# UL 521 — Which OSHA NRTL is recognized for which test standard: all 21 labs' scopes of recognition, cross-referenced

For UL 521, laboratory is FM Approvals; standard title is Heat Detectors for Fire Protective Signaling Systems, verified against its source on 2026-08-10.

- **Laboratory:** FM Approvals _(verified: appears in the quote below)_
- **Standard number:** UL 521 _(verified: appears in the quote below)_
- **Standard title:** Heat Detectors for Fire Protective Signaling Systems _(verified: appears in the quote below)_

## What the source says

> UL 521 | Heat Detectors for Fire Protective Signaling Systems |

## Source

- https://www.osha.gov/nationally-recognized-testing-laboratory-program/fm

Last verified: 2026-08-10. Review by: 2026-12-08.
Part of [Which OSHA NRTL is recognized for which test standard: all 21 labs' scopes of recognition, cross-referenced](https://referencesource.org/nrtl-recognition-scopes/).