My understanding is that laptops use a very high voltage power source for the LCD back-light, and that this can induce a current in other metal components in the vicinity. Happens on my Thinkpad quite often :)
This is most likely a leakage current issue. As part of the UL certification process there is a requirement for a maximum leakage current when the power supply is ungrounded.
The test is performed with all varieties of power outlet mistakes (hot and neutral reversed, only hot or only neutral connected, only one connected and reversed). It sounds like perhaps Dell switched power supplies and "forgot" to re-check with UL. I'm not sure what the leakage specification is for ITE equipment, but 3 mA can give you a pretty good jolt. I think around 8 mA it starts to get lethal.
The screws are probably signal-grounded and if you're touching something that's earth-grounded you provide the path for the return current. This is why getting a three-prong plug solves the issue. It could also be related to your house wiring/operating voltage/operating frequency.
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My understanding is that laptops use a very high voltage power source for the LCD back-light, and that this can induce a current in other metal components in the vicinity. Happens on my Thinkpad quite often :)
This is most likely a leakage current issue. As part of the UL certification process there is a requirement for a maximum leakage current when the power supply is ungrounded.
The test is performed with all varieties of power outlet mistakes (hot and neutral reversed, only hot or only neutral connected, only one connected and reversed). It sounds like perhaps Dell switched power supplies and "forgot" to re-check with UL. I'm not sure what the leakage specification is for ITE equipment, but 3 mA can give you a pretty good jolt. I think around 8 mA it starts to get lethal.
The screws are probably signal-grounded and if you're touching something that's earth-grounded you provide the path for the return current. This is why getting a three-prong plug solves the issue. It could also be related to your house wiring/operating voltage/operating frequency.