Relay Protection Type Code

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 NumbersANSI/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 Relay51 – Time Overcurrent Relay27 – Undervoltage Relay59 – Overvoltage Relay87T – Transformer Differential Relay87G – Generator Differential Relay86T – Lockout Relay for Transformer50N / 51N – Neutral Overcurrent RelaysSuffixes and LettersLetters 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 relay50/51 – 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 schemesCoordinating relays for selective trippingTroubleshooting faults and system eventsEnsuring 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.