Relay Protection Functions and Codes
Relay protection codes are standardized ANSI/IEEE device numbers used to identify the function of protective relays in electrical power systems.
Overview of Relay Protection Codes
Relay protection codes, also known as ANSI/IEEE device numbers, are used to identify the specific function of protective relays, circuit breakers, and other control devices in power systems. These codes are defined in ANSI/IEEE Standard C37.2, which provides a consistent numbering system for protective device functions, acronyms, and contact designations . Each number corresponds to a particular type of protection or control function.
Common ANSI Device Numbers
Some widely used relay protection codes include:
- 50 – Instantaneous overcurrent relay
- 51 – Time overcurrent relay
- 27 – Undervoltage relay
- 59 – Overvoltage relay
- 87 – Differential relay (87T for transformer, 87G for generator)
- 86 – Lockout relay
- 49 – Thermal overload relay Multifunction relays may combine multiple device numbers, such as 27/59, which indicates a combined under/over voltage relay .
Suffix Letters and Numbers
Suffixes are added to device numbers to provide more specific information about the relay application:
- 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 connection (e.g., 59N for neutral displacement protection)
- F – Field or fuse (e.g., protective fuse for a pickup transformer)
- Numbers – Distinguish multiple devices of the same type in the same system (e.g., 51-1, 51-2), .
Application in Power Systems
Relay protection codes are used in:
- Single-line diagrams – To indicate which protective functions are performed by each relay
- Relay settings and documentation – To identify the type of protection and its application
- Coordination studies – To ensure proper operation of relays during faults or abnormal conditions These codes are widely used in North America, while IEC standards use graphical symbols. Conversion tables exist to map ANSI device numbers to IEC symbols for international applications .
Key Takeaway
Understanding relay protection codes is essential for engineers and technicians working with power systems, as they provide a universal language for identifying relay functions, ensuring proper protection, and facilitating system documentation and troubleshooting.
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