NFPA 660 and Your Round Separator: Bonding, Grounding and the Flex-Connector Gap

A round vibratory separator is a steel or stainless machine, so it is easy to assume it is grounded. Look at how it is built, though. It sits on springs or mounts, it is connected to the spouts above and below by rubber or polyurethane sleeves, and its screens are clamped between rubber gaskets. Any of those can leave a metal part electrically isolated, with nowhere for static charge to go. Screening a combustible powder makes that a dust-ignition question. This guide covers vibratory separator grounding under the new consolidated standard, NFPA 660: what the standard means for a separator, where the isolation points are, and how to bond, ground and test the machine after maintenance.

This article is about static and dust ignition. For flameproof and Zone-rated machine construction, see ATEX and explosion-proof screeners. For the full list of separator hazards, from guarding to noise, see vibratory screening safety.

Why a screener builds static

NFPA 77 names sieving as one of the ways powder picks up charge, along with pouring, grinding, sliding and pneumatic conveying. Charge separates whenever powder moves across another surface, and a vibrating screen is a large surface with constant contact. In its pneumatic-conveying section, NFPA 77 says pipes and ducts "should be conductive (e.g., metal) or static dissipative and be grounded." The same principle applies to the equipment they connect to.

A safety bulletin from the Center for Chemical Process Safety puts the danger simply: "Generally, static has to accumulate on an ungrounded conductor (usually metal)." A grounded stainless screen frame lets the charge bleed away. A stainless frame isolated behind a rubber gasket can store it and release it as a spark.

Static also has a production cost. One OEM's technical note says that an isolated screen retards throughput, and static contributes to blinding and agglomeration. If a fine powder keeps blinding the screen and humidity changes make it better or worse, static may be part of the cause. Deblinding devices are covered in how deblinding systems work.

The three places a separator gets isolated

Walk around the machine and look for every place where metal meets something that isn't metal. On a typical round separator you'll find three.

1. The flex connectors at the inlet and outlets

Flex connectors exist so the vibrating body doesn't shake the fixed spouts. They are usually natural rubber, EPDM, neoprene, silicone, rubber-coated fabric or polyurethane. Standard rubber and polyurethane connectors are typically insulating unless the material is specified as conductive or static-dissipative, so check the spec. A non-conductive sleeve between the grounded spout and the separator body breaks the metal path at every inlet and outlet. OSHA's combustible-dust enforcement directive says non-conductive gaskets, seals and sleeves between pneumatic conveying duct sections "require appropriate use of grounding clips and jumper cables." That language is written for ductwork, but the same logic applies here.

2. The screen and frame gaskets

Kason's technical note on screen static is direct about it: "In a normal assembly, a rubber gasket isolates the pretension screen." The gasket keeps charge on the wire from draining to the frame. The fix it describes carries the electrical path from the tension ring around the outside of the gasket to the frame, which can then be grounded. Conductive gasket materials rated for food contact do exist; Sweco's aftermarket parts brochure lists one. If your screens sit in standard rubber gaskets, ask your screen supplier how the screen wire is bonded to the frame, or tell us your gasket and screen style and we'll tell you what we can supply.

Multi-deck machines add more of these joints. Each frame sits on a gasket under the clamp ring, so intermediate frames, the cover, and their discharge spouts and outlet connectors can all be isolated. We recommend treating each deck frame and the cover as its own conductor: each one needs a verified path to the base, checked frame-to-frame.

3. The spring supports or mounts

One OEM's technical note says that on circular machines with nonmetallic spring supports, the ground connection has to be made from the vibrating (free) body to ground, and in a hazardous location the stationary base should be grounded as well. If your machine sits on rubber mounts rather than steel coil springs, the vibrating body may have no metal path to the base at all. Steel springs don't settle it either: check whether the rubber or poly spring cups and locating boots they sit in isolate an otherwise steel spring. Measure it.

Bonding vs. grounding, and how to do both

The two words get used interchangeably, but they mean different things. The CCPS defines bonding as electrically connecting conductive objects to equalize their potential and prevent sparks. Grounding connects a conductive object to earth so the charge can dissipate. A separator needs both:

• Bond across every isolating part. Run a jumper from the separator body to the spout on the far side of each flex connector.
• Bond the screen wire to the frame where a rubber gasket isolates it, or use a gasket designed to conduct. On a multi-deck machine, carry the bond through each deck frame and the cover to the base.
• Ground the vibrating body. Run a dedicated conductor from the vibrating body, with slack for motion, to a building-steel ground point. In a classified area, ground the stationary base too.
• Don't rely on the motor's power ground. NFPA 77 recommends making static grounds to building steel rather than relying on power grounds or lightning protection. On a vibrating body, the motor cord also flexes constantly and isn't installed or inspected as a static bond, so run a dedicated static ground to building steel.

Installing the jumpers

• Use a flexible braided jumper with crimped lugs. A rigid strap that cracks after a few weeks of vibration is no good.
• Land each lug on bare metal with a star or serrated washer. On painted steel, scrape the landing point and recoat the joint once it's made up. On sanitary or washdown machines, don't drill product-contact surfaces; use an existing non-product-contact bolt or a ground tab, and pick a jumper material suited to washdown (stainless braid is one option for washdown).
• Leave enough slack for the machine's motion, and route the jumper away from pinch points.

Don't land the jumper on the band clamp over the rubber boot; the clamp sits on rubber and may itself be isolated. Land it on the spout and on bare metal on the separator collar.

One limitation: a jumper bonds the metal on either side of a rubber sleeve. It does nothing about charge on the sleeve itself. Whether your powder and connector call for a conductive or static-dissipative sleeve is a question for your Dust Hazards Analysis (DHA) team.

Re-checking bonding before restart

On a separator, the bonding connections are the ones maintenance disturbs most often. Screens come out, gaskets are replaced, clamp rings are loosened, and connector boots are swapped. Food and pharma lines are taken apart for washdown and clean-out, and sanitation crews often unhook a jumper during teardown to get a sleeve off and may not reconnect it. A powder-processing application note from a static-grounding equipment maker points out that this kind of disassembly is when bonding connections get left off or put back wrong. Vibration, flexing and corrosion wear them down between shutdowns.

What the readings mean:

• Metal bond path: no more than ~10 Ω copper / 25 Ω stainless. NFPA 77 says that range is typical in practice. Higher readings could indicate a lack of mechanical integrity, usually a loose connection or corrosion.
• Static-dissipative path: about 1 MΩ (10⁶ Ω) to ground is the commonly cited level for dissipating static from equipment. That is a dissipation level, not a pass mark for a metal jumper. Acceptance limits for a static-dissipative sleeve or gasket depend on the material, so take them from your DHA or the part's specification. If a braided strap reads in the thousands of ohms, something is wrong with it even though it is under a megohm.

How to run the check:

1. Zero (null) the meter leads.
2. Measure to a known ground reference: the building-steel ground lug the separator ground lands on.
3. Put the probes on bare metal on each side of each bond, not on paint, powder coat or product film. Probing a coating is a common cause of false high readings.
4. Measure body-to-ground and across each flex connector. On a multi-deck machine, also measure frame-to-frame and cover-to-frame, and from each screen's tension ring to its frame, where a gasket can isolate the screen.
5. Record each reading on the work order so drift shows up over time.
6. Have someone trained under your site's static/DHA program do the check and sign it off.

We couldn't find a published testing interval for separators. A sensible practice is to run the check after any work that disturbs those connections, washdown and clean-out included, and to put it on the restart checklist next to the clamp-ring check. If your separators were never bonded this way, do the retrofit at the next planned screen change, when the machine is already down and open.

What NFPA 660 changed, and what it didn't

NFPA 660, Standard for Combustible Dusts and Particulate Solids, 2025 edition, took effect on December 6, 2024. It replaced six separate standards: NFPA 61 (agricultural and food processing), 484 (combustible metals), 652 (combustible dust fundamentals), 654 (chemical, plastics and other particulate solids), 655 (sulfur) and 664 (wood processing).

The fundamentals that used to be in NFPA 652 are now Chapters 1 through 9, with a new Chapter 10 on emergency planning, as Processing magazine's January 2025 overview notes. The industry-specific rules are commodity chapters: 21 for agricultural and food, 22 for metals, 23 for sulfur, 24 for wood, and 25 for combustible dusts not covered elsewhere. A flour mill and a plastics compounder now read the same front half of the same book.

Powder & Bulk Solids, which followed the committee work, called it much more than a consolidation, with new definitions among the changes. What it did not absorb matters more here. NFPA 68 and 69 (explosion venting and prevention) and NFPA 77, the Recommended Practice on Static Electricity, are still separate documents. For the how-to of bonding and grounding, NFPA 77 (current edition 2024) is still the reference.

The DHA is still the core requirement under NFPA 660, and it has to be reviewed and revalidated at least every five years. Your DHA is the document that should already identify an isolated separator body. If it doesn't mention the screener's connectors, gaskets and mounts, raise that at the next revalidation.

Who actually enforces it

NFPA 660 is a consensus standard, not a federal regulation. It becomes enforceable when an authority having jurisdiction adopts it, usually through a state or local fire code. Insurers also write it into their requirements.

OSHA has no combustible-dust standard for general industry; its rulemaking was withdrawn in 2017. OSHA enforces combustible-dust hazards through its Combustible Dust National Emphasis Program (directive CPL 03-00-008, effective January 30, 2023), existing standards such as housekeeping and electrical, and the General Duty Clause. The directive says NFPA standards "shall not be cited as enforceable requirements" unless OSHA has incorporated them by reference, but treats them as evidence that a hazard is recognized and that a feasible fix exists. It predates NFPA 660, so it still lists 652, 654 and 61. The hazards haven't changed: it says ductwork should be "conductive, bonded, and grounded to dissipate static accumulation," and its inspection items include "static discharge potential due to lack of connectivity/bonding/grounding."

The other ignition sources on a screener

Static is one ignition source. A DHA for a screening step should cover the others too:

• Electrical rating. In a Class II (dust) location, motors and controls must be listed for the classification, Division 1 or Division 2. "Explosion-proof" is the Class I (gas and vapor) term; the Class II equivalent is dust-ignition-proof, and OSHA warns against confusing the two. In Division 2, the National Electrical Code also permits several totally enclosed motor types, such as TEFC with no external openings. Either way, the motor's surface temperature must stay below the ignition temperature of your dust, and no higher than 329°F for organic dusts that can dehydrate or carbonize. For motor problems, see vibratory motor problems and solutions.
• Tramp metal. OSHA's inspection list includes "tramp metal in feed or from upstream equipment," and its combustible-dust bulletin recommends separator devices to remove foreign material. Magnets or metal detection ahead of the screener are the usual catch for tramp metal in the feed. A torn screen adds its own: broken screen wire is tramp metal headed downstream, and austenitic stainless wire is only weakly magnetic, so magnets may miss it; a metal detector after the screener is a common check. See why round separator screens fail early.
• Dust release. Connectors and gaskets that leak put dust on surrounding surfaces, which is what turns a small event into a secondary explosion.

This article is general technical information, not a compliance determination. Your Dust Hazards Analysis, your authority having jurisdiction and your insurer set the requirements for your facility.

Frequently asked questions

Does NFPA 660 replace NFPA 652 and NFPA 654?

Yes. NFPA 660 (2025 edition), effective December 6, 2024, replaced NFPA 61, 484, 652, 654, 655 and 664. It did not replace NFPA 77, the recommended practice on static electricity, or NFPA 68 and 69.

Is NFPA 660 required by OSHA?

Not directly. OSHA has no general-industry combustible-dust standard. Its enforcement program says NFPA standards can't be cited as enforceable requirements, but it uses them as evidence that a hazard is recognized and that a feasible fix exists, under the General Duty Clause. NFPA 660 becomes enforceable when a local authority or an insurer adopts it.

Is a stainless vibratory separator automatically grounded?

No. The metal conducts, but rubber or polyurethane flex connectors, rubber screen gaskets and non-metallic mounts can isolate parts of the machine. Each isolating part needs a bond across it, and the body needs a ground.

Can I ground the separator through the motor's power cord?

It isn't recommended. NFPA 77 recommends static grounds to building steel rather than power grounds, and a motor cord flexing with the machine isn't installed or inspected as a static bond. Run a dedicated static ground instead.

What resistance reading should a bonding jumper have?

On an all-metal path, NFPA 77 says about 10 ohms or less for copper or 25 ohms for stainless is typical. A much higher reading suggests a loose or corroded connection. About 1 megohm is the generally accepted level for dissipating static, but that is a dissipation level, not a target for a metal jumper.

Do I need to re-check bonding after changing a screen?

Yes. Screen changes, gasket replacements, connector swaps and washdown teardowns are when bonding connections get missed. Measure across each connector and from the body to ground before restart.

Sources

• NFPA 660 effective date and scope; NFPA 652 fundamentals now Chapters 1 through 9, with a new Chapter 10 on emergency planning: Processing, "NFPA 660: What processors need to know", January 2025.
• Standards replaced by NFPA 660: Powder & Bulk Solids, "Ultimate guide to NFPA 660", March 2025.
• Chapter structure; NFPA 68, 69 and other supporting standards not consolidated: Fauske & Associates, "NFPA 660 and a Dust Hazard Analysis Case Study," Purdue P2SAC, Fall 2025.
• Substantive changes beyond consolidation (pre-publication coverage): Powder & Bulk Solids, "Update on NFPA 660", November 2023.
• Enforcement through the authority having jurisdiction: Processing, "NFPA 660: What every facility needs to know", August 2025.
• DHA revalidation every five years: ISHN, "Navigating NFPA 660", July 2025.
• OSHA enforcement, NFPA as evidence, ductwork bonding, jumper cables, inspection items: OSHA CPL 03-00-008, Revised Combustible Dust National Emphasis Program, effective January 30, 2023.
• Withdrawal of OSHA's combustible dust rulemaking: U.S. Chemical Safety Board, Rulemaking Watch.
• Bonding equipment to ground, separator devices for foreign material, Class II vs. Class I equipment: OSHA SHIB, Combustible Dust in Industry.
• NFPA 77 (2024) text on sieving as a charging mechanism, conductive or dissipative grounded pipes, ducts and equipment, resistance values and grounding to building steel, as quoted in a powder-processing application note and a grounding selection-criteria article.
• Definitions of bonding and grounding: CCPS Process Safety Beacon, December 2008. Static accumulating on ungrounded conductors: CCPS Process Safety Beacon, February 2021.
• Gasket isolation of the screen, grounding from the free body on non-metallic spring supports, static and throughput: Kason, "Overcoming Static Problems in your Vibratory Screen Separator," Screen Tips, June 1987, reprinted from Powder and Bulk Engineering.
• Conductive gasket rated for food contact (OEM aftermarket literature): Sweco, Separation Parts aftermarket brochure.
• Dust-ignition-proof terminology, Division 2 motor types, and surface temperature limits including 329°F for organic dusts: Powder & Bulk Solids, "Explosion-proof may not be the answer", 2014.
• Connector and gasket materials: ScreenerKing product pages, retrieved 2026-10-05.