What Is a Fusible Link?

A fusible link is a mechanical fire safety component that melts at a specific temperature to activate a protective action such as closing a fire damper or releasing a suppression system.

Author: Fusible Links Engineering Team  |  Published: 16 October 2025

Contents

  1. Definition
  2. Purpose in fire protection
  3. How a fusible link works
  4. Materials and temperature ratings
  5. Standards and certification
  6. Advantages
  7. Maintenance and replacement
  8. Frequently asked questions
  9. References

1. Definition

A fusible link is a heat‑responsive mechanical device designed to melt or separate at a predetermined temperature. When the thermal alloy reaches its rated melting point, the link releases a connected mechanism—such as a damper blade set, valve, or door—so that the protective action can occur.

Key point: Fusible links operate without electrical power. They provide a predictable, fail‑safe response based solely on heat.

2. Purpose in Fire Protection

The core function of a fusible link is to automatically activate fire safety equipment once ambient temperature exceeds a critical threshold.

  • Fire dampers and HVAC systems — closing blades to compartmentalise smoke and flame.
  • Fire doors and shutters — releasing closures to contain fire spread.
  • Fire suppression systems — triggering discharge mechanisms.
  • Industrial ovens and equipment — acting as thermal cut‑offs.
  • Sprinkler and deluge valves — enabling water or foam release (in certain assemblies).

3. How a Fusible Link Works

Most links comprise two metal elements joined by a low‑melting‑point alloy. The assembly is often held under slight tension by a spring or by the load of the closure system. When the surrounding temperature reaches the rated value—for example, 74 °C (165 °F)—the alloy melts, the bond separates, and the stored energy closes or releases the mechanism.

Example: In a fire damper, the link holds blades open under spring tension. At the rated temperature the link separates and the spring snaps the blades shut, preventing smoke and fire from travelling through ductwork.

A fusible link installed in a fire damper mechanism with blades held open by spring tension.
In-situ example: link maintaining the open position of damper blades until activation.

4. Materials and Temperature Ratings

Links use alloys with precisely controlled melting points. Common nominal ratings and typical uses include:

Nominal temperature (°C) Typical use Colour code
57 °CLow‑temperature environmentsBlue
68 °CStandard fire protectionRed
74 °CHVAC fire dampersSilver
100 °CIndustrial equipmentGreen
141 °CHigh‑heat applicationsOrange

Colour codes vary by manufacturer. Always follow the product’s certification and datasheet.

5. Standards and Certification

Fusible links used for life‑safety applications should comply with recognised standards and approvals, for example:

  • UL 33 — Heat‑Responsive Links for Fire‑Protection Service
  • FM 1121 — Factory Mutual Approval Standard for fusible links
  • NFPA 80 / NFPA 90A — Code requirements for opening protectives and HVAC systems
  • ISO 10294‑4 — Fire dampers: test methods and performance criteria
  • EN standards — European norms relevant to the associated system or device

Compliance note: Always verify the link’s approval (e.g., UL Listed or FM Approved) and ensure the temperature rating matches the environment and system specification.

6. Advantages

  • No power required — purely mechanical operation.
  • Fail‑safe behaviour under fire and power‑loss conditions.
  • Predictable activation at the specified temperature rating.
  • Cost‑effective and easy to integrate across systems.
  • Broad compatibility with dampers, doors, and suppression mechanisms.

7. Maintenance and Replacement

  • Inspect for corrosion, contamination, or physical damage.
  • Confirm the installed rating suits the local environment.
  • Replace damaged, contaminated, or aged links per manufacturer guidance.
  • Do not paint or coat links; coatings can impede heat transfer.

Typical practice is to inspect at least annually, or per the maintenance schedule of the connected system.

8. Frequently Asked Questions

What temperature does a fusible link melt at?

Common ratings range from 57 °C to 141 °C. The correct temperature depends on the system and environment.

Can a fusible link be reused after activation?

No. After operation the link must be replaced with a certified new part.

How often should fusible links be inspected?

At least annually, and immediately after any activation, in line with applicable codes and the system manufacturer’s instructions.

9. References

  • UL 33 — Heat‑Responsive Links for Fire‑Protection Service
  • FM 1121 — Factory Mutual Approval Standard for fusible links
  • NFPA 80 / NFPA 90A — U.S. codes for opening protectives and HVAC
  • ISO 10294‑4 — Fire dampers test methods and performance

Link these references to your licensed copies or official pages where appropriate.

Reviewed by: Certified Fire Protection Engineer (add name/credentials). Last updated 16 October 2025.