A relay protection number is a standardized ANSI/IEEE device number that identifies the specific function of a protective relay in an electrical system.

Overview

Relay protection numbers are part of the ANSI/IEEE C37.2 standard, which assigns numerical codes to protective devices to describe their function concisely and consistently across electrical systems . Instead of using long verbal descriptions, these numbers allow engineers, technicians, and utilities to quickly identify the type of protection a relay provides, improving clarity in diagrams, specifications, and communication.

Common Examples

• 50 – Instantaneous overcurrent relay, trips immediately when current exceeds a set threshold .
• 51 – Time overcurrent relay, trips after a preset time delay when current exceeds a set value .
• 50N / 51N – Instantaneous or time overcurrent relays for neutral or ground currents .
• 27 – Undervoltage relay.
• 59 – Overvoltage relay.
• 87T – Transformer differential relay.
• 59G – Ground overvoltage relay. Multifunction relays may combine numbers, such as 27/59, indicating a relay that provides both under- and overvoltage protection .

Purpose and Benefits

• Standardization: Ensures all parties (engineers, vendors, utilities) understand relay functions without ambiguity .
• Efficiency: Simplifies wiring diagrams and documentation by using concise numerical codes instead of lengthy descriptions .
• Clarity in Operation: Helps in selecting, configuring, and troubleshooting relays in medium and high voltage systems .

Application

Relay protection numbers are widely used in medium voltage (MV) and high voltage (HV) panels, often in combination with circuit breakers. They are powered by current transformers (CTs) and voltage transformers (VTs) to detect abnormal conditions like overcurrent, earth faults, or voltage deviations, and then signal or trip the breaker as needed . Letters can be added as suffixes or prefixes to specify the application more precisely, such as directional protection or ground-specific functions . In summary, the relay protection number is a universal code that identifies the protective function of a relay, making electrical system design, operation, and maintenance more standardized and efficient.