ANSI code for relay protection

ANSI device numbers are standardized codes used to identify the functions of protective relays and related devices in electrical power systems.

Overview of ANSI Device Numbers

ANSI device numbers are defined in ANSI/IEEE Standard C37.2 and are used to indicate the specific protective function of a relay or device in a power system. Each number corresponds to a particular type of protection or control function, and letters or suffixes can be added to indicate special applications or variations, such as neutral connection, ground, or generator-specific protection .

Common ANSI Device Numbers for Relay Protection

Some of the most frequently used ANSI device numbers include:

• 50 – Instantaneous Overcurrent Relay
• 51 – Time Overcurrent Relay
• 51N – Neutral Time Overcurrent Relay
• 50/51 – Overcurrent Protection (instantaneous and time-delayed)
• 46 – Negative Sequence Overcurrent Relay (phase unbalance protection)
• 27 – Undervoltage Relay
• 59 – Overvoltage Relay
• 59N – Neutral Overvoltage Relay
• 87 – Differential Relay
  • 87T – Transformer Differential Relay
  • 87G – Generator Differential Relay
• 86 – Lockout Relay
• 21 – Distance Relay (Impedance Relay)
• 32 – Directional Power Relay
• 64 – Ground Detector Relay
• 81 – Frequency Relay (81U for underfrequency, 81O for overfrequency) Suffix letters are used to specify additional details:
• G – Ground or Generator
• N – Neutral
• F – Field or Fuse
• X, Y, Z – Auxiliary devices or multiple instances of the same function .

Application in Power Systems

ANSI codes are widely used in single-line diagrams, schematics, and relay documentation to clearly indicate the protective function of each device. Modern microprocessor-based relays often perform multiple functions, and the ANSI number helps identify which function is active or being monitored. For example, a multifunction relay may include overcurrent, differential, and undervoltage protection, but each function is still referenced by its ANSI number for clarity .

Summary

ANSI device numbers provide a universal, standardized method to identify relay protection functions, ensuring consistency in design, operation, and maintenance of electrical power systems. Understanding these codes is essential for engineers, technicians, and operators working with protective relays and substation equipment .