ANSI/IEEE device numbers are standardized codes used to identify protective relays and related devices in electrical power systems, facilitating clear documentation and system design.

Overview of ANSI Device Numbers

ANSI/IEEE C37.2 defines device numbers to identify protective relays, circuit breakers, and other control devices in power systems. Each number corresponds to a specific function, such as overcurrent protection, voltage monitoring, or differential protection. Historically, each protective function had a dedicated electromechanical device, but modern microprocessor-based relays can perform multiple functions, often denoted as a multifunction device (11) in documentation .

Common Protective Relay Device Numbers

Some widely used ANSI device numbers include :

• 50 – Instantaneous Overcurrent Relay
• 51 – Time Overcurrent Relay
• 27 – Undervoltage Relay
• 59 – Overvoltage Relay
• 87T – Transformer Differential Relay
• 87G – Generator Differential Relay
• 86 – Lockout Relay
• 49 – Thermal Overload Relay Suffix letters or numbers are often added to clarify the application:
• G – Ground (e.g., 51G for ground overcurrent) or Generator (87G)
• N – Neutral connection (e.g., 59N for neutral displacement)
• T – Transformer (87T)
• F – Field or fuse
• Numeric suffixes (e.g., 51-1, 51-2) distinguish multiple devices of the same type in a system .

Multifunction Relays

Modern relays can combine multiple functions. For example, a 27/59 relay combines undervoltage and overvoltage protection. In such cases, the device number indicates all functions performed, and documentation like single-line diagrams uses these numbers to show the specific protective roles .

Directional Overcurrent Protection

Directional overcurrent relays require both current and voltage inputs to determine the fault direction. Unlike non-directional overcurrent relays, which trip when current exceeds a set value, directional relays trip the circuit breaker only if the fault occurs in the predefined direction, enhancing selective protection in complex networks .

Summary

ANSI/IEEE device numbers provide a standardized method to identify protective relays and control devices, ensuring consistent communication in design, operation, and maintenance of electrical power systems. Understanding these numbers, suffixes, and multifunction applications is essential for engineers working with protective relaying and system protection.