Background to the Ignition Risks of Electrostatic Charge

Background to the ignition risks of electrostatic charge
Working in dust explosive atmospheres carries the danger of causing an explosion of the dust-air mixture.

Storage silos, granaries or filters are typical places where hazardous dust atmospheres occur. Therefore, there are a variety of standards and best practice recommendations to ensure safety for man, machine and environment while working in these zones. The most important are TRGS 727, CLC / TR 60079-32-1:2015, TRBS 2152 and IEC TS 60079-32-1:2013.

Which dusts carry a high risk?

First of all, it is important to identify whether there is a potentially dangerous working environment due to explosive dust atmospheres. In order to investigate the explosiveness of a substance in dust/particle form, the IFA (Institute for Occupational Safety and Health of the German Social Accident Insurance) uses a standardized procedure, stated in ISO 6184-1. Using this method, the dusts examined were classified into dust explosion classes, based on the determined value.

This value describes the maximum increase in pressure caused by the ignition of the dust-air mixture in a 1 m³ container. The higher the value, the more explosive and faster the explosion expands. This also means: If one of the examined dusts is classified, it is proven as generally explosive when mixed with air.

In the following you will find an excerpt from the materials of the test series that were classified as generally explosive:

Wood, wood products, fiber: cotton, wood, jute, coconut fiber, peat, pulp
Food and feed: Flour, starch, rice
Coal, coal products: activated carbon, coke, pitch, soot
Plastics, resins, rubber: ABS, coating powder, rubber, wax, cell flour
Pharmaceuticals, cosmetics, and Pesticides: ascorbic acid, bath powder, herbicide, soap base
Intermediates, excipients: anthraquinone, flavor, lead, calcium, casein
Technical-chemical products: paints, light stabilizers, putty, washing powder
Metals, alloys: aluminum, bronze, iron, brass, magnesium
Other products: ash, binder, PVC powder, abrasive, toner, release agent

Electrostatic charge of particles and powders

In many manufacturing processes, the contact of dust/material with the equipment and conveyor is unavoidable. Due to the rapid impact and separation speeds, inadequate conductivity of the materials can lead to static charging of the material surface. This type of charging is due to an incomplete charge exchange during contact.

If it comes to a contact of two neutral materials, an exchange of electrons as a result of the different electron-binding forces takes place. When this contact occurs at high speed, the charges do not have time to return to their starting point and remain on the surface of the contacted material. As a result, on the material with higher electron-binding force occurs an excess of electrons, which means a negative surface charge. However, on the surface of the second material, an electron deficiency appears and thus a positive surface charge.

Due to the different surface charging, a high potential difference between the materials takes place which can result in sparks, strong enough to ignite the surrounding atmosphere.

Type of dust Value of MIE Example
very easily inflammable dusts < 1 mJ fine aluminum dusts
easily flammable dusts 1 … 10 mJ wax dust
moderately flammable dusts > 10 … 100 mJ sugar or milk powder dusts
hardly inflammable dusts > 100 mJ wheat flour, coal dust

How can the static charge be prevented?

Static electricity can be prevented by dissipating electrical energy to ground potential. Therefore, a sufficiently conductive connection to a designated grounding point is needed. The consensus of all standards confirms a sufficient conductivity below the maximum resistance value of in the discharge connection. However, only connecting the grounding cable is not enough to ensure a safe connection.

An unnoticed break in the cable or soiling at the contact point may be sufficient to reduce the conductivity of the compound so far that no or only insufficient charge can be discharged. As a result, a static charge may occur.

Modern grounding monitoring

Therefore, industry best practice highly recommends the use of a monitored grounding device. The user only has to attach the clamp to the object to be grounded.

Then, the grounding device performs and indicates a secure grounding connection to the user (e.g. via green LED‘s) and transmits the release electronically via the control outputs. During the work process, the device continuously monitors the quality of the grounding connection. If there is no sufficient connection the control outputs are blocked and the work process is interrupted by the loading control system.

In addition, modern grounding control devices have a self-diagnostic function to continuously monitor all safety-relevant functions and to clearly identify possible faults with fault codes. Thus, the function of the device is ensured and dangers due to malfunction of the device are avoided.

For the reliable grounding of big bags and trucks, it is of great advantage to using a grounding control device with additional object detection. This effectively prevents the misuse and bridging the system and therefore increases the overall safety level. 

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