Electricity leakage in external hdd case

[deviant-gv]

Hi,
I bought the following external HDD case.
http://www.ms-tech.de/eng/index.php/product/detail/pdcid/36/pdid/206
The case uses AC to 12V DC adapter.

When I use it, I feel in the surface , static electricity, not a spark but something like friction.
The same feeling is on the metallic surface of my laptop, when the HDD case is plugged in through USB3 interface.

I think about this issue, as i wouldn't like having a burned HDD or much worse a burned laptop....
What's your suggestions?

 

[Phillip Corcoran]

Buy a different enclosure for the hard drive, not the same brand as the current one.
I've never heard of that brand, all the enclosures I've ever bought (all well known brands) never caused that problem.

 

[fzabkar]

If the laptop's mains plug or the external enclosure's AC adapter have no earth pin, then the negative terminal often rises to half mains potential. Therefore the voltage could be 120VAC in 240VAC locales, or 60VAC in 120VAC areas. This is not dangerous, as the impedance is very high. You'll just feel a tingle.

In short, I would measure the voltages at the adapter's power plug on the DC side referenced to mains earth, and check the building's grounding.

 

[deviant-gv]

I think that neither laptop's AC adapter nor external enclosure' AC adapter have an earth pin...

 

[fzabkar]

That's probably why you feel the tingle.

Here is a typical ATX PSU design:

http://www.pavouk.org/hw/en_atxps.html

Notice the two 4n7 capacitors (C2 and C3) near the AC input at the top left corner of the schematic. Imagine what would happen if the SHIELD were not grounded. The junction of the two capacitors would float to a potential equivalent to half the mains voltage (ie half way between the Live and Neutral terminals, L & N). The impedance if each capacitor at 50Hz is about 700Kohm, so the max current draw would be about 0.35 mA from 240VAC.