An investigation of the occurrence of propagating brush discharges from Type B flexible intermediate bulk containers with liners

Flexible Intermediate Bulk Containers (FIBC) are generally divided into groups (A–D) based on their electrostatic ignition hazard. The key distinction between a Type A FIBC and a Type B FIBC is that the latter is incapable of producing highly incendive propagating brush discharges. This ability is normally evaluated by measuring the breakdown voltage, with the criterion for a Type B (according to the latest standard, IEC 61340-4-4:2005) being that the result must be less than 6 kV. However, the use of liners is specifically excluded, though in practice many powders transported in FIBC require a liner to be used, typically due to transport regulations. The presence of a liner clearly influences the breakdown voltage, and hence the propagating brush discharge risk from the package. The logical extension of the general Type B requirement would be that a liner with a breakdown strength of <6 kV could prevent the occurrence of propagating brush discharges; unfortunately, as the liner material and thickness are usually dictated by strength and compatibility requirements, a higher breakdown voltage is often unavoidable. Despite this, experiments on typical Type B FIBC/liner combinations together showed that even if the breakdown strength of the liner is >6 kV, propagating brush discharges may still not be possible from the bag/liner combination, though with thicker liners it was shown that propagating brush discharges could still occur. A relationship is proposed to identify the potential for a propagating brush discharge based on the FIBC fabric thickness, liner thickness and liner breakdown strength.