Sparking USB cable + blanking monitor?

[watercooled]

For some reason, when I connect my MyBook HDD to my PC and accidentally touch the case (which is bare metal) with the USB connector, a spark forms and the screen goes blank for a second or two. Also I've measured voltage from the USB cable to ground and it fluctuates between + and -0.1v on the readout - bear in mind this is a multimeter so voltages could be higher either way but I don't have an oscilloscope...
So, what could be causing this? I'm assuming a faulty MyBook/power adapter and I'm a little worried about the screen going blank - could this be something to do with the ground reference for the VGA cable being altered so the monitor has to re-sync?
Thanks

[stevie lee]

could be a loose wire inside the mybook.

I take it you have to plug/unplug the power connectror into the MyBook - what i'm thinking has happened is the wires on the back of the connector might have come loose, or worn through with the in/out plugging motion and one is now touching the case causing the spark.

its happened a few times on mobility scooters near the charging socket, could have happened inside the MyBook.

can you take it apart and have a look?

if there are worn through bits, black masking tape - the plasticy type stuff - is reccommended to be wrapped around the wires individually.

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one way to check if you don't want to dismantle the Mybook. Plug the MyBook together with the power adapter, but not into the wall, turn your multimeter to the continuity setting - the one that goes beep when you touch the ends together - touch one end on the MyBook, then the other onto each of the pins on the 3 pin plug. if it goes 'beep' then ones loose inside.

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feel free to ignore what i just typed. happy to help anyhow

[peterb]

There is probably a dodgy ground connection somewhere and there is a PD between the two connectors. Switched mode PSU's tend to be a bit leaky, especially the cheaper ones, but it usually isn't anything to worry about. It is almost certainly a capacitive effect (Which is why the multimeter doesn't pick up much), and if you earth yourself to the PC and then lightly touch the case of the HDD, you may sense something. (I say sense because you don't really feel anything). The HDD PSU is probably double insulated, so the case isn't connected to a true earth, but the PC is.

The screen blanking may well be caused by a brief lack of sync, caused by a fluctuation through the ground connectors, or just brief intereference caused by the flow of current.

I doubt it is anything to worry about.

[watercooled]

I dismantled the MyBook and, despite some marks on the case and some mangled metal tape, I couldn't find anything wrong with it inside. The drive is a WD3200AAJS if anyone wanted to know .
I tried the continuity test between exposed metal (USB port) and the AC side of the transformer and nothing but when I tried the negative side of the DC input to exposed metal there was a connection. I'm not sure if this is normal for double insulated equipment but surely it can't be good since there's obviously a PD between DC negative and ground so like you said a current flows when the USB cable is earthed to my computer.
BTW I've just checked another external HDD and the same is true so I'm guessing it's just an inferior power brick.
Thanks BTW

Edit: I've just tried some more power bricks for comparison and the same is true. I thought it was bad practice to dump power to ground. Wouldn't it be possible for this to trip an RCD in the breaker panel or not because the transformer isolates the supplies?

[stevie lee]

hmm. interesting about the power going to ground. i'll have to get back to you on that.

i'll point one of my co-workshop people at this thread, he might know more.

[watercooled]

Yeah there's definitely something going to ground from DC negative - there are sparks to prove it

[Agent]

Does the enclosure using a floating ground at all?

[watercooled]

If you mean the HDD, it's a MyBook and has a plastic enclosure but there are exposed metal parts such as the USB/1394 sockets which are directly connected to DC negative, so yes. Also a few times when I accidentally touch the socket with the tip of my finger when I'm also touching the connector I notice a tingling in my finger - not enough to be painful but enough to make me let go. If you mean the PC case it has proper ground, which I've tested.

[Agent]

What I suspect is that the PC and the MyBook are using different grounds, and that's causing the spark.

Some enclosed devices are starting to use floating ground more and more as they have the advantage of cutting down EMI leakage from the device.
Sadly this means that when there are devices with different grounds, the potential difference between them can cause issues

[Blastuk]

my very old mp3 player would do this with the usb cable sparking on the pc case.. easy solution, connect the usb cable before the power cable

[watercooled]

Yeah I think I'll do that - it won't prevent the ground loop but should stop the monitor blanking.