February 28, 2026
In Building Automation Systems (typically 24 VAC/DC extra-low voltage), actuator selection is not just about torque - it is about what happens when power fails.
SR Actuator (Spring Return / Fail-Safe)A Spring Return (SR) actuator is a motorized actuator that contains an internal spring mechanism that automatically drives the damper or valve to a predetermined safe position when power is removed.Also called: Mechanical Fail-Safe actuatorHow It FunctionsDuring normal operation -> BAS sends control signal (0-10 V / floating / 2-position) ->Motor drives actuator to commanded position ->Internal spring is compressed (energy stored)During power failure ->Supply voltage lost ->Spring releases stored energyActuator automatically moves to safe position (open or close) ->No controller command required -> Works even if BAS is downWhen to Use SR (Fail-Safe)Use SR wherever safety or equipment protection is critical:1. Smoke/fire dampers2. Outside air damper (freeze protection)3. Relief/exhaust safety dampers4. Generator room ventilation5. Critical AHU isolation6. Safety shutoff valvesWhy It Is Necessary Because in real buildings:1. Power failures Drop happen2. BAS controllers can fail3. Network communication can drop4. SR ensures the system moves to a known safe state automatically.2. NSR Actuator (Non-Spring Return / Non-Fail-Safe)A Non-Spring Return (NSR) actuator is a motorized actuator that has no spring mechanism and therefore remains in its last commanded position when power is lost.Also called: Non-Fail-Safe actuatorHow It FunctionsDuring normal operation -> BAS sends signal -> Motor positions damper/valve -> Position maintained electrically -> During power failureNo spring available -> Actuator stops immediately -> Device stays in last position -> No automatic safety movementWhen to Use NSRBest for comfort and non-critical control loops:1. VAV box dampers2. Zone control valves3. Comfort AHU mixing dampers (non-critical)4. Secondary CHW/HW valves5. Energy optimization loopsWhy NSR Is UsedBecause it offers:1. Lower cost2. Lower power consumption3. Smaller size4. Less mechanical wear5. Suitable for high-quantity devices (like VAVs)3. Fail-Safe Actuator A Fail-Safe actuator is any actuator that automatically moves to a predetermined safe position upon power loss.It can be implemented by:1. Mechanical spring (SR)2. Super-capacitor3. Battery backupImportant: All SR are fail-safe, but not all fail-safe are spring type.4. Non-Fail-Safe ActuatorA Non-Fail-Safe actuator is an actuator that does not automatically reposition when power is lost and remains at its last position. Equivalent to NSR in most HVAC applications.
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