For determining Clearances, is it allowed to reduce mains transients by using either external or internal surge protection devices?
More specifically you asked: I have a question concerning clearance requirement: 18.104.22.168.2.2, Determining AC mains transient voltages: “In general, clearances in equipment intended to be connected to the AC mains, shall be designed for Overvoltage Category II. Equipment that is likely, when installed, to be subjected to transient voltages that exceed those for its design overvoltage category requires additional transient voltage protection to be provided external to the equipment”. Question: Whether the sentence “shall be designed for Overvoltage Category II” is compelling despite the fact that the sentence begins with the words in general? Is it allowed to use lower mains transient value to reduce the clearance by using external surge protection module in mains supply side? How about internal surge protection?
In response, although it is unclear from your question whether the equipment subjected to the enquiry is equipment installed indoors or outdoors, it is noted that requirements in IEC 62368-1:2018 now consider both indoor and outdoor applications, whereas in IEC 62368-1:2014 it only covered indoor applications (with outdoor applications covered by IEC 60950-22). The outdoor locations requirements from IEC 60950-22 have been incorporated into IEC 62368-1:2018, including those that impact requirements for surge protection devices and consideration of transient voltages.
Sub-clause 22.214.171.124.2.2 in IEC 62368-1:2018 currently is clear that installed equipment likely to be subjected to transient voltages that exceed those for its design overvoltage category (e.g., OV Cat II) requires additional transient voltage protection to be provided “external” to the equipment, in which case, the installation instructions shall state the need for such external protection. Currently, in 126.96.36.199.2.2 IEC 62368-1:2018 does not mention use of such protection inside the equipment, although via an early proposal to IEC TC108 this may be further clarified in the next edition of IEC 62368-1 that such protection may be either inside or outside the equipment.
However, also keep in mind, subclause 188.8.131.52.2.1 states, transient voltages can be determined based on their origin, or can be measured in accordance with 184.108.40.206.2.5. If additional surge suppression to address reduced overvoltage category was provided inside the equipment, it is likely that measurement of actual transient voltages per 220.127.116.11.2.5 would be expected / required since the type of surge suppression installed externally typically is different than those installed internally. However, 18.104.22.168.2.5 also requires that surge suppressors internal to the equipment in circuits connected to the mains to be disconnected, which likely would invalidate the advantages of placing surge suppression inside the equipment for most applications.
IEC TC108 may need to provide some further clarifications on this topic directly in IEC 62368-1 or within IEC TR 62368-2.
As your questions really need to be considered in the context of a specific design, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html .