acdc coverter issue

[acdc]

Hi
Please would you help sort out an ac-dc converter issue that I cannot solve.
I have a ac-dc converter that powers a external hard drive.
I noticed a spark from the out-side of the usb cable when I touched it against the metal casing of my desktop computer when I went to plug it in (external drive into desktop).
The metal desktop casing is earthed so this meant that some power is passing through the metal end of the usb cable - not the pins inside the usb cable end but the out side metal end housing.
When I touched this metal end of the usb cable and the metal desktop computer case I got a slight tingle.
When I tested it with a voltmeter I found that it was 2.5 volt ac coming from that metal usb casing end.
when I traced it back I found that this 2.5 volt ac was originating from the negative side of the external drives ac-dc converter and passed through the external hard drive then out and down the usb cable.
I cant find anyone who knows if this is correct I feel that -----
There should not be any power from the negative out-put of an ac-dc converter and certainly not any ac power.
I also feel that there should not be any power of any type in the metal usb connection housing.
Please inform me
Thanks
acdc

 

[jimmc]

Quite normal for a switchmode supply I'm afraid, see this thread for an explanation...
https://www.eevblog.com/forum/index.php?topic=168.msg1511#msg1511

Jim