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The aim of this project is to verify the use of a Spark Test Apparatus (STA) for use at higher currents than currently set out in the standard IEC 60079.3. This was to be achieved through the use of a different electrode to that specified in the International Electrotechnical Commission (IEC) standard IEC 60079.3, for use in the standard Spark Test Apparatus to validate compliance of intrinsically safe circuits to the joint Australia/New Zealand (AS/NZS) standard AS/NZS 60079-11.
The two possible options available for a different electrode are an electrode of a different material or an electrode of the same material but different size. The literature review provided information on the use of copper wire for testing at higher currents. Based on this information and the request from IEC SC31G, the project was directed at investigating electrodes of the same material (tungsten) but of a different size. The Canadian Test Laboratory had produced a prototype Spark Test Apparatus making use of copper electrodes. The suitability and the sensitivity of their system has not been internationally accepted or published in international standards. At currents above 2A, resistive heating of the IEC specified 0.20mm diameter tungsten wire electrodes can cause an ignition and erroneously count as a spark induced failure. Since the testing and assignment of a temperature classification adequately addresses the heating effects of intrinsically safe equipment, the additional thermal ignition will unfavourably bias test results. It was therefore, decided to focus on the use of a larger diameter tungsten electrode.
Testing conducted at SIMTARS indicated that 0.38mm diameter tungsten electrodes could be suitable for currents higher than 2A, as no thermal ignitions were observed during tests up to 9.5A.