ANSI Coding Table for Relay Protection

ANSI device numbers standardize the identification of protective relay functions, with numbers and suffixes indicating specific protection types and applications.Overview of ANSI Device NumbersANSI (American National Standards Institute) device numbers are used to identify protective devices in electrical power systems, such as relays, circuit breakers, and instruments. These numbers are defined in ANSI/IEEE Standard C37.2 and are widely used in documentation like single-line diagrams and schematics . Each number corresponds to a specific protective function, and suffix letters provide additional context, such as connection type or device application.Common SuffixesG: Ground protection (e.g., 51G for a time overcurrent ground relay) or Generator (e.g., 87G for generator differential protection)T: Transformer (e.g., 87T for transformer differential protection)N: Neutral connection (e.g., 50N for neutral overcurrent relay)X, Y, Z: Auxiliary devices or additional functions Key ANSI Device Numbers for Relay ProtectionANSI NumberFunction DescriptionExample50Instantaneous overcurrent50N – Neutral overcurrent relay51Time overcurrent51G – Ground overcurrent relay59Overvoltage59B – Bus overvoltage relay27Undervoltage27 – Undervoltage relay46Phase unbalance46 – Negative sequence current protection87Differential protection87T – Transformer differential relay, 87G – Generator differential relay86Lockout relay86T – Transformer lockout relay32Directional power32 – Directional power relay21Distance protection21 – Distance relay for transmission lines78Out-of-step protection78 – Power swing or out-of-step relay Applications in Modern RelaysModern microprocessor-based relays can perform multiple ANSI functions in a single device. For example, a single relay may combine overcurrent, undervoltage, and differential protection, but each function is still referenced by its ANSI number in schematics and settings . This allows engineers to document and configure protection schemes consistently across substations and industrial systems.SummaryANSI device numbers provide a standardized method to identify and communicate relay functions, ensuring clarity in design, operation, and maintenance of electrical power systems. Understanding both the number and suffix is essential for correctly interpreting relay capabilities and protection schemes .