Relay Protection Types and Codes

Relay protection codes are standardized ANSI/IEEE device numbers used to identify protective relays and their specific functions in electrical power systems.

Overview of ANSI/IEEE Device Numbers

ANSI/IEEE device numbers are defined in ANSI/IEEE Standard C37.2 and are widely used in North America to identify protective relays, circuit breakers, and other control devices in power systems . Each number corresponds to a specific protective function. Historically, each device performed a single function, but modern microprocessor-based relays can perform multiple functions, often indicated by combined device numbers.

Common Device Numbers

Some of the most frequently used relay protection codes include :

• 50 – Instantaneous Overcurrent Relay
• 51 – Time Overcurrent Relay
• 27 – Undervoltage Relay
• 59 – Overvoltage Relay
• 87T – Transformer Differential Relay
• 87G – Generator Differential Relay
• 86T – Lockout Relay for Transformer
• 50N / 51N – Neutral Overcurrent Relays

Suffixes and Letters

Letters or numbers can be added to device numbers to indicate specific applications or conditions :

• G – Ground (e.g., 51G for time overcurrent ground relay) or Generator (e.g., 87G for generator differential)
• T – Transformer (e.g., 87T for transformer differential)
• N – Neutral (e.g., 59N for neutral overvoltage)
• F – Field or Fuse (e.g., protective fuse for a pickup transformer) These suffixes help clarify the exact function or location of the relay in the system.

Multifunction Relays

Modern relays often combine multiple functions into a single device. For example:

• 27/59 – Combined undervoltage and overvoltage relay
• 50/51 – Instantaneous and time overcurrent relay In documentation such as single-line diagrams, these codes indicate which protective functions are implemented, making it easier for engineers to understand system protection schemes .

Practical Use

Relay protection codes are essential for:

• Designing and documenting protection schemes
• Coordinating relays for selective tripping
• Troubleshooting faults and system events
• Ensuring compliance with industry standards By using ANSI/IEEE device numbers, engineers can communicate relay functions clearly across different systems and manufacturers, even in regions where IEC standards are also used . In summary, relay protection codes provide a standardized method to identify and document the protective functions of relays, ensuring reliable and coordinated operation of electrical power systems.