Definition
In FIBC, charge is generated by the contact and separation of product with conveying systems and the FIBC; a mechanism called tribocharging. In the context of FIBC, charging current is the rate at which charge is generated when FIBC are filled or emptied. The SI unit of charge is the coulomb, C. Rate of generation of charge is measured in coulomb per second, C/s, or more commonly amp, A (1 A = 1 C/s).
The importance of charging current in selecting safe static protective FIBC
One coulomb is a relatively large amount of charge. When powders or granules are tribochraged during FIBC filling or emptying operations, the amount of charge may be of the order of 10 nanocoloumb per kilogram (nC/kg). A full FIBC containing 1000 kg of product, if emptied rapidly without restriction, can be emptied in the order of 10 seconds. In this scenario charging current, IC, can be estimated as:
For highly chargeable products being emptied rapidly from FIBC, experimental evidence indicates that charging currents can be as high as 3 μA. Hence, this is the value that is used in the safety qualification testing of Type D FIBC specified in IEC 61340-4-4 (also see Proving the Safety of CROHMIQ). The concept is to test under worst-case conditions likely to be found in industry. Qualifying Type D FIBC under realistic worst-case conditions ensures that they are safe in all other conditions.
For Type C FIBC, IEC 61340-4-4: 2018 requires resistance to ground to be less than 1 × 108 ohm. In theory, this limits Type C FIBC to operations where charging currents do not exceed 3 μA. However, research from Japan[i] indicates that even higher charging currents do not produce incendiary discharges in Type C FIBC, provided they are securely grounded*.
*WARNING: Type C FIBC must be securely grounded before and during filling and emptying operations. Isolated Type C FIBC are extremely dangerous and are known to have caused explosions.
The breakdown voltage requirement for Type B FIBC specified in IEC 61340-4-4 ensures that propagating brush discharges cannot occur, even with charging currents exceeding 3 μA. It must be remembered that Type B FIBC are still at risk of generating regular brush discharges even at low charging currents. Type B FIBC, therefore, are only safe for packaging powders (sugar, flour, corn starch, etc.) with MIE greater than 3 mJ. There is a risk of ignition and explosion if Type B FIBC are used to package flammable powders, or any products containing residual solvents (e.g. toluene, xylene, etc.) or flammable gases (e.g. pentane), or any products emptied into flammable solvents.
Measuring charging current
In most cases, it is not necessary to measure charging current. As previously explained, the concept behind the testing as safety requirements specified in IEC 61340-4-4 ensure that static protective FIBC qualified to this standard are safe for most industrial applications.
If charging current is required to be measured, perhaps for a new process, or when handling specialist materials, the principle of the Faraday pail can be utilised.
The Type D FIBC Technology Monthly Newsletter for July 2024 explained the use of a Faraday pail to measure the charging current in an expandable polystyrene (EPS) manufacturing operation. The same measurement arrangement can be adapted and used to measure charging current for other FIBC filling or emptying operations.
A Faraday pail simply consists of a conductive container. The principle of operation of a Faraday pail is that any charge placed inside the container induces an equal charge on the surface of the container. If the container is connected to ground, charge is conducted to ground at a rate that is equal to the rate at which charge is entering the container. Charging current can be measured by placing an ammeter in the ground line.
In Texene Safety Performance Testing Laboratories, 55 gal. steel drums, without tops, are used as Faraday pails to check the charging current for the IEC 61340-4-4 ignition testing apparatus. 55 gal. steel drums are convenient to use when flow rates are a few kilograms per second. For faster flow rates, larger containers are required. Type C FIBC can be used as Faraday pails, provided they are of good quality with all conductive elements electrically interconnected.
Whatever type of conductive container is used to form the Faraday pail, it is important to ensure that the only electrical connection to ground is via the ground line through the ammeter. Otherwise, current can leak to ground and by-pass the measuring circuit. The container, therefore, needs to rest on, or be supported by insulating materials.
The measuring instrument (ammeter) used by Texene is a 6½-digit multimeter operating in DC current mode. Suitable instruments are widely available from Fluke, Keithley, and other manufacturers.
Below are some schematic arrangements showing ways that different types of containers can be utilised as Faraday pails.
(a) Measuring charging current in processes that fill FIBC at flow rates up to a few kg/s
(b) Measuring charging current in processes that fill FIBC at faster flow rates
(c) Measuring charging current when emptying FIBC
Precautions to take when measuring charging current
Charging current measurements as described above must not be done in areas where flammable gas or solvent vapours may be present (i.e. Zones 0, 1 or 2 / Class 1, Divisions 1 & 2).
If the product is known to be combustible or flammable, precautions must be taken to prevent the formation of dust clouds when making measurements.
All electrical connections, particularly the ground line connection to and from the ammeter must be securely made before any product flows. The ammeter must also be switched on before any product flows to ensure that internal connections are established to allow current through the instrument to ground.
Unshielded Faraday pails can be influenced by stray electric fields, which can cause measuring errors. Any insulating materials that may be electrostatically charged should be moved well away from the Faraday pail during measurements. All nearby conductors, including personnel, should be grounded.
It is recommended that experts in electrostatic measurements are consulted or contracted to carry out charging current measurements. The independent laboratories listed in the links below may provide assistance. Please note that this list is not comprehensive; there are other laboratories that offer similar services, which may be found via online searches.
[i] Yamaguma et al, J. Chem. Eng. Japan, Vol. 48, No. 8, 2015.