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Protective relays are a complex and crucial part of electrical engineering, especially in the solar and energy storage sectors. While they can seem overwhelming at first, they don’t have to be! This three-part series is designed to break down the basics of relaying for non-engineers working in the renewable energy industry.
**Intro to Relays #1 – What are Relays, CTs, & PTs?**
**Intro to Relays #2 – ANSI/IEEE Relay Device Numbers (see below on this page)**
**Intro to Relays #3 – What does SEL stand for?**
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### **What Are Relay Device Numbers?**
Protective relays use standardized numbers to describe their functions, replacing lengthy verbal explanations. These numbers follow the ANSI/IEEE C37.2 standard, ensuring consistency across the industry.
Using numbers instead of words offers several advantages:
- **Efficiency**: It’s faster to reference "59N" than "Overvoltage on the Neutral."
- **Standardization**: Everyone involved—engineers, utilities, installers—understands the same language, reducing errors.
- **Clarity on Drawings**: Numbers make diagrams more compact and easier to read, especially when relays perform multiple functions.
For example, a relay that handles phase overvoltage, undervoltage, frequency issues, ground overcurrent, and alarms would be labeled with multiple numbers instead of being described in full.
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### **Attention Engineers!**
If you enjoy deep technical discussions like this, you might be a great fit at Pure Power! Our team of 80 engineers has designed over 2,000 commercial and utility-scale solar projects. Working with such a knowledgeable group is a fantastic way to grow your career.
Check out our open positions today!
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### **Common Relay Functions in Solar and Energy Storage Systems**
Here are the most frequently used relay numbers in PV and energy storage systems:
| # | Name | Description |
|-----|-------------------------|-------------|
| 25 | Synchronizing Clock | Compares voltage, frequency, and phase angle between the grid and solar system before allowing connection. |
| 27 | Undervoltage | Triggers when voltage drops below a set threshold. |
| 32 | Directional Power | Detects reverse power flow and triggers accordingly. |
| 49 | Transformer Thermal | Activates when transformer winding temperatures exceed safe limits. |
| 50 | Instantaneous Overcurrent | Trips immediately when current exceeds a set limit. |
| 51 | Inverse-Time Overcurrent | Trips after a time delay based on current magnitude. |
| 52 | Circuit Breaker | Opens or closes a circuit; 52R also allows reclosing. |
| 59 | Overvoltage | Triggers when voltage goes above a defined level. |
| 74 | Alarm | Triggers visual, audible, or data-based alerts. |
| 79 | AC Reclosing | Controls automatic reclosing of AC circuits. |
| 81 | Frequency | Activates if frequency is outside acceptable ranges. |
| 86 | Lockout | Prevents operation until manually reset. |
| 87 | Differential Protective | Trips when there's a difference between measured currents. |
| 89 | Line Switch | A disconnect switch with additional features like shunt trip or aux contacts. |
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### **Letters That Modify Relay Numbers**
Some relay numbers include letters to specify additional functions:
| Letter | Function | Description |
|--------|------------------|-------------|
| R | Reclosing | Indicates the relay can reclose after tripping (e.g., 52R). |
| P | Phase | Refers to phase-specific functions (commonly default). |
| N | Neutral | Indicates the function is on the neutral conductor (e.g., 51N). |
| G | Ground | Indicates the function is on the ground (e.g., 51G). |
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### **Setpoints: The Important Details**
Relay functions aren't just about the number—they also require specific setpoints. These values (minimum, maximum, or both) are determined by engineers and vary depending on the project. They ensure the relay operates correctly under real-world conditions.
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### **Conclusion**
While relay device numbers may seem simple at first, they form the backbone of electrical protection systems. For developers and project managers, understanding the basics is key, but diving into the details isn’t always necessary. That’s where experienced engineers like those at Pure Power come in. If you need assistance with relays on your next project, reach out to us today. We’re here to help.