tlfong01 wrote: ↑Wed Jan 09, 2019 2:52 pm

*Ah, thank you. You said the thing I did not know how to express. Now let me summarize.*

1. There are two places you need to fit a fuse: (a) the 220VAC mains plug, let us call it the "mains fuse", (b) the application end, in my case, output of the 12VDC PSU. Let us call it the "application fuse".

2. I was going to use the common sense "power should be the same both sides law " to find if the application fuse is 2A, say, then the mains fuse should be 2A * (12 / 220) ~= 0.05A = 50mA

3. So I will fit a application fuse of 50mA (or the nearest I can find, perhaps 100mA).

...

Actually, there are three places.(a) mains fuse, correct. (b) INPUT to the 12VDC PSU "appliance fuse". (c) OUTPUT from the 12VDC PSU "12V fuse".

a) We have already determined this protects the external supply cable, rated 6A, so 3A is the most standard, but 1A, 2A, 5A are also suitable.

b) This protects the appliance itself from excessive damage (fire) under fault conditions, because the mains fuse will not adequately protect the internal mains wiring if it is 26AWG. This is also a mains fuse, 2A maximum. The minimum will be determined by the required 12V output current (translated by constant power calculation), plus some for losses, plus an "anti-surge" allowance for start-up conditions. So for 2A output at 12V, 100mA minimum (dodgy maths if you got 50mA). But a quick-blow 100mA fuse will probably blow every time you switch on. So 150mA Antisurge, or a 2A Quick blow will probably work (check "maximum inrush" current and time for given PSU).

c) Determined by the current carrying capability of the OUTPUT conductors and the PSU maximum current output. If the cables can safely carry more current than the supply can deliver, this fuse is probably unnecessary (Full answer depends on the failure modes of the PSU. Yuck.)