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For classification of electrical energy sources, is a single fault condition required to be performed on a single Y1 type capacitor connected between primary and secondary (and acting as reinforced insulation)?

Unless otherwise noted, the sub-clause references below are to UL / IEC 62368-1 Ed.3

 

In determining ES class per sub-clause 5.2.1, measurement under a single fault condition is required to be considered.  The single fault condition is defined in sub-clause 3.3.7.9, with specified details in sub-clause B.4 – faulting a reinforced safeguard is excluded, and capacitors complying with IEC 60384-14 and assessed according to 5.5.2 also are excluded from being faulted.

 

In your case of reinforced insulation between primary and secondary, and assuming (as the most likely typical example) an AC mains voltage up to 300 V and overvoltage category II, a single Y1 capacitor is allowed to bridge the reinforced insulation per G.11.3 and the associated table G.12.  Therefore, the Y1 capacitor is considered a reinforced safeguard and, since it also meets the criteria of the 4th dash in B.4.6, a single fault condition need not be applied to the capacitor.

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