1. 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 actuator

How It Functions
During 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 energy
Actuator automatically moves to safe position (open or close) ->No controller command required -> Works even if BAS is down

When to Use SR (Fail-Safe)
Use SR wherever safety or equipment protection is critical:
1. Smoke/fire dampers
2. Outside air damper (freeze protection)
3. Relief/exhaust safety dampers
4. Generator room ventilation
5. Critical AHU isolation
6. Safety shutoff valves

Why It Is Necessary
Because in real buildings:
1. Power failures Drop happen
2. BAS controllers can fail
3. Network communication can drop
4. 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 actuator

How It Functions
During normal operation -> BAS sends signal -> Motor positions damper/valve -> Position maintained electrically -> During power failure
No spring available -> Actuator stops immediately -> Device stays in last position -> No automatic safety movement

When to Use NSR
Best for comfort and non-critical control loops:
1. VAV box dampers
2. Zone control valves
3. Comfort AHU mixing dampers (non-critical)
4. Secondary CHW/HW valves
5. Energy optimization loops

Why NSR Is Used
Because it offers:
1. Lower cost
2. Lower power consumption
3. Smaller size
4. Less mechanical wear
5. 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-capacitor
3. Battery backup
Important: All SR are fail-safe, but not all fail-safe are spring type.

4. Non-Fail-Safe Actuator
A 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.