1 

An Evaluation of the Relative Safety of U.S. Mining Explosion-Protected 

Equipment Approval Requirements versus those of International Standards 

William Calder, dba Calder Enterprises, The Villages, FL 

David Snyder, NIOSH Mining, Pittsburgh, PA 

John F. Burr, NIOSH Mining, Pittsburgh, PA 

Introduction 

“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). 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. In general, MSHA’s 

regulations for explosion protection recognize 

two techniques: the use of explosion-proof 

enclosures (XP boxes) and 2-fault intrinsic 

safety for electrical and electronic equipment. 

For 2-fault intrinsically safe (IS) equipment, the 

U.S. mining industry requirements are unique in 

that the equipment is certified by MSHA. The 

criteria MSHA uses to evaluate equipment for 

intrinsic safety, as required by Title 30 of the 

Code of Federal Regulations (30 CFR) [30 CFR, 

Sections18.20(b), 18.68(a)(1), 19.1(b), 

20.1(c)(2), 22.6, 23.6, and 27.20(a)], is 

published in a document referred to as 

ACRI2001 (1). Outside of U.S. underground 

mining, many industries and countries accept 

equipment that is designed to a consensus-based 

international standard (ANSI/ISA 60079-11 is 

the U.S. version). The purpose of this study is to 

provide an overall assessment of the ANSI/ISA 

60079-11 standard on 2-fault intrinsic safety and 

the MSHA ACRI2001 acceptance criteria, and 

determine if the ANSI/ISA document can be an 

alternative to ACRI2001 while maintaining an 

equivalent or better level of safety for miners. 

There are many drivers for this study. First, 

immediately after the 2006 MINER Act (2) was 

passed, there was a perception in the industry 

that the unique MSHA requirements were 

contributing to delays or restricting the 

availability of equipment to meet the 

requirements of the Act. Additionally, the 

MINER Act required that mine operators submit 

an accident response plan for MSHA approval, 

to be reviewed every six months. MSHA is 

required to determine if these plans reflect the 

most recent credible scientific research and 

make use of currently commercially available 

technology. There is a concern, as expressed in a 

recent National Academy of Science (NAS) 

study (3), that the unique requirements for U.S. 

mining may be limiting the technologies 

available to coal miners and technology options 

for mine operators. Accordingly, the study 

recommended that the National Institute for 

Occupational Safety and Health (NIOSH) and 

MSHA re-examine their technology approval 

and certification processes to ensure they are not 

deterring innovation, and explore opportunities 

to cooperate with other international approval 

organizations to harmonize U.S. and 

international standards without compromising 

safety. Lastly, the National Technology Transfer 

Act (NTTA) and the accompanying Office of 

Management and Budget’s OMB-119 circular 

mandate the use of consensus-based standards 

by federal agencies. However, for U.S. mining 

operations, MSHA cannot accept such standards 
2 

unless a determination is made that such 

standards provide an equivalent level of 

protection for the miner, per the requirements of 

the 1977 Mine Act, the 2006 MINER Act, and 

30 CFR Part 18 regulations. 

In determining if the ANSI/ISA 60079-11 

consensus standard for intrinsic safety can be 

considered equivalent to the MSHA ACRI2001 

criteria, there are several approaches useful for 

analyzing these two documents and their effect 

on the safety of miners in their workplace. One 

is to do a micro-comparative analysis of the two 

documents to identify specific differences and 

their relative effect on miner safety. A second is 

to do a broader comparison of the level of safety 

provided by all the currently permitted 

explosion-protected types of equipment and 

assess their relative contributions to miner 

safety. A third would be even a broader 

approach of performing a functional safety 

analysis of an entire mine operation. This would 

be a macro-approach considering all factors and 

their relative importance in contributing to the 

safety of miners. In these contexts, the 

documents in question play a diminishing role in 

the perceived safety of miners as the scope of 

the analysis broadens. 

Since both documents address all of the criteria 

needed to evaluate a specific product to 

determine if it qualifies to be called intrinsically 

safe, NIOSH decided to use the micro-option and 

compare the two documents directly. To date, 

NIOSH has put forth considerable effort in 

performing a detailed comparison of the criteria 

given in the two documents (4). Since the 

ACRI2001 criteria and the ANSI/ISA 60079-11 

consensus standard both are applicable to the 

same explosion-protection technique (2-fault 

intrinsic safety), it was hoped that any 

differences could be reasonably resolved. 

However, the exercise identified many 

differences with either document being more 

conservative than the other on a variety of issues. 

The paper presented herein provides a 

conclusion regarding the level of protection that 

would be afforded the miner by accepting 

ANSI/ISA 60079-11, and focuses on three 

questions: What are the provisions of each 

document and how do the differences, as 

identified in the NIOSH-sponsored comparison, 

bear on mine safety? What provisions are in 

place to keep the documents current? What 

oversight is provided for equipment evaluated 

per the criteria in the documents? The third 

category needs to consider qualification of 

manufacturers, ongoing audits of 

manufacturers/products, and how changes to 

both standards and products are monitored and 

evaluated. In coming to its overall conclusion, 

the paper also considers alternative methods for 

comparing the level of protection, analyzes the 

integrity and currency of the standards, and 

discusses the oversight of standards 

implementation. 

Evaluation of the Differences in the 

Requirements 

To begin this evaluation, it is important to 

understand that both documents were developed 

from the same set of fundamentals, which is the 

essence of intrinsic safety as given in its 

definition, which states that electrical equipment 

and wiring designated as intrinsically safe shall 

be incapable of releasing sufficient electrical or 

thermal energy under normal or abnormal 

conditions to cause ignition of a specific 

atmospheric mixture. This definition raises three 

questions: 

 How much energy will cause ignition? 

 What are abnormal conditions? 

 How is a hazardous atmospheric mixture 

defined? 

All 2-fault intrinsic safety standards ever written 

on the subject were, in essence, providing the 

answers to these questions. Very simply, the 

energy question was answered with the 
3 

development of the ignition test apparatus, 

abnormal conditions were defined as two faults 

each of low probability after which no ignition is 

possible, and finally, the gas mixture question 

was fortuitously answered by research showing 

that the gas groupings developed with tests for 

explosion-proof enclosures correlated directly 

with arc ignition so the existing classification 

system could be used. There was and still is 

universal acceptance of these answers. This is 

why, fundamentally, all such standards could be 

viewed as being the same but with differences in 

detail. The world at large has solved these detail 

differences by bringing intrinsic safety experts 

from all interested countries together via the 

International Electrotechnical Commission 

(IEC), where all the different issues have been 

fully debated, proposals voted on, and ultimately 

producing an IEC document on intrinsic safety, 

IEC 60079-11. This standard has been adopted 

by all participating countries and is available to 

all others who wish to use it. It has been adopted 

with national differences by the United States 

via the American National Standards Institute 

(ANSI) and the standards developers 

International Society for Automation (ISA) and 

Underwriters’ Laboratories (UL). Furthermore, 

OSHA has recognized the current IEC 60079-11 

standard for years. 

Although the documents are similar in content 

having common roots, the comparative study 

showed many differences. As detailed in the 

unpublished comparative study by Homce et al., 

of the more than 360 sections in ACRI2001, 68 

were not relevant since the study was limited to 

portable products, 188 were effectively 

equivalent, 44 were less restrictive, and the 

remaining 66 were perceived to be more 

restrictive than the ANSI/ISA 60079-11 

standard. An analysis of these 66 items showed 

they were in 28 technical categories. Each of 

these categories was further analyzed as to their 

effect on safety. Of these, 11 were determined to 

be potentially substantive, but their effect on 

safety could not be readily established, while the 

remaining 17 were considered not to affect 

safety. 

Discussions of these differences with MSHA 

representatives resulted in a need to further 

address the issues to determine if irrefutable 

evidence could be found that supported the 

ANSI/ISA document’s criteria as providing at 

least the same level of safety. Although there 

was some success in tracing the roots of the 

criteria on two of the issues, as discussed later in 

the document, this demand was essentially 

impossible to fulfill as most of the criteria were 

developed forty to fifty years ago by standards 

developers who for the most part are no longer 

with us. Unfortunately, the supporting data and 

arguments for these criteria are not publicly 

archived and their whereabouts are unknown. 

There is one significant difference between the 

documents that presents a compelling argument 

for equivalent protection by ANSI/ISA 60079-

11, because the difference is directly related to 

the question of safety margins. In this case, the 

ACRI2001 document is less conservative. In 

both documents, the standard evaluation process 

to establish whether or not ignition is possible 

considers no faults (i.e., normal operation), the 

application of one fault, and the application of 

two faults producing the highest voltage/current 

levels. This provision specifies the application of 

safety factors applied to voltage or current to 

raise the maximum value of either to artificial 

levels as part of the evaluation. This in effect 

increases the energy in the circuit to a level that 

is the square of the factor applied to voltage or 

current because energy is proportional to the 

square of voltage and current. The international 

and the ANSI standard specifies a safety factor 

that is equivalent to 2.25 applied to available 

energy for the no-fault and one-fault analyses, 

and there still can be no ignition. By contrast, 

the ACRI2001 document specifies a safety 
4 

factor that is equivalent to 1.5 applied to 

available energy for the same fault conditions, 

and is therefore clearly less conservative for the 

no-fault and one-fault analyses. For the 2-fault 

analysis, both documents apply a unity safety 

factor. 

The difference in a safety factor of 2.25 and 1.5 

as applied to energy is substantial. The U.S. 

standards prior to adopting the IEC 60079-11 

criteria used the same safety factor as the current 

ACRI2001 document, but U.S. experts were 

unable to convince the IEC intrinsic safety 

committee that this was a reasonable 

consideration and hence established the more 

conservative safety factors in the latest version 

of IEC 60079-11. The difference in energy 

levels is significant because it is energy that is 

released in a spark that causes the ignition. U.S. 

experts at that time (as well as today) believed 

that the intrinsic safety criteria were already very 

conservative without using an excessive safety 

factor. This is very significant, for example, 

when considering allowable capacitance (which 

is a form of electrical energy storage) in an 

electrical circuit. If the maximum voltage is 10 

volts, applying the IEC 60079-11 safety factor 

of 1.5 times voltage (2.25 times energy) for nofault or one fault conditions to 10 volts brings 

the required voltage to 15 volts and the 

allowable free capacitance is greater than 80 μF, 

but less than 110 μF. Applying the ACRI2001 

factor of square root of 1.5 times the voltage (1.5 

times energy), the voltage becomes 12.25 volts 

and the allowable capacitance is greater than 

1000 μF, but less than 2000 μF. The difference 

between an allowable capacitance using the IEC 

based safety factor of approximately 100 μF vs. 

the ACRI based safety factor of 

approximately 1500 μF is very significant in 

establishing if ignition is possible. Therefore, in 

this example the IEC 60079-11 standard is much 

more conservative than the ACRI2001 

document. The large difference is due to the 

fact that the ignition curves are non-linear. The 

10 volt figure is a real possibility as that value is 

similar to those used in battery operated personal 

communication devices such as PDA’s or hand 

held communication radios (5). 

If one were to look at short circuit current limits 

in resistive circuits or inductance (inductance is 

another form of energy storage) limits relative to 

available current, the results are similar. This 

more than any other factor strongly suggests that 

the two documents can be considered equivalent 

in evaluating the relative safety provided by one 

against the other, with the ANSI/ISA 60079-11 

being considerably more conservative on this 

key provision. 

In summary, based on the results of the 

micro-comparative analysis and 

consideration of the differences, it is the 

opinion of the authors that the two 

documents provide an equivalent level of 

protection for a miner when used as the basis 

for approval of portable equipment. 

Alternative Methods to Compare the 

Level of Protection 

Equipment Protection Level Comparison 

An alternative way to view the level of 

protection afforded the miner is to consider the 

explosion protection that would be provided by 

equipment approved to the ANSI /ISA 60079-11 

standard as compared to the level of protection 

by other allowable explosion-protection 

techniques in U.S. gassy mines. Historically, 

explosion-protection techniques have been 

divided into groupings based on the area within 

the plant or mine where they are allowed to be 

used. Techniques within each group are assumed 

to provide a sufficient level of protection 

suitable for the degree of hazard in that area 

(zone). This “zone approach” for grouping the 

equipment protection techniques has been in use 

for more than 50 years, but the technical 
5 

justification for the groups is not well 

documented. To quote IEC 60079-0: “The 

installation standard, IEC 60079-14, allocates 

specific types of protection to specific zones, on 

the statistical basis that the more likely or 

frequent the occurrence of an explosive 

atmosphere, the greater the level of security 

required against the possibility of an ignition 

source being active.” More recently, the IEC 

refers to the groupings as “Equipment Protection 

Levels” (EPLs) and has provided more details as 

to the expected performance of the protection 

techniques for the various EPLs (6). 

Hazardous areas are divided into zones based on 

the degree of hazard. As shown in table 1, the 

probability or frequency of the atmosphere 

becoming explosive defines the degree of hazard 

and hence the zone. 

Table 1. Examples of zone designations and allowable explosion techniques 

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 

required

Zone 2 Between 10 and 

100

Non-incendive

Non-sparking

Limited energy

Enclosed break

Restricted breathing

Outby last open No protection 

required

NRZ Less than 10 No protection

required

Outby last open No protection 

required

In reference to table 1, Column 3 lists the 

explosion-protection techniques allowable in 

each zone. In terms of zone designations, Zone 0 

is more restrictive than Zone 1, and Zone 1 is 

more restrictive than Zone 2. NRZ indicates a 

“No Requirement Zone,” meaning that no 

explosion protection is required. The values 

assigned to frequency of the atmosphere 

becoming explosive have never appeared in any 

standard or regulation, but experts agree that if 

numerical values were applied, these would be 

fairly representative. In practice, the areas which 
6 

are required to be Zone 0, 1, and 2 are assessed 

somewhat subjectively as it is difficult to 

estimate the frequency with which an area might 

become potentially explosive. What is not 

shown on the table is that the IEC standards also 

allow combinations of Zone 2 explosionprotection techniques to improve the protection 

level of an equipment to make it acceptable for 

use in Zone 1 or Zone 0 (7). 

The IEC standards recognize four zones 

(including the NRZ), which accommodate 

thirteen different explosion techniques. Table 1 

provides some insight into the types of 

explosion-protection techniques that have 

become available over the years and the 

equipment design alternatives that might be 

introduced if the U.S. mining industry were to 

move towards IEC-based explosion-protection 

techniques. Of all the techniques, none of them 

have a better EPL than 2-fault intrinsic safety, 

but several of them have a better or equal EPL 

relative to XP enclosures. 

The U.S. mining industry recognizes two areas 

that are similar in concept to zones. In U.S. coal 

mining, everything “outby” the last open crosscut is treated as always non-explosive; therefore 

no explosion protection is required in that area. 

Everything “inby” the last open cross-cut is 

considered a potentially explosive area and 

generally requires either 2-fault IS or XP 

enclosure techniques in that area. It should be 

noted that ACRI2001 does allow encapsulation 

as part of an intrinsically safe system, but does 

not allow it as a stand-alone technique for higher 

power applications in the same manner as the 

IEC standards, so encapsulation is not included 

in the table for the U.S. mining industry. 

Based on MSHA requirements, as shown in 

Table 1, XP enclosures are considered 

permissible for use “inby” in U.S. gassy mines. 

Referring back to the IEC/ISA zones, the XP 

enclosures are considered to provide an EPL that 

is less than the protection provided by 2-fault 

intrinsic safety based on the IEC historical 

groupings. XP enclosures enjoy a safety record 

of over billions of hours of operation worldwide. 

Even so, all other factors being equal, from an 

EPL perspective, 2-fault intrinsic safety is 

considered to be several orders of magnitude 

less likely to fail than XP enclosures (8). 

In the context of equipment protection level, in 

addition to intrinsically safe equipment and 

equipment in XP enclosures, there is a special 

category that is neither of these, but is allowable 

in gassy U.S. mines (9). An example is a battery 

box that can weigh anywhere from less than 

1,000 pounds to in excess of 4,500 pounds, 

which suggests a significant amount of enclosed 

energy. The criteria for such boxes are that they 

be robust, being made from cold rolled steel or 

other material that has similar strength 

characteristics. The enclosure may have an 

insulating cover that would have to withstand 

pre-conditioning and impact tests. There are 

spacing requirements for exposed electrical parts 

such as terminals. Internal insulation is required 

on the cover if it is conductive as well as 

protective insulation as needed between batteries 

and the enclosure. The enclosure is required to 

have drains for water or electrolyte as well as 

significant vents for any gas emissions. 

Although such an enclosure is robust, 

nevertheless it is not an XP enclosure and would 

allow methane gas to freely enter were it 

present. Further, even though there are no 

normally arcing or sparking parts inside, a loose 

terminal or other failure mode could generate a 

spark with significant energy (ignition-capable) 

that could be an ignition source for a methane 

cloud. Therefore, if the use of a battery box were 

analyzed, the EPL would be considerably lower 

than for an XP enclosure, and much less than 2-

fault intrinsically safe equipment. 
7 

As can be seen from this example, if one were to 

use the EPL of the explosion-protection 

techniques currently allowed in underground 

coal mines as the basis for determining the 

equivalent level of “protection afforded the 

miner,” one could argue that, for comparison’s

sake, the baseline level of protection afforded 

the miner is established by the EPL of XP 

enclosures and other accepted practices. 

Therefore, 2-fault intrinsically safe 

equipment built to either standard would 

offer at least the equivalent level of 

protection, and in fact be greater than the 

current level of protection afforded the 

miner. Under this approach, differences 

between the ACRI2001 acceptance criteria and 

the ANSI/ISA document, even those that appear 

to be substantive at the micro-level, would be 

inconsequential relative to the level of protection 

afforded the miner when viewed from this 

macro-comparative approach. 

Functional Safety Analysis 

A still broader view of the level of protection 

afforded a miner could possibly be arrived at 

through a functional safety analysis. A 

functional safety analysis is a semi-quantitative 

method of analyzing an entire system. The IEC 

has developed standards for this type of analysis 

such as the one for Safety Instrumented Systems 

IEC 61508, which provides guidelines for 

quantitative assessment of the Safety Integrity 

Level (SIL) of a system. This type of analysis is 

similar in intent to the level of protection 

afforded the miner evaluation, in that a code of 

practice can be compared to a desired safety 

level. However, in the case of a functional safety 

analysis, the comparison is made to a welldefined safety level involving calculable 

probabilities, rather than trying to compare it to 

a previous standard or code of practice for which 

a safety level was never quantified. 

A functional safety analysis would represent the 

most complete analysis of all the factors that 

bear on the risk of an explosion, of which the 

EPL of the equipment is just one factor. Other 

factors include the frequency and duration of the 

atmosphere becoming explosive, the number of 

equipment and types in the potentially explosive 

atmosphere, the maintenance history of the 

equipment, the self-diagnostics capability of the 

equipment, how the mine is monitored for 

applicable explosion risks, how often and when 

the equipment is used, and other factors. Such an 

approach requires assessing the risk of each 

relevant part of the operational mine system and 

assigning numerical values to the risk (10).

Obviously, the risk and the resulting numbers 

would vary amongst mines, and such minespecific risk analysis is simply not a part of the 

current approach to mine safety analysis in the 

U.S. 

In a functional safety analysis, the main risk 

factors are quantified wherever possible, in 

which case the non-quantified EPL ranking 

discussed previously would be replaced with a 

statistical-based numerical probability akin to 

the EPL. The concept that there is a numerical 

probability associated with the failure of a 

protection system is captured in various terms 

such as: ‘Probability of Failure on Demand for a 

Safety Instrumented System’; or ‘Probability of 

Dangerous Failure per Hour for Safety of 

Machinery’ (11). It is generally recognized in 

the safety community that the probability of 

failure has a time dependency, such that the 

longer an equipment is in service without 

maintenance (that includes verification of the 

protection mechanism), the higher the 

probability of failure.7 The nature of the time 

dependency is given by the following equation: 

PF is proportional to 1 - e-t*lambda

where PF is the Probability of Failure and lambda 

is the failure rate, and t is the time in service since 
8 

the system, subsystem, or component (in our case 

the explosion-protection mechanism) was verified 

to be working properly. 

This equation captures the important role of 

maintenance and verification of explosionprotection systems as part of safety 

considerations. The equation suggests that an 

equipment with a relatively low failure rate 

(lambda), based on the design and component 

consideration, could actually have an 

unacceptably high probability of failure of the 

explosion-protection mechanism if the equipment 

is left in service for a long period of time (t) 

without verification and safety-related 

maintenance (perhaps because the equipment is 

designed without a means to verify the 

protection). Conversely, an equipment that may 

be considered to have a relative high failure rate 

(based on the explosion-protection technique and 

equipment design consideration) could 

conceivably have a very low probability of failure 

if the equipment has a means to readily verify the 

protection and the equipment is maintained and 

inspected often (such as XP enclosures in 

mines). This equation provides some insight into 

the pitfalls of focusing on the equipment 

standards without consideration of the broader 

operational mine considerations (such as time in 

service of the equipment) as a determination of 

the level of protection afforded the miner. 

While a functional safety analysis approach 

would be new to mining, it involves wellestablished analysis techniques such as Layers 

of Protection Analysis (LOPA) and Fault Tree 

Analysis (FTA) that are in common usage within 

the safety engineering community (12). 

Additionally, there is one particular type of 

equipment that was the subject of a lawsuit and 

has subsequently been allowed for use “inby” 

the last open cross-cut for certain mines by 

MSHA that deserves discussion. In rendering 

their decision (13), MSHA and the court applied 

a non-quantitative thought process very similar 

to what would be involved with a functional 

safety analysis. The equipment involved was the 

electronic total stations for mine surveying. Prior 

to this decision, the use of mechanical transits 

and steel tapes were required, which presented 

no explosion-protection risk at all since there are 

no electrical components involved. By contrast, 

the electronic total stations are battery powered 

and do not have any certification or circuitry 

specifically designed for explosion protection. 

Therefore, from a purely EPL comparison, the 

equipment by itself could not possibly provide 

the same level of protection. 

As would be the case with a functional safety 

analysis, MSHA and the courts considered 

broader aspects of the factors that affect the 

safety of the miners other than the equipmentlevel considerations. Although no certification 

of test data was presented, the arguments 

presented asserted that the equipment was nonsparking and non-thermal during operation 

(implying the same EPL as the IEC Nonincendive and Non-sparking standards would 

provide as listed in the Table 1), and that the use 

of these systems were essential to miner safety 

since the mechanical transits were no longer 

available. Further, it was successfully argued 

that if the mine only uses the equipment during 

non-mining periods and complies with certain 

stipulations, including continuously monitoring 

the atmosphere for methane, then the use of this 

equipment provides the same level of protection 

as the previous practice. This was a decision that 

was very difficult to make and came after many 

years of debate about the merits of the use of 

electronic stations, and the decision was 

ultimately forced by the court (14). 

This case may very well be a harbinger of things 

to come as the U.S. coal market shrinks and 

correspondingly less electronic equipment 

manufacturers are willing to design and build 

equipment specifically for a criteria that applies 

only to U.S. underground coal mines. 
9 

Quantitative functional safety analysis may 

provide a more reasonable path forward in this 

regard, but would require extensive training and 

research in the area of quantified risk 

assessment, agreement on assumed failure rates 

and calculation techniques, and concurrence on 

acceptable levels of risk. 

This discussion is important because it suggests 

that the line-by-line comparison of any two sets 

of intrinsic safety criteria and the exposing of 

differences between the two, whether they can 

be resolved or not, would have little impact on 

any subjective or quantitative approach to 

analyzing the safety of a mine system in terms of 

the probability of ignition. The detailed 

requirements given in the intrinsic safety criteria 

have little to no effect on these calculations 

because the fundamentals of the technique itself 

are the basis for the high level of safety afforded 

by this approach. Thus, a quantitative systems 

engineering approach to assessing the safety 

or level of protection afforded a miner would 

consider the ACRI2001 acceptance criteria 

and the ANSI/ISA document equivalent as a 

de facto assumption for such analysis. 

Integrity and Currency of Standards 

To make a standard truly effective, it must be 

developed and managed with an open process 

such as that described in the ANSI document, 

“Essential Requirements: Due process 

requirements for American National Standards” 

(15). In this document, ANSI asserts that due 

process consists of nine elements that apply to 

any activities related to the development of 

consensus for approval, revision, reaffirmation, 

and withdrawal of American National Standards 

(ANS). In this case, due process means that 

anyone having a direct and material interest in 

the subject has the right to present an issue and 

its justification, have that issue considered, and 

have the right to appeal. It is based on equity and 

fair play. 

Some of the more important elements of due 

process include openness for participation, 

balance of interests including actively seeking 

participants to achieve balance, providing public 

notice to announce the opportunity for 

participation, conducting appropriate votes and 

providing evidence that a consensus was 

achieved, and being receptive to appeals and 

dealing with them in an appropriate manner. In 

relation to standards evaluation, exercising such 

due process in the development and maintenance 

of a document provides a large measure of 

assurance that the document is technically sound 

and that the opportunity to participate is afforded 

to those who are materially affected by the 

contents. 

Another important aspect of standards 

development is ongoing maintenance to ensure 

their integrity and to keep them current by 

reflecting changes in technology as well as 

ongoing research results that justify altering any 

of the criteria in such standards. For example, all 

consensus standards recognized by ANSI must 

be reviewed at least every five years to ensure 

that the document is current and the criteria 

within are valid. 

The MSHA process for generating documents 

such as ACRI2001 is internal to MSHA and 

does not include inviting participation from 

others outside of MSHA who may be materially 

affected, although MSHA draws heavily on 

ANSI/ISA documents in its update process. 

Internal procedures direct appropriate levels of 

management to initiate the development of such 

documents as well as for ongoing maintenance 

of existing documents. The nature and frequency 

of the review process for existing documents is 

at the determination of appropriate management 

levels but is not supposed to exceed 5 years 

between such reviews (16). Approval of the 

results of any such work is also relegated to 

internal management. 
10

The open, inclusive, consensus process with 

regular formal updates required for ANSI 

recognition ensures that the standard 

represents the best science available. By 

adopting the ANSI/ISA document the miners 

should benefit by having the best safety 

technology at the earliest available date.

Oversight of Standards Implementation 

NIOSH has commissioned the preparation of a 

report, “Quality Assurance of Nationally 

Recognized Test Laboratories Using ANSI/ISA 

Standards for Certification of Intrinsically Safe 

Equipment” (17), which gives details of the 

ANSI and OSHA processes. As revealed in this 

report, the oversight performed on both 

standards writing organizations (such as the 

International Society for Automation (ISA) or 

Underwriter’s Laboratories (UL)) and 

Nationally Recognized Testing Laboratories 

(NRTLs) that associate with American 

Standards Institute (ANSI) and the Occupational 

Safety and Health Administration (OSHA), 

respectively, is significant. For a standard to 

become recognized by ANSI, the writing 

organization is required to function against a 

rigorous set of procedures as set forth by ANSI. 

In addition, those testing laboratories recognized 

by OSHA as NRTLs, who use these same 

ANSI-recognized standards to evaluate products, 

are governed by an equally rigorous set of 

procedures administered by OSHA. Both the 

ANSI and OSHA systems require audits of these 

organizations, focusing in particular on the 

performance against the procedures. 

If MSHA were to accept the IEC-based standard 

for evaluating intrinsically safe equipment, there 

are benefits that would accrue. For example, if a 

product had to be approved to both the IEC 

version and to the current MSHA criteria, it is 

likely the product designed to meet the MSHA 

criteria would be physically different from that 

designed to meet the ANSI/ISA IEC standard. 

The manufacturing process would have to 

accommodate both designs with one of them 

likely to be low volume (the MSHA version), 

resulting in higher manufacturing costs and a 

more expensive product. These costs would be 

avoided if a common standard were used. 

Another significant benefit is the oversight of 

both the manufacturers and their permissible 

products conducted by the NRTLs. NRTLs 

perform quarterly audits at the manufacturers’ 

facilities to ensure that the product being 

produced is exactly the same product that was 

found permissible. These audits review current 

production and the procedures used by the 

manufacturers to maintain control of the 

products. This is, of course, in addition to the 

oversight performed by MSHA. MSHA quality 

control auditing is focused not on the 

manufacturers but on the mining industry endusers. MSHA inspectors regularly inspect 

MSHA-approved equipment in service at 

operating mines and while it is being repaired at 

rebuild shops. MSHA engineers conduct random 

audits of new and repaired equipment at mine 

warehouses. 

All of the above makes a powerful statement 

in the quest to ensure the safe operation of 

electrical equipment in the workplace, 

whether in mines or above-ground 

operations. If MSHA were to accept an IECbased standard for evaluating intrinsically 

safe equipment, miners would benefit from 

an enhanced quality control program. 

Overall Discussion of Specific Differences 

in the Standards 

Historically, the several attempts to detail the 

root of the differences between the ACRI2001 

acceptance criteria and the ANSI/ISA document 

have only emphasized the difficulties 

encountered in such an exercise. There are 

several reasons for this, but the primary one is 
11

the subjective judgements used where a sound 

technical basis could not be established for many 

of the detail requirements. Many of the 

requirements in the IEC intrinsic safety standard 

are technically supported, such as the energy 

required for ignition, but the major contributor 

to the differences are the issues where subjective 

judgement was used. Examples of this are 

numerous: Should the safety factor applied to 

circuit analysis be 1.5 times the voltage and/or 

current, or should it be 1.5 times the energy? or 

should the current in a circuit having a 

protective fuse rate the maximum current that 

can flow at 1.7 times the fuse rating, or the 

current required to blow the fuse within 2 

minutes? These safety factors are similar, but 

fundamentally different, and either can be 

correct. The basis for most of the subjective 

criteria is lost in antiquity. Some of the issues 

can be traced, but for the most part the trail is 

not easily uncovered as many of the participants 

have died or have long since been retired and 

have no idea what became of the supporting 

documentation. Most of the information was in 

private files or even in public files that have 

either been stored in some obscure location or 

even discarded, since there were no rules about 

retaining such information. 

What did happen is that a large number of 

recognized experts of the world assembled via 

an IEC committee addressing intrinsic safety, 

and these experts agreed to a set of criteria that 

may not have been perfect, but resulted in a 

conservative protection technique that has 

withstood the test of time. Many of these experts 

had to abandon requirements developed in their 

own countries in order to come to a consensus. It 

is difficult to comprehend that such a collection 

of experts could be wrong. 

In considering the MSHA ACRI2001 document, 

it is clear that several of the subjective criteria 

were accepted, such as the spacing criteria 

applied between intrinsically safe and nonintrinsically safe circuits within an enclosure, or 

the thickness specifications for insulating 

materials, or the construction criteria for 

isolating components such as transformers. 

One specific difference that was pursued is the 

temperature limit relaxation for small 

components. NIOSH commissioned an in-depth 

study of this issue that resulted in a report, 

“Evaluation of the Technical Basis for Specific 

Provisions of the ANSI/ISA Intrinsic Safety 

Standards, Report 1, Small Component 

Temperature Ratings” (18). The research 

uncovered specific test data of tests performed 

both in England and Germany, with 

accompanying analysis that clearly 

demonstrated that a large variety of small 

components including fine wire can be heated to 

temperatures considerably higher than the 

autoignition temperatures of all the gases that 

were tested. From this example, it is clear that 

the ACRI2001 requirements are extremely 

conservative when considering small electronic 

components. 

A second issue given considerable study was the 

factor applied to fuse ratings for test purposes 

for components and circuitry beyond the 

protective fuse. NIOSH also commissioned a 

study of this issue, resulting in a report, 

“Evaluation of the Technical Basis for Specific 

Provisions of the ANSI/ISA Intrinsic Safety 

Standards -- Report 2, Fuse Factor Ratings and 

Other Issues” (19). In this case, the ANSI/ISA 

standard specifies a factor of 1.7 times the fuse 

rating to establish the downstream current to be 

used in testing, while the ACRI2001 document 

uses the current required to open the fuse in two 

minutes or less. This current is always at a 

higher level than when applying the 1.7 factor. 

The research into this was more obscure, but 

those involved in the process were able to 

provide the history. It was developed in England 

more than fifty years ago when several test 

programs were initiated to try to determine a 
12

reasonable factor to apply to fuses. The results 

were so highly variable that no technically sound 

conclusion could be drawn. The fuse experts at 

that time came to a subjective judgement that a 

1.7 factor would be a reasonable compromise 

and would represent a conservative approach 

based on the data they had. This number was 

proposed in the standards at the time and 

accepted by the European standards committee 

and ultimately adopted in the IEC standard. 

Years of experience have demonstrated that the 

1.7 factor is reasonable for the intended purpose 

of assessing thermal effects beyond the fuse. 

Literature addressing thermal ignition has 

always maintained that thermal ignition is a very 

inefficient process due the ideal conditions used 

to establish material auto-ignition temperatures 

vs. the conditions encountered in the normal 

environment. Because of the large difference in 

conditions, the practices in use are quite 

conservative when considering potential thermal 

effects. 

To resolve the fuse current issue on a technical 

basis would require a carefully contrived test. 

As stated above, our predecessors who tried to 

devise such tests 50 years ago were 

unsuccessful, and it would seem that the 

prospects for success today would be no more 

likely than the earlier attempts. At best, it would 

require testing literally thousands of units and a 

variety of types to get some kind of data to be 

able to compare the 1.7 factor to the two-minute 

criteria. Fuse manufacturers today cannot 

accurately predict fuse responses except 

generally, which is why they provide timecurrent blowing characteristics in plots that 

represent a best statistical fit in the tests that 

they perform. Fuse technology is not precise in 

that the rupture current has a range based on 

several factors such as temperature, the 

characteristics of the element, and the nature of 

the current increase such as a slow rise as 

opposed to a spike. 

These examples demonstrate that the issue-byissue comparison of the two documents may 

well be an exercise in futility. Yes, there are 

differences, but within the intrinsic safety 

concept, they are essentially irrelevant in 

affecting the potential for ignition to occur. 

Equipment that has been certified as meeting the 

IEC intrinsic safety criteria have logged millions 

of safe hours of operation in atmospheres far 

more dangerous than those posed in mines 

(Groups A, B, and C gasses as opposed to 

methane, a Group D gas). Further, in the context 

of a total mine structure, intrinsically safe 

equipment that would be used in mines pales in 

the danger it presents as compared to all of the 

other equipment permitted in mines. 

The exercise in futility is also due to the fact that 

the IEC criteria are based principally on 

European-developed standards because the 

initial draft was based on existing European 

practice at the time. There were actually few 

technical differences with North American 

practice with the exception of the safety factor 

issue – 1.5 on energy as opposed to 1.5 on 

voltage or current. The differences that did exist 

were principally based on practices steeped in 

local history. In North America, much of the 

routine test criteria such as dielectric strength 

tests or impact tests were taken from practices of 

UL or FM. These pretty much agreed with 

European practice as well. 

As demonstrated in this report, from the 

overall perspective of miner safety the 

differences between the ACRI2001 criteria 

and the ANSI/ISA 60079-11 standard are 

rather insignificant. The work (opinions, 

knowledge, experience, etc.) of the individual 

intrinsic safety experts representing several 

countries, including the US, which resulted in 

the ANSI/ISA standard cannot to be 

discounted. These experts have thoroughly 

vetted and upheld the standard in repeated 

reviews. 
13

Overall Findings Regarding Level of 

Protection 

All of the evidence to date would strongly 

suggest that there is an equivalent level of safety 

for miners when either the ACRI2001 

acceptance criteria or the ANSI/ISA document is 

used. For those areas where the IEC-based 

standard is less conservative, that delta is more 

than made up by the difference in safety factor 

applications where the ACRI2001 is far less 

conservative. The additional benefits to be 

derived from the NRTL-based oversight and 

quality control, as well as the potential for 

increasing the equipment available for use in 

the mines and reducing approval times, 

suggest that the overall level of protection 

afforded by the miner will not be reduced, 

and may be improved, by accepting the 

ANSI/ISA 60079-11 standard for portable 

equipment, as an alternative to ACRI2001. 

Because the scope of the micro-comparative 

study was restricted to stand-alone equipment, 

and it was used as the basis for our evaluation, 

the findings are restricted to portable equipment. 

However, the macro arguments presented herein 

apply to all IS equipment, and it is likely that 

with additional research our findings can be 

extended to other IS equipment. 

The findings and conclusions in this report are 

those of the author(s) and do not necessarily 

represent the views of the National Institute for 

Occupational Safety and Health. 

ABOUT THE AUTHORS 

William Calder: Mr. Calder is an independent 

technical consultant and has enjoyed a 

distinguished career in the electrical safety field 

having specialized in designing electrical 

equipment for use in classified (hazardous) areas 

and the development of the standards used to 

evaluate such equipment. Mr. Calder’s work 

experience includes a lengthy career at a leading 

manufacturer of process control instrumentation 

where he rose to Director of Product Safety and 

later served as an Assistant Vice-President in 

charge of all electrical engineering operations at 

a well-known product testing laboratory. Calder, 

a registered professional engineer, received his 

B.S. in Electrical Engineering at Worcester 

Polytechnic Institute and M.S.in Engineering 

Management from Northeastern University. He 

was involved with the development of the 

intrinsic safety standards, participating and 

playing a leadership role in both the U.S. and 

international committees for intrinsic safety 

through the Instrument Society of America (now 

known as the International Society for 

Automation) and as a member of the U.S. 

National Committee for the International 

Electrotechnical Commission that developed the 

intrinsic safety standards. 

David Snyder: Mr. Snyder is a senior engineer 

with the Office of Mine Safety and Health 

Research (OMSHR) of NIOSH, responsible for 

the scouting and development of mine safety and 

health technologies, and previously led the 

communications and tracking systems research 

under the MINER Act. Throughout his career, 

he has been on the leading edge of electronic 

systems technologies, including military 

communications and surveillance systems and 

commercial cellular networks. He was an early 

innovator in the areas of public Wi-Fi and 

shared tower site infrastructure. Before NIOSH, 

Mr. Snyder worked in the areas of 

telecommunications systems research, 

development, and technology integration and 

has held various positions including vicepresident and CTO of a large international 

company, director of engineering for a national 

wireless service provider, and CEO of a small 

start-up company. Earlier in his career, he 

worked as an engineer for mine operators in 

central Pennsylvania coal fields including 

Bethlehem Mines. He has a B.S. degree in 
14

Mining Engineering from Penn State University, 

an M.S. degree in Electrical Engineering from 

George Washington University, and is a 

registered Professional Engineer. 

John F Burr: Mr. Burr is currently a senior 

scientist with the NIOSH Office of Mine Safety 

and Health Research. Prior to joining NIOSH, he 

spent 42 years with CONSOL Energy 

spearheading the introduction and application of 

new technology into the underground coal 

mining industry. His projects included the 

electrical engineering design of mine power, 

communications, and monitoring systems. Much 

of this experience has focused on improving 

safety in the mines for personnel and equipment 

through the use of improved technologies and 

practices. He served on a variety of standards 

committees and has had leadership roles in 

professional society committees, and significant 

experience working with MSHA, UMWA, 

NMA, NEC, and BCOA on rule‐making related 

activities and special projects. He led a 

significant industry effort to implement the 

MINER Act of 2006 and collaborated 

extensively with NIOSH and MSHA on that 

work. 

REFERENCES

1. MSHA, Criteria for the Evaluation and Test 

of Intrinsically Safe Apparatus and Associated 

Apparatus (ACRI2001), 3rd Ed., November 4, 

2008. (Available at 

www.msha.gov/techsupp/acc/application/acri20

01.pdf) 

2. MINER Act Public Law 109-236. Mine 

Improvement and New Emergency Response 

Act of 2006. (Available at 

www.msha.gov/MinerAct/2006mineract.pdf.) 

3. National Research Council. (2013). Pp. 61-62, 

Recommendation 4, Improving Self-escape from 

Underground Coal Mines. Committee: Essential 

Components of Self-Escape. Board on HumanSystems Integration. Division of Behavioral and 

Social Sciences and Education. Washington, 

DC: The National Academies Press. 

4. Homce, G., “Certification of Intrinsically Safe 

Portable Electronic Equipment for Mining,” July 

2013, (unpublished internal report), Office of 

Mine Safety and Health Research, National 

Institute for Occupational Safety and Health. 

5. ANSI/ISA-60079-11, Annex A, Figure A.2, 

Group 1 Capacitive Circuits, February 15, 2013. 

6. IEC 60079-0, Annex D, Introduction of an 

alternative risk assessment method 

encompassing “equipment protection levels” for 

EX equipment. 

7. IEC 60079-26, Equipment with Equipment 

Protection Level (EPL) Ga 

8. Magison, E.C., Electrical Instruments in 

Hazardous Locations, 4th ed., ISA, 1998. 

9. Title 30, Code of Federal Regulations, Part 7, 

Subpart C, Battery Assemblies 
15

10. Sammarco J.J., “Programmable electronic 

and hardwired emergency shutdown systems: A 

quantified safety analysis.” Proceedings of the 

37th IEEE Industry Applications Society Annual 

Meeting. 2002:692-698

11. ISO 13849-1, Safety of machinery -- Safetyrelated parts of control systems -- Part 1: 

General.

12. 2008 Safety Integrity Level Selection: 

Systematic Methods Including Layer of 

Protection Analysis Hardcover – May 1, 2002 

by Edward M. Marszal and Eric William 

Scharpf

13. United States Department of Labor, Office 

of the Assistant Secretary for Mine Safety and 

Health, Petition for Modification, Case Nos. 

2010-MSA-1,2,11, and 12, “Order of Remand 

and Stay of Implementation of November 14, 

2013, Decision and Order,” Joe Main Assistant 

Secretary of Labor, 14 November 4, 2013 and 

modification to same on December 7, 2013.

14. Department of Labor Administrative Law 

Judge Michael P. Lesniak decision, April 11, 

2013

15. American National Standards Institute, 

“Essential Requirements: Due process 

requirements for American National Standards,” 

January 2015. (Available at 

www.ansi.org/essentialrequirements) 

16. DOCUMENT NO: NPOL1011, VERSION: 

2014-07-21, Center Document System, 

Approval & Certification Center, Mine Safety 

and Health Administration.

17. Calder, W., CDC-NIOSH-OMSHR Report, 

“Quality Assurance of Nationally Recognized 

Test Laboratories Using ANSI/ISA Standards 

for Certification of Intrinsically Safe 

Equipment,” February 5, 2014. 

18. Calder, W., “Evaluation of the Technical 

Basis for Specific Provisions of the ANSI/ISA 

Intrinsic Safety Standards, Report 1, Small 

Component Temperature Ratings,” November 4, 

2014. 

19. Calder, W., “Evaluation of the Technical 

Basis for Specific Provisions of the ANSI/ISA 

Intrinsic Safety Standards -- Report 2, Fuse 

Factor Ratings and Other Issues,” November, 

2014.