Electrostatics is a well-known everyday phenomenon in many industrial processes. Unfortunately, a dangerous, because electrostatic discharges can cause explosions.
Preventive explosion protection is characterized by the avoidance of potential ignition sources. Earthing systems provide the necessary assurance, especially when handling flammable or explosive substances, that static electricity is safely dissipated.
In our practical guide you will discover how electrostatic charges in hazardous areas can be effectively avoided and which earthing system is right for you.
At flammable and combustible product processing sites, the flow of gases, liquids, and solids is likely to generate electrostatic charges, such as filling and emptying containers, pumping, stirring, or mixing liquids. Due to their high potential for danger, electrostatic charges are considered as a possible source of ignition in potentially flammable and combustible atmospheres. The protection of employees and assets must not be left to chance.
The most effective and practicable method of preventing a fire or explosion from electrostatic charges is to exclude from the outset the electrostatic charging of equipment, vehicles or even people. Afterwards only conductive or dissipative objects or devices are to be used in potentially explosive areas. Depending on the ignition probability, these must be earthed or grounded. This allows electrostatic charges effectively and reliably dissipate from the EX atmosphere.
The grounding requirement also applies to people as well as to conductive or dissipative media, e.g. liquids or bulk goods
Grounding and equipotential bonding must be reliable and durable and withstand the expected stresses, in particular due to corrosion
Earthing terminals must be installed before starting work and remain in place until all dangerous discharges have been drained (pick-up devices or grounding clamps must be provided)
A separate operating instruction must be available for work on earthing and equipotential bonding in potentially explosive areas
People working in these areas must be instructed accordingly so that they know and use the equipment provided for earthing and equipotential bonding (it is especially important to note the typical ground faults, eg subsequent earthing of already loaded objects or equipment)
Equipment for earthing and equipotential bonding must be checked regularly (by qualified people for testing)
In practice, a number of procedural rules must be followed to protect against ignition by electrostatic charges. An important factor here is that electrically conductive system components (including mobile system components, people and vehicles) are connected to a "grounding source", which has been identified as a verified grounding point. Such a grounding point has a connection to the earth mass and discharges electrostatic charges from the plant parts to earth. The amount of charge generated by the process does not matter because the earth has an infinite capacity to balance positive and negative charges.
This connection of metal parts to ground point should have a maximum resistance of 10 ohms (see IEC 60079-32-1 section 184.108.40.206 Mobile Metal Parts).
To ensure adequate protection against electrostatic ignition, process equipment must be effectively grounded. Various systems are available for this purpose, which can be divided into three safety levels.
When selecting the suitable earthing system, operators should consider their risk assessment.
Often, grounding is achieved by means of single-core, passive earthing systems. These usually consist of a grounding cable and clamping pieces, which establish the contact between the object and the grounding point. Here you have the choice between screwable C-clamps and alligator clips, grounding clamps and clamps on the object side as well as eyelets as a connection option for earthing strips. However, it is not possible to ensure contact with the object with every pair of pliers. Plant components and equipment may contain paints, coatings, product deposits or rust layers which may prevent low-resistance contact between the earthing clamp and the components to be earthed. A stable electrical contact can be made in these cases only if the grounding clip can penetrate the contact-inhibiting layers. The necessary "bite resistance" is provided here by tested stainless steel pliers with tungsten carbide tips, which can penetrate through coatings and dirt. These ATEX and FM certified models ensure that they do not contain potential ignition hazards and have sufficient clip pressure and electrical continuity less than 1 ohm.
Passive systems are a low-cost grounding option for many standard applications. A disadvantage, however, is the assurance of contact. Not only degradations due to product deposits or protective coatings can lead to an unsafe connection, but e.g. also a bad condition of the cable connections, corroded ground connections or cable breaks. This requires a particularly high level of care in the application. It is the responsibility of the operator to regularly check the earthing systems for their safety. This includes the resistance check in the discharge path. However, since this often does not happen in practice, a bad ground connection is often not recognized immediately.
|+ Cost effective||– Repeated independent testing and monitoring of the earthing connection necessary|
|+ Sufficient for many standard applications|
In practice, safe grounding is not always carried out consistently and reliably. This also applies in particular to mobile objects (for example drums, containers or IBCs) that are always to be re-grounded. Earthing clamp contact may be affected not only by product deposits or protective coatings, but cable connections may also be rusted or otherwise in poor condition, so that an insecure connection may not be apparent at first glance. For efficient control, especially for larger hazards, active earthing systems with status display have therefore been developed. These monitor the connection with the object subject to electrostatic charging and optically signal to the employees whether there is a secure connection between the objects or with the tested grounding point. When the indicator turns green, employees know that they can safely complete the process. The grounding system continuously monitors the ground loop to ensure that the electrostatic charges generated by the process are safely dissipated from the potentially explosive atmosphere.
The ever-pulsing green LED on one of the brackets signals that there is a gap between the object in question, e.g. a barrel, and the site's verified grounding network has a resistance of 10 ohms or less. If the secure ground connection is interrupted, the LED indicator goes off. The employees can therefore clearly recognize that there is a risk of static discharge and that the filling process must be interrupted. Now the earth connection must be checked before the filling process may be continued.
People responsible for measures to protect against electrostatic ignition hazards usually have to choose between simple brackets and cables and monitored earthing systems. The latter offer advantages in many ways. On the one hand, they offer greater protection against electrostatic ignition hazards because they check the integrity of the connection with the system components. Furthermore, they offer the employees more security through the optical signal, whether an intact earth connection is present. But also the time required and the associated costs for a check of the secure connection can be minimized. These mobile grounding systems are particularly useful when manual intervention in the process prevents automation. An example of this are systems in which large quantities of drums and smaller containers are filled by hand with flammable liquids.
|+ High protection against electrostatic ignition hazards through permanent monitoring and optical indication of the safe connection||– No automatic locking function in automated processes|
|+ Safety for the employee|
|+ Time and cost savings through pure visual inspection of the contact|
In addition to a monitored grounding circuit, some grounding systems provide the opportunity to interact with the process. For this purpose, the earthing system has potential-free output contacts for communication with the process control or for direct coupling with devices which control the product flow or the processing of the product, such as, for example, Pumps, gate valves, stirrers or mixing plants. The connected devices will not be enabled until there is an intact connection with a maximum resistance of 10 ohms. This connection and the resistance value are continuously monitored.
If an insufficient connection of the system part is registered, then the process can be stopped by the corresponding signal of the earthing system. This will ensure that the equipment is grounded before the start of the charge generating process and that the process is stopped immediately if it is not. The activation of customer-side alarm systems in case of insufficient connection can also be realized. In addition, red and green LEDs indicate the earthing state according to the traffic light principle with "Stop / Go".
Earthing systems with integration into the process control offer maximum process reliability and protection against dangerous electrostatic charges. However, the acquisition costs are correspondingly higher compared to other systems. On the basis of a risk assessment, it should therefore be clarified whether company-specific conditions make the operation necessary - for example, in processes in which employees are not constantly on site to check the secure connection. In addition, processes that generate electrostatic charges often require manual intervention where automation of process control is not possible. An example of this are systems in which large quantities of drums and smaller containers are filled by hand with flammable liquids. In such cases, as well as with a limited budget, it may be possible to use grounding cables with monitoring function and optical display.
|+ Maximum safety for automated processes||– Purely stationary use, higher installation effort|
|+ Safety for the employee|
|+ Automated process release with intact, stop function with insufficient connection|
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