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Closing the Ex Engineering Loop on NEC 500 applications in the US

Author : Tony Scott, Managing Director, Global Ex Solutions

05 October 2020

On January 24, 2020 at 4:25 a.m., I was awoken by my garage door vibrating from the shock wave of an explosion, approximately 5 miles away. The explosion was from a manufacturing building that housed chemicals, in northwest Houston, Texas. Two plant workers and one local resident died later from their injuries.

Image: Shutterstock
Image: Shutterstock

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Some homes near the facility were blasted off their foundations, whilst others had collapsed ceilings, shattered windows, and bent garage doors. Since then, the US has suffered other, similar explosions, some of which have also been deadly. For the last few years, it seems that there has been disproportionate amount of fires and explosions from industry throughout North America than some other parts in the world.

Most companies in downstream and midstream industries in explosive (classified) locations in the US follow the NEC (National Electrical Code) which is updated every three years. The 500-516 sections of the code relate to explosive locations; the rest applies to other electrical practices. The NFPA (National Fire Protection Association) is best known for its sponsorship of the NEC. Adopted, but not a federal law in all 50 states, NFPA 70/NEC is the benchmark for safe electrical design and installation in order to protect people and property from electrical hazards.

The NEC 500 parts are used for classification purposes and categorises atmospheric explosion hazards into Class I (gas) Class II (dust) and Class III (fibers and flyings). NFPA 497 tables are also used for further information on gas properties. Two divisions are used to indicate the amount of risk in an explosive location. Division 1 is a location with a hazard likely under normal conditions and Division 2 is a location with a hazard under abnormal conditions. NEC 500 electrical and electronic equipment that are acceptable protection techniques for an installation in a Division 1 or Division 2 hazardous (classified) location include:

• Explosion proof Equipment (XP): like the Exd IEC 60079-01 concept, this protection technique is permitted for equipment in Class I, Division 1 or 2 locations.

• Purged and Pressurised: permitted for equipment in any hazardous (classified) location for which it is identified.

• Intrinsic Safety (IS): permitted for equipment in Class I, Division 1 or 2; or Class II, Division 1 or 2; or Class III, Division 1 or 2 locations.

• Non-incendive (NI): permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.

Tony Scott, Managing Director, Global Ex Solutions
Tony Scott, Managing Director, Global Ex Solutions

• Hermetically Sealed: permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations.

NEC 500 is more focused on the design and installation of the equipment listed above than its maintenance and inspection. A common assumption often is if the design and installation have been performed correctly and passed by the end user, sometimes with the agreement of an AHR (Authority Having Jurisdiction), the equipment is deemed acceptable for its lifetime.

From my experience, I would suggest that the lack of rigorous and consistent maintenance and inspections is contributing to an increase in accidents in hazardous locations. Chapter 5 of NEC requires equipment to be constructed and installed in such a way to ensure safe performance under conditions of proper use and maintenance. A separate document, NFPA 70B, the Recommended Practice for Electrical Equipment Maintenance 2019, is sometimes referred to, but the document lacks dedicated inspection tables and detailed guidance.

At present, formal procedures and requirements for hazardous location inspections have not been officially adopted by NEC 500 users in the downstream and midstream industries with explosive (classified) locations. Software is available that notes and records the exact NEC 500 inspection requirements which can address these inconsistencies. By adopting more rigorous and consistent inspection criteria and improving US-based training and competency, I believe that the engineering loop for NEC 500 Ex installation-maintenance-inspections can be closed. This would go a considerable way in mitigating the risk of explosions in the future.

About the author:

Tony Scott HNC BA (Hons) is the Managing Director of Global Ex Solutions. He has a wealth of experience in the design, installation, maintenance, and inspection of equipment in all sectors of the Ex industry. Tony has lead Ex teams, managed Ex projects and also taught various levels of IEC, ATEX, NEC500 and NEC505. For the last 12 years, Tony has consulted global organisations on all aspects of Ex applications, based mainly in the US but with some projects in the Middle East and Europe.


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