I recently bought a "Safecom" external hard drive housing, with USB & Firewire interfaces: 
http://www.safecom.cn/code/sub/category.asp?prdid=113&subcatid=12

I added a drive, connected it all up, and started formatting it.  After about three minutes there was a "Pop!" 
and it shut down.  I found the plugtop fuse had blown, and replaced it with another - no joy, the PSU was 
dead.

I took the whole thing back and had it swapped for another, and when I was about to plug things together, I 
had the PSU plugged in and switched on, but the drive-case power switch was off, and as I held the USB cable 
from the PC in one hand and the drive case in the other, I got a nasty tingle.  Not enough to hurt, but 
certainly uncomfortable.  I got out my trusty DMM and measured between the case and the USB cable:  95V AC!  I 
took the disk drive out of the case for safety and measured between the body of the case and mains Earth, and 
again there was between 90 and 104VAC.  I haven't measured the current-supplying capability of this "fault", 
but I may do that when I've sent this.

I assume there's a design fault with this PSU (which has all the approval markings, CE, UL, FCC and others) 
and I imagine it was enough to "blow" the first one, and as I didn't complete the connections, it hasn't had a 
chance to do so to this one.  I'm a little surprised that the RCD supplying the mains didn't trip, but I 
suppose if the short to Earth is on the output side, is isn't seen as a Phase/Neutral imbalance, and the fault 
mechanism could have been failure of a component in the PSU which then caused an overcurrent that blew the 
fuse and perhaps itself.

This is a "universal" supply, which accepts 100-240VAC, 50-60Hz, obviously an SMPS and is a small sealed block 
with an IEC 320 (Kettle Lead) socket for the input.  It puts out 5V and 12V at 3A each, via a mini-DIN 6-pin 
plug, with 2 pins for each of 0V, 5V, 12V.  The shell of this plug is isolated.  I tested the mains cable by 
plugging it into a mains circuit tester, and the connections are all present and correct (Earth Loop Impedance 
showed 0.7 ohm - not great but then it is through an extension lead).

So what do people think?  Any ideas what might have caused this apparent high-impedance connection to the live 
mains from the output 0V line?  Is it likely to be a design or manufacturing fault, given the 2:0 score so 
far? :-)

I suppose I'm left with taking it back for a refund, and trying to find another make that has a decent PSU 
with it...

Cheers,


Howard Winter
St.Albans, England


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