Update on NPFA 660 Standard for Combustible Dusts and Particulate Solids

The “current” NFPA 660 Standard for Combustible Dusts and Particulate Solids is a proposed standard that consolidates the 61, 484, 652, 654, 655, and 664 documents into a single standard.

November 15, 2023

11 Min Read
Jack Osborn
Jack E. Osborn, senior project engineer, Airdusco EDSImage courtesy of Airdusco EDS LLC

The current NFPA combustible dusts standards—61 (Food), 484 (Metals), 652 (Fundamentals), 654 (Chemical), 655 (Sulphur), and 664 (Wood)) remain in effect and should be used for interpreting current appropriate combustible dust hazards and related situations. Except for NFPA 652, each of these standards is considered a “commodity-specific or industry-specific” standard. NFPA 652 is still the fundamentals (of combustible dust) standard. NFPA 91 (Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids) will not be affected by the proposed 660 standard and will continue as an independent source.

The “current” NFPA 660 Standard for Combustible Dusts and Particulate Solids is a proposed standard that consolidates the 61, 484, 652, 654, 655, and 664 documents into a single standard. Until the proposed 660 standard is approved by the members of the NPFA it does not replace the current, individual combustible dust standards.  

Second Draft of NFPA 660

The proposed 660 standard is in its “second draft” stage and is open for public comments through January 4, 2024. After that date there will be committee meetings of each of the existing combustible dust standards to review the second draft public comments, Correlating Committee inputs, and Task Group reports. Upon completion of each of those five meetings (NFPA 655 is part of the 654 committee) a revised “chapter” (formerly a separate standard) with annex material will be presented to the Correlating Committee for a final review. After the review of the Correlating Committee the final draft of the proposed 660 standard will be subject to NITMAMs (Notice of Intent to Make a Motion) for revision (or even rejection) of the proposed standard. NITMAMs are also subject to public comment.

At the membership yearly meeting the NITMAMs regarding the proposed 660 standard will be voted on to determine if the membership wants to revise or reject the proposed standard. Certain actions may require a new, complete, cycle of the proposed standard (further delaying it for 3+ years – at least). Or it may be completely rejected, resulting in the existing combustible dust standards continuing as they are (they would then go through a revision cycle).

However, considering historical evidence, it is not likely the proposed consolidation standard will be rejected by the membership. If approved by the membership (with or without changes) the earliest projected effective date would be fall of 2025.

Basic Configuration of Proposed Standard

The basic configuration of the proposed standard is established (minor changes are possible) and described as follows:

(1) Chapters 1 through 9 cover the “fundamentals” of combustible dusts and particulate solids. Similar to what NFPA 652 presently covers and these chapters are the responsibility of the current 652 committee (with input from the other commodity-specific committees and Correlating Committee).

(2) Chapter 10 is reserved for an unknown possible future set of requirements (possible examples would Additive Manufacturing or Nano-Materials).

(3) Chapter 11 is for Agri and Food (formerly NFPA 61).

(4) Chapter 12 is for Combustible Metals (formerly NFPA 484).

(5) Chapter 13 is for Sulphur (formerly NFPA 655).

(6) Chapter 14 is for Wood (formerly NFPA 664).

(7) Chapter 15 is for “Combustible Dusts and Particulate Solids Not Otherwise Specified (formerly NFPA 654).

(8) The remaining chapters are for the Annexes:

a. Comprised of annex A through Z (Yes, 26 of them).

b. Annex A contains the annex material that was formerly in the original combustible dust standards, but revised and improved.

c. The remaining annexes include explanatory information that should be considered by the user of the standard. The information provided will assist the user in determining the best solutions and pathway for addressing existing combustible dust hazards.

More Than Just a Consolidation

An appropriate question at this time would be: What is such a big deal about consolidating six NFPA standards into a single standard? It is actually much more than a mere consolidation. Consider the following:

(1) This proposed standard will not exist in the future unless each of the six committees associated with their currently existing standards are allowed to retain their independent status. That is the reason for the chapters and their content.

(2) The proposed consolidation fully accepts that there will always be differences in the combustible dust hazards of wood verses food and agri products verses combustible metals, etc. The unique combustible dust hazards of each must be addressed now and into the future.

(3) The fundamentals (Chapters 1 through 9) must truly be “fundamental”. There has been a very strong effort by the members of all the committees to ensure the “fundamentals” chapters (1 through 9) provide only truly fundamental information. cover the “fundamentals” of combustible dusts and particulate solids.

(4) All the committees recognized that the user of this new standard requires as concise and understandable information as possible. As a result, significant improvements and clarifications have been provided in the content of the standard.

The following provides a description of the content and a limited summary of significant changes for each of the chapters (exclusive of the Annex information):

Chapter One is the Administration chapter. This chapter explains the purpose of the overall document and the individual chapters. It also includes typical “boilerplate” requirements regarding scope, liability, etc.

Chapter Two provides the listing of the references used in the standard. Considering the scope of the document, this listing is lengthy. The listing allows the user to further consult references used in the standard.

Chapter Three provides the list of fundamental definitions. This list is extensive and has been significantly improved with Annex information which clarifies the use in the various parts of the standard. A major effort has been made to ensure the user understands the defined terms used in the standard. It is important to note that each commodity-specific chapter also includes definitions that are unique to the chapter.

Chapter Four describes the overall Objectives of the standard. This has not changed significantly from the Objectives chapter in the current 652 standard but does include additional information to ensure coverage of the scope of the entire document.

Chapter Five is Hazard Identification. It is the author’s personal opinion that this chapter should be renamed “Identification of Material Hazards”. These requirements are definitely fundamental. One cannot determine the hazards and level of risks involved without a full understanding of the characteristics of the combustible dusts involved. This chapter covers the methods to determine those characteristics.

Chapter Six on the Performance Design option is basically unchanged (some clarifications). This chapter remains a viable option when no prescriptive solution is offered to a combustible dust or particle hazard.

Chapter Seven, Dust Hazards Analysis (DHA), describes the method to analyze your combustible dust and particle hazards at your facility and provides a plan to eliminate and/or mitigate those hazards. If you, as the reader, have an existing facility that hands and processes combustible dusts and have not performed the required DHA you must do so as soon as possible. This includes new and existing facilities and processes (for all commodity-specific industries). The author is personally and directly familiar with criminal (Federal Department of Justice) convictions of plant management personnel for ignoring existing combustible dust hazards that would be addressed by a properly completed DHA. This is no worthless requirement.

Chapter Eight, Management Systems, is another section the author would wish to rename to “Methods for Managing Hazards”. In general, this section covers methods that can be used to either eliminate or manage combustible dust hazards which do not involve physical changes. For example, this would be Management of Change (MOC), Emergency Planning, PPE, Housekeeping, Record Keeping, Training, etc.

If common sense is used, this fundamental section should be the epitome of common sense where a facility is handling and processing combustible dusts and particles. Explosions occur constantly, and the vast majority are avoidable, often by utilizing proper housekeeping. Explosions cannot occur if there is insufficient fuel. Plus, most major property damage and personal injury and deaths occur due to secondary explosions that are directly related to the unnecessary presence of combustible dust in the facility due to improper housekeeping practices. Proper housekeeping (and dust collection) represents a basic, simple, method to eliminate the fuel-hazard is missing from those catastrophes (e.g., Imperial Sugar, Didion, etc.). An employee with a broom and waste-pan or use of a centralized vacuum cleaning system is a powerful tool for explosion mitigation and control.

A facility that meets (and exceeds) the requirements of this chapter will go a long way to eliminating and/or controlling the hazards associated with combustible dusts and particles. Additionally, these are the least costly of the prescriptive methods for handling combustible dust and particle explosion, flashfire, and fire hazards.

Chapter Nine, Hazard Management: Mitigation and Prevention, is the chapter with the most changes associated with NFPA 660. This chapter basically covers the buildings and processes that are fundamental to facilities handling combustible dusts and particles. However, this is also an area where the inherent differences in the combustible dust exposures of food verses metals, verses wood, verses chemical, etc., come to the forefront.

Examples are plentiful. When considering mechanical feeding devices (e.g., screw feeders, belt feeders, vibrating feeders), bucket elevators, mixers/blenders, and conveyors the chapter refers you to the appropriate commodity-specific chapter. This is due to the inherent differences in bulk handing systems for the various commodities. Wood products do not behave like metals, etc. The commodity-specific chapters should be consulted for these devices.

Dust collection, however, is universally a requirement for all the commodity-specific combustible dusts and particles. Chapter 9 contains very important design and use information on these systems and should be provided for all such systems. However, it is vitally important to also consult with the same section areas of the commodity-specific chapters as there are many unique requirements that must be considered. An example would be metal dusts which represent unique problems when used with the wrong dust collection equipment. Such systems must include the requirements of the metals chapter.

Chapter 10 has been reserved--for an unknown requirement.

Chapters 11 through 15 cover the commodity-specific (61, 484, 655, 664, and 654) requirements. Each chapter will include administration, reference, definitions, objectives, etc., that are specific to that industry. Each of those are now “sections” instead of “chapters”. Each of these chapters includes information on the unique combustible dust requirements associated with their specific commodity.

Chapter 11 concerns the Agricultural and Food Processing commodity-specific industries. There are unique additions and modifications to the fundamental chapters provided. Examples are the ingredient transfer system and point-of-use dust collector equipment systems.  

Chapter 12 concerns Combustible Metals. This chapter must be consulted when combustible metals are involved. There are too many exposures that are not typical for other combustible dusts and particles that are applicable when metals are involved. Very small quantities of combustible metal dust can lead to severe explosions, flashfire, and fire hazards. There are many unique hazards that must be considered with combustible metals. Importantly, this chapter contains significant information on the safe practices for additive (i.e., 3-D) manufacturing.

Chapter 13 addresses the 655 standard on sulfur. This is a spinoff from NFPA 654. Sulfur is a highly combustible dust and solid due to its very low MIE (Minimum Ignition Energy) and MEC (Minimum Explosive Concentration). The author is aware of multiple explosions and injuries associated with this material. This chapter should always be consulted when sulfur, in any form, is present.

Chapter 14 concerns Wood Processing and Woodworking Facilities. This ranges from sawmills to furniture makers, etc. Wood is a major fire and explosion hazard that has unique problems and equipment systems that present specific combustible dust and particulate hazards (e.g., planers, hogs, saws, etc.).

Chapter 15 concerns “Combustible Dusts and Particulate Solids Not Otherwise Specified”. A laborious title, but properly descriptive as NFPA 654 used to be the “fundamental” chapter for any material not otherwise covered in the other standards. This is the NFPA 654 chapter. The original NFPA 652 borrowed heavily from NPFA 654. As a result, much of the nine fundamentals chapters have their basis from the original NFPA 654 standard. However, there is useful information on Additive Manufacturing, chemically unique hazards, etc.

The remaining 26 chapters are annexes. Annex A covers all of the same information, and more, of the original combustible dust standards. For example, if the user is looking for information on agri and food processing information from chapter 11 the annex source would be A11.xxx. The user is strongly advised to refer to Annex A when an asterisk (*) is provided in chapter 1 through 15 information. This explanatory information can keep the user from misunderstanding the scope and goal of the standard requirements.

The multiple additional annexes should be reviewed by the user to ensure that all appropriate sources are considered.

The remainder covers Annexes A through Z (literally). This is often an ignored part of the original standards and should be “required reading” when considering explosion, flashfire, and fire hazards. Annexes do not contain “required” information but do contain valuable and additional information that more fully explains what is required. The annex information often answers the questions generated by the previous requirements.


NFPA 660 is most likely to come into existence in late 2024, but perhaps later. There are other bridges to cross regarding the contents, configuration, and even its ultimate acceptance, but this information does represent the current status.

Jack E. Osborn, senior project engineer, Airdusco EDS. He has more than 45 years of experience in the concept, design, project management, start-up, and operations/maintenance for an extensive range of mechanical equipment systems, including dust collection, ventilation, bulk handling mechanical systems (pneumatic and mechanical), storage, weighing, mixing, batching, etc. He is also a member of all six NFPA committees on combustible dusts (61, 484, 652, 654, 664, and Correlating).

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