What is the difference between fail-safe and non-fail-safe actuators?

Fail-Safe vs Non-Fail-Safe Actuators — Selection and Code Requirements

The decision between fail-safe and non-fail-safe actuators is not a preference — it is driven by life-safety code, system protection requirements, and the consequences of the actuator's failure position. Selecting a non-fail-safe actuator in an application that requires fail-safe operation is a code violation that may not be discovered until the system is commissioned or, worse, until an actual emergency.

### How Fail-Safe Mechanisms Work

Traditional spring-return actuators use a pre-tensioned steel spring that is wound during normal motor operation. When power is removed, the spring releases its stored energy and drives the actuator — and the connected damper or valve — to the predetermined safe position. The spring must be sized to overcome the maximum torque or force required at the fail position, including any pressure differential across the damper or valve. Modern spring-return designs from Belimo use brushless DC motor technology that reduces holding current (the power required to keep the spring wound) to 0.5–1.0 W, addressing the historical criticism that spring-return actuators were energy-inefficient.

Alternative fail-safe technologies include Belimo's SuperCap (capacitive energy storage that drives the actuator to safe position on power loss, eliminating the mechanical spring entirely) and battery-backup systems. Both approaches offer the advantage of not requiring continuous power to hold position, unlike spring-return designs.

Non-fail-safe actuators simply stop at their last position when power is removed — "fail-in-place". They are mechanically simpler, less expensive (typically 30–50% lower first cost), and do not draw holding current, but they provide no safety function on power loss.

### Code-Driven Fail-Safe Requirements

The applications requiring fail-safe actuation are clearly defined in building codes:

- **Smoke Control Dampers**: NFPA 92 and IBC Section 909 require all smoke control dampers to fail to their predetermined fire-safe position on power loss. This is a life-safety requirement with no exceptions.

**Freeze Protection**: Outdoor air dampers serving water coils in cold climates must fail closed on power loss to prevent coil freeze damage. This is required by the IMC and is standard practice in ASHRAE Climate Zones 4 and above.

**Stairwell Pressurisation**: Dampers in stairwell pressurisation systems must fail open (or closed, depending on the design strategy) to maintain tenable conditions for occupant egress.

**Elevator Hoistway Venting**: Dampers serving elevator hoistway smoke relief must fail to the open position on power loss per ASME A17.1.

### When Non-Fail-Safe Is Acceptable

Non-fail-safe actuators are appropriate where the failure position does not create a hazard. Typical applications include VAV terminal unit cooling dampers (fail-in-place is acceptable — the zone may drift warm but no safety hazard is created), general exhaust fans in non-smoke-control applications, and heating-only valves in systems where overheating is the failure mode (not freezing).