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Classified Areas and Electrical Safety Measures

Any physical location in an industrial facility harboring the potential of explosion due to the presence of flammable process matter suspended in the air is called a hazardous or classified location. In this context, the label “hazardous” specifically refers to the hazard of explosion, not of other health or safety hazards2.


Classified Area Taxonomy

In the United States, the National Electrical Code (NEC) published by the National Fire Protection Association (NFPA) defines different categories of “classified” industrial areas and prescribes safe electrical system design practices for those areas. Article 500 of the NEC categorizes classified areas into a system of Classes and Divisions. Articles 505 and 5063 of the NEC provide alternative categorizations for classified areas based on Zones that is more closely aligned with European safety standards.

The Class and Division taxonomy defines classified areas in terms of hazard type and hazard probability. Each “Class” contains (or may contain) different types of potentially explosive substances: Class I is for gases or vapors, Class II is for combustible dusts, and Class III is for flammable fibers. The three-fold class designation is roughly scaled on the size of the flammable particles, with Class I being the smallest (gas or vapor molecules) and Class III being the largest (fibers of solid matter). Each “Division” ranks a classified area according to the likelihood of explosive gases, dusts, or fibers being present. Division 1 areas are those where explosive concentrations can or do exist under normal operating conditions. Division 2 areas are those where explosive concentrations only exist infrequently or under abnormal conditions4.

The “Zone” method of area classifications defined in Article 505 of the National Electrical Code applies to Class I (explosive gas or vapor) applications, but the three-fold Zone ranks (0, 1, and 2) are analogous to Divisions in their rating of explosive concentration probabilities. Zone 0 defines areas where explosive concentrations are continually present or normally present for long periods of time. Zone 1 defines areas where those concentrations may be present under normal operating conditions, but not as frequently as Zone 0. Zone 2 defines areas where explosive concentrations are unlikely under normal operating conditions, and when present do not exist for substantial periods of time. This three-fold Zone taxonomy may be thought of as expansion on the two-fold Division system, where Zones 0 and 1 are sub-categories of Division 1 areas, and Zone 2 is nearly equivalent to a Division 2 area5. A similar three-zone taxonomy for Class II and Class III applications is defined in Article 506 of the National Electrical Code, the zone ranks for these dust and fiber hazards numbered 20, 21, and 22 (and having analogous meanings to zones 0, 1, and 2 for Class I applications).

Within Class I and Class II (but not Class III), the National Electrical Code further sub-divides hazards according to explosive properties called Groups. Each group is defined either according to a substance type, or according to specific ignition criteria. Ignition criteria listed in the National Electrical Code (Article 500) include the maximum experimental safe gap (MESG) and the minimum ignition current ratio (MICR). The MESG is based on a test where two hollow hemispheres separated by a small gap enclose both an explosive air/fuel mixture and an ignition source. Tests are performed with this apparatus to determine the maximum gap width between the hemispheres that will not permit the excursion of flame from an explosion within the hemispheres triggered by the ignition source. The MICR is the ratio of electrical ignition current for an explosive air/fuel mixture compared to an optimum mixture of methane and air. The smaller of either these two values, the more dangerous the explosive substance is.

Class I substances are grouped according to their respective MESG and MICR values, with typical gas types given for each group:


  Typical substance  
Safe gap Ignition current
A Acetylene    
B Hydrogen MESG 0.45 mm MICR 0.40
C Ethylene   0.45 mm < MESG 0.75 mm 
  0.40 < MICR 0.80 
D Propane 0.75 mm < MESG
0.80 < MICR

Class II substances are grouped according to material type:


E Metal dusts
F Carbon-based dusts
G   Other dusts (wood, grain, flour, plastic, etc.) 

Just to make things confusing, the Class/Zone system described in NEC Article 505 uses a completely different lettering order to describe gas and vapor groups (at the time of this writing there is no grouping of dust or fiber types for the zone system described in Article 506 of the NEC):


  Typical substance(s) 
Safe gap
Ignition current
IIC Acetylene, Hydrogen
MESG 0.50 mm MICR 0.45
IIB Ethylene   0.50 mm < MESG 0.90 mm  
  0.45 < MICR 0.80 
IIA Acetone, Propane 0.90 mm < MESG 0.80 < MICR


Explosive Limits

In order to have combustion (an explosion being a particularly aggressive form of combustion), three basic criteria must be satisfied: sufficient fuel, sufficient oxidizer, and sufficient energy for ignition.

These three necessities are referred to as the fire triangle:

Remove any one (or more) of these elements from a location, according to the fire triangle, and you will successfully prevent the possibility of fire or explosion.

The fire triangle serves as a qualitative guide for preventing fires and explosions, but it does not give sufficient information to tell us if the necessary conditions exist to support a fire or explosion. For that, we need more quantitative data on the fuel, the oxidizer, and the ignition source. In order for a fire or explosion to occur, we need to have an adequate mixture of fuel and oxidizer in the correct proportions, and a source of ignition energy exceeding a certain minimum threshold.

Suppose we had a laboratory test chamber filled with a mixture of acetone vapor (70% by volume) and air at room temperature, with an electrical spark gap providing convenient ignition. No matter how energetic the spark, this mixture would not explode, because there is too rich a mixture of acetone (i.e. too much acetone mixed with not enough air). Every time the spark gap discharges, its energy would surely cause some acetone molecules to combust with available oxygen molecules. However, since the air is so dilute in this rich acetone mixture, those scarce oxygen molecules are depleted fast enough that the flame temperature quickly falls off and is no longer hot enough to trigger the remaining oxygen molecules to combust with the plentiful acetone molecules.

The same problem occurs if the acetone/air mixture is too lean (not enough acetone and too much air). This is what would happen if we diluted the acetone vapors to a volumetric concentration of only 0.5% inside the test chamber: any spark at the gap would indeed cause some acetone molecules to combust, but there would be too few available to support expansive combustion across the rest of the chamber.

We could also have an acetone/air mixture in the chamber ideal for combustion (about 9.5% acetone by volume) and still not have an explosion if the spark’s energy were insufficient. Most combustion reactions require a certain minimum level of activation energy to overcome the potential barrier before molecular bonding between fuel atoms and oxidizer atoms occurs. Stated differently, many combustion reactions are not spontaneous at room temperature and at atmospheric pressure – they need a bit of “help” to initiate.

All the necessary conditions for an explosion may be quantified and plotted as an ignition curve for any particular fuel and oxidizer combination. This next graph shows an ignition curve for an hypothetical fuel gas mixed with air:

Note how any point in the chart lying above the curve is “dangerous,” while any point below the curve is “safe.” The three critical values on this graph are the Lower Explosive Limit (LEL), the Upper Explosive Limit (UEL), and the Minimum Ignition Energy (MIE). These critical values differ for every type of fuel and oxidizer combination, change with ambient temperature and pressure, and may be rendered irrelevant in the presence of a catalyst (a chemical substance that works to promote a reaction without itself being consumed by the reaction). Most ignition curves are published with the assumed conditions of air as the oxidizer, at room temperature and at atmospheric pressure.

The greater the difference in LEL and UEL values, the greater “explosive potential” a fuel gas or vapor presents (all other factors being equal), because it means the fuel may explode over a wider range of mixture conditions. It is instructive to research the LEL and UEL values for many common substances, just to see how “explosive” they are relative to each other:


  LEL (% volume)  
  UEL (% volume) 
Acetylene 2.5% 100%
Acetone 2.5% 12.8%
Butane 1.5% 8.5%
Carbon disulfide 1.3% 50%
  Carbon monoxide  12.5% 74%
Ether 1.9% 36%
Gasoline 1.4% 7.6%
Kerosene 0.7% 5%
Hydrazine 2.9% 98%
Hydrogen 4.0% 75%
Methane 4.4% 17%
Propane 2.1% 9.5%



Note how acetylene has a UEL value of 100%. This means it is possible for acetylene gas to explode even when there is no oxidizer present. Some other chemical substances exhibit this same property (ethylene oxide and n-propyl nitrate being two more examples), where the lack of an oxidizer does not prevent an explosion. Some other substances have UEL values very close to 100%, hydrazine being one example at 98%. In these substances we see an important exception to the “fire triangle” rule that the elimination of any one element prevents combustion. With these substances in high concentration, our only practical hope of avoiding explosion is to eliminate the possibility of an ignition source in its presence.


Protective Measures

Different strategies exist to help prevent electrical devices from triggering fires or explosions in classified areas. These strategies may be broadly divided four ways:

Contain the explosion: enclose the device inside a very strong box that contains any explosion generated by the device so as to not trigger a larger explosion outside the box. This strategy may be viewed as eliminating the “ignition” component of the fire triangle, from the perspective of the atmosphere outside the explosion-proof enclosure (ensuring the explosion inside the enclosure does not ignite a larger explosion outside).

Shield the device: enclose the electrical device inside a suitable box or shelter, then purge that enclosure with clean air (or a pure gas) that prevents an explosive mixture from forming inside the enclosure. This strategy works by eliminating either the “fuel” component of the fire triangle (if purged by air), by eliminating the “oxidizer” component of the fire triangle (if purged by fuel gas), or by eliminating both (if purged by an inert gas).

Encapsulated design: manufacture the device so that it is self-enclosing. In other words, build the device in such a way that any spark-producing elements are sealed air-tight within the device from any explosive atmosphere. This strategy works by eliminating the “ignition” component of the fire triangle (from the perspective of outside the device) or by eliminating both “fuel” and “oxidizer” components (from the perspective of inside the device).

Limit total circuit energy: design the circuit such that there is insufficient energy to trigger an explosion, even in the event of an electrical fault. This strategy works by eliminating the “ignition” component of the fire triangle.


A common example of the first strategy is to use extremely rugged metal explosion-proof (NEMA 7) enclosures instead of the more common sheet-metal or fiberglass enclosures to house electrical equipment. Two photographs of explosion-proof electrical enclosures reveal their unusually rugged construction:

Note the abundance of bolts securing the covers of these enclosures! This is necessary in order to withstand the enormous forces generated by the pressure of an explosion developing inside the enclosure. Note also how most of the bolts have been removed from the door of the right-hand enclosure. This is an unsafe and very unfortunate occurrence at many industrial facilities, where technicians leave just a few bolts securing the cover of an explosion-proof enclosure because it is so time-consuming to remove all of them to gain access inside the enclosure for maintenance work. Such practices negate the safety of the explosion-proof enclosure, rendering it just as dangerous as a sheet metal enclosure in a classified area.

Explosion-proof enclosures are designed in such a way that high-pressure gases resulting from an explosion within the enclosure must pass through small gaps (either holes in vent devices, and/or the gap formed by a bulging door forced away from the enclosure box) en route to exiting the enclosure. As hot gases pass through these tight metal gaps, they are forced to cool to the point where they will not ignite explosive gases outside the enclosure, thus preventing the original explosion inside the enclosure from triggering a far more violent event.

A similar strategy involves the use of a non-flammable purge gas pressurizing an ordinary electrical enclosure such that explosive atmospheres are prevented from entering the enclosure. Ordinary compressed air may be used as the purge gas, so long as provisions are made to ensure the air compressor supplying the compressed air is in a non-classified area where explosive gases will never be drawn into the compressed air system.

Devices may be encapsulated in such a way that explosive atmospheres cannot penetrate the device to reach anything generating sufficient spark or heat. Hermetically sealed devices are an example of this protective strategy, where the structure of the device has been made completely fluid-tight by fusion joints of its casing. Mercury tilt-switches are good examples of such electrical devices, where a small quantity of liquid mercury is hermetically sealed inside a glass tube. No outside gases, vapors, dusts, or fibers can ever reach the spark generated when the mercury comes into contact (or breaks contact with) the electrodes:


The ultimate method for ensuring instrument circuit safety in classified areas is to intentionally limit the amount of energy available within a circuit such that it cannot generate enough heat or spark to ignite an explosive atmosphere, even in the event of an electrical fault within the circuit. Article 504 of the National Electrical Code specifies standards for this method. Any system meeting these requirements is called an intrinsically safe or I.S. system. The word “intrinsic” implies that the safety is a natural property of the circuit, since it lacks even the ability to produce an explosion-triggering spark6.

One way to underscore the meaning of intrinsic safety is to contrast it against a different concept that has the appearance of similarity. Article 500 of the National Electrical Code defines nonincendive equipment as devices incapable of igniting a hazardous atmosphere under normal operating conditions. However, the standard for nonincendive devices or circuits does not guarantee what will happen under abnormal conditions, such as an open- or short-circuit in the wiring. So, a “nonincendive” circuit may very well pose an explosion hazard, whereas an “intrinsically safe” circuit will not because the intrinsically safe circuit simply does not possess enough energy to trigger an explosion under any condition. As a result, nonincendive circuits are not approved in Class I or Class II Division 1 locations whereas intrinsically safe circuits are approved for all hazardous locations.

Most modern 4 to 20 mA analog signal instruments may be used as part of intrinsically safe circuits so long as they are connected to control equipment through suitable safety barrier interfaces.

A simple intrinsic safety barrier circuit made from passive components is shown in the following diagram7:

In normal operation, the 4-20 mA field instrument possesses insufficient terminal voltage and insufficient loop current to pose any threat of hazardous atmosphere ignition. The series resistance of the barrier circuit is low enough that the 4-20 mA signal will be unaffected by its presence. As far as the receiving instrument (indicator or controller) is “concerned,” the safety barrier might as well not exist.

If a short-circuit develops in the field instrument, the series resistance of the barrier circuit will limit fault current to a value low enough not to pose a threat in the hazardous area. If something fails in the receiving instrument to cause a much greater power supply voltage to develop at its terminals, the zener diode inside the barrier will break down and provide a shunt path for fault current that bypasses the field instrument (and may possibly blow the fuse in the barrier). Thus, the intrinsic safety barrier circuit provides protection against overcurrent and overvoltage faults, so that neither type of fault will result in enough electrical energy available at the field device to ignite an explosive atmosphere.

Note that a barrier device such as this must be present in the 4-20 mA analog circuit in order for the circuit to be intrinsically safe. The “intrinsic” safety rating of the circuit depends on this barrier, not on the integrity of the field device or of the receiving device. Without this barrier in place, the instrument circuit is not intrinsically safe, even though the normal operating conditions of the field device and receiving device are well within the parameters of safety for classified areas. It is the barrier and the barrier alone which guarantees those voltage and current levels will remain within safe limits in the event of abnormal circuit conditions such as a field wiring short or a faulty loop power supply.

More sophisticated active barrier devices are manufactured which provide electrical isolation from ground in the instrument wiring, thus eliminating the need for a safety ground connection at the barrier device.




In the example shown here, transformers8 are used to electrically isolate the analog current signal so that there is no path for DC fault current between the field instrument and the receiving instrument, ground or no ground.


1For that matter, it is impossible to eliminate all danger from life in general. Every thing you do (or don’t do) involves some level of risk. The question really should be, “how much risk is there in a given action, and how much risk am I willing to tolerate?” To illustrate, there does exist a non-zero probability that something you will read in this book is so shocking it will cause you to have a heart attack. However, the odds of you walking away from this book down and never reading it again over concern of epiphany-induced cardiac arrest are just as slim.

2Chemical corrosiveness, biohazardous substances, poisonous materials, and radiation are all examples of other types of industrial hazards not covered by the label “hazardous” in this context. This is not to understate the danger of these other hazards, but merely to focus our attention on the specific hazard of explosions and how to build instrument systems that will not trigger explosions due to electrical spark.

3Article 506 is a new addition to the NEC as of 2008. Prior to that, the only “zone”-based categories were those specified in Article 505.

4The final authority on Class and Division definitions is the National Electrical Code itself. The definitions presented here, especially with regard to Divisions, may not be precise enough for many applications. Article 500 of the NEC is quite specific for each Class and Division combination, and should be referred to for detailed information in any particular application.

5Once again, the final authority on this is the National Electrical Code, in this case Article 505. My descriptions of Zones and Divisions are for general information only, and may not be specific or detailed enough for many applications.

6To illustrate this concept in a different context, consider my own personal history of automobiles. For many years I drove an ugly and inexpensive truck which I joked had “intrinsic theft protection:” it was so ugly, no one would ever want to steal it. Due to this “intrinsic” property of my vehicle, I had no need to invest in an alarm system or any other protective measure to deter theft. Similarly, the components of an intrinsically safe system need not be located in explosion-proof or purged enclosures because the intrinsic energy limitation of the system is protection enough.

7Real passive barriers often used redundant zener diodes connected in parallel to ensure protection against excessive voltage even in the event of a zener diode failing open.

8Of course, transformers cannot be used to pass DC signals of any kind, which is why chopper/converter circuits are used before and after the signal transformer to convert each DC current signal into a form of chopped (AC) signal that can be fed through the transformer. This way, the information carried by each 4-20 mA DC current signal passes through the barrier, but electrical fault current cannot.

Go Back to Lessons in Instrumentation Table of Contents

Comments (1)Add Comment
written by BenjaminSunners, November 27, 2017
Wow, amazing article! I would love to use is as a part of my essay at best essay writing service uk reviews. Stay strong!

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