Electrostatics in hazardous areas: An underestimated hazard

Electrostatic grounding or bonding can be established using simple cables with suitable connection elements, such as clamps. However, it is important to remember that these grounding kits are subjected to heavy mechanical loads, especially in the case of temporary grounding. Often, they will fall to the floor and make their way beneath shoes or the tyres of forklifts and trucks.

However, the smallest mechanical damage to the conducting connection in the cable or clamp can lead to this safety equipment having no effect. Regular conductivity checks reduce the risk of this kind of damage going unnoticed. However, these checks involve additional work and it remains uncertain whether a loose contact would definitely be detected.

The contact between the clamp and the object to be grounded poses an additional risk. How safe is it if the clamp penetrates the coating or corrosion on the surface of a drum? Safe operation of the system is dependent on a number of factors, not least this contact.

The use of a monitoring device offers several advantages compared to passive grounding using a simple grounding kit. These devices establish and permanently monitor the connection between the equipotential bonding rail and the object to be grounded. This ensures that any absent or ineffective ground connections are safely detected. The status of the grounding is shown by an indicator lamp; additionally, on many devices, an additional contact can be used to stop loading directly or via an automation system.

A number of versions can be used to ground trucks and FIBCs – these additionally feature an object detection function. Once the devices are connected, this function enables them to detect whether they are connected to a truck or an FIBC – or whether the grounding clamp has only made contact with metal objects on the loading equipment. This detection system makes these applications significantly safer.

Electrostatic charges build up as soon as the liquids start moving, whether that is through pumping or mixing, or when loading or unloading containers. The extent of the charge is determined by the conductivity of the liquids, the characteristics of the tank and the flow speed. TRGS 727 contains detailed information about what to consider when handling liquids.

When loading road tankers, it must be ensured that grounding is performed before any covers are opened and before pipelines are connected. This grounding must have a resistance less than 1 M Ohm. Without additional grounding measures, this value will not be achieved. The connection to ground or equipotential bonding for the loading equipment must be present during the entire work process. The use of grounding equipment with a lock (i.e. automatic switch-off) is considered practical. On an international scale, IEC EN 60079-32-1 additionally requires the conductive connection between the connection point on the truck and the grounding point on the system to be less than 10 Ohm.

In the case of rail tankers, it can be assumed that sufficient grounding is established via the rails, due to their design. Despite this, the limiting values must also be complied with. In case of any doubt, additional grounding must be established.

Smaller containers (volume ? 1 m³) such as drums or IBCs should preferably be made of conductive or dissipative materials, which are connected to a ground potential before filling or emptying. The RIBCs consist of an insulating plastic tank, embedded in a metal frame. These must not be used for combustible liquids in explosion group IIC and IIB with a minimum ignition energy < 0.2 mJ. It must be ensured that versions that are specially designed for handling combustible liquids are used in this case. For instance, these have a cover and an outlet fitting made of dissipative material. The grid and outlet fitting must have a connection to ground.

When using these containers, it must be ensured that the measures for preventing electrostatic discharge are also implemented when non-combustible liquids are filled or emptied in an explosive atmosphere.

The grain size is a crucial factor in determining the ignition capability of bulk goods. The greater the grain size, the lower the ignition capability. As soon as the minimum ignition energy exceeds the value of 1 J, no additional measures to prevent electrostatic discharge must be taken. For finely-grained bulk goods, the specific resistance, humidity and conveying speed all determine the extent of the charge. It must be ensured that all conductive and dissipative parts of the system, including the containers, are grounded.

The use of flexible bulk goods containers are a special case in this regard. These containers are known as FIBCs (Flexible Intermediate Bulk Containers) or Big Bags. These containers are available in four different versions (A, B, C and D). Type A features no measures to reduce electrostatic charges. Types B, C and D feature design measures to prevent hazardous charges.

¹ When filling the FIBC only in conjunction with additional measures, e.g. purging with air.
² When filling and emptying the FIBC only in conjunction with additional measures, e.g. inertisation.
³ For the concentration of the solvent content, see also no. 6.2.
⁴ If the handled bulk goods determine the zone.
⁵ The restriction to only use a type C or type D FIBC for bulk goods with MIE < 3 mJ is based on the ignition capability of cone discharges. These can possess much higher energy in a type B FIBC than in a type C or type D FIBC. If a type B FIBC is used, the cone energy can reach a value of 3 mJ, even for bulk goods with a median value of just 0.055 mm. In a type C or type D FIBC, the median value of the bulk goods must be 0.27 mm or higher, so that the cone energy reaches 3 mJ. Typically, however, bulk goods that are this coarse have a MIE higher than 3 mJ.

Thanks to their structure, type B FIBCs prevent the build-up of propagating brush discharges because the breakdown voltage for the insulating material is less than 6 kV. Other forms of discharge, such as brush discharges or cone discharges, may still occur.

Type C FIBCs contain dissipative materials, which ensure that the leakage resistance is under a value of 10⁷ Ohms. This type of FIBC must be grounded. The use of a grounding monitoring device with object detection is recommended for grounding these FIBCs. Devices with this function monitor the correct connection to the Big Bag and ensure that operating errors are eliminated.

The principle behind type D FIBCs is based on limiting the charge to a safe extent, based on corona discharge. Evidence must be provided that no dangerous charges are generated. Conductive objects and people in the immediate surroundings must be grounded.

The movement of gases does not generate an electrostatic charge. Pneumatically conveying bulk goods is an exception to this statement. In this case, the specifications for conveying bulk goods must be observed.