​The Essential Guide to Temperature Sensors in Hazardous Areas

In industrial environments where flammable gases, combustible dusts, or ignitable fibers lurk, a single electrical spark can trigger catastrophic consequences. Petrochemical plants, paint manufacturing facilities, grain elevators, and numerous other settings require careful consideration when installing electrical equipment—particularly temperature sensors and transmitters.

But navigating hazardous area classifications, protection methods, and certification requirements can feel like deciphering a cryptic code. What exactly makes a device "explosionproof" versus "intrinsically safe"? How do you know which temperature sensor is appropriate for your specific hazardous environment?

This guide will demystify these crucial concepts and help you make informed decisions when specifying temperature sensors for hazardous areas.

Understanding Hazardous Areas: The Three Requirements for Danger

A hazardous area exists when three specific conditions converge:

1. Flammable materials are present in ignitable concentrations (gases, vapors, dusts, fibers, or flyings)

2. There's sufficient oxygen to support combustion

3. A potential ignition source exists (electrical equipment, mechanical sparks, or hot surfaces)

Typical hazardous locations include petrochemical facilities, spray finishing areas, aircraft hangars, grain processing plants, textile mills, coal mines, and dry cleaning operations. When designing systems for these environments, two key parameters determine the hazardous classification: the type of flammable material present and the probability of its presence.

Division vs. Zone: Two Classification Systems to Know

Two primary systems classify hazardous locations globally: the Division system (prevalent in the United States) and the Zone system (used internationally).

The Division System (US National Electrical Code)

The Division system categorizes hazardous materials into three classes:

• Class I: Flammable gases and vapors

• Class II: Combustible dusts

• Class III: Combustible fibers and flyings

These classes are further subdivided into groups based on explosive potential. For example, within Class I, Group A includes highly explosive acetylene, while Group D covers less volatile substances like propane, gasoline, and methane.

The probability of hazardous material presence is divided into:

• Division 1: Areas where hazardous materials may be present during normal operations

• Division 2: Areas where hazards arise only during abnormal conditions (like leaks or equipment failures)

As a rule of thumb, Division 2 applies when the probability of explosive materials is less than 1%.

The Zone System (International Electrotechnical Commission)

The Zone system, described in IEC 60079, is gaining worldwide acceptance and is recognized in the US National Electrical Code Article 505. It classifies gases into three groups (IIA, IIB, IIC) plus a fourth group (I) for underground methane.

Instead of two divisions, the Zone system uses three categories based on hazard probability:

• Zone 0/20: Areas where flammable atmospheres are continuously present (over 1000 hours/year)

• Zone 1/21: Areas where hazardous atmospheres may exist during normal operation (10-1000 hours/year)

• Zone 2/22: Areas where flammable atmospheres are unlikely or present only briefly (1-10 hours/year)

This more granular approach allows for greater flexibility in selecting protection methods, potentially reducing costs while maintaining safety.

Three Methods of Protection: Choose Your Strategy

Preventing explosions in hazardous environments incorporates three primary approaches, sometimes used in combination:

1. Containment: Let It Explode (Safely)

Rather than preventing explosions, containment methods like explosionproof (US) or flameproof (international) enclosures are designed to contain any explosion inside specialized housings, preventing flame propagation to the surrounding atmosphere.

A typical explosionproof assembly for temperature measurement includes an RTD or thermocouple probe, spring-loaded fitting, connection head, and transmitter. These components are engineered with features like extended spark gaps and specially designed covers to prevent flame escape.

2. Isolation: Keep Dangerous Atmospheres Away

Isolation techniques physically separate potential ignition sources from flammable atmospheres. Methods include:

• Continuously purging enclosures with pressurized "safe" air

• Oil immersion of electrical components

• Powder filling

• Hermetic sealing

3. Energy Limitation: Insufficient Power to Ignite

Perhaps the most elegant solution, energy limitation recognizes that sparks need sufficient energy to ignite gases. Many temperature measurement devices—including RTDs, thermocouples, and transmitters—operate at power levels below ignition thresholds.

Circuits may be classified as "intrinsically safe" when they're incapable of releasing enough energy to cause ignition under normal operation or fault conditions. These systems require no special housings but do need Zener diode barriers in signal lines to limit energy entering hazardous areas.

Intrinsically safe installations offer an increasingly popular and often more cost-effective alternative to explosionproof enclosures, particularly for temperature sensing applications.

Temperature Considerations: Don't Forget Surface Heat

Beyond electrical sparks, hot surfaces can also trigger explosions. All hazardous area equipment receives a temperature classification indicating its maximum surface temperature under normal or fault conditions at 104°F (40°C) ambient temperature.

Temperature codes range from T1 (maximum 842°F/450°C) down to T6 (maximum 185°F/85°C). This ensures the equipment surface will never exceed the ignition temperature of surrounding hazardous materials.

Choosing the Right Protection Method

The appropriate protection method depends on your hazardous area classification:

For Division 1 (Zone 0/1) areas:

• Explosionproof enclosures

• Intrinsically safe circuits (with appropriate barriers)

• Purged/pressurized systems (Type X or Y)

For Division 2 (Zone 2) areas:

• Nonincendive equipment (similar to intrinsically safe but without barriers)

• Hermetically sealed devices

• Non-sparking equipment

• Oil immersion

Purely passive devices like RTDs or thermocouples are generally safe for Division 2 areas in normal operation. However, their associated transmitters require careful selection.

Nonincendive vs. Intrinsically Safe: Understanding the Difference

Nonincendive devices are similar to intrinsically safe equipment but don't require barriers to guard against fault conditions. The rationale: in Division 2 (Zone 2) areas, the probability of simultaneous faults—a materials spill AND an electrical failure—is essentially zero.

The Zone system also recognizes "increased safety" equipment as an intermediate option between intrinsically safe and nonincendive approaches. This method employs constructional safeguards to avoid components that could arc or spark.

Ingress Protection: Don't Forget Environmental Factors

Beyond explosion protection, consider potential ingress of dusts, fibers, and fluids that could compromise reliability. Two rating systems help assess enclosure protection:

• IEC System: Uses "IP" followed by two digits. The first digit indicates protection against solids (0-6), the second against liquids (0-8).

• NEMA System: Uses type designations (1-13) for various environmental protections, plus special ratings for hazardous locations.

Certification: Who Says It's Safe?

Equipment for hazardous areas requires certification by recognized testing laboratories to verify compliance with standards. In the US, Factory Mutual (FM) certification carries the most weight, while European facilities must comply with ATEX Directive 94/9/EC.

Different countries have different testing authorities, including CSA (Canada), BASEEFA (UK), PTB (Germany), and many others. Reciprocity between authorities varies, so check that your equipment's certification is accepted in your location.

No matter the need, Mod-tronic has MINCO products that will fulfill your requirements.  For those needing FM or CSA certification we offer the following options:

• MINCO Explosionproof Flameproof Thermocouple FM/CSA Assemblies

• MINCO Explosionproof Flameproof FM/CSA RTD Assemblies

 

For those needing ATEX/CE/ICEx certification we offer the following products:

• MINCO Explosionproof Flameproof RTD Assemblies, ATEX, CE, ICEx Assemblies

• MINCO Flameproof Thermocouple Assemblies, ATEX, CE, IECEx Assemblies

Making the Right Choice for Your Application

Selecting temperature sensors and transmitters for hazardous areas requires careful consideration of:

1. The hazardous material type (gas, dust, or fibers)

2. Material classification (explosiveness grouping)

3. Probability of hazardous material presence (Division/Zone classification)

4. Temperature requirements (both process and maximum surface temperature)

5. Environmental protection needs (dust, water, corrosives)

6. Certification requirements for your jurisdiction

 

While explosionproof options provide robust protection, intrinsically safe circuits often offer more economical solutions for temperature sensing applications, especially when working with RTDs and thermocouples that inherently operate at low power levels.

By understanding these fundamental concepts, you can navigate the complex world of hazardous area equipment with confidence, ensuring both safety and operational efficiency in your temperature measurement applications.

Remember: Always consult local authorities and reference appropriate standards (NEC Articles 500-505 in the US, CEC Sections in Canada, or CENELEC Standards in Europe) when designing systems for hazardous locations. Safety in these environments is too important to leave to guesswork.