Determination of a Resistive PIS is described differently in Ed. 2 versus Ed. 3 of IEC 62368-1. Are they different test methods? How does one actually measure the power dissipated by a component or any other location?
More specifically you asked: How to understand the Resistive PIS exactly, because it says "dissipate power more than 15W" in the 62368-1 3rd edition while it state "have an available power exceeds 15W" in 2nd edition. My questions: 1) does it mean different test methods per 2nd and 3rd edition? 2) how to measure the dissipation power under normal and single fault conditions? 3) The definition says "location" while it says "any part in PS2 or PS3" in clause 126.96.36.199, so can we regard it as any two points in the PS2 or PS3 circuits or only consider of components like resistor, capacitors, inductors etc.?
In IEC 62368-1:2014 (Ed.2), sub-clause 188.8.131.52 specified the criteria to determine a Resistive PIS. Specifically, under a single fault condition the requirement inadvertently focused on the available power from the power source to the part rather than the power dissipated by the part in question. However, this Ed 2 criteria likely could result in the conclusion that every component in a PS2/PS3 circuit was a Resistive PIS regardless of whether an individual component actually dissipated excessive power that could cause overheating. (The definition of a Resistive PIS in 184.108.40.206 includes the statement “where a component may ignite due to excessive power dissipation.”)
In fact, IEC TC108 HBSDT discussed whether the criteria in 220.127.116.11 of Ed. 2 were appropriate in determining resistive PIS as intended by its definition per 18.104.22.168. IEC TC108 concluded the 22.214.171.124 criteria in Ed 2 was too onerous. Therefore, in the latest Edition No. 3 the criteria under single fault conditions have been revised to focus on the power dissipated by a part instead of the available power from the power source, which now is more aligned with the definition of Resistive PIS per 126.96.36.199.
In this public Q & A forum, and without an actual circuit and construction, it would be difficult to describe in great detail measuring methods of power dissipation under single fault conditions. Therefore, we recommend you contact UL for an in-depth engineering consultation based on your specific product details and information.
However, in general, power dissipated by a component, such as resistor, will be measured by first measuring current through the component and the voltage across the component while the equipment is in normal operation, followed by, while using the same measuring set-up, initiating any single fault condition that may increase the current or voltage through the same component. Applicable single fault modes are considered based on circuit analysis.
We note the term “location” is used in 188.8.131.52 since a PIS does not necessarily need to be a common component, although typically it is.