Protective relays represent a sophisticated domain within electrical engineering and contracting, often perceived as daunting. Yet, they don't have to be! This three-part series aims to introduce the basics of relaying to those in the solar and energy storage industries who aren't engineers.
Part 1: What Are Relays, CTs, & PTs?
Part 2: ANSI/IEEE Relay Device Numbers (available below)
Part 3: What Does SEL Stand For?
Relays are designed using standardized device numbers to describe their functions. Instead of descriptive phrases, we use numerical codes to outline a relay's role. These standards are outlined in the ANSI/IEEE C37.2 document.
Why Use Numbers Instead of Words?
Efficiency - They streamline the process of creating wiring diagrams and communication. For instance, saying "59N" is far quicker than "Over Voltage on the Neutral."
Standardization - All parties involved—from utilities to vendors—immediately recognize the intended functionality, minimizing errors.
Compactness - Given the multifunctional nature of relays, using numbers simplifies drawings. Consider a relay with "phase overvoltage & undervoltage, phase over frequency & under frequency, ground inverse time overcurrent, and alarm" functionalities. Writing out the full description takes up significantly more space than simply listing the corresponding numbers.
[Image description: A table showing various ANSI/IEEE relay numbers]
Attention Engineers!
If you're passionate about technical subjects like these, Pure Power could be a great fit for you. Our team of 80 engineers designs over 2,000 commercial and industrial (C&I) and utility-scale solar projects annually. Being surrounded by such expertise can significantly boost your career trajectory.
Commonly Used Functions in Solar and Energy Storage Systems:
| Number | Name | Description |
|--------|-----------------------|-----------------------------------------------------------------------------------------------|
| 25 | Synchronizing Clock | Compares utility and solar circuit voltages, frequencies, and phase angles to allow parallel connection if aligned. |
| 27 | Undervoltage | Activates when voltage drops below a preset threshold. |
| 32 | Directional Power | Triggers when power flow exceeds a specified limit in a given direction. |
| 49 | Transformer Thermal | Activates when winding temperature surpasses a set value. |
| 50 | Instantaneous Overcurrent | Trips when current exceeds a defined limit. |
| 51 | Inverse-Time Overcurrent | Trips when current exceeds a value for a set duration. |
| 52 | Circuit Breaker | Opens circuits; 52R indicates it can also reclose circuits. |
| 59 | Overvoltage | Activates when voltage exceeds a set threshold. |
| 74 | Alarm | Generates a visual, audible, or data alert. |
| 79 | AC Reclosing | Manages the reclosing or disabling of AC circuit interrupters. |
| 81 | Frequency | Triggers when frequency falls outside the acceptable range. |
| 86 | Lockout | Halts operation until manually reset. |
| 87 | Differential Protective | Activates upon detecting a discrepancy between two measured currents. |
| 89 | Line Switch | Refers to devices like disconnect switches, typically used only when electrical accessories are present. |
Additional Letters After Numbers:
These specify the exact nature of the function:
- R: Reclosing capability (e.g., 52R is a circuit breaker capable of opening and reclosing circuits).
- P: Phase-specific function (often implied unless stated otherwise).
- N: Neutral-focused function (e.g., 51N monitors neutral for unbalanced overcurrent).
- G: Ground-specific function (e.g., 51G monitors ground bonding for ground faults).
Setting Thresholds:
Beyond just identifying functions, setting minimum and/or maximum thresholds is crucial. These are determined by engineers and usually project-specific.
In summary, while the idea behind relay device numbers seems straightforward, it can quickly grow complex. Developers and project managers don’t need to delve into the technicalities to fulfill their roles. That’s where we come in—experienced engineers at Pure Power. Should you require assistance with the relays on your project, feel free to explore more options or contact us directly.
As someone who appreciates detailed technical content, consider joining our team. We’re always looking for talented individuals to contribute to our dynamic environment. Check out our open positions now!
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