NFPA 850 Recommendation

The Significance of Fast Depressurization Systems
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The National Fire Protection Association (NFPA) is an organization specialized in recommending best practices to prevent – or act against – fire. NFPA issues recommendations for all types of installations from buildings to substations.

In 2010, NFPA has updated its Recommendations 850 “Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations” and included the Fast Depressurization System.

The NFPA Recommendations 850 defines 4 criteria for Fast Depressurization Systems:

  • “Passive mechanical system”
  • “Depressurizes the transformer a few milliseconds after the occurrence of an electrical fault”
  • “Triggered by the dynamic pressure peak generated by the short circuit”
  • “Activates within milliseconds before static pressure increases”

The 2015 and 2020 editions have provided additional information, including a reference to an article from a group of experts analyzing an incident in which the FDS has prevented an explosion.

Interested in downloading:

Oil-filled transformer explosions and fires can be prevented in some cases, by the installation of a passive mechanical system, designed to depressurize the transformer a few milliseconds after the occurrence of an electrical fault. This fast depressurization can be achieved by a quick oil evacuation triggered by the dynamic pressure peak generated by the short circuit. The protection technology activates within milliseconds before static pressure increases, therefore preventing transformer explosion and subsequent fire…

Extract of the Annex A of the NFPA Recommendations 850


However, the NFPA 850 Recommendation includes definitions that rely on implicit knowledge regarding a set of complicated and extremely rapid physical processes, including dielectric breakdown in transformer insulating oil, the generation of internal arcs, the vaporization of the oil, the build-up of pressures on the transformer tank, and the resultant dangerously high stresses on the tank wall.

A study has been issued to clarify each point mentioned and is available for download free of charge here.

The TRANSFORMER PROTECTOR (TP) is a Fast Depressurization System as defined in the NFPA Recommendations 850. The TP has:

  • Been tested in High Voltage Laboratories
  • Operated in live conditions, preventing transformer rupture
  • Saved € hundreds of millions to their transformer owners
  • A Product Liability Insurance of € 15 million