Relay Protection Device Coding

Relay protection devices are coded using standardized ANSI/IEEE device numbers, with suffixes and multifunction designations to identify their specific protective functions and applications.

ANSI/IEEE Device Numbering System

The ANSI/IEEE C37.2 standard defines device numbers to identify the function of protective relays and devices in electrical power systems . Each number corresponds to a specific protective function, such as:

• 50 – Instantaneous overcurrent relay
• 51 – Time overcurrent relay
• 52 – AC circuit breaker
• 59 – Overvoltage relay
• 87 – Differential relay These numbers are used in single-line diagrams, schematics, and relay documentation to indicate the function of each device .

Suffixes and Special Designations

Suffix letters or numbers are added to device numbers to indicate specific conditions or connections:

• N – Neutral connection (e.g., 59N for neutral overvoltage)
• G – Ground or generator (e.g., 51G for ground overcurrent, 87G for generator differential)
• T – Transformer (e.g., 87T for transformer differential)
• X, Y, Z – Auxiliary devices or additional functions

Multifunction Devices

Modern microprocessor-based relays can perform multiple protective functions. These are often designated as Device 11, a multifunction device, and all functions must be clearly defined in the drawing legend, relay-setting record, or device function list . This ensures clarity in operation and maintenance.

Practical Coding Rules

1. Terminal and Lead Numbering: Standardized lead numbers and terminal connections are used to maintain consistency across relays and switchgear .
2. Color Codes: Multicore cables follow color codes to identify circuits for tripping, indication, and alarm functions .
3. Functional Requirements: Relays must be reliable, sensitive, and fast-acting, with the ability to discriminate between faults requiring immediate action and those requiring delayed operation .
4. Documentation: All relay functions, settings, and interconnections should be documented in schematics, legends, and commissioning records to facilitate testing, troubleshooting, and maintenance .

Summary

Relay protection device coding rules are based on ANSI/IEEE device numbers, with suffixes to indicate special connections or functions. Multifunction devices are designated as Device 11, and all functions must be documented. Standardized terminal numbering, lead identification, and color coding ensure clarity and reliability in protective relay systems, supporting proper operation, testing, and maintenance of electrical power systems .