Electronics Guide

Hazardous Location Classification

Hazardous locations are classified according to the type of hazard present and the likelihood of hazardous conditions occurring. The North American classification system defines Class I locations for flammable gases and vapors, Class II for combustible dusts, and Class III for ignitable fibers. Within each class, Division 1 indicates locations where hazardous conditions exist under normal operations, while Division 2 indicates locations where hazardous conditions exist only under abnormal conditions.

The international IECEx and ATEX systems use zone classifications. Zone 0 indicates continuous presence of explosive atmosphere, Zone 1 indicates likely presence under normal conditions, and Zone 2 indicates presence only under abnormal conditions. Similar zones exist for dust hazards as Zone 20, 21, and 22. These classification systems guide the selection of appropriate protection methods for equipment installed in each area.

Gas and vapor hazards are further characterized by their ignition properties. Gas groups classify materials by their minimum ignition energy and maximum experimental safe gap, ranging from Group D (propane, typical industrial gases) through Group C to Groups A and B (hydrogen, acetylene) with progressively stricter requirements. Dust groups classify combustible dusts by their ignition temperature and resistivity. The equipment rating must match or exceed the hazards present in the intended installation location.

Temperature classes indicate the maximum surface temperature of equipment under normal and fault conditions. The temperature class must be selected so that equipment surfaces remain below the ignition temperature of the hazardous material present. T1 through T6 classes in international standards and T1 through T6 in North American standards define progressively lower maximum temperatures. Equipment design must ensure that no accessible surface exceeds the rated temperature class under any operating condition.

Protection Methods

Intrinsic safety is the primary protection method for low-power electronic instrumentation in hazardous locations. Intrinsically safe circuits are designed so that any spark or thermal effect produced, either normally or under specified fault conditions, is incapable of causing ignition. This is achieved by limiting the energy available in the circuit through restrictions on voltage, current, and energy storage elements.

Intrinsically safe apparatus requires associated apparatus located in safe areas to provide the energy limiting functions. Barriers and isolators contain components that limit the voltage and current that can reach the intrinsically safe circuit, typically using Zener diodes for voltage limiting and resistors for current limiting. These devices provide the interface between intrinsically safe field devices and the control systems in safe areas.

Explosion-proof enclosures (flameproof in international terminology) contain any explosion that occurs within the enclosure and prevent the flame from propagating to the external atmosphere. These enclosures feature closely machined flanged joints that cool escaping gases below the ignition temperature. Equipment using this protection method can operate at higher power levels but requires robust enclosures with certified flame paths.

Increased safety, pressurization, encapsulation, and oil immersion provide additional protection methods for specific applications. Increased safety provides additional factors of safety against excessive temperature and sparking. Pressurization maintains the internal atmosphere above ambient pressure to exclude hazardous gases. Encapsulation embeds components in compound to prevent contact with hazardous atmosphere. Oil immersion submerges components in oil to exclude gases. The selection of protection method depends on the hazard classification, equipment type, and operational requirements.

Intrinsic Safety Parameters

The design of intrinsically safe circuits requires careful management of electrical parameters that determine ignition capability. The maximum voltage, maximum current, maximum power, and maximum capacitance and inductance are specified for each intrinsically safe circuit based on the gas group and temperature class.

Capacitive circuits store energy that can be released as a spark when contacts open or close. The maximum permissible capacitance decreases as the voltage increases and varies with gas group. Intrinsically safe circuit design must account for all capacitance in the circuit, including cable capacitance, which can be significant for long cable runs. The combined capacitance of the field device and cabling must remain below the limits for the system.

Inductive circuits can generate voltage transients when current is interrupted, creating sparks capable of ignition. The maximum permissible inductance depends on the circuit current and voltage, with limits varying by gas group. Energy stored in inductance must be managed through current limitation, protective components, or inherently safe inductance values. Cable inductance is typically less significant than capacitance but must still be considered.

The entity concept provides a structured approach to verifying intrinsic safety by comparing apparatus parameters to system limits. The intrinsically safe apparatus specifies maximum voltage, current, and power along with maximum allowable cable parameters. The associated apparatus specifies the voltage, current, and power limits it maintains. When apparatus parameters remain within limits accounting for cable parameters, the system is intrinsically safe.

EMC Considerations for Hazardous Locations

Electromagnetic compatibility requirements interact with hazardous location requirements in ways that can complicate equipment design. EMC protection devices such as capacitors, inductors, and suppressors can affect intrinsic safety parameters. Conversely, intrinsic safety barriers and energy limiting components can affect EMC performance. Successful design addresses both sets of requirements simultaneously.

Capacitors used for EMC filtering increase the stored energy in circuits, potentially compromising intrinsic safety. X-capacitors and Y-capacitors used in power line filters must be evaluated for their contribution to stored energy. In some cases, intrinsic safety requirements limit the capacitor values that can be used, constraining EMC filter design. Non-incendive capacitors or intrinsically safe capacitor values may be required.

Transient voltage suppressors protect equipment from electromagnetic transients but can fail in ways that compromise intrinsic safety. Zener diodes and TVS devices used in intrinsic safety barriers are specifically designed and tested for fail-safe behavior. EMC suppression devices added to intrinsically safe circuits must be evaluated for their behavior under fault conditions and may require approval as part of the intrinsically safe system.

Shielding for EMC can interact with bonding requirements for hazardous locations. Shield grounding must be implemented in ways that do not compromise the integrity of intrinsically safe circuits or create paths for fault currents that could cause ignition. The grounding scheme must satisfy both EMC requirements for shield effectiveness and hazardous location requirements for equipotential bonding.

Testing and Certification

Equipment for hazardous locations must be tested and certified by accredited laboratories before installation. The certification process evaluates both the protection method implementation and the electromagnetic compatibility of the equipment. Test procedures and acceptance criteria are defined in standards such as IEC 60079 series, UL 913, and FM 3610.

Intrinsic safety testing verifies that circuits remain non-ignitable under normal operation and specified fault conditions. Spark ignition testing uses standardized apparatus to determine whether sparks generated by the circuit can ignite test gas mixtures. Thermal testing verifies that surface temperatures remain below the rated temperature class. Fault testing applies component failures and wiring faults to verify that safety is maintained.

EMC testing for hazardous location equipment follows standard EMC test procedures with additional considerations. Emissions must be controlled not only for regulatory compliance but also to prevent interference with safety instrumented systems. Immunity testing verifies that electromagnetic disturbances do not cause the equipment to exceed its safe operating parameters or enter unsafe states.

Documentation requirements for hazardous location equipment are extensive. Control drawings define the configuration that maintains certification, including component specifications, assembly procedures, and wiring requirements. Any deviation from the certified configuration invalidates the certification. Users must maintain equipment according to manufacturer instructions to preserve the certification basis.

Installation Practices

Proper installation is essential to maintain the safety of equipment in hazardous locations. Installation standards including IEC 60079-14 and NEC Article 500 series define requirements for wiring methods, enclosure integrity, and protective device application. Installation inspections verify compliance with these requirements before equipment is energized in hazardous areas.

Intrinsically safe wiring must be segregated from non-intrinsically safe wiring to prevent the introduction of hazardous energy into intrinsically safe circuits. Cable colors, conduit marking, and physical separation ensure that maintenance personnel can identify intrinsically safe circuits. The cable parameters must remain within the limits specified on the control drawing, accounting for actual installed cable length and type.

Grounding and bonding in hazardous locations serve both electrical safety and static dissipation functions. Equipotential bonding ensures that all conductive parts are at the same potential, preventing sparks from static discharge. EMC grounding practices must be implemented in ways that support rather than compromise the equipotential bonding scheme.

Maintenance procedures for hazardous location equipment must preserve the certification basis while allowing necessary repairs and calibration. Hot work permits and safety procedures govern maintenance activities in hazardous areas. Replacement parts must match the certified specification, as substitution of uncertified components invalidates the equipment certification.

Wireless Systems in Hazardous Locations

Wireless communication and sensing systems present unique challenges in hazardous locations. The RF energy from wireless transmitters must be evaluated for its potential to cause ignition, separate from the intrinsic safety of the electronic circuits. Standards including IEC 60079-0 Annex G address RF ignition hazards.

Low-power wireless technologies such as Bluetooth, ZigBee, and WiFi can generally be designed for use in hazardous locations when properly configured. The transmit power must be limited to levels that cannot cause ignition, considering both the free-space field strength and the effects of nearby conductive surfaces that could concentrate fields. Antenna design and placement affect the maximum field strengths and must be evaluated.

High-power wireless systems including some industrial communication systems and radar may require explosion-proof or pressurized enclosures for the transmitter and antenna. The interface between the protected enclosure and the external antenna must maintain protection while allowing RF energy to radiate. Waveguide pressurization and flame-arresting antenna designs address this requirement.

EMC for wireless systems in hazardous locations must address both compliance with radio regulations and compatibility with safety instrumented systems. Spurious emissions from wireless devices must not interfere with process control or safety systems. Immunity of wireless systems to industrial electromagnetic environments ensures reliable communication for safety-related functions.

Safety Instrumented Systems

Safety instrumented systems (SIS) provide automatic protection functions that prevent or mitigate hazardous events. These systems must achieve specified safety integrity levels (SIL) that require extremely high reliability. EMC is a critical consideration for SIS because electromagnetic interference could cause spurious trips or, worse, failure to act when required.

IEC 61511 and IEC 61508 define requirements for safety instrumented systems including provisions for electromagnetic compatibility. Equipment used in SIS applications must demonstrate immunity to electromagnetic disturbances at levels appropriate to the installation environment. The effect of electromagnetic interference on system reliability must be considered in the safety integrity assessment.

Redundancy and diversity in SIS design provide protection against common-cause failures including electromagnetic interference. Multiple sensors using different technologies or physical principles may respond differently to electromagnetic disturbances. Voting logic that requires agreement between redundant channels before taking action prevents spurious trips from interference affecting a single channel.

Testing of SIS equipment for EMC typically exceeds standard commercial requirements, with higher immunity test levels and more comprehensive fault analysis. The interaction between functional safety certification and EMC certification requires coordinated demonstration that the equipment meets both sets of requirements throughout its intended operating life.

Regional Regulations

Hazardous location requirements vary significantly between regions, requiring attention to local regulations for equipment intended for global markets. North America, Europe, and other regions have distinct classification systems, protection methods, and certification bodies. Equipment may require separate certifications for each market.

North American requirements include NEC Articles 500-506 in the United States and CEC Section 18 in Canada. UL, FM, and CSA provide third-party certification. The Division system (Division 1 and 2) remains prevalent in North America, though Zone classification is increasingly accepted.

European requirements follow the ATEX Directive for equipment and the ATEX Workplace Directive for installations. Equipment must bear CE marking with the ATEX explosion-protection symbol. Notified bodies provide conformity assessment for most equipment categories. The Zone classification system is standard throughout Europe.

The IECEx system provides international certification recognized in many countries. IECEx certificates issued by certified testing laboratories can facilitate market access in participating countries. However, local requirements may still apply, and some markets require additional local certification even with IECEx certification.

Design Guidelines

Successful design of equipment for hazardous locations begins with clear understanding of the intended application and its hazard classification. Early determination of the protection method guides all subsequent design decisions. Involving certification bodies early in the design process can prevent costly redesigns needed to achieve certification.

Component selection for intrinsically safe circuits must consider both normal operation and fault behavior. Surface-mount components may have different fault behavior than through-hole equivalents. Component spacing must meet creepage and clearance requirements for the voltage class. Infallible components that cannot fail in dangerous ways simplify the safety analysis.

PCB layout for hazardous location equipment must maintain separation between intrinsically safe and non-intrinsically safe circuits. Galvanic isolation using approved isolating components provides the boundary. Creepage and clearance distances appropriate to the voltage levels must be maintained. Layout review by experienced engineers identifies potential certification issues before fabrication.

EMC design must be integrated with intrinsic safety design from the beginning. Filter component values must satisfy both EMC performance requirements and intrinsic safety limits. Shielding and grounding must support EMC goals while maintaining intrinsic safety barriers. Testing during development verifies that both EMC and safety requirements are being met.

Summary

Hazardous location EMC presents unique challenges that require specialized knowledge spanning electromagnetic compatibility and explosion protection. The potential consequences of ignition in hazardous atmospheres drive rigorous requirements for equipment design, testing, certification, and installation. Successful design addresses both EMC requirements and intrinsic safety requirements as integrated aspects of the overall equipment design.

The interaction between EMC protection devices and intrinsic safety parameters requires careful analysis and often involves trade-offs between EMC performance and safety margins. Understanding the regulatory frameworks and certification processes in target markets enables efficient design and certification strategies. As industrial automation extends into more hazardous location applications, the importance of competent hazardous location EMC engineering continues to grow.