Are You Using the Right Hose for Combustible Dust Vacuuming Applications?
Why Is It Important?
Significant electrostatic charge can be generated where nonconductive hoses are used for vacuum cleaning, vacuum transfer and pneumatic transfer applications. The development of electrostatic charge can lead to various types of electrostatic discharge inside of the hose. If the solid particulate being vacuumed is combustible in nature, the discharge can ignite a relatively dense dust cloud inside of the hose, causing an ignition.
What Properties Should my Hose Have?
International and European publications on electrostatic hazards  provide guidance with regard to properties of hoses used for this type of application. A suitably conductive or dissipative hose (sometimes referred to as antistatic) will have a leakage resistance to earth from anywhere on the inner surface of less than 108 ohms (100 megohms) as measured in accordance with ISO 8031 (and EN ISO 8031) . Where hoses are terminated with a metal nozzle (e.g. for vacuum cleaners), this will normally need to be separately earthed using a metal conductor. Where hoses incorporate metal wires for reinforcement, the wires must be earthed and bonded to the conductive components at each end.
 IEC/TS (and CLC/TR) 60079-32-1, "Explosive atmospheres Part 32-1: Electrostatic hazards, guidance", IEC (CENELEC).
 ISO (and EN ISO) 8031, "Rubber and plastics hoses and hose assemblies. Determination of electrical resistance and conductivity", ISO (CEN)
A Common Misconception
The hose I am using has metal reinforcing wires extruded in the liner and I have bonded the wire to conductive parts at each end, so there is no danger of charge build up. Fact: If the construction material of the hose is non-conductive, charge can build up on the inside surface of the hose and lead to propagating brush discharges of sufficient energy to ignite a combustible dust flowing inside the hose. The presence of the conductive wire actually makes this more likely.
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