Electrical Equipment Explosion Protection Research

General

“Explosion protection” refers to techniques used to minimize the potential for electrical and electronic equipment to create an ignition while operating in a hazardous location (HAZLOC). A HAZLOC refers to atmospheres containing flammable gases or vapors and/or combustible dusts. In particular, U.S. coal mines are required to use equipment in certain areas of the mine that has been approved by the Mine Safety and Health Administration (MSHA) for use in a methane and coal dust environment to limit the risk of the equipment creating an ignition-capable spark or a thermal energy ignition.

There are three basic means of providing explosion protection for equipment; these are, precluding the gas (explosive mixture) from reaching the circuitry, limiting the energy of the circuitry so that ignition is not possible, or containing the explosion to the area immediately surrounding the circuitry so that it cannot propagate into the surrounding environment.  The NIOSH sponsored research refers to the specific methods used to provide the explosion protection as “techniques” and the specifics of the techniques are typically documented in explosion protection standards.  Under OSHA and International Electrotechnical Commission (IEC) practices, the techniques allowable in a given location are usually identified by one of four zone designations.  The zones are defined by the relative risk of the atmosphere becoming explosive and other factors. The zones range from Zone 0, wherein the explosive atmosphere is expected to be present frequently or possibly continuously, to the NRZ (No Requirements Zone) where the risk of an explosive atmosphere being present during operation of equipment is expected to be negligible. The IEC recognizes 13 different techniques.

In general, MSHA’s regulations for explosion protection recognize two techniques, the use of explosion-proof enclosures (XP boxes) and 2-fault Intrinsic Safety (IS) for electrical and electronic equipment.  U.S. mining regulations further consider only two hazardous location categories (equivalent to zones), inby the last open cross cut (as well as several other specified areas) where the atmosphere is always considered to be potentially explosive, and areas not included in the first category (generally referred to as outby areas) which is similar to the NRZ.  The table below identifies the techniques and the associated zones for both the IEC and U.S. mining industry approach.

IEC/ISA and US NEC 505 Location designation	IEC/ISA and US NEC 505 Explosive Atm. (hrs/year)	IEC/ISA and US NEC 505 Techniques Allowable	U.S. Mining Industry Location designation	U.S. Mining Industry Techniques required
Zone 0	Greater than 1000	IS - 2 fault Encapsulation	Inby last open	IS - 2 fault Explosion Proof  (XP)
Zone 1	Between 100 and 1000	IS - 1 fault Flameproof (XP) Powder Fill Pressurization Increased Safety Oil Immersion	Outby last open	No Protection Req'd
Zone 2	Between 10 and 100	Non-incendive Non-sparking Limited Energy Enclosed Break Restricted Breathing	Outby last open	No Protection Req'd
NRZ	Less than 10	No Protecion Req'd	Outby last open	No Protection Req'd

In addition to the technical standards/criteria differences between U.S. mining and other industries, as it relates to explosion protection, there are several other notable differences as summarized in the table below.

Standard component	OSHA regulated industries	U.S. mining industry
Recognized techniques (standards)	13	2
Typical Number of Zones	4	2
Applicable Design Document	IEC/ISA standards	MSHA Approval Criteria
Equipment Approval Authority	NRTL Listed Equipment	MSHA
Equiment Approval Level	Component/sub-system	System
Industrial Implementation	Risk Based	Prescribed
Legal requirement for adopting new standards	Consensus standards unless proven unsafe	Use of new standards must be proven to be as safe as existing standards

These differences are substantial, and adopting IEC-based standards and practices in the U.S. mining industry would be a daunting task.  However, given the potential for increased availability of safety and health technologies that might be realized through acceptance of IEC based standards, NIOSH initiated an effort to better understand the issues associated with such a change.

Recent NIOSH sponsored efforts in the area of explosion protection research

The mission of the NIOSH research program for the mining sector is to eliminate occupational diseases, injuries, and fatalities among workers in the mining industries. Therefore, explosion protection research at NIOSH is focused on continually improving the safety of miners. Certain explosion protected equipment used in other countries and industries to improve health and safety are not available for use in the U.S. coal mining industry because this equipment has not been certified as Permissible (explosion protected) as required by U.S. mining law. Research into the differences in explosion protection requirements has been initiated to help understand the technical differences between equipment that meets IEC standards and US mining criteria. With this in mind, NIOSH chose to evaluate an explosion protection technique which both MSHA and the IEC recognize, two-fault intrinsic safety (IS). In fact, both the IEC standard and MSHA approval criteria for IS equipment evolved from the same original document (UL 913 4^th Edition 1988). Because the technique is essentially the same, the assumption was that this evaluation would be easy compared to the challenge of trying to evaluate the relative safety of completely different techniques that are not recognized by MSHA. To further reduce the scope and complexity of the analysis, it was limited to stand alone (portable) equipment to eliminate the need to resolve the distinctly different approaches used by MSHA and the IEC for approval of IS interconnected equipment (systems) and components.

The results of the comparison are available in the paper entitled “A Comparison of U.S. Mining Industry Criteria for Intrinsically Safe Apparatus to Similar IEC-Based Standards”. The summary conclusions of the effort were:

• The study suggested a potential short-term improvement in the quantity, diversity, and capabilities of IS equipment available to the U.S. mining industry
• A detailed comparison of MSHA and alternative IS construction and testing criteria, with respect to the impact on safety, was inconclusive
• A comparison of the safety records for MSHA IS criteria in the U.S., and IEC IS standards as applied in other countries, suggested that both are effective

As noted, the effort stopped short of coming to an assessment of relative safety for the two IS-criteria alternatives. This was largely due to the fact that many of the provisions have been in place for 40 years or more, and the reviewers were not familiar with the details surrounding the reasons that individual provisions were put in place. Thus a second effort was initiated to try to reach a conclusion about the relative safety of the competing criteria. The second effort proved to be as challenging as the first, but, ultimately a conclusion was reached. The resulting paper is entitled “An Evaluation of the Relative Safety of U.S. Mining Explosion-Protected Equipment Approval Requirements versus those of International Standards.”

In the course of the second research effort, a contract was initiated to also investigate two of the major IS criteria differences that delay or prevent the acceptance by MSHA of existing equipment that is approved to IEC standards. These reports are entitled “Evaluation of the Technical Basis for Specific Provisions of the ANSI/ISA Intrinsic Safety Standards, Report 1, Small Component Temperature Ratings” and “Evaluation of the Technical Basis for Specific Provisions of the ANSI/ISA Intrinsic Safety Standards – Report 2, Fuse Factor Ratings and Other Issues.”

As noted in the conclusions of the original effort, the gains in available equipment will be short lived unless MSHA accepts updates to the IEC standards on a continual basis as well as the standards themselves. The IEC standards are updated much more frequently than the MSHA standards, thus the consideration of the standards development process was thought to be essential. A contract was issued to evaluate the quality assurance relative to IEC-based standards usage in the U.S. The result of that effort is a paper entitled “Quality Assurance of Nationally Recognized Test Laboratories using ANSI/ISA Standards for Certification of Intrinsically Safe Equipment.”

Current Research

As discussed, the previous work culminated in a document which asserts, to the extent possible, an equivalent level of protection for portable stand-alone IS equipment certified to IEC-based standards when compared to MSHA-approved IS equipment. That document, “An Evaluation of the Relative Safety of U.S. Mining Explosion Protected Equipment Approval Requirements versus those of International Standards” is limited to a discussion of self-contained battery operated equipment in order to make the most straightforward case for the use of updated evaluation criteria. The current research is to expand the analysis to include additional types of intrinsically safe equipment such as devices that are line powered and/or interconnected to other equipment and components. Such equipment would include 120 V ac-powered “associated” equipment supplying intrinsically safe device(s), protective barriers suppling intrinsically safe device(s), and system approaches including the entity concept as described in the ANSI/ISA standards. The entity approach is an important concept because it permits the interconnection of apparatus even from different manufacturers without having each specific combination evaluated and certified. Evaluations and approval of each specific combination is currently required under the U.S. mining equipment approval process.

Future Direction of Explosion Protection Research

 The fundamental question to be answered, in the context of current U.S. mining law, is how can new explosion protection techniques be introduced without reducing the level of protection afforded the miner? Such a comparison has proven very difficult to make without consideration of all factors contributing to the risk of an explosion, which include not only the equipment protection technique, but the reliability and effectiveness of the mine ventilation, monitoring of the atmosphere, and maintenance of the equipment, among other things. It is clear that the two techniques explicitly recognized by U.S. mining law provide some of the highest level of explosion protection available on an equipment basis, but does limiting explosion protection approvals to these two techniques preclude the introduction of new technologies that could improve safety and health in other areas? For instance, could more practical and affordable atmospheric monitors with integrated systems shutoff be developed using alternate explosion protection techniques rather than the techniques currently used in the U.S. mining industry? If so, then it is possible that improved monitoring, in more areas of the mine, with better automated equipment shutoff to eliminate hazards could be realized. NIOSH will be considering such questions in the future as the mining technology program progresses. If the answer to these questions proves to be yes, then methods may be needed to assess the safety of systems on a more complete basis, such as those put forth in the IEC 61508 standard for Safety Instrumented Systems, which provides guidelines for quantitative assessment of the Safety Integrity Level (SIL) of systems.

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