Currently the highest capacitance allowed for Group I (underground coal mining) intrinsically safe equipment is lower than that allowed for Group II (petrochemical industry) intrinsically safe equipment, despite the fact that the energy required to ignite Group I gases is higher than for Group II.

The current standards are based on capacitance curves developed in early British standards that have been transferred into tables. The researchers said transferring the maximum capacitance values from curves to tables has "highlighted numerous interpolation errors", which they now hope to correct.

The ACARP-funded project, Determination of Permitted Maximum Capacitance Values for Intrinsically Safe Equipment, seeks to correctly define reliable values for the level of electrical capacitance allowed.

NSW DPI senior engineer Mohammed Abdelkrimi said that with current capacitance standards, operators and designers of equipment for underground coal mines are limited in the equipment that can be used.

“They are constrained to use values that are less than what they normally would use, and it limits them in the design, it limits them in the number of equipment that they might be able to connect, it's just very restrictive for them," he said.

“The project is about determining capacitances for Group I that are in line with requirements of the standard."

The determination of capacitance having a safety factor of 1.0 for Group I involves charging a capacitor to a certain voltage, then discharging the capacitance into air containing 8.3% methane, the most easily ignitable mixture found in underground coal mines. The test is repeated, with increasing voltages, until ignition occurs.

The capacitance that has a safety factor of 1.5, the requirement for IS equipment, is then derived from the results of safety factor 1.0, by dividing the maximum voltage by 1.5.

Sample tests have already shown significant differences between the current capacitance standards for underground coal mines and actual safe levels of capacitances.

Under the current standards the maximum safe capacitance given for a voltage of 15.5V is 71µF, whereas in sample tests the maximum safe capacitance value was found to be 120µF. Similar differences were recorded for voltages of 14V, 15V and 16.2V.

Based on these results, the researchers said it would be possible to increase the capacitance available to the industry, and therefore "increase the safe useable applications of intrinsically safe equipment", allowing equipment designers more flexibility in creating intrinsically safe equipment, simplifying compliance assessment and reducing the time and cost of bringing new intrinsically safe equipment to the market.