When IS is not safe - Part 1

Staff Reporter

Questions raised about varying testing procedures and industry safety remain unanswered.

On the day before Christmas last year apprehension became alarm as

underground coal mine managers in New South Wales received their second safety alert in as many days. It stated that intrinsically safe power supply equipment used at many mines was no longer intrinsically safe and had to be removed.

The resulting impact on mines ranged from minor disruption to complete shutdowns. The chain of events that culminated in the safety alerts has since been the subject of much conjecture in the industry.

In attempting to determine precisely what happened, Australia’s Longwalls approached safety inspectors, equipment manufacturers, testing bodies, Standards Australia, NSW and Queensland government departments, and electrical engineers. Almost without exception those approached were only prepared to discuss what “they thought they knew” off the record.

What quickly became clear though was that testing procedures used to determine whether a piece of equipment is intrinsically safe (IS) for use in underground mines — in other words, whether the equipment will cause an explosion under certain conditions and threaten miners’ lives — were the focus of most of the hypothesising.

Australia has two accredited testing laboratories: New South Wales-based Londonderry Occupational Safety Centre (LOSC), and SIMTARS in Queensland. Both work to standards specified by Australian Standards.

Austdac 12V power supply units originally certified and approved in 1995 by Londonderry, and carrying Mines Department Approval (MDA), were installed in many local mines. The range was subsequently extended to include 14V, 18V and 24V supplies, used in potentially hazardous environments to power longwall and other mining machines, environmental monitoring systems and other underground equipment.

In December, several Austdac power supplies caused a gas ignition in a test situation and were pronounced unsafe. Ramifications of this testwork continue to be felt throughout the industry.

Australia’s Longwalls obtained documents in which Austdac Pty Ltd outlined the situation to mines in late December, over the signatures of Austdac’s commercial director Simon Cole and marketing director, Geoff Watkinson. According to the letter “tests were carried out late on 23 December 1998 by the WorkCover Authority in the presence of the senior electrical inspector of the New South Wales Department of Mineral Resources and Austdac representatives”

What happened in the test room must have caused consternation to those present. The approved Austdac power supplies were tested with a “spark test apparatus” and two important issues became immediately apparent.

First, according to the NSW DMR alert, “an explosive mixture of gas was ignited with an output current significantly lower than 1 ampere. The apparatus cannot be considered intrinsically safe and apparatus and cables connected to the power supplies may cause the ignition of flammable gases.” In other words, the power supplies failed a seminal test of safety and were potentially an accident waiting to happen.

Secondly, it looked as if the power supplies in question had not previously undergone testing using the spark test apparatus. According to several IS professionals who spoke to Australia’s Longwalls, a spark gap test is the only way to test such power supplies.

Exactly why re-testing of the power supplies became so imperative in late December is unclear. According to Londonderry, “test staff detected potential areas of non-compliance with the Australian Standard for intrinsic safety in the existing equipment and so a re-assessment and additional testing were carried out. The re-assessment process indicated there were major non-compliance issues that could impact on safety by causing the risk of an explosion”

According to a NSW DMR information circular issued in January, the approved Austdac power supplies were being assessed for “supplementary approval and certification”. However, this seems unlikely given Austdac’s letter to the mines which also said: “Austdac wishes to stress that it had no prior indication whatsoever of any non compliance of its apparatus until tests were carried out late on 23 December 1998.”

The presence of both the inspector and Austdac personnel at the December 23 testing was unusual, said one industry source, while the NSW DMR rapid response smacked of crisis management. Clearly, something had been overlooked or had gone wrong and all parties involved suddenly found themselves with a problem on their hands.

Chris Aguis, program manager product certification at Standards Australia, said confidentiality rules prevented him discussing any specifics but that “standards do change — processes change”

“Obviously if there’s a new standard, if it’s felt that something done previously under all good intentions and using known technology and processes of the day, and then subsequent to that it’s been identified that maybe we didn’t apply the right approach or something’s not as secure as we thought it was, under duty of care you can’t bury your head in the sand and say I hope no-one picks that up,” he said. “Standards are not static documents, they are constantly evolving.”

It is a reasonable argument, but not germane here. The standards laid out for testing IS power supplies, as detailed in AS2380.7, first came out in 1987 and have remained largely unchanged, barring a few minor amendments. In part, the problem seems to lie with differing interpretations in applying the methodology that govern testing procedures for IS equipment.

Testing procedures for IS equipment

There are two ways to test IS equipment, according to the Australian Standard AS2380.7. The testing lab can apply either a theoretical test or a physical test to the equipment. The theoretical test involves a “desk-top” exercise in which the tester uses a circuit diagram to examine the stated parameters of the equipment. He then extrapolates the equipment’s potential performance in a theoretical explosive atmosphere and checks the result against what is known as published ignition curves. The ignition curves are international measures that show, only in theory, whether a piece of equipment might explode under certain conditions. If the equipment comes close to the curves, a physical test is deemed appropriate.

It is generally acknowledged that testing IS equipment is problematic, especially with modern types of circuitry. According to Londonderry, “there are many difficulties in unequivocally establishing their compliance with intrinsic safety requirements. It can be hard to apply assessment techniques to check outputs, but equally, testing using the spark test apparatus can give inconsistent results. Some certification bodies shy clear of certifying such equipment”

Australia’s Longwalls understands that using a “desk-top” exercise for the power supplies in question is not the appropriate method of testing. This is because they are non-linear power supplies, which use active electronic components, or semiconductors, rather than resistors to limit current. IS professionals confirm that non-linear power supplies should not be tested against published ignition curves and require, in every instance, physical testing. Although AS2380.7 allows for two alternatives in testing IS equipment, the two tests are not equivalent.

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