SMPS connundrum

[KSP]

Hi all

Recently had a unit go bang without tripping the mains RCD so I decided to investigate. Turns out it wasnt the power supply that actually went bang, but the motherboard. What looks like a decoupling capacitor (SMD ceramic) had for some reason gone short and driven the +24v down to earth. Obviously the capacitor was the failure point, smoke, noise, bad smells etc. etc.

I thought I would check the PSU brick to make sure it was still fine. Its a CUI Inc VSBU-120-D524A (5V & 24V OP)
http://www.cui.com/product/resource/vsbu-120-dual.pdf

Upon plugging it up to the mains and measuring the outputs I was getting my 5V but not my 24V. Checking the unit I removed it from I saw that the input connector to the board had live and neutral switched. I tried connecting it like this and like magic I get my 5V and 24V. Why is this? I can't get my head around this at all.

The datasheet states the input connectors pins 1, 2, 3 should be live, neutral, earth respectively. So why does it not work properly unless live and neutral are reversed? And why does only the 24V drop and not the 5V?
And finally, could this reversal be the reason that the current surge was not picked up by the RCD?

  

[SeanB]

Line and neutral are interchangeable, especially as this could be used in a country where the socket polarity is not guaranteed as to which is live or neutral, so both have to have the same robust insulation with respect to the protective earth.

The 24V not showing at all times could be due to a protective device operating, or a cracked solder joint or something like that. You might want to check the secondary side for failure in the 24V side, something there is broken.