Powdered granular or pelleted materials can produced large amounts of static electricity. Electrostatics is a surface phenomenon and for a given mass of powder the total surface area and therefore the propensity to generate and store electrical charge is very high.

In industrial situations involving standard FIBCs there are primarily three ways in which potentially hazardous levels of electrostatic charge can be generated.

When filling an FIBC, product transferred to the bag is often in an electrostatically charged condition.  Bulking the product within the bag can intensify the volume charge density resulting in a high electric field radiating from the bag walls.

Emptying product from the bag will also generate charge due to triboelectrification (frictional charging) between the product and the bag wall.

Finally, simply handling, cleaning or rubbing the outside surface of the bag may also generate static charge.

 Once electrostatic charge has been generated or accumulated by any of the mechanisms described above a situation can arise in which electrical discharges can occur.

There are a number of different forms of electrostatic discharge possible from standard (non-antistatic) FIBCs.  These are:
(i)Brush discharges
These are low energy discrete electrostatic discharges which can occur from insulating surfaces such as the bulked product or the bag wall.  The maximum discharge energy is around 4 milliJoules.
(ii)Cone discharges
These can occur across the surface of the bulking material in the bag.  Cone discharges can be more energetic than brush discharges and their limiting energy is dependent on the diameter of the bag being filled as well as the particle size of the product.  Pellets can produce cone discharges of higher energy than fine powders.

(iii)Propagating brush discharges
This type of discharge can occur when ions are created at powder bulking.  Some of these are repelled towards the bag wall and can accumulate there, often causing opposite charge to appear on the outer bag wall.  Propagating brush discharges can have a content (up to 1 Joule) and can readily ignite solvent vapours and powder clouds.

Under certain conditions, other electrostatic discharge mechanisms may also appear:
X  an electrically insulating bag (standard polypropylene) which has become contaminated with a conducting substance such as water, may also be capable of producing spark discharges from the contaminated area to any nearby earth.
X  conducting objects, including personnel, which are in the vicinity of a charged bag and are not properly grounded can also become electrostatically charged by induction and depending on their proximity to other grounded objects could produce spark discharges.

If the bag receiving the powder is made from a conducting material which is connected to earth, then brush and propagating brush discharges from the bag fabric are no longer possible.  External bag contamination and external object charging by induction are also no longer relevant.  If the bag is not earthed or becomes disconnected from earth then energetic spark discharges become possible.

The energy stored on a conducting object depends on its capacitance and the square of the voltage to which it is raised, and it is easy to envisage spark energies of a few tens of mJ for conducting patches on insulating bags, people and movable objects in the area.  The spark energy from a charged, electrically isolated conducting FIBC could be considerably greater, and perhaps over 100 mJ.