Conductive threads are often incorporated into the fabrics used for FIBCs (as they areinto personnel protective clothing) for use where flammable atmospheres may be present.The Type C construction of FIBCs www.fibca.cn requires that these threads be bonded to earth. It is noteasy to feel confident in achieving this over extended operating lifetimes. There is the viewthat the Type D construction, which does not require earth bonding, can be acceptable.Critical aspects in the design of unearthed Type D bags are whether an electrostatic dischargeto a bag containing charged material can get rapid access to sufficient electrostatic energy toprovide a source of ignition of any flammable materials nearby (gases or dusts). The chargeto feed an electrostatic discharge could come from material within the bag, but more probablyfrom the outer bag surface. Charge on the outer surface of the bag will probably prevent anysignificant electrostatic discharges from within the bag. The prime concern is hence withcharge on the outer surface. If the bag surface is a good quality insulating material then aninitial discharge to an earthed probe approaching the charged bag will spread outwards overthe charged surface until the radial electric field at the edge of the discharge can no longersupport further propagation. By making the surface ‘resistive’ the radial propagation of thedischarge can be limited and the opportunity for incendive discharges greatly reduced.Making fabrics ‘resistive’ within the appropriate range (around 108ohms [1]) would not beeasy. Use of ‘conductive threads’ of appropriate resistivity is likely to achieve a similarcontrol much more easily. The threads are likely to limit radial propagation of the dischargeso long as the resistivity of the threads prevents charge being drawn in rapidly from the net ofthreads. Clearly interconnected metallic threads would be quite inappropriate!

　　The use of threads with ‘core conductivity’ is attractive as such threads have a high‘resistivity’ and will not interconnect, except to a very minor extent by capacitance at anythread crossovers. A problem with this type of thread is to know their characteristics becauseyou cannot reliably get access to measure thread resistance. Work is to be reported at theIEEE-IAS meeting in Rome in October [2] will show how the resistive characteristics ofconductive threads in fabrics, and the influence of any antistat surface treatment, can bemeasured non-invasively. The approach is to measure the variation of shielding performanceas a function of frequency. Figure 1 below shows measurements on a few practical materials.The materials here all show a reduction of shielding attenuation to near zero by 2MHz or so.Where similar measurements are made for fabrics including metallic conductive threads theshielding performance depends on the amount of metal thread present and is clearlyindependent of frequency over this range.

　　What is now needed is experimental matching of the variation of shielding performancewith frequency to the chance of ignition by electrostatic discharges. The model proposed forassessing materials is plausible - but is in need of quantitative confirmation.