	Control	Older control schemes
5	Stopping device	Shuts equipment down	Machine control	Motor stop circuits
6	Starting circuit breaker	Connects a machine for starting	Machine control	Motor starters
7	Rate-of-change relay	Reacts to how fast a quantity changes	Protection	ROCOF schemes
8	Control-power disconnecting device	Isolates the control supply	Auxiliary	Control supply fuses and switches
9	Reversing device	Reverses direction or phase rotation	Machine control	Reversing starters
10	Unit sequence switch	Steps a unit through its sequence	Control	Multi-unit plant
11	Multifunction device	One unit performing several numbered functions	Control	Numerical relays and IEDs
12	Overspeed device	Detects excessive speed	Machine control	Turbines, motors
13	Synchronous-speed device	Detects that speed has reached synchronous speed	Machine control	Synchronous machines
14	Underspeed device	Detects speed below a set value	Machine control	Turbines, motors
15	Speed or frequency matching device	Matches speed or frequency before paralleling	Generator	Generator synchronising
16	Data communications device	Handles data links between devices	Auxiliary	Gateways, Ethernet switches
17	Shunting or discharge switch	Shorts or discharges a circuit	Switching	Field and capacitor circuits
18	Accelerating or decelerating device	Controls how quickly speed changes	Machine control	Drives
19	Start-to-run transition contactor	Moves a motor from starting to running connection	Machine control	Reduced-voltage starters
20	Electrically operated valve	Valve operated by a solenoid or motor	Control	Plant auxiliaries

ANSI Device Numbers 21–40: Protection and Generator Functions

No.	Device name	Plain-language meaning	Category	Typical application
21	Distance relay	Trips when measured impedance falls inside a set reach	Protection	Transmission lines, feeders
22	Equalizer circuit breaker	Connects equalising paths	Switching	DC and older schemes
23	Temperature control device	Keeps temperature within limits	Auxiliary	Panel heaters
24	Volts-per-hertz relay	Detects too much voltage for the frequency	Protection	Transformers, generators
25	Synchronism-check device	Permits closing only when voltage, phase and frequency match	Generator	Generators, bus-ties
26	Apparatus thermal device	Detects abnormal equipment temperature	Auxiliary	Transformer winding temperature
27	Undervoltage relay	Operates when voltage falls below its setting	Protection	Buses, motors, feeders
28	Flame detector	Senses presence of flame	Auxiliary	Burner systems
29	Isolating contactor or switch	Isolates a circuit for safety or maintenance	Switching	Auxiliary circuits
30	Annunciator relay	Gives a visible or audible warning of a condition	Auxiliary	Alarm panels
31	Separate excitation device	Connects a separate excitation source	Generator	Excitation systems
32	Directional power relay	Detects the direction of power flow	Protection	Generators, interconnections
33	Position switch	Shows when a part reaches a set position	Auxiliary	Switchgear mechanisms
34	Master sequence device	Governs a series of operations	Control	Automated schemes
35	Brush-operating or slip-ring short-circuiting device	Lifts brushes or shorts slip rings	Machine control	Wound-rotor machines
36	Polarity or polarising-voltage device	Sets or monitors polarity	Control	Special schemes
37	Undercurrent or underpower relay	Detects loss of load	Protection	Motors, pumps, generators
38	Bearing protective device	Detects bearing temperature or distress	Auxiliary	Rotating machines
39	Mechanical condition monitor	Detects vibration or mechanical faults	Auxiliary	Rotating machines
40	Field relay (loss of excitation)	Detects loss or abnormal level of field current	Generator	Synchronous generators

ANSI Device Numbers 41–60: Protection and Switching Functions

No.	Device name	Plain-language meaning	Category	Typical application
41	Field circuit breaker	Connects or removes the field supply	Generator	Excitation systems
42	Running circuit breaker	Connects a machine for normal running	Machine control	Motor circuits
43	Manual transfer or selector device	Lets an operator select a mode or supply	Control	Local/remote selectors
44	Unit sequence starting relay	Starts the next unit in a sequence	Control	Multi-unit plant
45	Atmospheric condition monitor	Detects smoke, fumes or other hazardous conditions	Auxiliary	Substations, plant rooms
46	Reverse-phase or phase-balance current relay	Detects unbalanced or reversed phase currents	Protection	Motors, generators
47	Phase-sequence or voltage-balance relay	Detects wrong phase order or unbalanced voltage	Protection	Motors, incomers
48	Incomplete-sequence relay	Detects a start sequence that did not finish	Machine control	Motor starters
49	Machine or transformer thermal relay	Models heating to protect against overload	Protection	Motors, transformers
50	Instantaneous overcurrent relay	Trips with no deliberate delay on high current	Protection	Feeders, motors, transformers
51	AC time overcurrent relay	Trips after a delay that shortens as current rises	Protection	Feeders, motors, transformers
52	AC circuit breaker	Makes and breaks the AC circuit, including fault current	Switching	All AC switchgear
53	Exciter or DC generator relay	Controls build-up of excitation	Generator	Older excitation systems
54	Turning-gear engaging device	Engages turning gear on a shaft	Generator	Turbine-generators
55	Power-factor relay	Operates at a set power factor	Generator	Synchronous machines
56	Field-application relay	Applies field at the right moment	Generator	Synchronous motors
57	Short-circuiting or grounding device	Shorts or earths a circuit	Switching	Maintenance earthing
58	Rectification failure relay	Detects a rectifier fault	Control	Rectifiers, chargers
59	Overvoltage relay	Operates when voltage rises above its setting	Protection	Buses, generators
60	Voltage or current balance relay	Detects imbalance between two circuits	Protection	Capacitor banks, VT supervision

ANSI Device Numbers 61–80: Monitoring and Protection Functions

No.	Device name	Plain-language meaning	Category	Typical application
61	Density switch or sensor	Detects insulating-gas density	Auxiliary	SF6 switchgear
62	Time-delay stopping or opening relay	Delays a stop or open command	Control	Sequence control
63	Pressure switch or detector	Detects abnormal pressure or gas surge	Protection	Transformers, GIS
64	Ground protective relay	Detects an earth fault	Protection	Generators, transformers
65	Governor	Controls prime-mover speed or output	Generator	Turbines, engines
66	Notching or jogging device	Limits starts per hour or allows jogging	Machine control	Motors
67	AC directional overcurrent relay	Overcurrent protection that looks at current direction	Protection	Parallel feeders, ring mains
68	Blocking or power-swing blocking relay	Blocks tripping during power swings	Protection	Transmission lines
69	Permissive control device	Allows a control action when conditions suit	Control	Interlocks
70	Rheostat	Variable resistor	Auxiliary	Older control circuits
71	Liquid or gas level switch	Detects level	Auxiliary	Transformer oil, tanks
72	DC circuit breaker	Makes and breaks a DC circuit	Switching	Battery and DC systems
73	Load-resistor contactor	Switches a load resistor in or out	Switching	DC and braking circuits
74	Alarm relay	Gives an alarm output	Auxiliary	Substation alarms
75	Position-changing mechanism	Moves a device between positions	Switching	Draw-out breaker drives
76	DC overcurrent relay	Overcurrent protection for DC circuits	Protection	DC systems
77	Telemetering device	Sends measured values to a remote point	Auxiliary	SCADA transducers
78	Phase-angle or out-of-step relay	Detects loss of synchronism between systems	Protection	Generators, interconnections
79	AC reclosing relay	Recloses a breaker after a trip	Protection	Overhead lines, feeders
80	Liquid or gas flow relay	Detects flow, or loss of flow	Auxiliary	Cooling systems

ANSI Device Numbers 81–94: Frequency, Differential and Auxiliary Functions

No.	Device name	Plain-language meaning	Category	Typical application
81	Frequency relay	Operates on over- or underfrequency	Protection	Grids, generators
82	DC reclosing relay	Recloses a DC circuit breaker	Protection	DC systems
83	Automatic selective control or transfer relay	Switches automatically between supplies	Control	Bus transfer schemes
84	Operating mechanism	Stored-energy or motor drive for a switch	Switching	Breaker mechanisms
85	Pilot communications relay	Uses a communication channel for protection signals	Auxiliary	Teleprotection
86	Lockout relay	Latches a trip and blocks closing until reset	Protection	Transformers, buses, generators
87	Differential protective relay	Trips when current in and out of a zone does not balance	Protection	Transformers, buses, generators, lines
88	Auxiliary motor or motor-generator	Drives auxiliary equipment	Machine control	Pumps, fans, motor-generator sets
89	Line switch (isolator)	Isolates equipment; not normally used to break load	Switching	Substations
90	Regulating device	Holds a quantity such as voltage steady	Control	Tap changers, AVRs
91	Voltage directional relay	Responds to voltage direction	Protection	Special schemes
92	Voltage and power directional relay	Responds to voltage and power direction together	Protection	Special schemes
93	Field-changing contactor	Changes the field connection or value	Generator	Excitation systems
94	Tripping or trip-free relay	Starts or assists a trip command	Protection	Trip circuits

Numbers 95 to 99: these are for specific applications that no other number covers. IEEE C37.2-2022 also adds modern acronyms alongside the numbers, so check the current standard for exact assignments before you quote them.

How the Device Numbers Are Grouped

We grouped numbers 1 to 94 by their main job. Protection is the largest group, which is why most day-to-day work revolves around a small set of numbers. This grouping is our own editorial one, so another author may place a few numbers differently.

94
numbers

• Protection: 27 (29%)
• Auxiliary and monitoring: 17 (18%)
• Control and sequencing: 16 (17%)
• Machine control: 13 (14%)
• Generator and synchronising: 11 (12%)
• Switching and isolating: 10 (11%)

The switching group includes numbers 52 and 89. To see how breakers differ in practice, read our guide to types of circuit breakers or the comparison of MCB, MCCB and ACB.

ANSI Device Number Suffix Letters and Their Meanings

Letters added after a number narrow down what the function measures or protects. So 50 is “instantaneous overcurrent”, while 50N is the same function connected to neutral current. Suffix meanings depend on context and sometimes on the manufacturer, so always check the relay manual.

Suffix	Common meaning	Example	Note
N	Neutral or residual	50N, 51N	Measures earth current through the neutral or residual connection
G	Ground, or generator	51G, 87G	Ground in overcurrent codes; generator in differential codes
P	Phase	50P, 51P	Phase-fault elements
T	Transformer	87T	Can also mean time delay in other codes
B	Bus	87B	Busbar zone
L	Line	87L	Line current differential
M	Motor	87M	Context dependent
U	Under (or unit)	81U	Underfrequency in 81U
O	Over	81O	Overfrequency in 81O
BF	Breaker failure	50BF	Backup if a breaker does not clear a fault
X, Y, Z	Auxiliary devices	27X	Usually auxiliary relays tied to the main function

Difference Between 50N, 50G, 51N and 51G

All four are earth-fault overcurrent functions. The difference is where the current signal comes from. N elements usually use the residual current of the three phase CTs (or a neutral CT). G elements usually use a dedicated ground CT, such as a core-balance CT around the cable. The numbers still carry the timing: 50 trips without deliberate delay, 51 trips after a time delay. Some manufacturers use the letters the other way round, so confirm against the relay manual.

Combined ANSI Device Numbers

Numbers are often written together because one relay performs several functions. A slash separates them.

Label	Meaning	Where it is used
50/51	Instantaneous and time-delayed phase overcurrent in one relay	Feeders, motors, transformer incomers
50N/51N	Instantaneous and time-delayed neutral overcurrent	Earth-fault protection on solidly or resistance earthed systems
50G/51G	Instantaneous and time-delayed ground overcurrent	Cable feeders with a core-balance CT
27/59	Undervoltage and overvoltage functions	Buses, generators, grid interconnections
81O/81U	Overfrequency and underfrequency	Generators, load shedding
87T/50/51	Transformer differential with overcurrent backup	Power transformer protection panels
50BF	Breaker-failure protection	Substations with a bus-wide trip scheme
64REF / 87N	Restricted earth-fault protection	Transformer windings (naming varies by vendor)

ANSI Device Numbers by Protected Equipment

This matrix shows which numbers are commonly found on each type of equipment. It is a guide to what you may see on drawings, not a design rule. The actual scheme depends on the equipment, system and utility requirements.

Equipment	Commonly used numbers	Notes
Generator	25, 27, 32, 40, 46, 49, 51V, 59, 64, 81, 87G	32 detects reverse power; 40 detects loss of field; 87G is generator differential
Power transformer	24, 49, 50/51, 51N, 63, 87T	63 covers gas or sudden-pressure devices; 87T is the main fast protection
Motor	37, 46, 47, 49, 50/51, 66, 87M	46 and 47 catch unbalance and wrong sequence; 49 models heating
Line or feeder	21, 50/51, 67, 79, 87L	21 on longer lines; 79 on overhead lines where faults are often temporary
Busbar	50BF, 86, 87B	87B trips all connected breakers; 86 holds the trip
Capacitor bank	27, 50/51, 59, 60	60 is often used for unbalance; scheme varies with bank design

Many of these functions are fitted inside switchgear panels, where the relay, breaker (52) and isolator (89) sit together in one lineup.

ANSI Device Numbers vs IEC 61850 Logical Nodes

IEC 61850 does not replace ANSI numbers with one symbol each. It describes functions as logical nodes, which are software building blocks that digital substation devices use to share data. Each logical node has a four-letter name, such as PDIS for distance protection. ANSI numbers describe what the function is. Logical nodes describe how it is modelled and communicated.

ANSI function	IEC 61850 logical node	Function
21	PDIS	Distance protection
50 / 51 / 67	PTOC (some vendors use PIOC for 50, PDEF for earth-fault direction)	Overcurrent protection
27	PTUV	Undervoltage protection
59	PTOV	Overvoltage protection
81O	PTOF	Overfrequency protection
87	PDIF	Differential protection
50BF	RBRF	Breaker-failure protection
79	RREC	Auto-reclosing
52	XCBR	Circuit breaker
89	XSWI	Isolator or switch

These are representative examples, not a universal one-to-one conversion. Mappings differ between manufacturers and between relay models, so use the relay’s own IEC 61850 conformance document when you need exact names. ANSI numbers and IEC 61850 normally sit side by side in modern relays: the number appears on drawings and settings, and the logical node is used in the communication model.

ANSI Numbers vs IEC Graphical Symbols

IEC graphical symbols (IEC 60617) draw a device on a diagram, for example a relay coil or a breaker. An ANSI number says what the device does. Drawings can use both together, such as an IEC symbol for a relay labelled “51”. European and IEC-led projects tend to lean on symbols and logical nodes; North American, Indian and many international projects use ANSI numbers widely.

How to Read ANSI Device Numbers on a Single-Line Diagram

A single-line diagram shows the main equipment as one line. Device numbers appear in small circles or boxes next to the breaker, transformer or feeder they act on. The number tells you which protection function is present, and the suffix tells you what it watches. Primary protection (such as 87T) acts fastest on its own zone, while backup protection (such as 51 or 50BF) acts later or covers a neighbouring zone.

Worked Examples of Relay Labels

Label	How to read it
50/51	Base functions 50 and 51: instantaneous plus time-delayed phase overcurrent.
50N/51N	The same pair, with suffix N: both use neutral or residual earth current.
87T	Base 87 (differential) with suffix T: transformer differential protection.
87T/50/51N	Transformer differential, plus phase instantaneous and time-delayed overcurrent, plus neutral time-delayed overcurrent, usually all in one IED.

To decode any label, split it at the slashes, read each base number from the list, then apply the suffix. Our guide on fuse vs circuit breaker is a useful companion if you want to understand what happens after the relay sends its trip signal.

Common Misconceptions About ANSI Device Numbers

Misconception	Reality
They are only used in the United States.	They are used worldwide, including India and the UK, alongside IEC practice.
Each number is a separate physical relay.	One modern IED can host many numbered functions.
Every number from 1 to 99 has a defined function.	The standard assigns 95 numbers; 95–99 are for specific applications.
A suffix letter always means the same thing.	G, T, B and others change meaning with context.
ANSI numbers and IEC 61850 logical nodes are identical.	They describe different things and do not map one-to-one.

Frequently Asked Questions About ANSI Device Numbers

What are ANSI device numbers?

They are standard numeric codes from IEEE C37.2 that identify the function of a protection, control or switching device in a power system.

What is the ANSI device number for an instantaneous overcurrent relay?

It is 50.

What is the difference between ANSI 50 and 51?

Device 50 trips with no deliberate delay when current passes its setting. Device 51 trips after a time delay that gets shorter as current rises.

What does ANSI 87 mean?

It means differential protection. The relay compares current entering and leaving a protected zone and trips if they do not balance.

What is an ANSI 86 lockout relay?

It is a latching relay that trips the connected breakers and blocks reclosing until it is reset by hand or remotely.

Why is a circuit breaker called device 52?

Because IEEE C37.2 assigns number 52 to the AC circuit breaker. The number is a convention, not a calculation.

What does the N suffix mean?

It usually means the function uses neutral or residual current, as in 50N and 51N.

What does G mean in ANSI relay numbers?

It usually means ground in overcurrent codes such as 51G, and generator in differential codes such as 87G.

What is ANSI 79 used for?

It is automatic reclosing. It closes a breaker again after a trip, which helps on overhead lines where many faults clear themselves.

What does ANSI 81 mean?

It is a frequency relay. 81O acts on overfrequency and 81U on underfrequency.

What is the IEC equivalent of ANSI device numbers?

There is no exact one-to-one equivalent. IEC 61850 uses logical nodes such as PTOC, PDIS and PDIF, and IEC 60617 provides graphical symbols.

Are ANSI device numbers used in India?

Yes. They are widely used on Indian utility, EPC and industrial drawings and relay settings, often alongside IEC practice.

How many ANSI device numbers are there?

IEEE C37.2-2022 assigns 95 device numbers and 22 acronyms.

Conclusion

ANSI device numbers give engineers a shared shorthand for what each device does. Learn the core set first (21, 27, 50, 51, 52, 59, 81, 86 and 87), then add suffixes and combined labels as you meet them on drawings. Bookmark this page as a quick reference, and see our overview of protection relays for the hardware behind the numbers.

Disclaimer: this is a general reference. Relay selection, settings, coordination, testing and commissioning must be carried out by qualified protection engineers in line with IEEE C37.2-2022, local regulations, utility requirements and project design documents.

Sources: IEEE Standards Association: C37.2-2022; Wikipedia: ANSI device numbers; IEC 61850-7-4 preview.