Should the cumulative effect of small parts be considered for the exemptions in sub-clause 6.3.1 of IEC 62368-1?
More specifically you asked: How is the exemption in 6.3.1 of IEC 62368-1 (2014) applied? "parts having a size less than 1750mm3" - each part is compared to the limit - or all identical parts are added together and the result compared to the limit? In my case, I have 16 dust caps on F connectors. Each cap measures 910mm3 - what is the verdict?
In response, it should be noted that the requirements in sub-clause 6.3.1 constitute Basic Safeguards against risk of fire hazard. The purpose of the Basic Safeguard is to mitigate a risk of self-ignition, primarily by limiting temperatures to not greater than 90 % of their spontaneous ignition temperature limit, or use of HB rated materials if the parts are located external to the Fire Enclosure. Two of the exceptions to the main requirement in 6.3.1 cover small components if the parts have a size less than 1750 mm3, or the parts have a mass of combustible material of less than 4 g. IEC 62368-1 does not provide details about cumulative effect restrictions; therefore general principles need to be considered.
The requirements in sub-clause 6.3, including exceptions, rely on the Fire Triangle concept as one of its Hazard Based Safety Engineering principles, which for fire requires 3 key elements: oxygen, heat and fuel. Reducing the amount of combustible materials is acceptable method of preventing fire ignition and spread. In order to consider combustible material as negligible fuel, the overall construction of the product needs to be considered, including the size, count, location, orientation and separation of components. The IEC TC108 Hazard Based Standard Development Team (HBSDT) took these considerations into account when developing 6.3.1.
Regarding the example in your question, as dust cups are individual parts, typically with separation between other similar parts, dust cups on F connectors in typical constructions may be considered as separate components when applying the exceptions in 6.3.1, not requiring a cumulative effect.
For an in-depth consultation you are encouraged to contact UL, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html.