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Current Sensing Relay (CSR) in HVAC & BMS – Can It Detect Belt Breaks or Shaft Jams

September 11, 2025

Here’s the truth 👇
🔹 Belt Broken (Fan Blower)
(**) Possible with adjustable CSR
Measure normal fan motor running current (e.g., 3.2 A).
Set CSR threshold slightly lower (say 2.0–2.5 A).
If belt snaps, motor runs unloaded (~1.0 A) → CSR drops out → BMS raises “Fan Fail” alarm.

🔹 Shaft Jam / Stuck
(**) Not suitable for CSR.
Jam causes current to rise (locked rotor condition).
CSR only checks ON/OFF against a setpoint, not “too high current.”
Correct detection = Thermal overload relay or VFD fault feedback.

🔹 Models Commonly Used in BAS
Veris H908 / H909 (adjustable, split-core)
Functional Devices (RIBXGTF)

💡 Rule of Thumb:
CSR → Motor proof + belt detection (adjustable type only).
Overload relay / VFD → Shaft jam, overload, protection.

(*) In BMS, using the right feedback device ensures reliable alarms, reduced downtime, and smarter controls.

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