RE: [OT] USB voltages and grounds

["Keith Doxey" <ukha@xxxxxxxxxxx>]

Hi Paul

> I've caused Windows to hang a number of times when plugging in
> USB cables and had the spark :(
>
> Is there nothing else that can be done? Seems pretty poor design?
>
> Some of my kit is powered from sockets on opposite sides of the
> room (some through UPS's, filtered power strips etc - quite a
> mess of 50+ plugs and transformers!!!). Will this contribute to
> the problem?
>

It shouldnt matter because all your sockets are on the same ring main so
have the same earth. The problem is caused by floating Switch Mode power
supplies which are double insulated which means they dont have an earth
connection so any leakage just appears on the output. They should be easy
to
spot as they will have a logo with a square inside a square and if they are
wall wart types will almost certainly have a plastic earth pin.

Thinking about this a bit more, if you have several such items linked
together the leakage will be accumulative to the point where it can give
you
nasty nip, if you were then to try connecting this to something grounded
the
spark would be more intense.

The only possible solution I can think of is to have all the USB
connections
permanently made, or use a USB switch box which will at least maintain the
grounding of the shields even the the device is effectively
"unplugged"

One other thing you can try.... because the PC is grounded it is almost
certainly the kit you are connecting that is floating. Perhaps you could
connect up the USB connection between Kit and PC with the KIT DISCONNECTED
FROM MAINS. Once the two items are bonded together reconnect the mains to
the piece of kit and power it up. Inconvienient I know but I cant think of
any other alternative other than bonding the chassis of each item together
prior to connecting them up by USB

Keith




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