the macmini has no earth ground in the power cable. Were do you think the static electricity go ?
All of us are familiar with walking across a rug, reaching out to touch a doorknob, and getting zapped by a charge of static electricity, what's technically known as electrostatic discharge, or ESD. For most of us, it's annoying; for some, dangerous (fireworks and explosive makers have to take special precautions to avoid static sparks); and for the sensitive electronics inside a PC, static can be a computer-killer.
For most day-to-day PC use, static isn't much of a problem, but the chances of problems go way up if you pop open your computer's case to add RAM, upgrade your CPU or hard drive, or plug in a new sound card or graphics.
A little background: Static electricity is much more common than you might think, and most of it is created by a process called triboelectrification, when two materials touch (your fingers and your PC keyboard, for example) and then move apart or rub. Electrons are exchanged, and one object becomes electrically positive; the other electrically negative. When you touch another object with an opposite charge, or a ground (neutral charge), electrons flow.
The amount of voltage involved in static electricity sounds impressive. Walk across that rug and touch a grounded metal object, and the voltage can be in the 10,000-to-12,000-volt range. (If you think back to your high school physics class, you'll remember why static voltage isn't life threatening. Its amperage is miniscule. And it's amps, not volts, which are dangerous.)
There are many variables involved in how much static voltage triboelectrification creates, including the materials involved and the humidity. Low humidity causes static shocks with more kick. But for PC upgrades, the important thing to remember is that while a static shock must be 3500 to 4000 volts before you can feel it, it's the voltage below that level that is common, and insidious. It's entirely possible that you'll open up your PC, plug in an add-in card or some RAM, never have any sensation of static, and still have zapped the electronics. That's because the integrated circuits can be damaged or destroyed by static voltages as low as 400 volts.
What's worse is that the component you installed may appear to be fine, but days, weeks, or months later your PC may lock up or start acting strangely. A dead board or RAM module is easy to diagnose if it doesn't work immediately after you install it, but low-voltage static charges can also cause latent damage, destroying a few gates out of the millions in a typical integrated circuit. That damage can be almost impossible to diagnose, and may not cause problems for a long time. It's also quite possible that you might never take precautions to avoid static and yet never lose a component to static damage.
To avoid the ravages of static damage, your body and the components you're working with (add-in card, RAM, PC case, and so on) must be at the same electrical potential. And the easiest way to do this is to make sure that all static charges are drained to ground, an object connected to the Earth, which can harmlessly absorb the static charge. Until recently, that wasn't difficult. Since all standard AC wiring includes a common ground, you used to be able to ground yourself by touching the case of your PC while it was switched off but still plugged into the wall outlet. However, since today's PCs have voltage flowing through their motherboards whenever they're plugged in (5 volts direct current are used for switching the PC on and off), it's all too easy to accidentally short something and zap your motherboard, without static being involved at all. That's why it's essential that PCs be unplugged when you work with them.
If you're daring and careful, you can still keep yourself and the computer at the same electrical potential by constantly touching the case while installing an upgrade, but it's a juggling act. And if you accidentally touch something with a different electrical potential--such as the tabletop the computer is sitting on--all bets are off.
Using a ground is still the most effective way to minimize the potential damaging effects of static electricity, although you'll need to invest in some additional components Here's how to do it: