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I have an external hard drive with its own power. When I touch the usb cable against the case of my Mac I get little sparks. I have tried it on a PC but don't seem to get the same sparks

Why might this be?

Update:

The hard drive is using it's own power, so an earthing fault seems possible, except that it happens with an external hard drive and Mac Mini at home, and a different external hard drive and 2 separate Mac Pros at work , so an identical fault seems less likely

Update2:

A voltmeter shows a voltage difference, occasionally over 20v but it disappears too quickly to be sure.

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Could it simply be because the mac is all-metal and the PC is probably partly plastic? – Console Jun 3 '10 at 11:25
Same happens with me with IBM Lenovo T500 and a Chieftec BlackBox external HDD. Don't ask me why. :/ – Shiki Jun 3 '10 at 14:55
@Console .... No it could not, metal does not generate electricity by itself. Actually it would be more likely that plastic would build up a static charge ... metal will not, since it is highly conductive. – Casey Jun 4 '10 at 6:03
@Casey: That metal is conductive was kind of my point...if there is a difference in grounding or feedback from the powered drive you wouldn't get sparks to a plastic chassis, but you would to a metal one. Still, just a guess. – Console Jun 4 '10 at 14:11
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3 Answers

up vote 3 down vote

It could be because one or the other is improperly grounded. This could be serious and/or dangerous, so be careful. It could also be harmless. Given an apple product against some random external hard drive enclosure, I'd suspect the enclosure, but you should probably get it checked out.

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If your USB device is not smoking, then it is very safe to assume that the spark is harmless. – Casey Jun 3 '10 at 12:37
@Casey Normally I would agree, but he said that the external hard drive had its own power, which most likely means it has an external adapter which plugs into a wall outlet. If there's a grounding fault, there is potential for this to be quite dangerous, and I'd much rather he err on the side of caution. You don't toy with electricity. – Darth Android Jun 3 '10 at 13:04
I have the same behaviour with an USB hub with external power. I use it for several months and so far I haven't observed any negative effects. So I'm believing it is the harmless and not the dangerous case... – ashcatch Jun 3 '10 at 14:00
@ashcatch He can always use a voltmeter and ammeter to see what levels we're talking about here, and hope that they don't change. – Darth Android Jun 3 '10 at 14:21
There is an easy way to prove if this answer is wrong. Connect up a USB cable and then touch it to a grounded surface, if there is no spark, then the device is properly grounded. – Casey Jun 4 '10 at 6:01
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up vote 2 down vote

My guess is because Apple USB ports can source double the current than PCs (1100mA vs 500mA). There is more info about it here: http://support.apple.com/kb/HT4049

On some newer Intel-based Macs, such as the MacBook (13-inch, Late 2007), when a device requiring more than 5V and 500mA is connected, the port with that device connected to it becomes a high-powered port capable of offering up to 1100 mA at 5 V. That port will continue to operate as a high-powered port until the device is removed.

Don't worry about the spark, it is not issue, since the Mac USB ports will not let more current through it than they can handle. Even if you shorted the 5V line directly to ground.

The physical property that causes a spark is the capacitance on the power terminals of your USB device. At 0V (uncharged) all capacitors behave as shorts (and draw high current). Once the capacitor is charged, it can treated like a open connection, and current will stop flowing to ground.

The benefit, is if there is a drop in voltage from the USB port, the capacitor will begin providing power back into your USB device. This cause the voltage that the USB device "sees", to be cleaner than what is possibly coming out of the PC.

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Same happens on Lenovo. Any thought on that? – Shiki Jun 3 '10 at 14:56
This explanation seems most likely, but note that it is the hd usb port, not the Mac, so the extra power would not be a factor – David Jun 3 '10 at 14:57
@david, so the cable is plugged into the Mac and the sparks come from the HD port, or vice-versa? Basically doesn't matter which side the cable is plugged into, since all you need to do is complete the circuit to get the spark – Casey Jun 3 '10 at 19:40
@casey The cable is plugged into the HD port, and sparks when touched to the Mac case. I agree with your explanation, but the extra power of the Mac USB can't be a factor – David Jun 4 '10 at 9:43
Although it does occur to me that the power terminals are not being connected, the 'ground' part of the usb connected to the HD is touching the case of the Mac, which is presumably also grounded – David Jun 4 '10 at 9:47
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up vote 0 down vote

This sounds like Static Electricity and I would stop fooling around with it. It's easily possible that it fries your HDs controller or causes other problems, because as you've seen it's possible that the voltage rises above several hundred Volts or even worse (you need ~1000V to bridge 1mm of air).

As Darth Android said, this is most likely a grounding issue. Please see the Wiki-Article for further information.

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You can't get static by rubbing a 0v connector against a metal case – David Jun 7 '10 at 11:08
@david.sykes: If I remember right, static is nothing more then charge difference between objects (pretty similar to a capacitor). So it doesn't matter if it is a powered cable or not, the difference tries to clear itself. – Bobby Jun 7 '10 at 11:48
@david.sykes: Oh, and he never set that he 'rubbed' it at the case, which would of course not generate any static charge. – Bobby Jun 7 '10 at 11:55
A metal connector is coming in contact with a metal case, and repeatedly generating sparks each time it touches. This is not static electricity as I understand it – David Jun 11 '10 at 7:14
@david.sykes: Oh, repeatedly...that changes some things. In that case I'm dropping out, sorry. I mean, I still see a static electricity/grounding problem here, because I can't imagine another cause. As you said, it's a computer case and a 0V connector, so there shouldn't be any voltage at all. – Bobby Jun 11 '10 at 9:24
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