Powder Volume Resistivity Testing

BS EN ISO IEC 80079-20-2 / BS EN 60079-32-2: 2015 / PD CLC TR 60079-32-1

The test is conducted by passing a predetermined voltage between two electrodes of known geometric size through a test material of known volume, under the control of the specific test conditions. Generally, direct resistance is measured, and the resistivity is calculated.

Many materials’ surfaces absorb atmospheric moisture or attract it to the surface of their particles. The addition or removal of this moisture often has a marked effect on the resistivity of the material. Therefore, preconditioning and testing are typically performed under controlled relative humidity conditions of 25 (± 5) % and 23 (± 2) °C.

The test classifies a material using the following standardised boundaries:

As per BS EN ISO / IEC 80079-20-2: 2016 Explosive atmospheres Part 20-2: Material characteristics – Combustible dust test methods, powders may be grouped according to their resistivity as follows:

• A resistivity of 1 × 103 Ω·m or less shall be a Group IIIC conductive dust.
• A resistivity greater than 1 × 103 Ω·m shall be a Group IIIB non-conductive dust.

As per PD CLC TR 60079-32-1:2018 – Explosive atmospheres. Electrostatic hazards guidance, powders may be grouped according to their resistivity as follows:

• low resistivity powders, with volume resistivity’s ρ ≤ 1 MΩ m;
• medium resistivity powders, with volume resistivity’s 1 MΩ m < ρ ≤ 10 GΩ m;
• high resistivity powders, with resistivity’s ρ > 10 GΩ m.

The volume resistivity of a powder dictates how efficiently charge migrate through a material by electrical conduction. The higher the volume resistivity value, the more resistive the powder is. High resistivity powders will accumulate and retain charge presented to them in all situations.

Low and most mid-range resistivity materials (conductive and static dissipative) will dissipate charge, providing it has a good path to earth. This process can be achieved by handling these powders in a well earthed environment (earthed conductive or static electricity dissipative containers, silos, hoppers, tools and plant equipment).

The build-up and retention of charge on a powder, object or equipment possess the biggest threat if the charge is suddenly released in the form of a spark discharge, which can cause an ignition of a flammable atmosphere. Therefore, it is imperative to know and understand the electrostatic properties of your powders to ensure they are handled correctly to prevent discharges and potential ignition of flammable atmospheres.