“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 #2 – Risk of Energy.
Risk of Energy Definition: A “Risk of Energy is considered to exist within a product if the user can access low voltage, hazardous energy circuits (>240VA available). Emphasis is placed on “available” energy – if the circuit is bridged by conductive jewelry or hand-held tools, the ensuing short-circuit will cause all available energy in the circuit to pass through the necklace, ring, bracelet, watch, or hand-held tool. With available energy over 240 VA, the jewelry/tool will overheat faster than the user can respond causing severe burns! A Risk of Energy is defined as:
Risk of Energy > 240 VA available energy
Risk of Energy – Not All Standards: Risk of Energy is a hazard that is referenced in only a few product safety standards. However, in the United States, manufacturers have “ultimate liability”. Ultimate liability means that product manufacturers can be held liable for injuries suffered by the user regardless if the product meets the safety standards and is certified. It therefore behooves all product manufacturers to consider all possible hazards, even if they are beyond the scope of the safety standards that apply to their product. With this in mind, all product designers should at least know the definition for a Risk of Energy and how to prevent the user from being exposed to such a hazard.
Risk of Energy – Accessibility: The first element of learning Risk of Energy protection is to understand the term “accessibility”. In order for the user to be subjected to an energy hazard, the user’s jewelry or tool must touch an electrical circuit. Ability to touch an electrical circuit is typically referred to as “accessibility”. This is critical to understanding Risk of Energy and it provides us with the first set of important compliance design questions:
To answer these questions we need the product safety standards. Product safety is not intended to be a subjective process. Determining whether the user can access the circuit should not be dependent on what jewelry or tool is used. Rather, the product safety standards specify “accessibility probes” that are used to determine what circuits are considered to be user accessible. There are different accessibility probes in different standards.
Many compliance professionals are familiar with the “International Finger” which is based on an adult male hand size. There is also a “UL Finger” that is much smaller as it is based on the hand size of a women and, a set of “Child Finger Probes” representing two different age groups of children, with the probe including a child’s arm. In addition, some standards include probes that represent a basic tool that is typically found in the product’s user environment (paper clip, screw driver, etc.). There is also a test chain that simulates a user’s jewelry drooping into a product vent to see if it can “access” a hazardous circuit. Be sure to check your product safety standards to identify what accessibility probes and conditions of use apply to your products (i.e. force limitations, limited use guidelines, etc.).
It should be noted that most safety standards require opening and removing all parts that can be removed or opened without tools before applying the accessibility probes. This can include using a Finger Nail probe to try and pry open snapped together parts. And for products with electrical connectors for communication or accessories (USB, headphones, etc.), these connectors are assumed to be accessible regardless of the type of connector. This is very important – all circuits connected to external connectors are usually considered user accessible (other than certified power receptacles).
Risk of Energy – Determination: To determine if a user accessible circuit is considered a Risk of Energy, measure the energy in the circuit under all conditions of loading (including short-circuit) to determine if the circuit is capable of supplying greater than 240 VA (for 1 minute). If the circuit is considered a Risk of Energy, you must limit the available energy in the circuit to below 240 VA. This is typically done by including an overcurrent protection device in the circuit – a device rated to limit the current such that the maximum VA “available” is less than 240 VA – use a fuse, PTC, or other current limiter. Any device that reliably limits the circuit below 240 VA is acceptable. However, unlike Risk of Shock, only one level of protection is required. This means that you do not have to meet the Risk of Energy requirement with any fault conditions. Whereas with Risk of Shock, you must meet the limits during a single fault condition.
Risk of Energy – A Real Hazard? There are some that question whether 240 VA can create a hazard. Recent research by a technical committee suggests that 240 VA may not lead to heating sufficient to cause burns. Informal testing done by a CertifiGroup engineer found that the specific jewelry is a major factor – some chain link designs prevented conductivity and heating, other chains had too much resistance to conduct sufficient current for heating.
Regardless of the results of formal or informal research, there are a few facts that may outweigh this awareness,
at least for now:
Risk of Energy – Summary: In summary, low voltage accessible circuits with available energy exceeding 240VA is considered an unacceptable hazard in some standards. As with Risk of Shock, the 1st step in applying the Risk of Energy requirements is determining circuit “accessibility” using the accessibility probes from the product safety standards that apply to your product. To protect the user, the product should be designed to prevent access to circuits that have greater than 240 VA available, while also limiting the energy in user accessible circuits below this limit.
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