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When AV / ICT equipment utilizes an a.c. rated fuse in a d.c. circuit, what level of investigation is required to determine if an a.c. rated fuse is acceptable for use in a d.c. circuit?

The preferred position regarding any fuse relied upon for safety is that the fuse be suitably rated for the application. Therefore, manufacturers are encouraged to use d.c. rated fuses in d.c. circuits.


However, components may be used outside of their ratings if it can be determined that the use of a component outside of its rating does not introduce a hazard during the operation or servicing of the equipment, i.e., application of concepts per the 4th dashed paragraph of sub-clause 4.1.2 of IEC 62368-1:2018 and UL 62368-1:2019 (Edition No. 3).


To acknowledge the difference in nature between a.c. and d.c. circuits, a.c.-only rated fuses used in d.c. circuits should demonstrate their capability for use in d.c. applications by means of performance-based type test program (which also may include a periodic follow-up test program).  


(These considerations are documented in a previous UL Practical Application Guideline (PAG), 2.7-2, AC rated fuses used in DC circuits, which was associated with UL / IEC 60950-1, but which the principles still remain valid.)


In the PAG an exception is allowed for fuses that are UL Listed / Recognized for 120 V a.c. minimum and that are used in circuits having a maximum 36 V d.c.  Based on published literature and empirical research performed by UL SMEs in the fuse industry sector, for d.c. circuits with a nominal open circuit voltage of 36 V d.c. maximum, d.c. arcing is not  considered sustainable at these voltages. Therefore, a.c. fuses rated minimum 120 V a.c. are found capable of performing the intended protection of these circuits safely and consistently.


In the IECEE CB Scheme for IEC 62368-1, accepting NCBs may have additional requirements for acceptance and evaluation. Therefore, based on the specific construction, there may be differing approaches for compliance with IEC 62368-1 versus a certification program like UL Mark, GS Mark etc. 


As there appears to be a specific, detailed construction that needs review / analysis, 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 .  Via this consultation we can review the specific details of use of a a.c. rated fuse in a d.c. circuit in your product / construction, and discuss the potential additional test and follow-up service requirements, if any.


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