UL 991 PDF

1 Scope

1.1 These requirements apply to controls that employ solid-state devices and are intended for specified safety-related protective functions.

1.2 These requirements address the potential risks unique to the electronic nature of a control. Equipment or components employing an electronic feature shall also comply with the basic construction and performance requirements contained in the applicable end-product or component standard. These requirements are intended to supplement applicable end-product or component standards and are not intended to serve as the sole basis for investigating the risks of fire, electric shock, or injury to persons associated with a control.

1.3 These requirements do not cover controls covered by end-product standards in which an electronic control investigation is specified.

1.4 Sections 9 – 22 contain standardized test methods for investigating the performance of an electronic control when subjected to particular environmental stresses. The suitability of each test to a given control shall be determined by the end-product standard(s). Determination shall include an assessment of:

1. a) Whether the control will be exposed to a particular environmental stress in its application, and

2. b) Whether the response of the control to a particular environmental stress is relevant to its intended safety-related protective function in its application.

Summary of Topics

This revision of UL 991 dated December 27, 2024 includes the following changes in requirements:

– Clarification of requirements if no critical components are found in a failure mode and effects analysis: Figure 6.1, 6.3, and 23.4.

– Alternate test methods for the environmental stress tests: 11.1.1, 12.1, 13.1, 14.1.1, 15.1.1 – 15.1.4, 15.1.1A, 15.2.1A, 15.2.2A, 15.2.3A, 15.2.4A.