Hazards of Static Electricity in Ex Zones
Prevent the ignition hazards of electrostatic charging with standard-compliant grounding
Table of contents:
Little attention is often paid to electrostatic charging as a potential ignition source in ex zones. But a look at the energy released in the spark discharges clearly shows how high the risk of ignition really is.
Therefore, in this article you will learn how electrostatic charging occurs and which legal requirements you should meet in order to prevent electrostatic charging with standard-compliant grounding.
In addition to the physical background of electrostatic charges, we also show you examples of the real ignition hazards that can arise in everyday work environments.
A detailed description of the physical background and ignition hazards in dust-ex atmospheres can be found in the following white paper:
How does static electricity arise in ex atmospheres?
In a large number of manufacturing processes as well as filling and emptying processes, there is unavoidable, intensive contact between the material and the equipment, machines, and transport facilities. Intensive contact means that the surfaces are max. 10nm apart.
From a physical point of view, two different materials with their respective electron binding forces meet at this moment. At the moment of intensive contact, electrons are exchanged between these two materials in order to create an electrically balanced state between the materials. If this contact is broken again, the electrons are normally retransferred.
What are the ignition hazards of static electricity in ex atmospheres?
The charged surfaces and materials then strive to achieve charge neutrality. To put it simply, they want to get rid of the excess charges. The easiest way is to discharge the excess charges towards ground potential.
The easiest way is to discharge the excess charges towards ground potential. If a sufficiently conductive connection is established between one of the charged surfaces and the ground potential (e.g. by an employee wearing conductive shoes), a sudden charge exchange takes place.
Depending on the strength of the accumulated charge, this discharge can also take place before actual contact with the charged surface. This then leads to a spark discharge, which releases uncontrolled energy into the surrounding ex-atmosphere and can cause an explosion. In this case, even the charging of smaller objects is already sufficient to cause an ignition-effective discharge.
A small container of up to 50 l can already have an ignition energy of up to 3 mJ.
Putting this in relation to the minimum ignition energy (MIE) of flammable gases and vapors, it becomes clear how high the risk of explosion is:
Minimum ignition energies
of selected gases and vapours
An electrostatic discharge can also have an ignition effect in pure dust-air mixtures and mixed zones of gas and dust hazardous areas.
Here, however, the MIE of the atmosphere is particularly dependent on the median particle size of the dust and the mixing ratio between gas and dust atmosphere.
A general statement is made by the classification of the professional association for raw materials and chemical industry in Germany, which divides the dust into a total of 4 different classes according to their minimum ignition energy:
How can electrostatic charges be prevented?
It is therefore evident that electrostatic charging must be prevented, particularly in potentially explosive atmospheres, in order to prevent it from becoming an ignition source for explosions.
Electrostatic charge can only accumulate on an object if the object is isolated from ground potential or if there is such a high resistance between the object and ground potential that the resulting electrostatic charges cannot be dissipated quickly enough. The electrostatic charge on the object increases over time until it is suddenly discharged.
This means that a conductive ground connection can help. By connecting the object to a designated grounding point (e.g. the potential equalization rail), the electrical charges can be dissipated immediately and do not accumulate on the object.
Which rules must the grounding connection in ex zones comply with?
From a European point of view, the standard IEC/TS 60079-32-1 should be observed for standard-compliant grounding. It describes process sequences and measures to contain hazards due to electrostatic charging and also provides calculation examples for determining important parameters such as the potential ignition energy of the work process.
Electrostatic hazards – guidance
It states that a ground connection with a resistance value of 10^6 Ω is generally sufficient. However, deviating from this value, the regulation specifies different discharge values as well as instructions for special applications such as the grounding of big bags or tankers, which must be observed separately. For example, higher resistance values are considered sufficient for type C big bags, while monitored grounding is recommended for grounding trucks.
However, depending on which country you come from, local regulations or company rules may also apply.
What are the advantages of monitored grounding devices?
Basically, a simple, unmonitored grounding connection with high-quality and robust grounding clamps and cables, such as the TIMM Bite grounding series, is sufficient to comply with the maximum leakage resistance.
However, unnoticed damage to the cable, a break in the cable core or contamination at the contact point can be enough to reduce the conductivity of the connection to such an extent that no or only insufficient charge can be dissipated.
This means that electrostatic charging is still possible.
It is therefore sensible to use monitored grounding with a grounding control device. The user only has to attach the grounding clamp to the object to be grounded.
The monitoring device then shows the user a safe ground connection (e.g. via an LED display) and transmits the release signal electronically via the control outputs to the process control.
During the work process, it constantly monitors the quality of the grounding connection.
If the connection gets separated or is not sufficient, the device interlocks the control outputs to stop the work process and signals the danger via a red LED display.
For the reliable grounding of trucks and big bags, it is also sensible to use grounding devices with object recognition, which can distinguish whether the grounding clamp is really connected to the correct object (truck, big bag) or whether it was operated incorrectly.
Doing so, permanent releases (e.g. by clamping the clamp on a piece of metal) can be prevented.
You can find an overview of our portfolio of monitored grounding devices at: More details