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ELEKTROSTATISCHE AUFLADUNG IN EX-BEREICHEN ZUVERLÄSSIG VERHINDERN

Explosive atmospheres require special protective measures – this sentence applies in particular to the invisible hazards of electrostatic charging. To many, this field appears to be a closed book and is not easy to understand due to the large number of variables. Yet, regulations and standards help to make the right decisions and to effectively protect against electrostatic hazards in explosive atmospheres.
 
In this article, we will show you which requirements result from this for electrostatic grounding in explosive areas, which regulations must be adhered to, and how modern grounding for explosive areas should be implemented.

Wann tritt elektrostatische Aufladung in der Praxis auf?

Elektrostatische Aufladung kann während der unterschiedlichsten Arbeitsprozesse erzeugt werden und sich dann auf Gegenständen oder Materialien ansammeln.
 
Je größer die Ansammlung wird, desto wahrscheinlicher ist die Entladung der Spannung in Form einer Funkenentladung – eine extrem gefährliche Situation in explosionsgefährdeten Bereichen.
 
Typische Aufladungsprozesse bei der Arbeit in explosionsgefährdeten Bereichen sind unter anderem:
 
  1. Elektrostatische Aufladung beim Umgang mit Flüssigkeiten
    > Befüllen und Entleeren von u.a. Tankwagen, Fässer, IBC
  2. Elektrostatische Aufladung beim Umgang mit Schüttgütern
    > Befüllen und Entleeren von u.a. Silo-LKW, FIBC, Fässer
Da der der Materialfluss flüssiger und fester Materialien oftmals mit sehr hohen Geschwindigkeiten erfolgt, beispielsweise der pneumatische Transport von Schüttgut oder das Befüllen von Tankwagen, kommt es zu einer Kontaktaufladung zwischen dem transportierten Material und den Transportanlagen.
 
Ist eines der Anlagenteile isoliert angebracht, hat also keinen oder nur ungenügenden Kontakt zu einer Erdverbindung, können sich die elektrostatischen Aufladungen ansammeln. Je größer die angesammelte Energie, desto höher wird der Drang, diese Ladungen zum Erdpotenzial abzugeben.
 
Diese Möglichkeit bietet sich immer dann, wenn sich ein leitfähiges, geerdetes Bauteil oder Objekt in der Nähe des isolierten Bauteils befindet. Beispiele hierfür sind unter anderem andere Anlagenteile, Metallfässer, Werkzeuge oder auch die Arbeiter selbst. Über diesen Weg können die überschüssigen elektrischen Ladungen in Sekundenbruchteilen zum Erdpotenzial abgeleitet werden.
 
Wird ein solches Objekt in die Nähe der angesammelten Energie gebracht, kommt es häufig zu einem unkontrollierten Ladungsübergang, einem elektrischen Funken. Dieser Funken bildet einen leitfähigen Kanal durch die Luft, in dem die elektrischen Ladungen abgeführt werden. Die hierbei freiwerdende Energie ist eine gefährliche, hochenergetische Zündquelle für die Atmosphäre des explosionsgefährdeten Bereiches.
 
Einen Überblick über die physikalischen Grundlagen der elektrostatischen Aufladung finden Sie in unserem rechtsstehenden Whitepaper und dieser bebilderten Übersicht.

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:

Timm EUS-2 overfill prevention controller international standard IECEx

IEC TS 60079-32-1 (2013)

Explosive Atmospheres – Guidance

BAUA-Logo

TRGS 727 (2016)

Avoidance of ignition hazard due to electrostatic charges

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.

TIMM Grounding Control Device EKX-4

Monitored grounding with object detection for explosive atmospheres

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.

 

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