Got an Air Handling Unit (AHU) that needs a fan-safety shutdown? Need to isolate field devices from your building automation controller? Or maybe your middle name is Alan? If any of these sound familiar, you’re in the right place. (And if all three are true…well, our mind-reading technology is definitely working.)
In this blog, we’re diving into how to safely and effectively isolate control signals and protect your equipment—because when it comes to building automation, proper isolation isn’t just best practice, it’s essential for safety, reliability, and code compliance. We’ll break down the concepts, show you how it’s done, and point you toward the right tools to make it simple.
At Functional Devices, we specialize in creating products that take the complexity out of building automation. From relays and current sensors to power supplies and RIB combos, our solutions are designed to save you time, space, and headaches on the job site. If you’re ready to streamline your next project, contact our team. We’re here to help you build smarter, safer systems.
How RIB Relays Simplify Fan Safety Circuits
Fan safety shutdown circuits typically require monitoring fan operation and responding to failure conditions. RIB relays provide:
- Isolated contacts. Safely interface low-voltage controls with high-voltage devices.
- Flexible inputs. AC or DC field devices can trigger alarms or shutdown signals.
- Field-adjustable settings. Thresholds and trip points can be configured to match fan motor specs.
- LED indicators. Visual confirmation of relay status for faster troubleshooting.
By isolating the control signal from the fan motor circuit, RIB relays reduce risk to your building automation system and prevent false trips caused by voltage fluctuations or transient loads.
Typical Wiring Configurations
There are multiple ways to configure a fan-safety shutdown using RIB relays:
- Single Fan Shutdown. A single relay monitors one fan motor and triggers an alarm or cuts power if the motor fails.
- Multiple Fans/Branch Shutdown. One relay can monitor multiple motors or inputs using parallel contacts, allowing for coordinated safety responses.
- Remote Monitoring Integration. Relay contacts can feed back into a BMS or PLC, enabling alerts, logging, and automated sequences.
Wiring diagrams should always ensure that the relay’s coil voltage matches the control signal, and the contact ratings match the motor load. RIB relays support a variety of input and output voltages, making them ideal for retrofits or new installations.
Now let’s dig into the details of one of our most popular models.
RIBMNLB
The first product in Functional Devices’ line of Fan Safety Alarm Circuits is the RIBMNLB. RIBMNLB combines all the relay logic to facilitate fan status, fan safety control, and damper actuator control. It is intended for use in a circuit that will control fan start/stop and fan safety shut-down circuit monitors three critical inputs:
- Low-limit freeze protection (to stop fan and remove power from damper actuator)
- Static pressure (to monitor for hi/low pressure condition)
- Smoke detector/fire alarm
Dry Contact Inputs
|
Isolated Dry Contact Outputs
|
|||||
|---|---|---|---|---|---|---|
| Low Limit | Static Pressure | Smoke Detector/FA | Master Relay | Low Limit | Static Pressure | Smoke Detector/FA |
| CLOSED | CLOSED | CLOSED | CLOSED | CLOSED | CLOSED | CLOSED |
| OPEN | x | x | OPEN | OPEN | x | x |
| x | OPEN | x | OPEN | x | OPEN | x |
| x | x | OPEN | OPEN | x | x | OPEN |
| OPEN | OPEN | OPEN | OPEN | OPEN | OPEN | OPEN |
Other Fan Safety Alarm Circuits
We offer a range of other Fan Safety Alarm Circuits to meet your needs. Only have two inputs? Take a look at the RIBMNLB-2. Have 6 normally open inputs? Take a look at RIBMNLB-6NO. Need BACnet capabilities? Take a look at RIBMNWLB-7-BC. While all these devices offer similar functionality, having a device that's sized correctly and has the correct logic directly out of the box can save on valuable space and time. If you're interested, be sure to check out the linked datasheets to see all the extra features such as latched inputs and 24V sourced outputs.
Best Practices for Installation
These best practices ensure the system responds promptly in the event of fan failure and prevents damage to connected equipment:
- Mount relays in accessible panels for maintenance.
- Verify load voltage and current ratings before connecting fan circuits.
- Use shielded wiring if the fan’s environment has significant electrical noise.
- Confirm the relay’s response time matches your safety requirements.
Updated for 2025: Enhancements in Fan Safety Circuits
Modern AHUs often include VFDs, variable-speed fans, and remote sensors. RIB relays remain compatible, but consider:
- Integration with DC and AC signals. New building automation controllers may output DC signals that still reliably trigger standard RIB relays.
- Expanded monitoring. Combining RIB relays with current sensors provides both status monitoring and load protection.
- Smart BMS feedback. Digital outputs from RIB relays can now feed directly into AI-enabled energy management systems for predictive maintenance.
These updates make RIB relays a future-proof solution for safe, reliable fan operation in complex facilities.
FAQ About Fan Safety Alarm Circuits
Q: Can a single RIB relay monitor multiple fans?
A: Yes. Relays can be wired in parallel for multiple fan motors, provided total load and contact ratings are not exceeded.
Q: What is the fastest response time I can expect from a RIB relay in a fan-safety circuit?
A: Typical response times are milliseconds, but consult the datasheet for your model to ensure it meets your motor protection requirements.
Q: Can I use RIB relays with VFD-controlled fans?
A: Yes. RIB relays isolate control signals and can be triggered by VFD outputs, provided voltage and current specifications match the relay’s coil ratings.
Q: Are RIB relays suitable for both AC and DC inputs?
A: Most RIB relays are compatible with either AC or DC control voltages. Confirm coil voltage and polarity requirements in the datasheet for your specific model.
Protect AHUs with Functional Devices
Whether you’re protecting your AHU, isolating field devices, or just confirming your middle name is Alan, the key takeaway is the same: proper isolation keeps your systems safe, reliable, and code-compliant. Using the right relays and accessories can transform a tricky install into a straightforward solution—no guesswork required.
Functional Devices is here to make that process even easier. With decades of experience and a lineup of RIB relays, power packs, and control solutions, we’ve got the tools and expertise to help you get it right the first time. Explore our fan safety alarm circuits or reach out to our team today; we’re ready to help you power, protect, and perfect your next project.