Safe grounding of Big Bags

HOW TO PREVENT ELECTROSTATIC CHARGES WITH MONITORED GROUNDING OF FIBC TYPE C

In this article you’ll get to know, which different types of Big Bags exist, how great the danger of electrostatic charges really is, what electrostatic charges causes and which legal requirements you need to obey in order to safely ground your FIBC type C.

In addition, we’ll show you the best practice for the grounding of big bags, derived from a real accident that was examined by the swiss accident insurance institution.

We also show you, which advantages monitored big bag grounding comes with and to which features of monitored grounding devices you should pay attention.

This Whitepaper gives you a comprehensive overview of the best practice for grounding of Big Bags Type C and how it helps you to fulfill the operators responsibility.

What are the different types of big bags and where are they allowed to be used?

FIBC, commonly known as Big Bags, come in 4 different types which determine their usability and certification for ex-zones. These different types are known as type A, type B, type C, and type D.
 
Big Bags type A do not have any protection against electrostatic charges and are therefore not allowed for use in potentially explosive atmospheres. However, they are often used in not explosive atmospheres, such as construction sites. Big Bags type B look very similar, the used material however has a breakdown voltage of less than 6kV and therefore prevents from high energetic propagating brush discharges.
 
Big Bags type C have a net of dissipative threads woven into the material which dissipates electrostatic charges to designated grounding tabs and needs to be grounded during use. Big Bags type D are completely made from a special fabric which limits the electrostatic charges to a harmless level.
 
As mentioned earlier, the certification for ex-zones differs between the four types. For choosing the correct big bag variant for your situation and ex-zone, the minimum ignition energy (MIE) and the designated ATEX zone must be taken into consideration.
Big Bag types and their approved areas of use according to DIN EN IEC 61340-4-4

Big Bag types

and their approved areas of use according to DIN EN IEC 61340-4-4

For example, big bags type A are only allowed in non-ex atmospheres while big bags type B are only allowed for pure dust-ex zones. If the MIE of the dust is equal or less than 3 mJ or gas- and dust-ex atmospheres may arise at the same time, only big bags type C and D are allowed.

What causes electrostatic charges during (un-)loading of Big Bags?

(Un-)filling processes with big bags often leads to the so-called triboelectric effect. Due to the very high flow rates of the (un-)loaded material, there are very short contact times between the material and the fabric of the big bag. In that moment an exchange of electrons takes place between these components.
After separation, the electrons don’t have enough time to get back to the material where they started. Over time the electrons accumulate on the material with higher electron binding forces, while the number of electrons on the other material declines. This results in different electrical potentials – electrostatic charging takes place.
If the potential difference is too high, the accumulated energy may discharge with a high energetic spark. The energy which is released in this spark is often more than enough to ignite hazardous atmospheres and therefore cause serious damage to the plant and environment.

How does the grounding of Big Bags type C help to prevent this danger?

The formation and accumulation of electrostatic charges is possible, because the charge buildup is higher than the conductivity of the big bag fabric. That means, the more isolated an object (i.e., the big bag) is from earth potential, the easier it can get electrostatic charged. Because of this, dissipative and conductive objects in ex-zones must be connected to earth potential in order to safely dissipate electrostatic charges. 

Big bags type C are traversed by a net of conductive threads which dissipate the electrostatic charges from every point of the big bag to the designated grounding tabs. The operators have to use these grounding tabs to establish a conductive and high-quality grounding connection.

Which requirements does the grounding connection of Big Bags type C need to meet?

In regards to big bag grounding, the DIN EN IEC 61340-4-4 is the most kown and followed standard.

Timm EUS-2 overfill prevention controller international standard IECEx

DIN EN IEC 61340-4-4

It is the international set of rules which describes the technical requirements for big bags, test methods, and the correct use of all four types.

It states that big bags type C must be grounded during the whole work process. 

In addition, the grounding connection must not exceed a limit of 100 MΩ – the operators have to make sure that this limit value is complied with and therefore the electrostatic charges can be dissipated safely. However, depending on which country you come from, local regulations or company rules may also apply.

In Germany, for example, the TRGS 727 is the main standard for prevention from electrostatic charges in hazardous areas and needs to be taken into consideration. But these regulations, like TRGS 727, are often already harmonized with the DIN EN IEC 61340-4-4, making it easier for operators and safety engineers to find the correct procedure.

What is the best practice for safe grounding of Big Bags type C?

In order to derive the best practice for big bag grounding it is sensible to look at a real working accident which was investigated by the swiss accident insurance institution.
 
In this case, plastic additive should be filled from a big bag type C into a downstream silo via a hopper.
 
A cable reel was available in the work are to ground the big bag and its grounding clamp was to be attached to the big bag. But allthough this was available, a fatal explosion occured during the emptying process.
 
The results of the investigation showed that the employees forgot to attach the clamp to the big bag, leaving it completely isolated from earth potential. This caused the electrostatic charges to build up which then discharged into the surrounding dust atmosphere, igniting the explosion.
Grounding Control Device EKX-FIBC with object detection for Big Bags Type C

Grounding Control Device EKX-FIBC

with object detection for Big Bags Type C

If the grounding cable would have been connected to the big bag properly, no explosion would have occurred – no explosion due to electrostatic charges without an ignitable discharge. That’s why best practice should build on it.

Firstly, despite the working instruction, there was no procedurally need to connect the grounding cable to the big bag. Also, if though the cable would have been attached according to the regulations, the quality and resistance of this grounding connection would still be unclear. This depends on various factors such as the condition of the grounding cable or the quality of the big bag.

It is therefore appropriate nowadays to use monitored grounding devices such as the TIMM EKX-FIBC for grounding big bags type C. Such devices continuously monitor the correct grounding of the big bag and its quality and can be interlocked into the loading control system via their relay and NAMUR control outputs. For example, warning and release signals can be sent to the loading control. However, the use of optical or acoustic displays is also possible. In the work accident discussed, it could be conceivable, for example, to use a flap above or in the hopper that only opens when the grounding device sends a release signal. In addition to the work instructions, there would then also be a process-related need to connect the grounding cable – and the risk of an explosion would be significantly reduced.

When selecting your grounding device, make sure that it has a functioning object detection in order to prevent incorrect operation and manipulation. Such object recognition can distinguish whether a big bag is actually connected or whether the clamps are simply clamped onto a piece of metal in order to achieve permanent release. It only signals filling permission if the Big Bag is grounded in accordance with the standards – a clear plus in terms of safety and at the same time it makes it easier to monitor the work processes.

 

You are unsure if and how you need to implement eletrostatic grounding in your application?

Or would you like to learn more exact details about the best practice? Feel free to reach out to our team of experts.
 

We look forward to helping you.