On 7/24/2014 11:35 AM, Baron wrote:
aeio prodded the keyboard

I noticed a tingling from contact with a USB lead of an external
Hard Drive.

Investigating this led to the PSU, the plug of which has a high
voltage on the outer connector compared with house earth. It was
varying between 70 and 110V.

(I did not check the voltage of the output which I have no reason to
doubt was correct - 12V I think it should be)

The PSU is a three pin plug. Presumably the earth connection is not
being used.

Is the PSU faulty?

Check the outer shell to earth pin with an ohmeter.

I'd expect that the output is nominally floating with respect
to the AC input port.
There usually are caps between line side and DC for EMI reasons.

I had a computer supply with a bad cap in the 5V standby supply.
Caused the GFI to trip.
Never did figure out the relationship between the cap
and the symptom, but the cap fixed it.