Electrical Equipment Explosion Protection Research


“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 and other gassy 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. It should be noted that the alignment in the table between U.S. mining designated areas and the IEC zones is based solely on aligning the allowable techniques between the two approaches. Unlike the IEC definition for zones, the terms “Inby” and “Outby” are not defined in terms of the probability or likelihood of an explosive atmosphere, rather they are based solely on the physical location in the mine.

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

Inby last open

IS - 2 fault
Explosion Proof  (XP)

 Zone 1

Between 100 and 1000

IS - 1 fault
Flameproof (XP)
Powder Fill
Increased Safety
Oil Immersion

Outby last open

No Protection Req'd

Zone 2

Between 10 and 100

Limited Energy
Enclosed Break
Restricted Breathing

Outby last open

No Protection Req'd


 Less than 10

No Protection 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.

Explosion Protection Consideration

OSHA regulated industries

U.S. mining industry

Recognized techniques (standards)



Typical Number of Zones



Applicable Design Document

IEC/ISA standards

MSHA Approval Criteria/30 CFR

Equipment Approval Authority

NRTL Listed Equipment


Equipment Approval Level



Industrial Implementation

Risk Based


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 and associated equipment 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 industry. 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 was 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 4th 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 IEC-based 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 this additional 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 U.S. version of the 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.” The American National Standards Institute (ANSI) is responsible for coordinating and managing the overall development of consensus standards in the USA as well as coordinating international activities such as with the IEC standards acceptance. ANSI designates an Accredited Standards Developer (ASD) for the review and acceptance of an IEC standard as a U.S. American National Standard; in this case the International Society of Automation (ISA) for the intrinsic safety standard. Hence the “ANSI/ISA” designation for U.S. adopted versions of the IEC standards.

After the successful effort to determine safety equivalency for battery powered stand-alone intrinsically safe equipment, a new effort was initiated to determine the equivalency for interconnected equipment and systems. This effort expanded that previous study to all equipment including non-self-contained battery operated equipment, mine powered “associated” equipment supplying IS devices, IS barriers supplying IS devices, and system approaches including the entity concept allowed 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. Evaluation and approval of each specific combination is currently required under the U.S. mining equipment approval process. The resulting paper, “Intrinsically Safe Systems: Equivalency of International Standards Compared to U.S. Mining Approval Criteria,” was published by IEEE. The paper concluded that the MSHA criteria and the U.S.-adopted version of the IEC standards provide equivalent protection for such equipment. This conclusion combined with the other papers extend the equivalency finding for all two-fault intrinsically safe equipment categories.

Due to the rapid pace of electronics technology change, the IEC standards are updated frequently. Thus there is a concern, as noted in the conclusions of the original effort, that 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. Given that the IEC standards are updated much more frequently than the MSHA criteria, 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.”

Considerations of other Techniques

As mentioned earlier, of the 13 explosion protection techniques that are used in other industries, there is only one other than 2-fault intrinsic safety that is recognized by MSHA. That is the use of explosion proof (XP) boxes, these are referred to as flameproof (FP) containers internationally (IEC 60079-1); the purpose of which is to limit the explosion to inside the container so it does not propagate into the surrounding environment.

Acceptance of flameproof as an equivalent to MSHA XP criteria (30 CFR 7 and 18) would require making an equivalent level of protection determination. Unlike the case with the comparison of IS standards, the principle differences which bear on the fundamental protection mechanism are significantly more conservative in the MSHA standard as identified in report the Department of Labor, Mine Safety and Health Administration, “Report of Investigation, Part 6 Equivalency Review and Comparison: MSHA and IEC Explosion-proof Enclosure Standards” (MSHA, December 2005) Therefore one cannot readily conclude that FP offers an equivalent level of protection relative to XP.

Additionally, unlike the situation for two-fault intrinsic safety where all other U.S. industries, besides mining, use the IEC standards; not all U.S. industries have adopted the IEC flameproof standard for all applications. Those industries which still use the Class and Division approach to categorizing different environments use the criteria specified for XP boxes in ANSI/UL 1203, which is much more similar to the MSHA XP criteria than it is to the IEC Flameproof criteria. A comparison of the Explosion Proof (XP) AN/UL 1203 requirements to the IEC Flameproof (FP) requirements is provided in the Intertek document “Explosionproof v Flameproof: Understanding the differences between ANSI/UL 1203 and IEC 60079-1.” (Select the title from the list of publications for access.)

It would appear that accepting IEC FP instead of MSHA XP would require either a regulatory change to accept that FP has a “sufficient” level of protection, or some quantitative definition of level of protection that determines the differences between MSHA XP and IEC FP are not significant relative to the safety improvements that may be gained through increasing the availability of equipment or through some other benefit not directly related to the standard. Any of these solutions would almost certainly require a radical change to the regulations and/or changes to the approval process for use of electrical equipment in coal mines.

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 could hazards and/or the potential for explosions be reduced by improved monitoring, in more areas of the mine, with better integrated equipment shutoff capability? If the answer to these questions are yes, in order to realize the advantages, then a different means of determining the equivalent level of protection on a more complete basis may need to be adopted. Potential methods include system level methods 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.

Page last reviewed: 9/24/2018 Page last updated: 9/24/2018