Hi All,
Thanks for the replies. The 3.3V rails are surely ELV and hence need functional isolation in between them but what I'm concerned about is that the clearances are calculated considering the overvoltage category. The circuit is a series circuit with the mains supply and although there a the 3.3V shunt regulation, I think I anyway have to consider the 3.3V rail as being on an overvoltage category II section (Example, in the EN 61010 standard, when a secondary voltage is concerned, taking the output from an isolating transform is seen differently from when taking the output from an (example) buck regulator since that the possible transient seen on the output due to mains overvoltage is different and hence, clearances are listed differently in the standard (Table 6 vs Table K.17)). If this is so and the 3.3V are to be considered on an over voltage category II, Table 2K indicates a clearance of 1.0mm for a function isolation of <71V. Having said that, I did see what two other competitors have done and I've seen that they actually placed a polygon pour throughout the whole PCB which is connected to mains Live and only had a clearance of 0.5mm to everything. But the PCBs are not really from reputable companies and I don't really like the idea off having 0.5mm only between Live in and Live out lines. As I see it, although they are both Live lines, when the load is OFF, the whole 230VAC mains will be dropped across the line switch and hence I must keep the appropriate functional isolation (from table 2K results in 1.5mm at 420V peak). Am I right?
As what T3sl4co1l pointed out, some components don't even have the mentioned isolation but here is how it become confusing. The commonly used BTA16 triac is rated at 600Vpeak with non-repetitive peak voltage of 800V. Considering the 600Vpeak, the clearance between the pins should be 3mm for functional insulation but the leaded version has a pitch of 2.4mm which makes it impossible to maintain the required clearance on the PCB (even if operated at mains voltage, its still not possible to maintain the 1.5mm clearance between the pads on the PCB). Yet this component passes UL1557.
Now basically I have two question remaining:
1. Figure 2H shows the different isolations to maintain between different circuits (I think this is the diagram Phoenix refereed to). The first line shows what to maintain between Primary circuit and similar circuit or conductive part. Does the 'similar circuit or conductive part' means between primary circuits irrespective of the voltage difference in the sense that, between my Live-in line and my Live-out line I can keep minimum of 1.00mm functional isolation and not the 1.5mm as listed for a voltage difference of 420V? (this would agree with the competitor's PCBs I've seen although they maintained 0.5mm and not 1mm).
2. Should I consider the 3.3V rail as also being connected to an Over voltage category II section? If so, the consequences would be that table 2K requires a clearance of 1.0mm while if not (due to the input MOV for example), the minimum of 0.2mm can be maintained. (Note , table 2M gives clearance for secondary circuits and places the burden on the reader as to which over voltage section to apply considering the measured secondary transient over voltages. But a secondary circuit is defined as one derived from an isolating transformer and hence does not apply here).
Edit: On thinking regarding the clearances, I came to realize that being a serially connected product having Live-in and Live-out, if an over voltage is to happen and an arc/short is produced, this would not happen directly between Live and Neutral as in standard mains connected products but will happen between the same Live wire, resulting in the voltage being developed across the load (which would have its own safety isolation clearance). Could this be and accepted argument and hence being the reason why competitors left minimal clearance between the Live-in and Live-out?
Thanks