Electrostatic discharge and combustible dust – when does ESD cease to be a “nuisance”?

Static electricity brings to mind a “small zap” when touching a door handle. In installations handling combustible dusts, however, that “small zap” can turn a filter, silo, or conveyor into an ignition point. Different electrostatic discharges carry different energy levels — and not every electrostatic discharge is capable of igniting a dust cloud. In this article, we bring order to the topic: what types of discharges we encounter in practice, which of them pose a real threat to dusts in the ATEX context, and how to reduce risk through design and operation — before the “spark” stops being a metaphor.

Why are electrostatic discharges particularly dangerous with dusts?

With combustible dusts, static electricity is dangerous because the energy of typical discharges often exceeds the minimum ignition energy of many dusts. In industries such as woodworking, food processing, pharmaceuticals, plastics, and metals, ignition of a dust cloud very often starts with ESD — especially when the process generates charges while simultaneously creating conditions for a cloud to form.

For ignition to occur, a dust cloud needs “only” two things: a concentration above the MEC (Minimum Explosible Concentration) and an ignition source with energy equal to or greater than the MIE (Minimum Ignition Energy). And here the problem begins, because typical objects in a facility can release energy that doesn’t look dangerous — until you compare it to the MIE.

Example electrostatic discharge energies from objects commonly found in practice:

• a person – approx. 90 mJ,
  
• a bucket – approx. 10 mJ,
  
• a road tanker – approx. 2000 mJ.
  

And now the other side of the comparison — minimum ignition energy of various dusts:

• aluminium – approx. 10 mJ,
  
• sugar – approx. 30 mJ,
  
• flour – approx. 50 mJ.
  

The conclusion is uncomfortable but simple: in the world of dusts, static electricity is not a “comfort issue”. It is a source of energy that is directly sufficient to ignite many dust-air mixtures. 

That is why in the ATEX approach we do not treat ESD as an afterthought — but as a fully valid ignition scenario that must be described, calculated, and “closed” with technical measures.

MIE of dusts vs. ESD energy – a simple comparison that makes a difference

If we know the MIE of a dust, we can quickly assess which electrostatic discharges are critical and which typically lack the energy to cause ignition. MIE (Minimum Ignition Energy) is the minimum spark energy capable of igniting a dust-air mixture. That is precisely why MIE is so practical — it allows us to stop guessing.

For sensitive dusts (e.g. fine metal powders), MIE can be < 10 mJ. For typical organic dusts (flours, sugars, starches), it is usually tens of mJ. MIE increases as particle size and moisture content increase — but that does not mean the issue disappears. It only means the scenarios change.

The key step in the analysis looks like this:

• take the MIE of the specific dust (from testing or documentation),
• compare it with the energy of the potential discharge (discharge type + geometry + process conditions),
• select protective measures appropriate to what can actually ignite the cloud.

This is exactly the moment when it is worth looking deeper into the parameter itself — because without it, ESD risk assessment tends to be merely “descriptive”. 

At Atex Doradztwo, we integrate this into a single process: explosion hazard assessment + ignition source identification + selection of protective measures. That way, MIE is not just a “curiosity” — it is a parameter that drives design and operational decisions.

Types of electrostatic discharges in practice – what we encounter in dust installations

In dust installations, we primarily distinguish single-electrode discharges (on insulators and in clouds) from two-electrode discharges (between conductors) — and the latter typically deliver the highest energy. There are more academic classifications, but in a facility what matters is the question: does this have enough energy to ignite our dust?

The simplest, most practical classification looks like this:

Single-electrode discharges (one “electrode” + air):

• brush discharges,
  
• propagating brush discharges (Lichtenberg discharges),
  
• cone discharges from accumulated dusts.
  

Two-electrode discharges (between two conductors):

• classic spark discharges,
  
• corona discharges (typically at sharp edges and high voltage),
  
• other configurations that in practice ultimately come down to the problem of potential difference and the discharge path.
  

In the following sections we focus on the discharge types that most frequently do the “ignition work” with dusts: sparks, propagating brush, and cone discharges. Classic brush discharges and corona are also discussed, but with an honest note on when they are less critical for “pure” dust clouds.

Spark discharges – the simplest scenario that most often ends badly

Spark discharges are dangerous because they can carry energy many times higher than the MIE of most dusts. An electrostatic spark is a discharge between two conductive objects at different potentials — for example, a charged installation component and a grounded structure.

In practice, this is not a “laboratory” phenomenon. Typical scenarios from dust-handling facilities include:

• an ungrounded metal component inside a filter, cyclone, or hopper,
• a flexible hose with metal elements but no continuity of grounding,
• a person or forklift that is not bonded yet touches the structure in an explosion-hazardous zone,
• intermediate containers, drums, and FIBCs — where “insulation” is created by paint, contamination, or a loose clamp.

When we compare discharge energies such as “a person approx. 90 mJ” against dusts with MIE values of 10–50 mJ, it becomes clear that in many cases a single touch at the wrong moment is enough. That is why in dust zones the foundation is grounding and equipotential bonding — applied consistently, checked, measured, and not just “done at some point in the past”.

If you want to close this topic practically, it connects very well with grounding as a protective measure — we encourage you to read the article on why grounding is so critical in Ex zones.

Brush discharges, propagating brush, and cone discharges – three phenomena that must not be lumped together

Not every “static crack” carries enough energy to ignite a dust cloud — which is why we must distinguish the discharge type and the conditions under which it occurs. This distinction helps avoid two mistakes: underestimating real risk and “design panic” that ends in excessive costs without any improvement in safety.

Brush discharges

These occur on insulating surfaces (plastics, films, linings) when they become charged relative to their surroundings. Their energy is typically in the range of 1–4 mJ.

For “pure” dust clouds, this is often insufficient to ignite the mixture — but that does not mean the topic goes away. Brush discharges can be significant when:

• there are hybrid mixtures (dust + flammable gas/vapours),
  
• the process involves dusts with an extremely low MIE,
  
• insulating surfaces are large and charges accumulate over a long time.
  

Propagating brush discharges

This is where things get serious. Propagating brush discharges occur on thin insulating layers over a conductive substrate (e.g. a plastic coating on metal). Under the right conditions, a discharge can reach energies of hundreds of mJ or even joules.

This is a scenario that is often underestimated, because “it’s just paint/a lining”. Yet at the same time, it is one of those cases where static electricity can carry an enormous energy margin above the MIE of dusts.

Cone discharges

These occur during the filling of silos and vessels with dusts, as charge accumulates in the pour cone and on the walls. Their energy is typically estimated at tens of mJ — which is already within the range that “matches” the MIE of many organic dusts.

Cone discharges are particularly relevant in:

• high filling rates,
  
• pneumatic conveying,
  
• FIBCs and intermediate hoppers,
  
• plastic walls or insulated surfaces.
  

That is exactly why in an ATEX analysis it is not enough to say “ESD exists”. We need answers to: what type of ESD, where, with what energy, and whether our dust is sensitive to it.

What this means for a facility – actions that genuinely reduce the risk of dust ignition

The most effective strategy against electrostatic discharges combines three things: grounding, material control, and control of charge-generating processes. Each element delivers something on its own, but only together do they close off the scenarios.

In practice, we implement this step by step:

• grounding and equipotential bonding of everything conductive:
  
  • silos, filters, cyclones, conveyors, vessels, pipelines,
    
  • bag and FIBC filling stations,
    
  • mobile items (drums, trolleys),
    
  • personnel control via footwear/flooring in zones where relevant,
    
• deliberate selection of materials and coatings:
  
  • limiting large insulating surfaces in dust zones,
    
  • caution with linings and coatings inside equipment, as these can build the propagating brush scenario,
    
• control of processes that generate high charges:
  
  • silo and vessel filling rates,
    
  • pneumatic conveying, transfers, drops, separation, drying,
    
  • limiting areas where dust forms stable clouds near potential discharge points.
    

On top of this comes the data-driven foundation: testing of dust explosion parameters (including MIE, MEC, Kst, Pmax) and matching them to ESD scenarios. Without knowing the MIE, it is difficult to honestly say which discharges are critical and which are merely “noise” in the analysis.

At Atex Doradztwo, this is exactly what our advisory work is built on: explosion hazard assessment, ignition source analysis, technical recommendations, and updating ATEX documentation — so that protective measures are proportionate to the energy of possible discharges, not chosen “just in case”.