The 35kV Di-electric test and the portable fire extinguisher


    As we all know, water and electricity do not mix well yet traditionally - particularly during the 20th Century - water based extinguishers were often operated by an acid/alkaline reaction resulting in a discharge which was a rather good electrolyte. Coupled with the rudimentary electrical installations of the time and their limited safeguards and it was a recipe for disaster!

    Vaporising liquids, then CO2, and later Dry Powder were all developed for safe use on live electrical supplies and by the 1970’s in a bid to improve the safety and efficacy of water-based extinguishers manufacturers (particularly in France) developed the water spray extinguisher, enhancing its performance by the addition of synthetic foam solution (AFFF). As well as enhanced performance on Class A & Class B fires, it was considered that a spray discharge of droplets of water rather than a solid jet may offer protection against electric shock when discharged on live equipment.

    Around this time the various European Standards and the then British Standard for the manufacture of extinguishers BS5423 (1980) introduced a formal test for electrical safety of water-based extinguishers that has been carried over into the current British & European Standards BS EN 3 – the 35kV di-electric test.

    What is the 35kV di-electric test?

    A 1m x 1m metallic plate is hung by insulators and connected to an AC transformer that runs 35,000V between the plate and earth. The extinguisher to be tested is arranged so that its nozzle is directed at the centre of the plate at a distance of at least 1m. An ammeter is connected between the handle of the extinguisher and earth and between the nozzle and earth. The extinguisher is then discharged against the plate and the current recorded.

    If the current between the handle and earth, and between the nozzle and earth, is not more than 0,5 mA (the limit for perception by humans and a tenth of that typically required for muscular reaction) at any time during the complete discharge of the portable fire extinguisher, then the test is passed.

    An extinguisher that passes should be marked to indicate that it is suitable for use on live electrical equipment e.g.: "Suitable for use on live electrical equipment up to 1000V at a distance of 1m".

    An extinguisher that fails or isn’t tested (such as water and foam extinguishers discharged as a jet and some non-kitemarked extinguishers which have not been submitted for testing) must be marked “WARNING: Do not use on live electrical equipment”

    Why are many water-based extinguishers on the UK market not marked either way?

    Originally, BS5423 allowed marking for direct use, and the original version of the standard governing extinguisher provision & maintenance (BS5306-3:1985) allowed the provision of water-based extinguishers for electrical risks where no risk of pooling and conduction across wetted surfaces was likely. For much of the 1980’s Foam Spray extinguishers were marked to be used on electrical fires and confidence in the test was so high that at the end of the decade British Rail started phasing in Foam Spray extinguishers not only to replace water, but also Halon extinguishers which were provided for electrical risks on trains and in buildings.

    However, there was some resistance to this marking as it was perceived that it could cause confusion because there were still large numbers of water jet and foam branch pipe extinguishers in use that couldn’t pass the test, as well as spray-based extinguishers that were not being tested.

    So, for most of the last 40+ years 35kV test passed extinguishers in the UK are marketed as safer if used close to electrical equipment or if ‘accidentally’ sprayed on them and have little user information on electrical safety beyond that they have passed the test (assuming all users actually know what the test is), whereas in Europe all are clearly marked as safe on electrical fires up to 1000V.

    Manufacturers of Water Mist extinguishers mark their extinguishers as suitable for electrical fires but not their water and foam spray models despite thermal passing the same test based on the fact that the current passed on the 35kV test is usually even less than the spray extinguishers and that the mist discharge, most of which evaporates, is unlikely to pool and form wetted surfaces of conduction.

    Can a 35kV test passed extinguisher be used for electrical risks or not?

    In the UK BS5306-8 still advises that they are not provided specifically for this use and that CO2 or Powder be provided (although in the Irish Republic no such differentiation exists in their standard IS 291).

    This does not alter the fact that an extinguisher passing the test is deemed safe for electrical risks and so in theory your risk assessment could decide it was acceptable, especially if using Water Mist.

    However most electrical equipment is by its nature enclosed and the seat of the fire thus shielded from the extinguishing agent. As a result, a water mist or spray, or even accepted agents for electrical fires such as powder, will struggle to penetrate the enclosure and thus may fail to extinguish the fire. Even if the fire is put out, the secondary damage from the water (or powder) to the equipment could be significant.

    Therefore, for greater effectiveness and no confusion over electrical safety CO2, which being a gas can flood an enclosure and find the seat of the fire, remains the best choice in many situations.

    Fire Depot has been the UK's favourite fire safety supplier for over 50 years, we know the fire protection and prevention business inside out. Our experienced team can offer advice and guidance about any of our fire safety products. For expert help and advice, please contact the Fire Depot team on 0330 999 2233, email us at visit to see our full range of fire safety products.

    The information contained within this blog is provided solely for general informational and educational purposes and is not intended as a substitute for professional advice. Before taking any actions based upon this information, we advise the reader to consult any and all relevant statutory or regulatory guidance and where felt necessary to consult a qualified fire or industry regulation professional. The use or reliance on any information contained herein is solely at the reader’s risk.

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