Relay protection circuits are numbered using the ANSI/IEEE C37.2 standard, where each number identifies a specific protective function, often with suffixes to indicate application details.

Overview of ANSI/IEEE Device Numbering

In electrical power systems, protective relays and devices are assigned standard numbers according to the ANSI/IEEE C37.2 standard. This system allows engineers to quickly identify the function of a relay or device in diagrams, schematics, and control panels . Each number corresponds to a specific protective function, such as overcurrent, undervoltage, or differential protection. For example:

• 50 – Instantaneous overcurrent relay
• 51 – Time overcurrent relay
• 27 – Undervoltage relay
• 59 – Overvoltage relay
• 87 – Differential relay

Use of Suffixes

Suffix letters or numbers are added to clarify the application or connection of the device:

• G – Ground (e.g., 51G for time overcurrent ground relay) or Generator (e.g., 87G for generator differential relay)
• T – Transformer (e.g., 87T for transformer differential relay)
• N – Neutral connection (e.g., 59N for neutral overvoltage)
• X, Y, Z – Auxiliary devices or specific functions These suffixes help distinguish between similar functions applied to different equipment or system points.

Multifunction Relays

Modern microprocessor-based relays can perform multiple protective functions in a single device. In such cases, the relay may be represented by a combination of device numbers, such as:

• 27/59 – Combined undervoltage/overvoltage relay
• 50/51 – Instantaneous and time overcurrent in one relay Even when multiple functions are integrated, each function retains its ANSI device number for clarity in documentation and single-line diagrams .

Practical Application

Relay numbering is widely used in:

• Single-line diagrams – to indicate which protective functions are present
• Control schematics – to identify relay contacts and trip circuits
• Maintenance and troubleshooting – to quickly locate and understand relay functions By following the ANSI/IEEE numbering system, engineers ensure consistent communication and documentation across power systems, regardless of manufacturer or relay type .

Key Takeaway

Relay protection circuit numbering standardizes the identification of protective devices, making it easier to design, operate, and maintain electrical power systems. Numbers indicate the function, while suffixes specify the application, and multifunction relays combine multiple numbers to represent all functions performed by a single device.