Big Bags in the Ex zone – types A, B, C or D, and why can a single mistake lead to an explosion?

In many industrial facilities, big bags are the primary method for transporting and storing bulk materials – from sugar and flour, through plastic granulates, to chemical dusts and metals. The problem is that during filling, emptying, and transporting these bags, large electrostatic charges are generated.

In installations where flammable dusts or gases are present, even a small electrostatic discharge can carry enough energy to ignite an explosive atmosphere. That is why selecting the appropriate type of FIBC (Flexible Intermediate Bulk Container) is critical in ATEX environments.

In audit practice, it is very common to encounter situations where a facility uses the wrong type of big bag in an Ex zone. Sometimes this results from misinterpretation of standards, sometimes from a lack of awareness of the differences between big bag types A, B, C, and D. Unfortunately, one such mistake can lead to a near-miss event, and in extreme cases – to an explosion.

Why do big bags generate electrostatic charges?

Big bags generate electrostatic charges primarily during the flow and friction of bulk material particles against the bag surface.

During technological operations such as:

• filling the bag from a silo,
  
• transferring material to a container,
  
• transport by forklift,
  
• emptying through a discharge spout,
  

there is intensive contact between dust particles and the polypropylene (PP) fabric. This process results in separation of electric charges.

In practice, this means that:

• the bag surface can reach potentials of several kilovolts,
  
• charges can accumulate on the fabric surface,
  
• under certain conditions, electrostatic discharges occur.
  

In industrial environments, several types of such discharges are distinguished:

• spark discharges,
  
• brush discharges,
  
• propagating brush discharges (PBD),
  
• cone discharges in dust clouds.
  

Some of these can reach energy levels sufficient to ignite an explosive atmosphere. For comparison:

• MIE (Minimum Ignition Energy) of many organic dusts is 10–100 mJ,
  
• metal dusts can have MIE below 10 mJ.
  

In such a situation, a single electrostatic discharge may be sufficient to cause ignition.

That is why, in analyses performed during explosion hazard assessments, one of the key elements is the identification of transfer operations and bagging systems.

What are the differences between big bag types A, B, C, and D?

Big bag types differ in their ability to control and dissipate electrostatic charges.

In industrial practice, four basic types of FIBC bags are distinguished.

Big bag A

Big bag A is the simplest type of bag made from polypropylene fabric without antistatic properties.

Characteristics:

• no protection against static electricity,
  
• possibility of brush and propagating discharges,
  
• cannot be safely used in Ex zones.
  

Type A may only be used when:

• no flammable dusts are present,
  
• no flammable gases or vapors are present,
  
• an explosive atmosphere cannot form.
  

In practice, this means that big bag A should not be used in ATEX installations.

Big bag B

Big bag B limits the occurrence of propagating brush discharges, but does not eliminate all electrostatic discharges.

Bags of this type:

• are made from material with a lower breakdown voltage,
  
• reduce the risk of PBD discharges.
  

However, they can still generate:

• brush discharges,
  
• a potential ignition source for gases or vapors.
  

Therefore, big bag B may only be used in dusty environments, provided that:

• no flammable gases are present,
  
• a gaseous atmosphere is not possible.
  

In many facilities, a common misinterpretation occurs – using big bag B in installations with solvents. In such cases, the level of risk is far too high.

Big bag C

Big bag C is a conductive bag that must always be grounded during technological operations.

The bag structure incorporates:

• conductive fibers or tapes,
  
• a system for dissipating electrostatic charges.
  

There is one condition for safe use:

big bag C must be connected to ground before filling or emptying begins.

If this condition is not met:

• the bag behaves like type A,
  
• ignition protection practically disappears.
  

In ATEX audits, it is very common to encounter situations where:

• a grounding point exists,
  
• but operators do not connect the grounding cable.
  

This is a classic example of a near-miss event.

When is big bag D used and why is it often confused with type C?

Big bag D is an antistatic bag that dissipates electrostatic charges without the need for grounding.

Unlike type C:

• it does not require physical connection to ground,
  
• it uses material that dissipates electrostatic charges.
  

The most commonly used fabrics are of the type:

Type D – fabric (e.g. Crohmiq).

The mechanism of action is based on:

• controlled dissipation of charges across the material surface,
  
• limiting the generation of spark discharges.
  

Type D bags may be used in environments containing:

• flammable dusts,
  
• flammable gases and vapors,
  

provided that the bag surface is not contaminated with conductive material, such as:

• water with salts,
  
• metallic dust,
  
• conductive oils.
  

In industrial practice, it sometimes happens that:

• big bag D is treated as a universal bag,
  
• its conditions of use are ignored.
  

That is why in safety documentation – such as the Explosion Protection Document (EPD) – it is necessary to precisely describe:

• the type of bags used,
  
• operational procedures,
  
• the method of ignition risk control.

Most common mistakes when using big bags in ATEX installations. What to watch out for?

The biggest problem in industrial facilities is not a lack of technology, but the incorrect use of the right type of FIBC bag.

During explosion safety audits, several recurring risk scenarios are frequently identified.

The most common mistakes include:

• using big bag A in an installation with flammable dust,
  
• using big bag B in an environment with solvent vapors,
  
• failure to connect grounding with big bag C,
  
• treating big bag D as a universal solution,
  
• lack of operational procedures for personnel.
  

A particularly dangerous situation arises when the technological installation involves a dust cloud with a low minimum ignition energy (MIE).

For example:

• sugar: MIE approx. 30 mJ,
  
• flour: MIE approx. 30–60 mJ,
  
• aluminum dust: MIE even < 10 mJ.
  

For comparison, electrostatic discharges can generate energy exceeding 100 mJ, meaning that in many cases ignition energy is reached with a large margin.

In practice, this means that an incorrect choice of big bag can create a direct ignition source for an explosive atmosphere.

How to assess electrostatic risk in big bag operations?

Electrostatic risk assessment should be part of a full explosion hazard assessment carried out for the technological installation.

The analysis covers several key elements of the technological process.

In engineering practice, the following are analyzed:

• physicochemical properties of the dust,
  
• ignition parameters of the material,
  
• type of FIBC bags used,
  
• method of transport and filling,
  
• presence of a gaseous atmosphere.
  

Key dust parameters include, among others:

• MIE – minimum ignition energy,
  
• MIT – minimum ignition temperature,
  
• LIT– layer ignition temperature of dust,
  
• Kst – dust explosion index.
  

For example:

• MIT of a dust cloud is often 400–600°C,
  
• LIT of a dust layer may be 250–350°C.
  

These parameters allow determination of:

• whether an electrostatic discharge can ignite the dust,
  
• whether installation surfaces constitute an ignition source,
  
• whether the equipment used meets ATEX requirements.
  

That is why in safety documentation – such as the Explosion Protection Document(EPD) – operations involving FIBC bags should be described in detail.

Example of a near-miss event – when does a big bag become an ignition source?

In many industrial facilities, potentially explosive events do not end in an explosion simply because one element of the explosion triangle was missing.

During one analysis of a food powder transport installation, a near-miss situation was identified.

The scenario was as follows:

1. an operator was emptying a big bag C,
   
2. the grounding cable had not been connected,
   
3. the material had an MIE of approx. 30 mJ,
   
4. a dense dust cloud had formed in the installation.
   

During the transfer, the operator noticed sparking on the surface of the bag.

In this case, no explosion occurred because:

• the dust concentration was below the lower explosive limit,
  
• the process lasted only a short time.
  

However, from a safety analysis perspective, this was a situation in which:

a real ignition source had formed in a potentially explosive atmosphere.

Such events are very often the first signal that the explosion safety management system requires corrective action.

Why should big bag selection be part of ATEX documentation?

The type of big bags used should be unambiguously specified in the explosion hazard analysis and in the Explosion Protection Document.

This documentation should include, among other things:

• a description of transfer operations,
  
• identification of ignition sources,
  
• requirements for FIBC bags,
  
• grounding procedures,
  
• instructions for operators.
  

The absence of such records often leads to situations where:

• the purchasing department selects a cheaper type of bag,
  
• operators are unaware of the grounding requirement,
  
• the installation operates with uncontrolled ignition risk.
  

That is why in engineering practice, FIBC bag selection should be the result of a technical analysis, not a logistics decision.

Technical support – when is it worth conducting an ATEX audit?

If big bags are used in a technological installation, it is worth verifying whether their use complies with explosion safety requirements.

In practice, audits show that many facilities:

• use the wrong type of bag,
  
• have no grounding procedures in place,
  
• do not include transfer operations in the EPD.
  

At ATEX Doradztwo, we assist in such situations by providing:

• explosion hazard assessment,
  
• preparation of the Explosion Protection Document (EPD),
  
• ATEX directive compliance audits,
  
• HAZOP, SIL and PL analysis,
  
• identification of ignition sources in industrial installations.
  

This enables facilities to:

• reduce explosion risk,
  
• meet legal requirements,
  
• avoid costly production downtime.

A proper electrostatic risk assessment makes it possible to avoid situations where a routine transfer operation becomes a potential ignition source.