Safe Grounding in Explosive Atmospheres
How do grounding systems in explosive atmospheres protect against electrostatic hazards?
Table of contents:
When does electrostatic charging occur in practice?
Electrostatic charging can be generated during a variety of different work processes and can then accumulate on objects or materials. The greater the accumulation, the more likely it is that the voltage will discharge in the form of a spark discharge – an extremely dangerous situation in potentially explosive atmospheres.
Typical charging processes that occur while working in explosive atmospheres include:
- Electrostatic charge when handling liquids
> Filling and emptying of tank trucks, drums, IBCs
- Electrostatic charge when handling bulk materials
> Filling and emptying of, among others, silo trucks, FIBC, drums
Because the flow of liquid and solid materials often occurs at very high speeds, for example the pneumatic transport of bulk materials or the filling of tank trucks, the transported material and the transport equipment do tend to come into contact with each other.
If one of the system components is isolated, i.e. has no or insufficient contact with a ground connection, the electrostatic charges can accumulate. The greater the accumulated energy, the higher the need to discharge these charges to the ground potential.
This option is always available when a conductive, grounded component or object is located near the insulated component. Examples include other equipment parts, metal drums, tools, or even the workers themselves. The excess electrical charges can be discharged to the ground potential in a fraction of a second through this path.
Placing such an object near the accumulated energy often results in an uncontrolled transfer of charge, an electrical spark. This spark forms a conductive channel through air to dissipate the electrical charges. The energy released in this manner is a dangerous, high-energy ignition source for the atmosphere of an explosive environment.
How does grounding help to protect against electrostatic charging in explosive atmospheres?
The by far most effective method is to ground all conductive objects in the explosive environment. In this case, the resulting electrostatic charges are safely discharged to ground potential without being able to accumulate on the object – and therefore no longer pose an ignition hazard. This ground connection should be in place for the entire time of the charge-generating operation and should be adequately conductive to ensure safe grounding in the hazardous area at all times.
Work instructions and limit values specified in standards, directives and guidelines help those in charge of process plants to implement this grounding connection correctly. In addition, many of the specifications can be used as models for in-house regulations.
One of most important directives for grounding in explosive atmospheres is:
The quintessence of this standard is to ensure that charges of equipment parts, of the transported substances but also of the persons working in the explosive atmosphere are prevented. Performing grounding in accordance with standards is also considered the most straightforward and effective way to prevent the dangers of electrostatic charging.
As such, conductive objects that present the potential risk of electrostatic charge should generally be grounded, and the ground connection used in the process should generally not exceed a discharge resistance of 1 megaohm (106 ohms).
What is the difference between grounding and equipotential bonding?
In the aforementioned standards, reference is made to both grounding and equipotential bonding in the explosive atmosphere to prevent electrostatic charging. Yet, these two measures do in fact follow different objectives. This is why it is so important to understand the difference between the two terms.
While grounding involves establishing a conductive connection from the object to the ground potential, equipotential bonding refers to a conductive connection of two or more objects. The purpose of this is to prevent charges of different strengths from forming on the individual components, which in turn prevents a potential spark over between these parts.
Yet, even if a functioning equipotential connection is established between the objects, this does not imply that there is also a dissipative connection to the ground potential. This can have the effect that the objects continue to have the ability to electrostatically charge. If a conductive object, such as a worker or a tool, comes in close contact with these equipment parts, the accumulated energy can discharge despite the existing equipotential bonding and ignite the atmosphere in the explosive environment.
This is why: Even if equipotential bonding has occurred, it must be ensured that at least one point of the assembly has a conductive connection to ground potential. This is the only way to prevent electrostatic charging.
How can I establish a safe ground connection in an explosive atmosphere?
The general rule is that if a metallic connection line (cable) is used, a dissipative ground connection should be established. As long as the connection is not being monitored, it must be sturdy enough to maintain the maximum discharge resistances over a long period of time and to withstand mechanical or weather influences. Therefore, depending on the application, a cable cross-section of 4-10mm² is sufficient for an unmonitored grounding.
In addition to the possibility of unmonitored grounding, it makes sense to install automatic grounding monitoring, especially in explosive environments. The use of monitored grounding systems such as the TIMM EKX-4 provides some safety but also practical advantages and represents the recommended best practice of the mentioned standard.
These devices ensure a ground connection in compliance with the standards and also monitor compliance with the stipulated limit values. As a result, grounding problems, such as an undetected cable break or a dropped grounding clamp, are reliably detected and the corresponding hazards are prevented by an immediate process interruption.
There are also benefits for the worker during daily use. Status lights allow him to read the grounding status at any time and detect a potentially dangerous situation at an early stage. If the device is integrated into the system control, it can also help to ensure that work and safety instructions are adhered to. Since there is no process release until a proper ground connection is made, the worker on site is forced to use the safety device.
Grounding devices with integrated object detection are particularly suitable for this application. These devices detect the electrical characteristics of the connected object and compare them to defined reference values. This effectively prevents the ground clamp from simply being connected to a metallically conductive point in the system or bridging the ground connection. This helps to prevent any manipulation or bypassing of safety devices.
These devices are used especially frequently when loading and unloading trucks, FIBCs, IBCs and drums in explosive atmospheres.
You are unsure if and how you need to implement electrostatic grounding in your application?
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