In G.11, Capacitors and RC Units, there do not appear to be rules to determine the minimum number of capacitors required, with only "examples" given in G.11.4. Examples are not requirements, so are there other compliance criteria?
More specifically you asked: Is there any Interpretation document or Decision on G.11.4? The wording "examples" should be used only for clarification of already stressed requirements. G.11 gives requirements at subclauses 11.1, 11.2, 11.3, then it gives only examples in 11.4. There is not any hazard-based safety process or algorithm that introduces safety requirements for which the engineer is able to determine the number of Y capacitors without those supplied "examples" inside G11.4. The only requirements on characteristics of capacitors are in Table G.8 and are based on r.m.s working voltage. The examples are without any HBSE justification and are in contrast with the requirement of use of r.m.s. voltage included in Table G.8. In particular 22.214.171.124.2 specifies that short-term conditions and non-repetitive transients are not taken into account. This is again in contrast with pairs of Table G.9 - Table 26 for transient voltages, Table G.10 – Table 27 for peak working voltages and Table G.11 - Table 28 for temporary overvoltages. As required in 126.96.36.199 the test voltage for Y capacitors is to be determined as inside 188.8.131.52. There are not specifications or HBSE procedures that relates such determined test voltage to the topologies supplied by G.9, G.10, G.11 examples. They cannot be used to determine if one construction using Y capacitors is suitable for safeguards. In particular counterexamples against supplied examples cannot be used as reasons of non-conformity.
In response, unfortunately, there is no formal interpretation document for G.11.4 of IEC 62368-1:2014. The only available material is in IEC 62368-1 and IEC TR 62368-2.
However, IEC TC108 agreed that the past examples in Tables G.9 - G.12 in IEC 62368-1:2014 were causing more confusing than being helpful, so therefore G.11.4 (now G.11.3) was simplified in IEC 62368-1:2018.
The current intent is that basic requirements in IEC 60384-14 are to be used for selecting suitable capacitors, supplemented by IEC 62368-1:2018 Table G.12 and its Rules for application of the Table, including Rule 1, which allows one Y2 capacitor used in cases where 2.5 kV is required.
IEC 62368-1:2014, Table G.9, Table G.10, Table G.11 are based on basic principles provided in clause 184.108.40.206 and 220.127.116.11, with additional consideration of Table G.8. As a result, it is required to compare type test impulse test voltage and type test r.m.s test voltage of the capacitor with the test voltage calculated according to 18.104.22.168 method 1, 2 and 3.
For example, assume power system is 240 V ac mains, single phase, overvoltage category II, and a Y2 capacitor is connected between Line and Protective Earth (Basic Insulation).
Based on Table G.8, type test impulse test voltage for Y2 capacitor is 5 kV peak, type test r.m.s. test voltage of Y2 capacitor is 1.5 kV rms.
Based on 22.214.171.124, the test voltage of electric strength is calculated :
a) Method 1 – Required withstand voltage = 2.5 kV. Test voltage 2.5kV dc or peak, > 1.5 kV rms * 1.414, < 2 * 1.5 kV rms * 1.414, thus two Y2 is required.
b) Method 2 – Peak working voltage <= 0.5kV. Test voltage 0.5kV dc or peak, < 1.5 kV rms * 1.414, thus, one Y2 is required.
c) Method 3 – Nominal mains system voltage 240V ac rms. Test voltage 2kV dc or peak, < 1.5 kV rms * 1.414, thus one Y2 is required.
As a result, normally two Y2 capacitors would be required, but revised Rule 1 now allows one Y2 capacitor to be used in such cases where 2.5 kV would be required (per Table G.12 of IEC 62368-1:2018).
The actual electric strength test is the basic primary requirement, and the G.11.4 / Tables G.9 to G.11 is the example application that can be exempted from the basic requirement of the electric strength test. As you pointed out, even if the construction does not complies with the example of Table G.9, the product can still comply with requirement by basic electric strength test.
Please see also previous question “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?”, in which we discussed the changes in Clause G.11 in the third edition.https://62368-ul-solutions.com/engineers/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.-why-the-difference/
The above answer is based upon the current state of the requirements in the published standards, IEC 62368-1:2014 (Ed.2) and 2018 (Ed.3), and reflects discussions within IEC TC108 to this point in time. However, IEC TC108 actively is discussing the need for further improvement of 126.96.36.199 and associated Annex G.11, and in fact has identified some areas likely needing more work. Therefore, we anticipate further revision in the next amendment or edition. Please also consider that IEC TC108 remains open for suggestions on how to further improve the standard. Therefore, if any industry member believes additional improvement is necessary, we suggest working through your National Committee for IEC TC108 to provide your proposals and technical rationale, thus contributing to the process.