ANSI Device Function Numbers: A Complete Guide

Staring at a power diagram full of cryptic numbers? Electrical schematics with codes like “50/51” or “87G” can be intimidating. This guide demystifies ANSI codes, helping you specify, buy, and operate equipment with confidence.

What Are ANSI Device Function Numbers?

Why do engineers use numbers instead of plain English? Trying to coordinate projects across borders becomes a mess of translations. This standard, used for nearly a century , solves that problem, creating a universal language for all power system professionals.

These codes are officially defined by the IEEE C37.2 standard, titled “Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations”. Each number, from 1 to 99, represents a specific function of a device or protective relay within a power system.

Whether you are looking at a single-line diagram, a switchgear specification sheet, or a protective relay’s setting file, these numbers consistently identify a component’s job. This standardization is critical for:

• Interoperability: Ensuring that equipment from different manufacturers can be understood and integrated.
• Safety: Clearly and unambiguously identifying protective functions to prevent accidents.
• Efficiency: Streamlining the design, commissioning, and maintenance process.

A device number defines a function, not the device itself. A single, modern digital relay (a multifunction device, ANSI Code 11 ) might perform dozens of these functions simultaneously, such as 50, 51, 27, and 59.

The “Big Three”: Understanding Overcurrent Protection (Codes 49, 50, 51)

Overcurrent is the #1 enemy of electrical systems. But what’s the difference between thermal, instantaneous, and time-delay protection? Choosing the wrong relay is a costly, dangerous mistake. This section breaks down the three most critical codes.

ANSI Code 49: Thermal Overload Relay

Function: The “Thermal Overload Relay” is your system’s defense against sustained, low-level overloads.

• What it does: This function simulates the heating effect of current. It protects equipment like motors and transformers from damage caused by running slightly above their rated capacity for too long.
• How it works: It operates on a slow “thermal curve.” It will intentionally ignore short, minor spikes in current (like a motor starting), but it will trip if the high current persists and threatens to “cook” the equipment’s insulation.
• Buyer Application: Essential for motor protection circuits, generators, and transformers where thermal damage is the primary concern.

ANSI Code 50: Instantaneous Overcurrent Relay

Function: The “Instantaneous Overcurrent Relay” is your system’s airbag.

• What it does: This function provides immediate protection against massive, severe fault currents, such as a direct short circuit.
• How it works: It has no intentional time delay. As soon as the current exceeds a very high, predefined setpoint, it trips the breaker instantly (typically in milliseconds) to prevent catastrophic damage, fire, or arc flash.
• Buyer Application: This is a non-negotiable safety function. It is used for short-circuit protection on all main feeders, breakers, and critical loads.

ANSI Code 51: AC Time Overcurrent Relay

Function: The “AC Time Overcurrent Relay” is the “smart” overcurrent protection.

• What it does: This function trips based on an inverse-time curve. The higher the overcurrent, the faster it will trip.
• How it works: Unlike the “all-or-nothing” Code 50, a Code 51 relay introduces a calculated delay. This allows it to “ride through” temporary, harmless events like motor startup inrush current. However, it will reliably trip on persistent faults that Code 49 might be too slow for.
• Buyer Application: This is the most common form of overcurrent protection for feeders and branch circuits. It allows for system coordination, ensuring the breaker closest to the fault trips first, leaving the rest of the system online.

The Power Combo: Why “50/51” is Everywhere

On most diagrams, you won’t see 50 or 51 alone; you’ll see “50/51”. This common combination packages two functions into one protective device.

• The 51 (Time Overcurrent): Handles the “problem” currents (overloads).
• The 50 (Instantaneous): Handles the “disaster” currents (short circuits).

This dual-function protection is the industry standard for feeders and switchgear, providing comprehensive defense against the full spectrum of overcurrent threats.

A Comprehensive Quick-Reference Guide (Codes 1-99)

Beyond overcurrent, dozens of codes define a system’s operation. Scrambling to find a specific function for voltage, frequency, or reclosing? This comprehensive reference guide organizes all 99 codes so you can find what you need, fast.

ANSI Device Function Numbers Quick Lookup Tool

Master, Control & Logic (Codes 1-25)

This group includes master elements, control relays, and devices that manage system startup, shutdown, and synchronization.

1: Master Element

2: Time-Delay Starting or Closing Relay

3: Checking or Interlocking Relay

4: Master Protective Relay

5: Stopping Device

6: Starting Circuit Breaker

7: Rate-of-Change Relay

8: Control-Power Disconnecting Device

9: Reversing Device

10: Unit-Sequence Switch

11: Multifunction Device

12: Overspeed Device

13: Synchronous-Speed Device

14: Underspeed Device

15: Speed or Frequency Matching Device

16: Data Communications Device

17: Shunting or Discharge Switch

18: Accelerating or Decelerating Device

19: Starting-to-Running Transition Contactor

20: Electrically Operated Valve (Solenoid)

21: Distance Relay

22: Equalizer Circuit Breaker

23: Temperature Control Device

24: Volts-per-Hertz Relay

25: Synchronizing or Synch-Check Device

Protection & Monitoring (Codes 26-48)

This set covers a wide range of protective functions for voltage, current, power, and the physical condition of equipment.

26: Apparatus Thermal Device

27: Undervoltage Relay

28: Flame Detector

29: Isolating Contactor

30: Annunciator Relay

31: Separate Excitation Device

32: Directional Power Relay

33: Position Switch

34: Master Sequence Device

35: Brush-Operating or Slip-Ring Short-Circuiting Device

36: Polarity Device

37: Undercurrent or Underpower Relay

38: Bearing Protective Device

39: Mechanical Condition Monitor

40: Field (Loss-of-Excitation) Relay

41: Field Circuit Breaker

42: Running Circuit Breaker

43: Manual Transfer Device

44: Unit-Sequence Starting Relay

45: Atmospheric Condition Monitor

46: Reverse-Phase or Phase-Balance Current Relay

47: Phase-Sequence or Phase-Balance Voltage Relay

48: Incomplete-Sequence or Blocked-Rotor Relay

The Overcurrent & Breaker Family (Codes 49-75)

This critical group includes the overcurrent codes (49, 50, 51) as well as breakers, switches, and relays related to system state.

49: Thermal Overload Relay

50: Instantaneous Overcurrent Relay

51: AC Time Overcurrent Relay

52: AC Circuit Breaker

53: Exciter or DC Generator Relay

54: Turning Gear Engaging Device

55: Power-Factor Relay

56: Field Application Relay

57: Short-Circuiting or Grounding Device

58: Rectification Failure Relay

59: Overvoltage Relay

60: Voltage or Current Balance Relay

61: Density Switch

62: Time-Delay Stopping Relay

63: Pressure Switch

64: Ground Protective Relay

65: Governor

66: Notching or Jogging Device (Startup count limit)

67: Directional Overcurrent Relay

68: Blocking or Out-of-Step Relay

69: Permissive Control Device

70: Rheostat

71: Level Switch

72: DC Circuit Breaker

73: Load-Resistor Contactor

74: Alarm Relay

75: Position-Changing Mechanism

System Operation & Advanced Protection (Codes 76-99)

This group covers advanced protection, automation, and communication functions.

76: DC Overcurrent Relay

77: Telemetering or Speed Relay

78: Phase-Angle Measuring or Out-of-Step Relay

79: Reclosing Relay

80: Liquid or Gas Flow Relay

81: Frequency Relay (Often split into 81O for Over-Frequency and 81U for Under-Frequency )

82: DC Reclosing Relay

83: Automatic Transfer Device

84: Operating Mechanism

85: Pilot Communications Relay

86: Lockout Relay

87: Differential Protective Relay

88: Auxiliary Motor or Motor-Generator

89: Line Switch

90: Regulating Device

91: Voltage Directional Relay

92: Voltage and Power Directional Relay

93: Field-Changing Contactor

94: Tripping or Trip-Free Relay

95: Trip-Circuit Monitor

96: Transmitter / Alarm Initiating Device

97-99: Reserved for Specific Application

Beyond the Numbers: Decoding Suffixes and Prefixes

Seeing “87T” or “51G” adds another layer of confusion. These letters aren’t random or optional. They are crucial modifiers that specify exactly what the relay is protecting and how it functions. This section decodes that shorthand.

While the number defines the function (e.g., 87 = Differential Protection ), the suffix defines the application.

Here are some of the most common suffixes you’ll see:

G: Ground. (e.g., 51G is a time-overcurrent relay for ground faults, not phase faults).

N: Neutral. (e.g., 51N is a time-overcurrent relay connected to the neutral).

P: Phase. (e.g., 51P is a time-overcurrent relay for phase conductors).

T: Transformer. (e.g., 87T is a Transformer Differential Protection relay ).

B: Bus. (e.g., 87B is a Bus Differential Protection relay ).

G: Generator. (e.g., 87G is a Generator Differential Protection relay ).

BF: Breaker Failure. (e.g., 50BF is an instantaneous overcurrent relay used specifically for a breaker failure protection scheme).

Understanding these combinations is essential for reading one-line diagrams. A relay labeled 87T is fundamentally different in its settings and application from one labeled 87G.

How This Knowledge Helps You Buy the Right Equipment

Why does a B2B buyer or operator need to know this? You risk purchasing non-compliant, unsafe, or over-spec’d equipment. Understanding these codes ensures you procure the exact protection your system needs and can “talk the talk” with engineers.

When you receive a Request for Quotation (RFQ) for switchgear or a transformer, it will be filled with these codes. Your ability to match these requirements to product data sheets is what separates a successful bid from a costly mistake.

Specify with Precision: Do you just need 50/51 protection, or does the application require a full differential (87) scheme?

Avoid Over-Specifying: Why pay for a complex multifunction relay (11) with 20 functions when the application only calls for a simple 59 (Overvoltage)?

Ensure Compliance: Knowing that “49: Thermal Overload” is required for a motor means you can ensure the motor control center you purchase has that specific, certified protection.

Conclusion

ANSI codes are the global language of power system protection. Mastering overcurrent codes 49, 50, and 51 is key, but understanding the full system ensures safety and efficiency. Use this guide to specify your next project with complete confidence.

Frequently Asked Questions (FAQ)

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