Protective relays are a complex and critical part of electrical engineering, especially in the solar and energy storage industries. While they might seem intimidating at first, understanding the basics can make all the difference. This three-part series is designed to introduce non-engineers to the fundamentals of protective relaying, helping you better grasp how these systems work. **Intro to Relays #1 – What Are Relays, CTs, & PTs?** **Intro to Relays #2 – ANSI/IEEE Relay Device Numbers (below on this page)** **Intro to Relays #3 – What Does SEL Stand For?** --- ### **What Are Relay Numbers?** Protective relays use standardized device numbers to describe their functions instead of using full descriptions. This system is outlined in the ANSI/IEEE C37.2 standard, which ensures consistency across the industry. Using numbers rather than words offers several advantages: 1. **Efficiency**: It's faster to reference a number like “59N” than to write out “Overvoltage on the Neutral.” 2. **Standardization**: Everyone from engineers to installers understands what each number means, reducing confusion and errors. 3. **Clarity on Drawings**: Numbers make diagrams more compact and easier to read, especially when multiple functions are involved. For example, instead of listing all the functions of a relay like “phase overvoltage & undervoltage, phase over frequency & under frequency, ground inverse time overcurrent, and alarm,” you can simply label it with the corresponding numbers. --- ### **Why Use Numbers Instead of Words?** In the fast-paced world of solar and energy storage, clarity and speed are essential. Relay numbers allow teams to communicate effectively and efficiently. Whether you're reviewing schematics or discussing project requirements, knowing these numbers helps ensure that everyone is on the same page. --- ### **What Numbers Are Commonly Used in Solar Projects?** Here’s a list of the most frequently used relay functions in photovoltaic (PV) and energy storage systems: | # | Name | Description | |---|----------------------|-------------| | 25 | Synchronizing Clock | Compares voltage, frequency, and phase angle between the utility and solar system to enable safe grid connection. | | 27 | Undervoltage | Triggers when voltage drops below a set threshold. | | 32 | Directional Power | Detects power flow in a specific direction (e.g., reverse power). | | 49 | Transformer Thermal | Activates if transformer winding temperature exceeds limits. | | 50 | Instantaneous Overcurrent | Trips immediately when current exceeds a defined value. | | 51 | Inverse-Time Overcurrent | Trips after a set time if current remains above a threshold. | | 52 | Circuit Breaker | Opens or closes a circuit; 52R includes reclosing capability. | | 59 | Overvoltage | Triggers when voltage goes above a set limit. | | 74 | Alarm | Alerts through visual, audible, or data signals. | | 79 | AC Reclosing | Controls automatic reclosing of an AC circuit. | | 81 | Frequency | Triggers if frequency is outside acceptable ranges. | | 86 | Lockout | Prevents operation until manually reset. | | 87 | Differential Protective | Detects differences between measured currents. | | 89 | Line Switch | A disconnect switch, often used with accessories like shunt trips. | --- ### **Letters That Follow Relay Numbers** Sometimes, letters are added to relay numbers to further define their function: | Letter | Function | Example | |--------|----------------|------------------------------| | R | Reclosing | 52R = Circuit breaker with reclose capability | | P | Phase | Often implied by default (e.g., 50, 51) | | N | Neutral | 51N = Overcurrent on neutral | | G | Ground | 51G = Ground fault detection | These additional characters help distinguish between similar functions and improve precision. --- ### **Setpoints: The Key to Proper Operation** Relay functions aren’t just about identifying the type of protection—they also require precise settings. These setpoints, such as voltage thresholds or time delays, are determined by engineers based on the specific needs of the system. They vary from project to project and must be carefully configured to ensure safe and reliable performance. --- ### **Conclusion** While the concept of relay device numbers may seem simple at first, it quickly becomes more intricate as you dive deeper into the details. However, you don't need to be an expert to understand the basics. For those who want to focus on project management or development, having a team of experienced engineers—like those at Pure Power—can make all the difference. With over 2,000 projects completed, we specialize in delivering high-quality solutions for solar and energy storage systems. If you're looking for expert guidance on your next project, reach out today. Let us help you navigate the world of protective relays with confidence.

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