“Designing for Compliance” is critical to getting your product certified on the first try. Designing for Compliance requires owning, knowing, and applying the standard(s) while designing your product. However, in order to successfully read and understand the standard, you have to know the intent of the requirements. Our “Designing for Compliance” series of whitepapers will educate you on “The 6 Hazards of Product Safety”. The intent of the requirements in all UL/CSA/EN/IEC safety standards is to protect the user from the “6 Hazards of Product Safety”. This whitepaper covers Hazard #3 – Risk of Energy.

Risk of Fire Definition: All electrical circuits, other than those at Class II voltage/power levels, are considered to involve a “Risk of Fire”. Due to the potential for catastrophe as a result of a fire, the standards take the safest approach and assume that all products can catch fire at some point over their useful life. The requirements in the product safety standards are therefore focused on requiring design principles that limit the chances for a fire to start, spread, and escape from the product.

Risk of Fire – Only A Hazard If It Leaves the Enclosure: The potential for an electrically induced fire always exists within an electrical product. The sole concern with fire in the product safety standards is whether it can spread from the product to the building contents and structure, at which point it would be a threat to the building occupants. The product safety standards generally do not care if a product burns and turns to ash inside the enclosure, as long as fire does not get out of the product’s enclosure. (There is generally no concern with smoke in the product safety standards other than smoke that is hot enough to spread fire.)

Risk of Fire: A 3-Step Approach: To help insure that any product fire is contained within the product’s enclosure, the safety standards require a 3-step approach:

  1. Limit the likelihood that combustible materials will catch fire in the first place by maintaining operating temperatures that are below the material’s temperature rating. And,
  2. Limit the likelihood that any fire will spread internally and grow by limiting the flammability of combustible materials – by requiring UL flame ratings for printed circuit boards, wiring, insulating materials, and internal plastics. And,
  3. Limit the likelihood that fire can get out of the enclosure by controlling vent size and location near combustible materials so that any fire does not spread through the openings – the primary concern is a fault condition causing a fire that drips flaming particles onto the supporting surface. In addition, higher flame ratings are required for all plastic enclosures to insure fire containment.

We then perform fault testing to verify that there is no “Risk of Fire”. One of the pass/fail criteria when performing fault testing is whether there was a “Risk of Fire” as a result of the fault condition = did fire escape the enclosure during testing? To make this determination, the standard specifies conducting fault tests with the product sitting on tissue paper and covered with cheese cloth. The tissue paper and cheese cloth are used as the “fire indicators” – should either of these materials discolor or char, the results are considered a “Risk of Fire” which is a test failure.

Fire Hazard Protection: Let’s now dig deeper into the Fire Hazard Protection design principles that accomplish the 3 elements of Risk of Fire protection. Based on the definition of a Risk of Fire, the ultimate goal is to design the product for fire containment – during fault testing, the product can burn internally as long as the fire does not escape the enclosure.

We can take the 3 elements of Risk of Fire protection, and translate each into the design principles that achieve the intended level of protection.

1) We can limit the likelihood that the product will catch fire by:

2) We limit fuel to a fire so that any fire is unlikely to grow by:

3) Design the enclosure so that any internal fire will not escape the enclosure:

Risk of Fire – Summary: In summary, Risk of Fire can certainly be viewed as one of the most important of the 6 Hazards of Product Safety. The potential for a single product to risk many lives by causing a structural fire must be considered in the Risk Analysis of electronic products. Compounded by the fact that product fires are dramatic and, in today’s social media era, the results of a single event can be shared with millions of potential customers. Yet there are no 100% fault free components meaning there are no 100% fault free products. As such, it is important that you fully understand and apply all the principles of fire containment as a means to help mitigate the inevitable potential for a fire in your product.

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