Designing Safe Electronics for Explosive and Hazardous Environments

Designing electronic products for Explosive Atmospheres and Hazardous Locations presents unique challenges. In addition to the obvious environments such as coal mines and petrochemical plants, explosive environments also include places with explosive dust (flour, metal, sugar, wood) and explosive airborne fibers (e.g. cotton). here are also areas in which the atmosphere is normally non-hazardous but hazardous conditions could be foreseen under abnormal conditions.

To ensure safety, equipment must be designed to be safe under these conditions during normal operation as well as fault conditions. Regulations in most of the world require that devices undergo testing and certification before being put into use. While self-certification may be legal in some areas it is not recommended for both liability reasons and because customers may refuse to accept the product.

The EU follows the ATEX directive (Directive 2014/34 EU - ATEX 'Product' Directive) while North America follows HazLoc guidelines (NFPA 70). The IEC publishes the IEC 60079 standard which specifies the general requirements for construction, testing and marking of Ex Equipment and Ex Components intended for use in explosive atmospheres. UL provides the UL 913 standard for Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1, Hazardous (Classified) Locations. Finally, IECEx is a global system designed to harmonize the various standards. In the future most equipment will follow IECEx. The various regulations and standards use the terms “Class,” “Division”, “Zone”, and “Group” to differentiate the various hazards. ‍

Core Design Principles

There are two general design principles that are employed to ensure safe operation in hazardous locations:

• Explosion proof: This approach ensures that the equipment is sealed and rugged. Safety is achieved by preventing the ingress of flammable gas or dust as well as preventing egress of a flame. Essentially isolating/insulating the device from the environment.

• Intrinsic safety: This approach limits the energy present in a system to ensure that it is insufficient to cause ignition under normal or fault conditions. This means that the device is designed to be incapable of generating a spark or enough heat for ignition even under single or multiple fault conditions while in the hazardous environment.

Explosion Proof Design

There are several methods to accomplish explosion proof design, but all generally employ use of non-sparking materials (e.g. aluminum, brass, copper, plastics), high temperature tolerance, preventing gas/dust ingress and stopping flame egress. While approaches are all based on the same basic principles, they differ significantly in cost and serviceability.

• Encapsulation/potting seals everything in an electrical and thermal insulator. A simple solution for some applications but equipment cannot be serviced and troubleshooting failures becomes challenging.

• Oil-filled, sand-filled, glass-bead filled. Suitable for certain applications and sometimes serviceable but messy.  

• Flame proof design can withstand internal ignition, but more importantly, the design prevents a flame-front from propagating out of the enclosure. This design is more challenging but may be necessary in some cases. One example is an oxygen sensor which monitors a potentially explosive atmosphere while the sensor needs to operate at temperatures over 600°C.  

• Pressurized/Purged equipment contains a non-explosive gas at a positive pressure relative to its surroundings. Typically requires monitoring and active management to maintain correct pressure.

Intrinsically Safe Design

The main premise of Intrinsically Safe (IS) design is that the design is incapable of generating a spark or heat capable of igniting a vapor or flammable dust. IEC 60079-11:2023 / UL913 specify the construction and testing of intrinsically safe equipment. Some of the key principles are:

• Intrinsic safety begins with architecture. Limiting the energy after a design is done is very costly. Understanding the energy constraints and product requirements from the outset and architecting accordingly is a necessity for a successful project.

• Energy Limitation: for circuits to restrict available energy below ignition thresholds, total capacitance, inductance and relevant resistance must be selected to prevent energy storage or release above certain thresholds.  

• Voltage & Current Control: Maximum allowable values can vary by classification. Typical IS designs operate at low voltages (under 30V) and limited current (mA range) to ensure safe operation.  

• Zener Barriers and Isolators: Limit voltage and/or currents that reach hazardous areas.

• Redundant Safety Mechanisms: If a fault can adversely affect the safety of the system, it is called a “countable” fault. Redundant protection circuits ensure that a one or more countable fault doesn’t compromise the safety of the system. These could include multiple series or parallel elements, depending on whether they could fail open or shorted.  

• Fail-Safe Fuses and other components: select components that fail in a safe manner, preventing sparks or excessive heat. These parts often come with their own certification.

• Thermal Management to prevent hot surfaces that could cause ignition, even under fault conditions. This often impacts size and surface areas.  

• Accessories such as battery packs may also need to be intrinsically safe.

• Electromagnetic immunity: external interference should not induce an unsafe condition in the device.

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Notified Body

When certifying an ATEX design, working with a known Notified Body or Nationally Recognized Testing Lab (NRTL) that specializes in the ATEX certification process can make the process smoother and less daunting. While they cannot directly tell you what changes to make or implement, they are critical in helping understand the violations, interpreting the applicable specifications, and guiding your design into compliance.

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Conclusion

Designing for ATEX or HazLoc environments requires careful consideration of the applicable hazards, regulations and standards from the very start of the design process.  Regardless of the approaches selected, producing a safe design will drive significant tradeoffs. Limiting power to produce an IS design may drive functionality and use cases. Thermal management, redundancy and explosion-proofing drive the overall size. Material selection can drive schedules and limit the supplier selection pool. Accessories and use cases also need to be considered early.

Leveraging and working closely with a Notified Body early in the process will help you understand the applicable standards allowing you to properly design a solution. In the best case, failure to consider all this can lead to significant overruns and delays. As engineers, we always need to remember that the worst case could lead to loss of life if our designs do not perform safely.