In the Table G.9 of IEC 62368-1:2014(Ed.2), two Y2 capacitors are required for B/S insulation for 300 V OVC II, which is not in line with Table 1C of IEC 60950-1. What is the rationale for this difference?
More specifically you asked: According to the Table G.9, for OVC II circuit with mains supply voltage of 300Vrms, two Y2 capacitors are required in series for bridging basic or supplementary insulation in the circuit. This is not in line with Table 1C of IEC 60950-1 and also with current practice, where one Y2 capacitor between primary circuit and PE is enough. Why is that the requirement?
This issue has been discussed significantly in IEC TC108. As a result, in IEC 62368-1:2018 (Ed.3), one Y2 capacitor now may be used in cases where 2.5 kV of electric strength is required (per 22.214.171.124). (Refer to the Rule 1 in Table G.12 of IEC 62368-1:2018(Ed.3)) The rationale for this change is below.
As a base requirement in 126.96.36.199, it specifies that capacitors shall pass the electric strength test of 188.8.131.52, taking into account the total working voltage across the capacitors. Compliance can be determined by electric strength test per 184.108.40.206, or by pre-selection of appropriate capacitors certified to IEC 60384-14 according to the classifications/ratings per IEC 60384-14. (Note, in IEC 62368-1:2018, G.11.4 and previous Tables G.9, G.10, G.11 and G.12 have been removed as they were causing confusion when compared to the base information in IEC 60384-14. Now there only remains Table G.12 (formerly Table G.8 in IEC 62368-1:2014.))
In the pre-selection scenario, it was noted that the type test r.m.s. test voltage (1.5 kV RMS) of Y2 capacitor given in Table G.8 (62368-1:2014) is less than the required withstand voltage per 220.127.116.11 in the application where 2.5 kV peak is required (1.5k x 1.41 < 2.5k), while IEC 60950-1, Table 1C referred to Table 5B for the type test voltage for which the Y2 capacitor had to comply. It is our observation that this is why two Y2 capacitors were initially indicated in Table G.9 (62368-1:2014, row covering up to 300 Vrms mains, OVC II, Basic insulation) for the application mentioned above, while Table 1D in the legacy 60950-1 indicated “one” Y2 capacitor for the same row.
In the development of IEC 62368-1 Ed. 3, in addition to the deletion of G.11.4 and the associated tables, Table G.12 (Table G.8 in Ed. 2) was revised to indicate an exception to the pre-selection requirements in the table, i.e., one Y2 capacitor may be used in cases where 2.5 kV ES is required. Thus in IEC 62368-1:2018 the requirement for Y2 capacitors is consistent with the legacy 60950-1 standard.