Determination of the electrical resistance of dust samples – comparison and evaluation of different methods

Status:

completed 12/2011

Aims:

Analysis of sources of ignition is of great importance for the risk assessment of processes in the area of explosion prevention and protection. The formation of ignition sparks resulting from the electrostatic charging of bulk goods is closely related to these goods' electrical resistance.

Determination of the electrical resistance of dusts permits prediction of electrostatic charging during separation and deposition processes, or where dusts are conductive, of the risk of short-circuit currents caused by their deposition upon current-carrying components. The literature currently describes two measurement methods (EN 61241-2-2, IEC 93). The values produced by the methods are not comparable. The first stage of the project examined which methods can be used to determine reproducible values under laboratory conditions. The influence of further dust properties, such as moisture, grain size, powder density, etc., upon the measured resistance values was studied, and the metrological limits of the methods were also determined.

Activities/Methods:

Six specially selected test dusts were used to study the two standard methods by means of comparative measurements. Based upon the experience gained to date, a new test cell was designed which can be used to produce reliable measurements even in the teraohm range. All three methods were then applied to the test dusts, and the results interpreted statistically.

The influence of pressure and indoor atmospheric humidity were then studied systematically in the climatic chamber and in the desiccator.

Results:

Comparison of three different test cells with the use of selected standard dusts for determination of the specific electrical resistance of dust layers yielded close correlation between the channel test cell to EN 61241-2-2, "Electrical apparatus for use in the presence of combustible dust - Part 2: Test methods - Section 2: Method for determining the electrical resistivity of dust in layers" and a newly developed concentric test cell employing annular electrodes. The scatter of the measured values is caused by the stochastic orientation of the dust particles in the test cell. In the piston test cell to IEC 93/TRBS 2153, the weight of the electrode combined with compressible deposits gives rise to a systematic error which can lead to deviation by a factor of ten from the results obtained by the two other methods.

The influence of the atmospheric humidity upon the specific resistance during measurement was proved conclusively by long-term studies. The kinetic characteristics of these processes are dependent upon the substrate surface of the dust. This has a considerable influence upon the variation in the specific resistance over time. In four of the standard dusts studied, additional consolidation of the dust layer led to a reduction in the specific resistance. Measurement voltages above 100 V may trigger chemical and physical processes in the dust layer which give rise to orientation of the dust particles in the electric field and possibly even discharge.

The annular test cell will replace the method to IEC 93 employed at the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) for determination of the electric resistance of dust layers. This will improve the validity of the test results and the consultancy provided by the IFA's prevention services.

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