Most teams that handle combustible powders have the basics covered. The plant is bonded and earthed, the operators wear dissipative footwear, the metalwork all reads continuity back to a verified earth point. On paper, the static problem is solved. And then a silo ignites during a routine fill, and everyone stands around the incident report asking how, because everything was earthed.

I have seen versions of this conversation more than once. The honest answer is usually that the charge nobody managed was never on the metalwork in the first place. It was inside the powder.

This is the part of electrostatic safety that gets skipped, and it is exactly the part that matters most in solids handling. EN 1127-1 lists static electricity as one of the thirteen recognised ignition sources, and in powder handling it is one of the harder ones to see coming.

Earthing and bonding deal with charge sitting on conductors. They do almost nothing about the charge a powder carries with it as it pours into a vessel, and that charge can produce a discharge that earthing was never able to touch.

Why earthing alone cannot control static electricity in powders

When we classify hazardous areas for gases, conductivity is mostly somebody else’s problem. With powders, electrical resistivity becomes one of the key parameters determining how quickly electrostatic charge can dissipate or accumulate.

How does powder resistivity affect electrostatic charging?

IEC 60079-32-1, the guidance standard for electrostatic hazards, splits bulk materials into three bands. Low resistivity powders sit at or below 1 MΩ·m. Medium runs from there up to 10 GΩ·m. High resistivity is anything above that. The catch is that genuinely low resistivity powders are rare in practice.

People assume a metal powder is conductive and therefore safe, but an oxide film forms on the surface fairly quickly and the bulk stops behaving like a conductor. Carbon black is one of the few materials that stays low. For most of what moves through a process plant, you are working in the medium or high band, which is where charge relaxes slowly and accumulates.

Why minimum ignition energy should be based on the finest powder fraction

The second number that matters is the minimum ignition energy (MIE), and here is a trap worth naming. You cannot assess a powder on its average particle size. The fines decide the hazard. The standard is explicit that the ignition assessment should be based on the finest fraction that may be present, which in practice means sieving a sample through a 63 micron screen and testing that. Ignitability climbs as particle size falls, and the fines are also the fraction that goes airborne and forms the cloud. Assess the bulk and you will flatter yourself. Assess the fines and you get the truth.

What is minimum ignition energy for combustible dust?

Minimum ignition energy (MIE) is the lowest electrical spark energy capable of igniting a specified combustible dust cloud under defined test conditions.

MIE is an important parameter in electrostatic risk assessment because it helps determine whether an electrostatic discharge could provide sufficient energy to ignite the dust.

Particle size matters because finer material can be easier to disperse into an airborne cloud and may exhibit different ignition behaviour from the bulk product. The representative fraction used for testing should therefore reflect the combustible dust that can realistically arise within the process.

“Earthing and bonding deal with charge sitting on conductors. They do almost nothing about the charge a powder carries with it as it pours into a vessel”

When are special electrostatic controls needed for combustible powders?

There is a useful rule of thumb buried in the guidance. If the minimum ignition energy is above 1 joule and there are no flammable gases or vapours in the mix, special measures against static are usually not needed. The exception, and it is an important one, is where propagating brush discharges are possible. For the great majority of organic powders, dusts that ignite below 1 joule, you are firmly in the territory where this article applies.

Which electrostatic discharges can ignite combustible dust?

Not every spark you can feel on a door handle is capable of lighting a dust cloud. The guidance separates the discharge types by how much energy they can deliver, and the differences are the whole reason cone discharges deserve their own conversation.

Spark discharges from isolated conductors

Spark discharges are the familiar ones, a jump between two conductors at different potentials. These absolutely can ignite dust, and they are also the easiest to design out. Compare the stored energy against the powder’s minimum ignition energy, then earth everything that can hold a charge: equipment, product handling lines, and people. This is the discharge that conventional earthing and bonding actually defeats, which is probably why so many sites believe the job ends there.

Brush discharges from insulating surfaces

“Assess the bulk and you will flatter yourself. Assess the fines and you get the truth.”

Brush discharges come off insulating surfaces and look alarming, but the current understanding in the standard is that brush discharges cannot ignite combustible dust, provided there are no flammable gases or vapours present. That proviso matters. A solvent-wet powder changes the picture entirely. On their own, dry dusts shrug off brush discharges.

Cone discharges during powder silo filling

Cone discharges are the ones to worry about. When you fill a silo with a charged medium or high resistivity powder, the charge does not distribute itself politely. It concentrates in the heap building up at the bottom, the cone, and discharges flash across the surface of that cone as more material lands on it.

These can and do ignite powders. The critical point for plant designers is that the discharge happens within the product, at the bulking point, in a place that no earthing strap reaches. You have to stop the charge arriving, or stop the cloud being ignitable when it does.

This is why proving earth continuity to the silo shell does not by itself demonstrate that electrostatic ignition hazards associated with the charged bulk powder have been controlled.

Propagating brush discharges from insulating layers

Propagating brush discharges are the most energetic of the lot, routinely above 1 joule, and they occur where a highly insulating layer sits against a conductive backing, the classic example being an insulating liner inside an earthed metal vessel. If you are running high resistivity powders against insulating films, this is a second mechanism that earthing will not save you from.

Why doesn’t earthing prevent every electrostatic discharge?

Earthing is highly effective at preventing dangerous charge accumulation on conductive plant, equipment and isolated metal objects because the charge can flow to earth.

The situation is different when charge is retained within an electrically resistive powder or on an insulating material. In these cases there may be no sufficiently conductive path through which the charge can dissipate.

Electrostatic risk assessment must therefore consider where the charge is generated, where it accumulates and which discharge mechanisms are physically possible rather than relying solely on earth continuity measurements.

How to control static electricity during silo and container filling

Once you accept that the dangerous charge is in the powder, the control strategy shifts from “bond everything” to “manage the material and the process.” The standard offers a practical menu, and most plants can pick several items from it.

Increase the rate of electrostatic charge dissipation

The first lever is the material itself. Raising the conductivity of the bulk, sometimes through additives or coatings, lets charge relax faster than it accumulates. Humidification can do the same job: around 70% relative humidity at 23 °C is the figure quoted, although it loses effectiveness with high-speed conveying and warm product, and it can ruin the flow properties of some powders, so it is not a free win.

Reduce electrostatic charging during powder transfer

The second lever is the handling. A lot of charge generation is self-inflicted by the way material is moved. Reducing the proportion of fines, since wear and abrasion manufacture their own hazard, limiting dispersion by favouring dense phase conveying over dilute phase, dropping the conveying speed and throughput, and preferring gravity transfer to pneumatic transport where the process allows it: each of these takes energy out of the system before it can become a problem.

Conductive and antistatic hoses for powder conveying

Where pneumatic transport is unavoidable, the hoses should be conductive or antistatic with a leakage resistance below 100 MΩ.

Assess the receiving silo and filling conditions

The third lever is the vessel assessment itself, and this is where the guidance gets quantitative. For filling silos and containers, the standard provides decision paths keyed to the powder’s resistivity band, and they hang on measurable field strengths. The electrical field in the dust cloud should stay below 500 kV/m.

At the silo wall, the radial field limit is 3 MV/m. Geometry helps you here: smaller vessels are inherently safer, and the assessment treats a volume under 100 cubic metres, or a diameter under 3 metres, as a lower risk case. If your vessel is larger than that and running high resistivity product, the cone and the cloud both need to be assessed properly rather than assumed away.

“Earthing, bonding and dissipative footwear are necessary and they are not the finish line.”

None of this is exotic. It is mostly about knowing which band your powder sits in, measuring the fines honestly, and choosing a filling method that does not generate more charge than the material can shed.

How electrostatic risk assessment affects hazardous area classification

Dust hazardous zones 20, 21 and 22

The electrostatic assessment is not a standalone exercise. It feeds the area classification, the dust zones 20, 21 and 22 set out in IEC 60079-10-2, and the area classification in turn dictates the equipment that is allowed to be in that space. A silo where a cone discharge cannot be excluded is a silo where the instrumentation, the level sensors, the cameras and the portable devices that operators carry all have to be rated for the zone they sit in, with dust environments typically calling for protection by enclosure, type “t”, under IEC 60079-31.

That last point tends to be where the paperwork meets reality. It is one thing to classify a zone on a drawing. It is another to make sure that every device that crosses into it, including the phone in someone’s pocket and the camera used for an inspection, is certified for the job. The discharge assessment and the equipment selection are two ends of the same chain, and a gap at either end is a gap in the whole thing.

Do phones and portable devices matter in dust hazardous areas?

Hazardous area classification applies to equipment brought into the classified area as well as equipment permanently installed there.

That means cameras, inspection equipment, communication devices and other portable electrical equipment must be suitable for the zone in which they are used.

A robust hazardous area management system therefore needs to control not only fixed instrumentation but also temporary and portable equipment introduced during inspection, maintenance and routine operations.

Why earthing is necessary but not sufficient in powder handling

The reason cone discharges deserve attention is not that they are common. It is that they are the failure mode that survives a plant doing everything else right. Earthing, bonding and dissipative footwear are necessary and they are not the finish line. In solids handling, the charge that gets you is the one riding in on the product, and the only way to manage it is to understand the powder before it ever reaches the vessel.

For silo and container filling, a complete electrostatic assessment must therefore address the chargeability and resistivity of the powder, its ignition sensitivity, the transfer method, possible discharge mechanisms and the explosive atmosphere in which those discharges could occur.

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Frequently Asked Questions about Static Electricity in Powder Handling

Why can static electricity be dangerous during powder handling?

Movement and contact between powder particles and process equipment can generate electrostatic charge. If that charge accumulates and produces an energetic discharge in the presence of a combustible dust cloud, it may provide an ignition source.

Does earthing eliminate static electricity in powders?

No. Earthing is essential for conductive equipment and isolated metal objects but it cannot necessarily remove charge retained within a resistive powder. Powder electrostatic hazards therefore require additional assessment.

Why can a silo still have an electrostatic ignition risk if it is earthed?

An earthed silo shell can prevent dangerous charge accumulation on the conductive vessel itself while charged powder entering the silo may still retain electrostatic charge. Certain discharge mechanisms can therefore occur within or around the powder bed independently of the vessel’s earth connection.

What is powder resistivity?

Powder resistivity describes how strongly a bulk powder resists the flow of electrical charge. Powders with higher resistivity tend to dissipate electrostatic charge more slowly and may therefore allow greater charge accumulation.

What is minimum ignition energy?

Minimum ignition energy or MIE is the lowest electrical spark energy capable of igniting a particular combustible dust cloud under specified test conditions.

Why are fine powder particles important when assessing dust ignition risk?

Fine particles can disperse readily into airborne dust clouds and may exhibit different ignition behaviour from the coarser bulk material. The representative material used for ignition testing should therefore account for the fine combustible fraction present in the process.

What is a spark discharge?

A spark discharge occurs between conductive objects at different electrical potentials. Spark discharges can contain sufficient energy to ignite combustible dust and can usually be controlled by effective bonding and earthing of conductors.

What is a brush discharge?

A brush discharge occurs from a charged insulating surface. Its ability to ignite an atmosphere depends on the nature of the atmosphere and discharge.

What is a cone discharge?

A cone discharge is an electrostatic discharge associated with charged bulk powder accumulating within a silo or container. Charge within the powder bed can create electrical fields and discharges around the powder surface during filling.

Why does earthing not prevent a cone discharge?

Cone discharges originate from charge accumulated within the bulk powder rather than simply from an isolated conductive object. Earthing the vessel therefore does not necessarily provide a path for that charge to dissipate.

What is a propagating brush discharge?

A propagating brush discharge is a high-energy electrostatic discharge that can occur when charge accumulates across a highly insulating layer associated with a conductive backing. It can represent a significant ignition hazard in explosive atmospheres.

Does pneumatic conveying generate static electricity?

Yes. Contact and separation between powder particles, air and conveying surfaces can generate significant electrostatic charge during pneumatic transport.

Is dense-phase conveying better for reducing static electricity?

Lower conveying velocities and reduced particle dispersion can in some circumstances reduce charge generation compared with higher-velocity dilute-phase conveying. The electrostatic risk should still be assessed for the actual powder and conveying system.

Can humidity reduce static electricity in powders?

Increasing humidity can improve charge dissipation for some materials and processes but its effectiveness depends on the powder, temperature and handling conditions. Humidity can also affect powder flow and product quality so it is not universally suitable.

What is IEC 60079-32-1?

IEC 60079-32-1 provides guidance on identifying and controlling electrostatic ignition and shock hazards in equipment, products and industrial processes.

What are dust zones 20, 21 and 22?

Zones 20, 21 and 22 are hazardous area classifications used for locations where combustible dust atmospheres may be present. The classification reflects the likelihood and duration of the explosive dust atmosphere.

Does hazardous area classification apply inside a silo?

Potential explosive dust atmospheres inside and around powder-handling equipment may require hazardous area classification based on the likelihood and duration of the dust cloud.

Do phones and cameras need to be suitable for dust hazardous areas?

Electrical and electronic equipment introduced into a classified hazardous area must be appropriate for that zone and the applicable explosion-protection requirements. This can include portable inspection and communication equipment.

What should be assessed when filling a silo with combustible powder?

The assessment should consider powder resistivity, minimum ignition energy, particle-size distribution, expected dust concentration, filling method, conveying velocity, silo geometry, insulating materials, possible discharge mechanisms and hazardous area classification.

What is the most important lesson about static electricity in powder handling?

Earthing and bonding remain essential but they address only part of the electrostatic problem. A complete assessment must consider the charge generated and retained by the powder itself as well as conductive plant and equipment.