Typical "wall wart" power supply, the LV cord had become damaged where it exits the grommet, resulting in a short. There is no fusing on either side of the transformer, not even an overtemperature cut-out on the primary.
In this particular case the primary burned open circuit before any more damage could be done, but the xfmr was very hot.
This particular unit was plugged into a power strip and thus had 13A protection at least, but this type of poorly designed unit could just as easily be plugged directly into an outlet on a ring circuit protected at 32 amps.
This wall-wart may be made with fuseable magnet wire (dictionary spelling would be fusible, but I see fuseable more often). In the state the supply is in right now, following the "output loading" test that was accidently performed, it should still pass a 3000 VAC hipot test from primary to secondary (2000 VAC for 120V equipment). If not, then someone messed up. Are you sure there isn't a thermal fuse up against the primary winding under the first two layers of tape?
Paul: thanks for the additional citations. I think I'll just stick with good old American radials (sounds like a tyre commercial!) I have no idea if my sparky's Navy education included UK wiring; I would guess not!
Paul, I note on the case, the brand of US Robotics, is this a US made product designed for the UK market? Second question, How hard was it to open the case?, I remember posting a thread about these devices some time ago and the problems I had with actually getting the plastic case open to investigate what had happened to the power supply unit on my cordless drill charger. I note that yours actually has screws in it, not the common plastic welding.
is this a US made product designed for the UK market?
It's the P.S.U. for a 3Com/U.S. Robotics external modem, about 8 or 9 years old. This appears to be a specific international version of the modem different from the North American version -- Only has CE etc. approval marks not UL/CSA/FCC, documentation and onboard help systems don't include American/Canadian options, and so on. Says Made in Hungary on the sticker.
The P.S.U. is the one specifically for the U.K., Irish Republic, Malta, etc. No clue as to its country of manufacture.
How hard was it to open the case?
Not hard at all after I put a large screwdriver between the sections and just snapped the threads out!
Yes, it was screwed together, but with screws having a peculiar head for which I had no tools. Why can't they just use regularly slotted or Philips on something like this?